What source do you have for this assertion? I have not thuroughly researched the Wilwoods, but they appear to be the same Superlite 6s I have on my Stealth for the fronts, and I'm not sure which 4 pots for the rear? The 6 pots are quite impressive IMHO. Is this just a dust sheild issue, or is there imperical data out there on the Wilwoods?

This is one of the common misconceptions. Getting into ABS is not necessarily fast, and in fact is often fairly slow. Being fast is about balancing forces in a smooth fashion. But that is a whole different discussion. Getting into ABS also does not mean your brakes are operating at their maximum performance, it actually means they have failed.


Warning: Dean's Soapbox mode enabled. Preaching begins in the next paragraph. You have been warned.

And actually, brake selection is lagely about modulation, or the systems ability to create high levels of friction without locking up. I know that sounds contridictory, but it isn't. This is where heat capacity, cooling and pad compound come in. I'll try and explain with a exagerated example.

When a rotor is extrmely hot, it actually deflects under the pressure of the pads. This is bad! So at the leading edge of the pad, a slightly thicker rotor is continiously being compressed. As I understand, this area of interferance is where lockup actually begins, and where if you do not release the lockup the pads can actually fuse/weld themselves to the rotor. If instead, the system as a whole, but mostly the rotor stays cooler and therefore deflects less, minimizing this interferance area, you will see an increase in braking performance. This area is also why in most any fixed caliper multi piston per side configuration, the leading piston will be smaller, allowing the leading edge pressure to be lower than the mid to trailing edge.

Thsi is why I keep emphesising that the diamter of your rotor is not very important, but the width and cooling fins are! Even thermal mass doesn't do you any good if you can't cool it.

Look at the many NASCAR series... As far as I know, they are all running under a 12" rotor.(11.75"x1.25" I believe is most common). Do any of us really think we in our weekend warrior roles can hold a candle to the demands they put on their brakes? Other series run even smaller diameter brakes. If bigger was better, don't you think some series somewhere would run them?

Braking is not about stopping the rotor, it is about stopping the car which whether you believe it or not is done by the tires! Dispating energy without locking up is how you do that. getting into ABS means your brake system has failed, not that it is running at maximum performance. I'm not saying ABS is bad, only that is not a valid measuring point.


Soapbox mode disabled.

I am far from an expert on this, and would love to hear other technical opinions on this becasue there is certianly more I can learn on the subject. I just feel obligated to speak up when I see/hear these types of automotive myths being discussed.

Good soapbox rant Dean!

My point regarding the whole ABS thing wasn't to say that hitting the ABS is the end-all limit of braking. My point was that at some point, considering the tires were using, "better" brakes aren't going to actually help us to stop faster.

Once your brake setup feels good, doesn't fade, and allows you to brake at the limits of your tires, there isn't any reason to upgrade anymore!

OK, I have my instructor hat on now. How do you know you are at the limits of your tires?

In most cases, braking isn't about stopping. It is about slowing. How the brake slows the car under differnt circumstances including but not limited to speed, pedal pressure, temperature, pad depth, etc... Can you describe the ideal brake system? You only refer to feel, and fade as parameters. Can you be a little more specfic?

I'm not at the limits of my tires, which is why I don't need a huge brake kit yet. I just want a stiffer pedal, primarily to allow me to brake harder and still have proper pedal alignment for heel-toe.

Now, if I were able to lock all 4 tires at the same time (or the fronts just slightly before the rears) during straight line braking from high speed, then I'd say that I'm "at the braking limits of the tires". At that point, my bias is just right, so I am using all my available traction to slow the car. And if the pedal feel is such that I'm comfortable and modulation is predictable allowing me to brake just above the locking threshold, and if the brakes don't fade, then I'd say that my brakes couldn't be improved.

Dean,

Cup teams use small diameter rotors because NASCAR mandates a maximum wheel diameter of 15"; their packaging constraints within such a small wheel are what limit them to relatively small braking systems compared to what teams in most road racing series use.

Sorry, you are right, I was thinking of NASCAR only at the time. I do think it is all the NASCAR series, not just the cup, but that isn't really important and I may well be wrong on that count as well.

Most production car based series are limited to a 17" wheel which keeps to 14" rotors, though there is often a weight penalty for using larger rotors. Many open wheel series have 13-15" wheels, so they are on 12s, and smaller. WRC uses about 12" on everything but tarmac where they use whoping 14.4"ers.

I guess my point was we don't push them anywhere as hard as these guys do, and they run for many more miles etc.

:D :D :D Ha, you have fallen into my trap... It was kind of a trick question to get you to think about it. :D :D :D

If all 4 wheels lock, you are at the limits of the brakes, not the tires necesarily. This is where people get confused. If instead of locking, the brakes continued to apply even greater decelerating force without locking the pad to rotor contact, you would not actualy reach the limits of the tires until the tires started to substantially slip while still rotating. Unfortunately, this is often closely followed by brake lockup because the reduced torque from the tire/road loss of friction shifts the balance and the pad/rotor interface locks.

I'm not saying any particular brake system is not capabe of reaching the limits of a given tire prior to locking, only to change focus from the brakes to the tires and get you to think about how limitations in the brake system could be percieved as using all the tire's traction when it may not be. The equations for all of this are way out of my league.

Perhaps an example would help. Pretend you had a set of infinitely strong cross drilled rotors and the calipers have infinitely hard pins instead of pads connected to the pots. These binary brakes would be on or off, applying either zero or infinite friction instantly to the "pad"/rotor interface causing it to clock, this locking of the brake system is what I mean by a failure which prevents the tire from rolling causing the loss of traction at the tire/road interface.

This is why pad materials are so critical and why spreading force over an area instead of a point allows a brake system to apply braking force while still allowing rotation.

The Wilwoods flex like Hanz and Franz under heavy pressure. Causes poor pedal feel. I had only heard this as a rumor until recently, but then a customer came into S-Squared complaining of poor pedal feel. 3 liters of bleeding later, it still felt like crap. Finally, during the last bleed session, Nate noticed the calipers flexing about 15 degrees before full pedal pressure had been reached. These were the 4 pot calipers, btw, which should be actually stiffer than the 6 pots.

15 degrees on what axis?
Which 4 pots? Wilwood makes two or three different 4 pot calipers.

Why do you feel the 6 pots would be less stiff? The 6s are a much larger caliper in every dimension with a different configuration than the 4s.

Also, I am unaware of a Wilwood 4 pot application for the WRX from Wilwood. The 6 pot Billet Superlite front kit from Wilwood is here: http://www.wilwood.com/products/kits/sl6bbhk/index.asp
The rear is a 4 pot kit based on the Billet Dynalite 4 pot caliper. It is only in their PDF file. http://www.wilwood.org/2003kitcatalog.pdf

I think someone else must have built a 4 pot kit using Wilwood parts that you saw or heard about, or they discontinued because I do not see a pert number for it in their stuff.Sounds like somebody threw together something with the wrong parts for the application.

