Lowering & suspension parameter discussion

[Nick Koan]

Originally Posted by cody

I thought it had to do with "long travel suspension" that Subaru advertises. Don't Subarus have longer suspension components with the pivot point near the center of the vehicle? Sorry I'm not familiar with suspension terminology. I'm looking for a diagram to illustrate what I'm talking about.

Well, "long travel suspension" would be spring and shock rates as Austin mentioned, and not a factor of "geometry". The term "geometry" more applies to the relative mounting points of the control arms, strut, axle, etc. and their relation to each other.

[cody]

I just remember this flash demo on SOA's site that doesn't seem to be there anymore. It illustrated that the suspension pivoted from the center of the vehicle and therefore the camber was not as adversley affected by uneven surfaces...thus making Subarus safer. Sorry if I'm not making sense.

[AtomicLabMonkey]

Originally Posted by cody

I thought it had to do with "long travel suspension" that Subaru advertises.

"Long travel" is a generic phrase for a spring & shock package with, simply, a lot of travel. Typical production cars will have about 6-7" of total bump & droop travel; if you look at a desert race truck, it might have 20" or more. This allows you to run fairly soft spring & shock rates which cushion the impact shocks from racing across dunes & gulleys at 80mph...

Originally Posted by cody

I just remember this flash demo on SOA's site that doesn't seem to be there anymore. It illustrated that the suspension pivoted from the center of the vehicle and therefore the camber was not as adversley affected by uneven surfaces...thus making Subarus safer.

Pretty much no suspension design will ever physically pivot from the center of the vehicle, it's impossible due to packaging constraints with everything else in the car... engine, fuel tank, passenger compartment, etc. Even F1 cars don't have their suspension links mounted in the center of the car. That picture you posted is pretty typical, and you can see clearly where the inboard pivots are.

Now, maybe they were talking about the suspension instant centers, which would be a different story.. I don't know.

[sperry]

The reason you see MacPherson struts on rally cars (and stuff like monster trucks and off-road racers) is because they need a way to let the suspension travel long distances.

Imagine that Double A-Arm in a configuration that would allow say a foot of travel... the arms would