Dean,

I think I have to disagree with some of what you said. If a tire locks up, by definition the caliper has applied more braking torque, and hence more force opposite the direction of tire rotation, than the tire was capable of developing for its given (instantaneous) normal force and coefficient of friction with the road surface. Take a given tire with a given downforce, section of road, and coefficient of friction of 1.0 which locks up at a braking torque of 1000lb*ft. If you strap a different tire on the car with a coefficient of friction of 2.0 on that section of road, and apply the exact same braking torque of 1000lb*ft, the wheel will now not lock up; it will now take 2000lb*ft to lock up the wheel. I think this pretty much defines being deceleration-limited by the tires, not the brakes.

I think maybe what you were trying to say was more concerning how effective a braking system (and operator) is at modulating force near the limit of what the tire can produce?

They opened up like a clamshell. 15 degrees is just me typing before thinking- who knows if it was more or less than that, but it is quite noticeable. I said 4 pots are stiffer than 6 pots, typically, based on StopTech's White Paper on caliper build and design. And it may have been the 6 pot kit- my brain is mush lately. :P I believe it was the Willwood-built Perrin kit, but I don't remember for sure. Aaron saw it as well, maybe he can clarify my details.

Yes and no. I did actualy use the word modulation in the first sentance of my soapbox, but...

Take my Pin and Hole example. The brake interface transitions from dynamic to static friction before the tire transitions from rolling/static friction to dynamic of skidding. the tire itself provides the delay between the two events. In all but a "perfect" system one of the two events must happen first, and that is what I am getting at.

An incrediable amount of money goes into just designing pads with different friction charecteristics. Some transition from dynamic to static friction more easily than others at a given load and temperature.

In many ways, ABS is a primitave way to make up for other shortcomings in a brake system after the fact. With a soft enough sidewall, and a fast enough ABS system the tire may never even transition to dynamic friction even though the brakes are swithcing back and forth from static to dynamic as they lock and unlock. The tire never locks even though the brakes do repeatedly.

Do you see what I mean by the brakes failing/transitioning before the tire?

Oh, and I keep forgetting to mention something most people don't realize.

Bigger brakes with more piston area corresponds to a SOFTER, not firmer pedal.

And there is actually a fairly good spiel on scoobymods, though they don't talk about modulation as much as I'd like.

http://www.scoobymods.com/forums/showthread/t-1122.html

Ok, I see what you were getting at now.

Alright Dean...

I'm a little confused by this "trap" I fell into. I understand what you're saying about the brake's transition from dynamic to static friction being able to lock tires that would otherwise still have rolling friction to spare. But if you read my post again:

I mentioned being able to modulate the brakes to stay above locking up the tires. Perhaps I wasn't explict enough, but it was my intention to describe the situation you later fleshed out: brakes that decelerate the car at the maximum dynamic friction of the tires. At that point, your brakes cannot be better... they are not the bottleneck in your braking performance.

Let's look again at your "binary brake" system. That extreme example actually demonstrates that *any* brake that can lock the rotor can be used to brake "at the limits of the tire" provided that they can be modulated fast and accurately enough. As you stated, you can theoretically lock the rotor and unlock it w/o the tire itself ever locking up as long as you unlock the rotor before the tire's sidewalls run out of flex. Using this knowledge I could actually design a computer controlled brake that would lock and unlock thousands of times a second to bring the tire to its limit, and threshold brake without ever locking up a tire. (Although you'd heat the bejesus out of the tires... in fact the brake rotor wouldn't heat up at all... those tires would have to generate all the heat that results from stopping the car!! )


The same goes with regular brakes. Lets say I've got a crappy pad that has a very quick and harsh transition from dynamic to static friction. i.e. the pad would cause the tires to lock at a deceleration rate less than the tires "maximum" rolling friction. You would argue that a "better" pad with a less harsh transition would prevent this from happening, and thus would improve the overall brake system. While I won't disagree, I want to point out that a better driver, or a super-fucking-good ABS is also a solution. If I were to be able to modulate those crappy pads, they'd also allow tire-threshold braking, just as in our example binary brakes.

What's my point? Well, I'm not sure. :lol: Given the choice of harsh transition or smooth transition pads, I'd clearly go with the smooth pads since they make driving at the limit easier, however I'm not exactly sure how to find out which pads those are... it's not like that's labeled on the box. Frankly, I just want brakes that stop well, resist fade, and don't feel like I'm stepping on a wet sponge.

Maybe we're beginning to drastically overcomplicate things. Let's get back to basics for this thread.

Scott wants advice and opinions on what to do with his brakes. Since Scott is a real person, with a real job and real bills and a real life (such as it is :P) he has only so many options. These would seem to include:

subaru 4 pots
Stoptechs
STi Brembos
Wilwoods
Rotora


Of those, I personally would choose either StopTech or the 4 pots, basing the last choice on cost, ultimate functionality, and secondary effects such as wheel concerns. 4 pots require replacing one set of wheels with a set that costs roughly $250. The Stoptechs require a 3rd set of 17s for winter, let's call that $450 for rotas. Stoptechs also cost about twice as much as 4 pots. Both have excellent fade resistance. Both retain good "stopping power." Personally, I think that the performance ceiling of a StopTech kit may be a little higher, but value-wise, 4 pots are king.

OK, now we are talking... I agree with you completely! And rereading, maybe you didn't fall into the trap exactly, but you did go where I wanted you to, the tire to road interface.

Everybody gets so caught up in the brakes they forget about the tire characteristics, and that the tire actually stops the car, not the brakes. As we go to stiffer and stiffer sidewalls to achieve better handling, we take some of the tolerance out of our brake systems. Anything we can do to minimize the spikes in the brake system aid the tire in staying hooked to the pavement at the limit. Have you ever heard tires under ABS? In many cases, they chirp about 8 times per revolution as the brake locks, and releases causing the tire to slip.

More pad area(calipers), better pad compounds(Pads), cooler temperatures(better rotors and/or ducting) and to an extent, more piston area(calipers) all contribute to better brake performance at the limit. OK, large diameter rotors does to, but I hate to admit it. :D

Pads are often the cheapest of the above followed by rotors and finaly calipers. This is why I sugested what I did originally, what you had already done, pads, and rotors. If you have exhausted those options for the most part and are still not happy, the only place to go is calipers. I still think if you can get a used set of JDM/CDM 4 pots cheap, that would be a great option. If they don't meet your needs, they would be easy to unload to someone else with little loss.

Wow, I wish I was involved in this thread a little earlier. I'm going to try and address several points on this page of the thread. I fear it's going to be a little lengthy...

Dean,

The Stoptech kit is only a front upgrade system. The pistons are custom sized to the rotor AND the WRX platform to move brake bias 10% more towards the rear over stock. Stoptech hardly throws bigger parts at anything (unlike many of it's competitors). They design, engineer and extensively test every application before it becomes available for sale. Stoptech uses the same caliper (ST-40) for most of it's applications. However, they machine unique piston sizes for each of the applications. Stoptech has 14 different piston sizes they can utilize to generate optimum brake bias, pedal travel and pad bite characteristics. Not taking piston size into consideration in your analysis ignores one of the more critical factors in a braking system.

BAN SUVS,

Reducing front brake torque is an excellent thing, not a bad thing for the WRX. The WRX comes with too much front brake bias, as do all production cars. Moving some of it rearward helps improve front/rear brake bias, which will reduce stopping distances. With the stock front calipers and rotors, I was able to consistently lock the front brakes with Hoosier R3S03 race tires as well as with front downforce via a splitter. The WRX does not need more front brake torque! There is no such thing as adding more stopping power, unless the original system cannot lock the brakes. What you can do is increase brake torque with less pedal effort. However, when considering a brake system, you do not want a system that generates very high brake torque with very low pedal effort. Systems like that are very hard to modulate at the limit. A well designed system will take some effort to lock the brakes. As long as the effort is not fatiguing, the higher effort provides the driver a wider modulation range, making it easier to use the brakes to keep the tires on the edge of adhesion.

You also mentioned that you have yet to lock the brakes at the track. One of two things is happening. You are using ABS, which prevents the wheels from locking, or you are not braking hard enough. Unless of course you have faded the brake pads and/or fluid, which would make it impossible to lock the wheels after a few laps. But even with R compound tires, you should have no issues with locking the front wheels on the track while the brake pads and fluid are still within their operating temperature range.

Dean,

The Wilwood calipers are known to be some of the most flexible calipers in the aftermarket. See this chart here: http://www.stoptech.com/technical/ca...ctionchart.htm
Company W is Wilwood. The higher the caliper flex, the worse the pedal feel, because the effort you are putting into the pedal is being wasted by flexing the caliper. Very similar to flexible rubber brake lines. Since this is widely known, I feel confident that the flexing described in this thread above is a normal occurance in a Wilwood caliper, not a hobbled together caliper made of Wilwood components.

You also mentioned that getting into ABS is not necessarily fast, and is often fairly slow. This is entirely dependant on the ABS braking system and you will find that in the higher classes of sports cars and performance cars, the ABS systems are excellent and hard to outperform. They react extremely fast and keep the tire rotating at a speed that provides optimal adhesion.

Your soapbox conclusions are mostly inaccurate. The trailing piston is larger to counteract the debris field and slight outgassing that starts developing at the leading edge of the pad and continues along the length of the pad. More force is applied to the rear of the pad to counteract this and provide even pad wear.

The melting point of iron is 2,800F. Brake rotors will glow at 1,200F. Brake rotor iron does not become maleable until it reaches the 1,700F - 1,800F degree range, however, it will undergo a loss of tensile strength at 1,600F, but will not deform. There is a phenominon known as judder associated with straight vaned rotors, where the leading edge of the pad will set up a pressure wave between vanes and will cause slight pedal kickback, but this is cured by using a curved vane rotor, which also improves cooling efficiency.

The majority of OEMs and many aftermarket BBK vendors will use rotor iron with a tensile strength of 18,000psi. This is also known as dampened iron and is used to reduce rotor noise and decrease NVH issues. This is a softer iron. Stoptech, as well as the top racing brake rotor manufacturers use iron with a tensile strength of 40,000psi, which dramatically increases it's strength all the way up to the melting point, in which case both rotors turn into puddles.

Saying that piston size differences in the caliper is designed to eliminate distortion of the rotor is completely inaccurate.

You used NASCAR as proof that larger rotors are not required. This is an unfortunate example, since on many race tracks that NASCAR runs on, they never even use the brakes. The answer to your question is yes, many weekend warriors will dramatically exceed the demands put on the brakes over NASCAR drivers. Other tracks in NASCAR, like Bristol, put a huge toll on the brakes. The cars use a much wider rotor on those tracks, but do not use a bigger diameter rotor because of NASCAR mandated wheel diameter and rotor diameter. Using NASCAR as a technological example is horrible because their rules have locked them into very old technology. The formula for NASCAR is not to produce the fastest, most technological racing in the world. It is designed to produce the closest, most cost effective racing possible.

That is why true road race cars, like the Daytona Prototypes, ALMS Prototypes, Indycars, Grand Am GT and GS and such use huge diameter rotors. These cars are much more technologically advanced than anything you would find in the NASCAR series and they all use big rotors. I'll get into why larger rotors are better at the end of this post.

On to the next point. When all four tires lock simultaneously after gradually increasing brake pressure without any steep transients, you can say that the brake system is perfectly balanced. You mentioned the difference between locking the brakes before reaching the maximum traction of the tires. This is wrong. Your example of an infinitely hard pin example, first of all, doesn't work they way you explain, and secondly is completely irrelavent to how brakes work.

A brake system will apply more and more force to the pads, creating greater and greater friction, slowing the wheel and tire combination more and more. There is never a case where the friction coefficient between the brake pad and the rotor will instantaneously jump to a level to suddenly lock the brakes. Brake systems do not work that way. There is never an instantaneous on-off. There is always a ramp. You cannot weld a pad to a rotor.

The state of a rolling tire versus a locked tire is dependant on the coefficient of friction between the road surface and the tire versus the braking force applied to the rotor based on pedal pressure. It is a linear progression that rolls off as the tire adhesion is exceeded. As a matter of fact, maximum braking occurs when the tire is rotating just slightly slower than road speed.

The characteristics of how quickly the coefficient of friction between the pad and the rotor increases is dependant on pad material. Some pads can come on very quickly, others have a more gentle ramp. NONE of them will lock a wheel prior overcoming the coefficient of friction between the tire and the road.

Furthermore, brake pads are not designed to transition from dynamic to static friction. They are designed to provide a stable coefficient of friction in a given temperature range and pressure range. Quite a bit of effort has gone into the "release" characteristics of a pad, but that is as you are coming off of the brakes, not adding pressure to the locking point. Given a stable braking system regardless of bias, locking of the brakes is a driver error.

You mentioned that bigger brakes with bigger pistons create a softer pedal. This is not true. Larger pistons will create a longer pedal, because you have to displace more fluid before the pads touch the rotor. Once the pads do touch the rotor, the pedal movement is based solely on the flexibility of the system. A very stiff system will not have a softer pedal with larger pistions. Finally, with a larger rotor and larger pistons, it will require less pedal effort to generate the previous brake torque. This is not a softer pedal, it is lighter pedal effort.

In your closing comments, you said that more pad area, better pad compounds, more piston area and hating to admit it, larger rotors contribute to better brake performance. As written, this is inaccurate. What you are saying is that if you increase the size of everything, your braking performance will improve. Absolutely not! What you've done is drastically increase the brake torque, which may not be a good thing for brake bias or pedal feel. You have to take everything into consideration to design an appropriate brake system for an application.

If you increase front rotor diameter, you will need to reduce piston size to prevent too much front brake bias. If you just increase the rotor size front and rear, the original size pistons may be too big, creating a brake pedal that is far too light and difficult to modulate. All of the components that make up a brake system must be carefully analyzed when making changes, and an increase in one parameter may require a decrease in another parameter.

Finally, I want to explain why a larger diameter rotor is better, even though you don't like the idea. A larger diameter rotor requires less clamping force to generate the same brake torque as a smaller rotor, since the torque is applied further away from the center of the wheel. Less clamping force means less heat is generated per square inch over the face of the rotor. Less heat is a good thing. Also, the air speed in the vanes of a larger diameter rotor is faster than a smaller rotor, since the larger rotor is passing through more air per revolution, allowing it to draw heat away from the rotor more quickly. Smaller rotors are built wider to help flow more air, but they are also built with thicker steel, because brake temps will be higher (due to higher clamping forces required to slow the tire) and the additional mass is used to strengthen the rotors to run at the higher temperatures.

One of the major benefits to a larger rotor is the ability to modulate the brakes closer to the edge of the tire's adhesion. This is because there are more rotor inches passing through the caliper. This gives you higher "resolution" with the larger rotor. It makes the brakes easier to modulate, keeping the tire consistently closer to the limit of adhesion.

To your credit, you did mention that the tire stops the car and the tire's characteristics are very important. Somehow along the way, the information provided to you led you astray and you contradicted yourself several times as well as used analogies that do not relate to braking systems.

Please don't be offended by any of my comments. You said yourself you are not a brake expert and can stand to learn a few things. I am not a brake expert either, but 11 years of roadracing experience and working closely with brake manufacturers have provided me with a solid working understanding of brake systems.

Let me know if you have any questions.

Gary
Sheehan Motor Racing
www.teamSMR.com

WHOA. Thanks for such a informative post Gary.

Wow! Thanks for the info Gary.

Also, here is some interesting information (and misinformation) regarding monobloc brake calipers...

http://www.stoptech.com/whitepapers/monobloc3.htm

Gary
Sheehan Motor Racing
www.teamSMR.com

Not offended, a ton of questions and comments and clarrifications that I just finished typing but aparently my session timed out, so I will have to retype it over the weekend since it is gone forever!!! Stupid computers! :)

<OT>

This board sucks for very long posts. If you're working on a long post, make sure to edit in a text editor, then hit reply and paste in your response. I've been burned before too.

</OT, please resume the brake discussion!>

Oh no! Dean, sorry to hear about that. I know how frustrating that is.

As a matter of fact, I was worried about my computer crashing, so I copied to Notepad.exe and saved just in case. I'm glad I did. I wasn't aware that a session could time out on SECCS. I don't think I would have typed all that in again!

I'm looking forward to your questions this weekend.

Gary
Sheehan Motor Racing
www.teamSMR.com

MORE OT:

You can change the session length in the forum admin, General Admin section, then "Configuration." I think the default setting is 3600 seconds. Doubling or tripling it would probably do the trick.

Wow Gary, your expertise is most welcome here. Thanks for clearing some things up! Unfortunately, I think you've just eliminated StopTechs as a possibility for use on my car. Let me explain...

I have a 2001 Impreza RS. I've never faded or locked up my brakes on the track Using FHI 4 pot fronts, OE pads, rotors, and lines with Ate SuperBlue fluid. This is probably because I jsut haven't had to use my brakes that much yet- all of my track experience is at Thunder Hill, where I only have one hard braking point, and that's on the back stretch. I only get up to about 90-95 and brake to 35 or so, and my system is easily able to handle that. On the front straight I only drop from <100 to about 80-85. That track is simply faster than my car. On to why I can't use StopTechs now-

I am doing a full STi 6 speed swap, including the R180 rear end and hubs. Those hubs require the use of either STi 2 pot rears or STi Brembos. I've decided against the Brembos due to cost and limitations on wheels. That leaves me with the old STi 2 pots. Since I have STi 4 pots up front, this seems a natural combination to me (I don't know how closely matched my existing MC and bias valve are to this setup, but it can't be worse than any non-Subaru setup I suppose). I know the pedal will be somewhat softer, but that's the breaks... so to speak. :P Anyways, should my 4 pot setup prove incapable of handling the heat from increased speeds at the track (forgot to mention- I'm also doing a compete motor build. Basically an STi motor with a VF22) I was planning on getting StopTechs. I love the company's attitudes towards the enthusiast community, their openness with their testing and with providing concrete proof of their claims, and heck, I even know someone who works there now. But alas, adding them would destroy my brake system. I would end up with far too MUCH rear bias, and I am nowhere near skilled enough to drive a car that behaves that way. The only way I could run StopTechs at this point would be to have custom piston sizes made.

BAN SUVS,

Well, you're going to love Stoptech even more. I just got off the phone with Bob. They will customize the piston sizes of the front calipers to work perfectly with whatever rear calipers you are going to use. All you have to do is tell them the rear rotor diameter and the rear piston sizes and they will do the rest. Is that service or what?!

They are waiting for your call :)

Gary
Sheehan Motor Racing
www.teamSMR.com

Excellent! Now, if I hadn't already spent upwards of $20k on this car in the past few months, he wouldn't have to wait so long. :P I'm going to see how far 22b takeoffs can get me with good pads and fluid first.

Oh, and this is probably better off in another thread, but you mentioned the R3S03s... have you tried the R3S04s yet? And how long does a set last you typically?

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Edited slightly 2/22 9:00am pacific to fix some typos and unclear language.
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OK, last time I wrote this response, I went down your list, I think I will do it different this time..

I too wish you had been involved in this discussion earlier. We actually agree on most of the things we are discussing, so I will ignore most of those and stick to the questions you have raised in my mind, and the areas we disagree to some extent.

I am trying not to be defensive, or offensive, but hope we can all learn something from this discussion.

First, a couple easy questions before we get into the deep stuff...

I am going to concede the bigger disks are better for all the reasons you listed. I think I have been biased in favor of width since my wider/flat/directional vaned but same diameter kit on my Stealth was so much superior as compared to the not as wide/cross drilled/directional vaned but larger diameter kit that I put on my A4. I realize they are completely different systems, and it is likely that one is just a better fit for the vehicle than the other, but it has warped my impressions.

My question to you on that would be all other things held constant; would you prefer 10% more diameter, or width? OK, maybe this isn't easy...

You describe the Stoptech front only upgrade system for the WRX as shifting 10% bias to the rear. With a bigger diameter rotor, and I assume a higher coefficient of friction pad than stock supplied with the kit, the only way I can see this could be done is with significantly less piston area than stock. Do you have the stock vs. Stoptech piston area and piston circle diameter? I just don't recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock.

Now on to the theory stuff.

I don't know your background, but for the most part, I come from a geeky science/physics/engineering background. Often in that world, they take things to extremes such as my Infinite CF pin and hole example. I realize it isn't how brakes normally work, but it serves a purpose.

You said "When all four tires lock simultaneously after gradually increasing brake pressure without any steep transients, you can say that the brake system is perfectly balanced."

My example was to address precisely the transients you mention.

Let me start by describing the picture as I see it. We have two friction interfaces working against each other through the semi flexible lever that is the wheel and tire. The brake interface has two states, dynamic friction as the wheel continues to spin, and static when it stops/locks. The tire/ground interface is in a very weird world called rolling friction that is a combination of static and dynamic friction that transitions to mostly pure dynamic friction when the tire stops rotation while it is still in motion.

Both of these friction interfaces have a torque curve they go from coasting to lock up. Where those two torque curves intersect is the instantaneous point where the brake locks or the tire stops rotation.

I don't think either of these curves is linear as you describe, but I may not be reading this sentence correctly. "The state of a rolling tire versus a locked tire is dependant on the coefficient of friction between the road surface and the tire versus the braking force applied to the rotor based on pedal pressure. It is a linear progression that rolls off as the tire adhesion is exceeded." Otherwise, I think we agree on this.

My thought right or not is that whichever curve is the steepest, or more likely has the highest rate of change in slope at that point is the interface that could stand to be improved the most. A bad brake system will have a steep curve at that point, a bad tire will have a steep curve at that point.

You also state that "A brake system will apply more and more force to the pads, creating greater and greater friction, slowing the wheel and tire combination more and more." again, I agree with this, but then you say "There is never a case where the friction coefficient between the brake pad and the rotor will instantaneously jump to a level to suddenly lock the brakes. Brake systems do not work that way. There is never an instantaneous on-off. There is always a ramp." While I agree with the first sentence, there must be a instantaneous change from dynamic to static friction, or else the brake would not be able to ever actually stop the car.

My pin/hole example is the ultimate example of a bad brake system as it is nothing but a single infinite transient. It instantaneously applies an infinite brake torque through an infinite coefficient of friction as the pin stops the rotor. At that instant, the torque curve is a straight vertical line from 0 to infinity as the "brake" is applied. Some non zero time later through the flexible lever that is the tire, the tires torque curve follows curving initially as the tire gives way until it goes into a skid as the sidewal etc. runs out of elasticity.

I realize it is extreme, and not remotely real world, but IMHO it shows how a brake system that has transients, or is "grabby" for whatever reason can force a tire lockup earlier than necessary.

This is where I got into the ABS making up for a bad brake system and where Scott came up with the idea of the ultimate ABS system that would run my pin/hole brake system in which the tire would end up absorbing close to 100% of the energy of stopping. Again, a ridiculous example, but educational none the less as we see that the energy must still go somewhere and how an excelent ABS system can improve a less than optimal brake system.

I think we both agree that a huge amount of money has gone into pad formulation to minimize spikes, enhance release, provide smooth coefficient of friction curves, etc. But you can still by something close to a cow turd at your local Kragen and put it into your stock caliper with a single small pot and small pads that closely resembles a C clamp. :D Even though you can probably lock up a tire and/or get into ABS with this turd and C clamp brake system, I think we would both agree a better system can stop your car faster.

I agree a good ABS system can stop a car faster than most humans, but it remains to be seen if the WRX system falls into the category of "sports cars and performance cars" you describe. I was also referring to overall vehicle balance at the time when I said getting into ABS was not necessarily fast. IMHO, standing the car on it's nose every time you hit the brakes is not always the best way to get around the track.

I do not have first hand experience with this, and weld was probably a bad word, but I have heard tell of where probably the binding agent in the compound in the pad became fused to the rotor and required significant force to be dislodged.

This of course is my segway into the rotor and pad discussion. :)

It is really hard not to get defensive about some of this, so bear with me...

To the best of my knowledge, everything is compressible to some extent, rotors, pads or diamonds. Substances do not have to be molten to do so. Unless I am mistaken, the modulus of compression for steel is 1/160x10^9 N/m^2. Yes, this is really small, but it is not zero. Brake pads also have a non zero modulus of compression which undoubtedly varies based on compound. All of these materials are also subject to thermal expansion, but that is a whole other topic.

This compression, along with the deformation due to flexing between the vanes is probably the reason for the Jutter you describe, and some of the transients you see in some brakes.

Every friction example I can think of has the potential for more heat at the leading edge then on the remaining friction surface. That is usually due to some material compression and/or sloughing off of one or both of the materials as the leading edge molecules crash into the oncoming molecules of the surface it is in contact with.

There are a ton of things that go into the design of a caliper and the rest of a brake system. I would guess that reducing leading edge pressure and therefore temperatures are contributors to this design decision in addition to the debris issues you describe.

I think we are splitting hairs trying to differentiate between a softer and lighter pedal. Perhaps I was using a specific braking term incorrectly, but we appear to have meant the same thing, pedal pressure, not the distance the pedal travels.

And when I basically said bigger everything improves braking performance, I was referring to a single brake, not the entire car where bias and other issues come into play.
To be honest, I had not considered the possible feel issue with increase in piston area. On reading another article, it appears drivers are better at modulating to a point with a firmer pressure (non light) pedal. Would you agree with this?

Hey, at least I got that the tire stops the car right... :lol:

Sorry if any of this doesn't make sense. It is late, and I probably haven't proofread it as well as I should...

[edit] If it still doesn't make sense, I can no longer blame it on being tired :)

It's also dependant on the instantaneous normal (downward) force on the tire, from weight and any aero download, since the normal force acting with the coefficient of friction is what enables the longitudinal braking force at the tire/road to be developed. Fbraking = CF * Fnormal. A tire with the same CF with the road obviously won't be able to sustain the same braking force over the crest of a sharp rise at 100mph as it will on a level road, since there's less normal force on the tire.

Dean, I am glad you are willing to concede. Acknowledgment of denial is the first step in recovery! :p

To answer your first question, I would always take 10% diameter over width if I could fit it under my wheel. While a wider rotor will provide additional cooling, a larger diameter rotor generates less heat to begin with. Less heat generated is less heat to shed.

You said you don’t recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock. That’s because you’ve been looking at kits that are not sized properly to the application, like the kit that went on your A4. If it had larger front pistons and larger diameter front rotors, it moved even more braking bias to the front of an already front biased brake system. The end result would be a system that locks the front brakes WAY too early as well as have a longer brake pedal and lighter pedal pressure. Nasty.

The stock Subaru WRX calipers have pistons that are 42.5mm in diameter per my calipers. The Stoptech ST-40 calipers have a leading edge piston of 36mm and a trailing edge piston of 40mm. Here is an aftermarket brake kit company selling a kit that is properly designed for the application with larger front rotors and smaller front pistons.

My college degree is in electrical engineering. I have had plenty of physics courses thrown in as well. I will not discuss theory with you on how brakes work. I will tell you how brakes work. There will not be any need to discuss an infinite CF pin and hole example. It does not apply. If you have questions on my explanation, feel free to ask. But don’t bring theoretical extremes into the discussion.

STOP. There are no transients in the coefficient of friction between the brake pads and the rotors other than the direct input of line pressure from the driver. The transient between dynamic friction to static friction between the brake pads and rotor is IRRELEVENT because the tire has been “locked up” for a considerable and measurable amount of time before the wheel actually stops rotating. I will explain.

The tire stops the car. A tire has a limited amount of adhesion while rolling. As you add brake pressure and use the tire to slow the car, you approach the limit of the tires adhesion. As you continue to add brake pressure, the tire will actually start to rotate slower than road speed. Just as this starts to happen, maximum tire adhesion has been reached. As you further add more brake pressure, the tire continues to slow in relation to road speed. As the delta between actual road speed and the rotational speed of the tire increases, the tire loses adhesion as it transitions to sliding friction. It will get to a point where the speed difference between the road and the tire becomes so great that the friction between the tire and the road is the same as if the wheel were actually stopped, even though the tire is still rolling. For all intents and purposes, the tire is locked because minimum friction is being generated between the road and the tire.

Throughout this entire process, the coefficient of friction between the pads and the rotors HAS NOT CHANGED. There is no instantaneous transient of dynamic friction to static friction between the pads and the rotors. It is a constant dynamic friction between the pads and rotor based on increasing brake pressure and decreasing tire/road friction. When a wheel has finally locked (i.e. – stopped rotating) it is very late in the game and traction between the road and the tire has been long gone for quite some time. The difference in the coefficients of friction between a wheel rotating at 5mph in dynamic friction versus a wheel that has stopped rotating in static friction is absolutely negligible when the road speed of the vehicle is 100mph. The part that you have to understand and accept is that the driver has control of the rotational speed of the tire from full roadspeed to full lock, regardless of the speed of the vehicle. The reason it is practically impossible to control that is because the friction between the tire and road rolls off VERY fast as the tire begins to rotate slower than road speed. It has NOTHING to do with the transition of the friction characteristics of dynamic friction vs. static friction of the pads to rotors because a static state has not been reached.

In summary, the friction curve between the tire and the road will drop dramatically as the tire slows below road speed and becomes flat from a given rotational speed all the way to a fully locked wheel sliding along the pavement. The friction curve between the pads and the rotor stays very consistent throughout the speed range of the rotating tire. There isn’t a transition in the coefficient of friction between the pads and the rotor that would CAUSE a tire to lock, other than driver input.

No, the worst braking system in the world works the same way because the tire/road interface rules all. Your infinite transient does not apply. The tire will always reach it’s minimum traction long before the tire actually stops rotating. Continuing to add brake pressure will speed up when the wheel actually locks.

It’s not extreme. It doesn’t apply. You are not understanding the friction curve between the tire and the road. In addition, we are not talking about stomping on the brake pedal as hard as humanly possible as an effective braking method. Neither of these scenarios are applicable to describing brake systems.

NO! The ultimate ABS system would keep the tire rotating just slower than roadspeed. The on-off nature of the ABS system would cycle the brakes between road speed and a tiny bit too slow for maximum traction. The average would be a tire rotating just below roadspeed in the maximum adhesion window of the tire.

Your example using the pinhole brake system would alternate between near maximum adhesion with the tire at road speed to absolute minimum adhesion with a locked wheel. That average adhesion would be DRASTICALLY lower than the average ABS system in production today. Today’s ABS systems do not sense lock-up. They sense a speed delta in the tire to itself and to the other tires, but the tire is always rotating.

Are you seeing why the pin and hole analogy doesn't work?

Yes, but a cow turd will still stop your car with the same principals. Brakes do not exhibit a dramatic increase in friction between the pads and rotors as rotor speed decreases. Your car comes to a smooth stop with the application of constant brake pedal pressure. It does not have a sudden jerk of deceleration as it approaches zero speed. Do not confuse the rebound of the stored energy in the springs once the car reaches a stop as a sign of increased braking force.

The WRX does not fall into this category. It’s ABS system is not the best.

Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking). ABS will actually compensate for an unbalanced system by allowing all four wheels to do the maximum amount of work possible.

You can transfer pad material from the pad to the rotor. It’s called pad deposition and it occurs when you apply a very hot pad to a very hot rotor when the rotor is stopped. But this is a result of a stopped wheel, it does not cause a locked wheel by welding itself to the rotor when the wheel is rotating.

Okay, so don't get defensive of your theory. Just accept the facts I am about to give you. I have explained that differential piston sizes are there to compensate for the debris field that forms along the pad surface. Additional force on the trailing piston is required to force the pad through the debris field and make the entire surface of the pad usable. This is not a theory of mine. It is what I have learned directly from brake manufacturers, brake designers and brake engineers. If you don't agree with what I have stated above, stop theorizing and start talking to the people that design and build brakes.

I was not splitting hairs. There is terminology in the braking world that are used to describe the characteristics of a brake system. A soft pedal and a light pedal are two different things. Regardless of what you implied, it’s important to clarify this for the other readers of the thread, because what you described was not what you were experiencing.

But you can’t speak about a car’s four brake system in those kind of generalities. They don’t work that way.

The driver’s ability to use the brake system effectively is critical to the operation of the brakes. This thread is discussing improving brake performance and interface to the driver is very important. Yes, a pedal that requires higher effort is easier to modulate, as long as it doesn’t require so much pressure as to be fatiquing. Think about it, with your hand on a scale, is it easier to control a range from zero to an ounce or from zero to 10 pounds? It’s much easier to select any point between zero and 10 pound than between zero and one ounce. However, if you changed it to control between zero and 100 pounds with your hand, it isn’t feasible.

I’m ready to answer questions you may have on any of the above.

Gary
Sheehan Motor Racing
www.teamSMR.com

Well put Gary, I think you've help me understand braking systems a bunch more. I think the failure in our earlier discussion was a result of a brake-centric train of thought over a tire-centric train of thought. Basically we were tracing the effects of the brakes on the tires, rather than the effects of the tires on the brakes. Couple that with an incomplete understanding of a tire's dynamic to static friction transition, and I think you can see where the pin-in-hole brakes came from. :lol:

Taking a step back regarding the sizing of pistons in the front brakes to match an existing rear caliper, how is that different from using a brake proportioning valve? I'm not quite sure how a valve would work on a WRX due to the RF-LR/LF-RR dual circuit nature of the WRX's brake system (two valves?), but I infer that they're quite popular for tuning the brakes on solid-rear axle cars that use a front/rear circuit. Is there some drawback to using a valve? It seems the ability to adjust brake bias would be very useful! Plus, you could potentially lower your initial brake purchase costs by not having to have custom pistons up front, just get the less expensive, mass produced calipers, and adjust the bias w/ a valve.

I appreciate your further description of brake operation, especially the tire dynamics.

I'm sorry you don't want to discuss theory, torque curves etc., but that is your choice.

Unless I am mistaken , the single 45mm stock piston has less surface area than 2x36mm and 2 x40mm Stoptech pistons. Unless my math is wrong, that is 222mm^2 vs. 2 x 177.6 + 2 x 197.4 = 750mm^2. You didn't provide piston ring diameter, but it is undoubtedly larger for the larger rotors which only going to make it worse. How does this shift bias 10% to the rear?

You may want to discuss leading edge temperatures and pressures a little more with your “brake manufacturers, brake designers and brake engineers” to better understand the "facts" behind leading piston size.

I quote from http://www.stoptech.com/whitepapers/glossary/t.htm

“Uneven wear of brake pads caused by geometry, by the difference in temperature between leading and trailing edges and/or by lack of stiffness in the caliper.”

In another article, they mention debris etc., but it only effects the system to “some extent” . Leading edge temperature is the primary issue.
From: http://www.stoptech.com/whitepapers/...ons_122701.htm

“The trailing area (portion) of the pad, to some extent "floats" on the entrapped gasses and particulate matter generated from the leading portion of the pad. The leading portion of the pad will always be hotter than the trailing portion and so will correspondingly, wear faster - resulting in a pad that is tapered when viewed from the edge. This phenomenon is termed "longitudinal taper".

The differential in heat generated across the pad surface, leading to trailing, is characteristic regardless of caliper and pad design. This is why all racing calipers and most high performance street calipers have differential piston bores. Most high performance pads also feature a tapered leading edge”

A good analogy I found in another technical paper on brakes might help explain it: http://www.dietersmotorsports.com/tech/2000/1-00.html

“The relationship between the pistons, brake pads, and rotors is not as simple as it seems. The caliper must load the brake on the trailing edge of the pad. This is done so the pad bites into the rotor evenly for more stopping power. Think of it as if you were moving a 100-pound bag of sand across a dirt lot. If you pull it behind you, the front of the bag will be up and skim across the top of the sand (greatest load to the rear), leaving an even trail behind you. If you get behind the bag and try to push it (greatest load to the front), you will cause the front of the bag to dig into the dirt and create a hole. In the case of the brakes, the leading edge of the pad would then see greatly increased uneven wear.”

Enough of that.

I can't believe you said this:

”Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking).”

Perhaps you consider every corner entry where you want to set the front end of the car, or need to scrub a small amount of speed trail braking.

Can you honestly tell me that every time you press the brake pedal, you are trying to reach maximum braking potential, ABS or not for however short of a time?

Dean,

The WRX has two 42.5mm pistons in the caliper. The Stoptech caliper uses one 36mm piston and one 40mm piston. You do not factor in all four pistons in the area, only two. This is because the two piston floating stock caliper applies the same force to both sides of the rotor (i.e.-a 2 piston floating caliper equals a 4 piston solid mount caliper with equal piston sizes).

All of your references to the differential in temperature across the pad is because the trailing edge of the pad is impeded by the debris and outgassing. More force is required at the trailing edge to force it through the debris.

In discussing the fast way around the track, you are correct, there are instances where you do not brake at the maximum because of balance of the car, etc. But our entire discussion to this point has been discussing maximum braking ability of a system. So, in all of the major braking zones where the intent is to scrub off a significant amount of speed, I use the brakes to the maximum of the tire's ability. Standing on it's nose. As I transfer straight line braking for turning, I am still braking at the limit of the tire just prior to lock-up.

Very short braking areas where there is only a little bit of speed to scrub, you don't hit the brakes to the point to upset the car. But you wouldn't be utilizing ABS to have the car stand on it's nose as you described either.

Take Sears Point. There are six "stand the car on it's nose" braking zones and only two places where you do not brake at absolute maximum, because you don't need to shed that much speed.

You didn't mention anything about the majority of the discussion revolving around how the tire traction breaks away quite a long time before wheel lock-up. That was actually the most important topic in this thread. Did you understand what I was trying to convey?

Gary
Sheehan Motor Racing
www.teamSMR.com

I'm not going to speak for Gary, (since he is more than capable of doing that for himself :lol: ) but assuming that the racetrack is made up of discreet straights and corners, and the racecar is of sufficient power to weight ratio (therefore having a higher speed at the end of every straight than the car is capable of cornering at), the car should absolutely be braked at maximum capacity leading up to every single corner. This is fairly obvious. Now, when you look at some real world racetrack examples that have many combination corners, and get out there in a production car that doesn't have typical racecar-like acceleration & speed, then it's a toss-up as to what will be required on different parts of the track. If you come out of a slow hairpin and then lead into a large-radius sweeper just a second or two later, the car might just not be going fast enough entering the sweeper to require more than a gentle tap on the brakes to get it to turn in sufficiently. I think this is what you were getting after, Dean?

Scott,

I forgot to answer your question. The prop valve is a crutch for a system that isn't sized properly. Manufacturers don't want to design brand new components for every vehicle. It's cost prohibitive. A prop valve reduces line pressure to the output line of the valve. It's typically done to reduce the amount of rear brake.

The WRX has two (in the same package). They allow line pressure to build equally until a given pressure is reached. Once that pressure is reached, the output pressure "knees" to a less steep ramp as input pressure is increased. Essentially it will allow the rear brakes to do more work at light brake application, but ensures that the fronts do more work as brake pressure is increased, keeping the car safe.

Gary
Sheehan Motor Racing
www.teamSMR.com

Stock WRX calipers are 2 pots both on one side of the caliper.

I think Gary's point was "If you need to slow down, do it as fast as possible." Which makes sense to me... the more time you spend at top speed the faster your laps will be. Don't waste time slowing down at less than the maximum braking ability of the car.

I don't think he was talking about using the brakes to transfer weight to help the car turn, and he explicity said he wasn't talking about trail braking. Just when you need to slow down, which should be in a straight line braking area for the most part.

Edit: I dunno why this ended up so far down the thread.. it was posted right after Dean's calculations!

Sorry, my math was in error. I typed 45, not 42.5 into my calculator. 42.5 results in 209.7 x 2 = 419.4 and the Stoptechs comes down to 375. This is 89% the surface area. I don't have the piston circles, but the Stoptechs appear to be 328mm rotors, and stock are approximately 290mm. This is 13% more rotor diameter. So we have 89% the piston area and 113% the diameter. I am still having a hard time understanding how this can shift 10% bias to the rear. Any thoughts?I'm sorry, I disagree, and Stoptechs own documentation and physics appear to disagree as well. As I understand it, debris and outgassing are mostly a thing of the past with modern pad formulation and manufacturing processes... You clearly have access to people and resources I don't. Given these documents, would it be possible for you to discuss this with the enginerers at Stoptech?I believe this discussion started as a discussion about brake options to increase feel and minimize fade. And It's good to see that you agree that balance etc. is an issue when not in a maximum braking zone as was my original statement.I said "I appreciate your further description of brake operation, especially the tire dynamics", and I do. Since you don't wish to discuss the intersection of torque curves, transients, etc. I'm not sure there is anything left to discuss on that front.

Dean,

Your replies are sounding more and more defensive. This is not an attack on you. Please try to stay objective.

I will go back and read the whitepapers you posted regarding offset piston sizes as well as follow up with the Stoptech folks. It's possible they didn't feel the need to go into all of the engineering details with a "dumb race car driver." (that's a self description, by the way)

Just to be clear, I meant chassis balance, not brake balance. Brake balance is insignificant when not braking at the maximum.

I don't know how to interpret that. It may mean that you still believe your pin/hole theory is applicable. Or not. It's not clear to me.

I tried for quite some time to figure out what you were doing with your math. I'm still not sure I know what you were trying to calculate.

The pressure exerted on the pad is the brake line pressure multiplied by the surface area of the pistons. Lets assume that brake line pressure and the cf between the pad and rotors are constant for both systems. The area of a circle is pi*r^2. For the stock pistons, that's 3.1415 * 21.25mm^2 = 1,418mm^2. Since there's two pistons, the total piston area is 2,837mm^2. The Stoptech is 3.1415 * (18mm^2 + 20mm^2) = 2,274.5mm^2.

Given the same cf brake pads and the same brake line pressure, that gives the stock system a brake torque constant of 290mm / 2 * 2,837mm^2 = 411,365mm^3. The Stoptech system has a brake torque constant of 328mm / 2 * 2,274.5mm^2 = 373,018mm^3. The difference is (411,365mm^3 - 373,018mm^3) / 373,018mm^3 = 10.28%. The Stoptech system is 10.28% less than the stock system. Unless I screwed up my math somewhere.

Regardless of how this thread was started, I entered into it because I saw several issues with the way you were discussing brake systems (i.e.-wider rotors better than larger diameter rotors, NASCAR as an example to back that up, poor brake design causing lock-up prior to the tires adhesion limits being exceeded, braking and ABS described with a pin and hole analogy, etc.) My only purpose was to help you and the readers of this thread understand how brakes work.

When I read your last post, it seemed you were more interested in showing where I may be inaccurate regarding sidebars rather than digging further into how brakes work. Perhaps I used the wrong word when I said I don't want to talk about theory. What I really meant was I don't want use your theories of extreme analogies to attempt to describe a braking system. I am interested in further discussion of torque curves, etc.

Gary
Sheehan Motor Racing
www.teamSMR.com

All I can add to this is that I have made the choice to go with Alcon brakes. The SWRT uses them and there is no way in hell they will use anything second best on that car. They have some of the top guys working on that car and more money then god so that is hands down where my money is going. :D

Zero26D,

Out of curiousity, how much are you spending on the exact same Alcons that the SWRT is using and what will you be using them for?

It's also important to point out that a rally brake system does not require the same thermal capacity as a roadrace brake system.

Gary
Sheehan Motor Racing
www.teamSMR.com

Just because SWRT uses a certain setup, that doesn't make it the best for your needs. We've already had the NASCAR example. NASCAR teams have tons of money, but their brakes are limited by series rules and the fact that most ovals don't require heavy braking. Similarily, WRC competition and rules may not allow for a brake system that will be useful on the street. I don't know the details on the Alcon's used by SWRT, but I would certainly find out before deeming them the best brakes. Not to mention the likelihood that they'll cost way too much.

Hopefully he got the Tarmac version. Gravel/snow brakes have to fit under 15" wheels. :-p They're little better performance-wise than 4 pots, although I'm sure the build quality is super. Tarmac brakes use something like a 14" disc, vented, slotted, all the bells and whistles. Do WRC cars allow water-cooled brakes? I had a good time once going through the AP caliper catalog... nifty stuff in there. Loved the line about F1/Indy car calipers... "please call for information on ordering." Ha :lol:

Thanks. I really do appreciate all your real world experience with this as well as the technical insight and I would like to understand the theory as well. Please let us know what you find out.Agreed…I’m not certain my example is entirely without merit, :) but do agree it is not how brakes work under normal conditions. I do think your description of the tire friction and release was very helpful to my understanding. It would be interesting to understand what the torque curves for both these friction zones is just to see what the look like on top of each other.Thanks, your math looks 100% correct, I appreciate you going through the exercise. My math was entirely in error because the idiot behind the keyboard entered the wrong formula into the very first Excel cell and then replicated it for the remaining cells… pi^2r. Oops.

I apologize for becoming defensive. My bad math combined with my bad interpretation of some of your comments led me astray. I personally have a much better understanding now of how all this works together now. I am even more convinced that I want a better ABS system, but oh well, just keep learning to drive better I guess. Do you know if the STI system is any better?

P.S. I hope we can get your insights into swaybars and other suspension goodies next. But that is for another thread.

Dean,

I have no experience with the STi ABS system. Or the WRX ABS system for that matter. We removed the ABS system completely at the start of the season.

I have heard lots of people complain about Subaru ABS, though.

Gary
Sheehan Motor Racing
www.teamSMR.com

It's especially bad on snow-covered ice or loose surfaces where locking the tires would actually help. I tried stopping once from 5mph with 15 feet of room before the stop sign and ended up sliding out into the street, and I had my snow tires on! :shock: You'd think the ABS wouldn't ever just disable the brakes, especially at such a low speed.

I miss my Thunderbird's ABS... it cycled much quicker than the Subaru system, and seemed be much less finickey about triggering when you don't need it. Part of that may be due to the T'Bird's 4000+ lb weight... but overall it just felt and worked much better.

If you'd like to ditch the ABS Dean, you can simply pull the ABS fuse from the inside fuse box. In fact, lots of people have installed a switch in there to allow easy on/off of the ABS. 'Course, I'm not sure about all the side-effects... like switching it on/off while driving, or what your insurance company will say if you get into a wreck w/ the ABS disabled.

It must have been better than what they put in the Mustangs... its ABS is always disconcerting, the pedal gets harder and it "feels" like it's grinding bare metal on metal somewhere.

I've been told that the '96+ Mustangs got the good ABS. I'll bet that's what my '95 TBird had, since my '95 also had the 4.6L motor that went into the '96 Mustang, while the Mustang lingered on the 5.0.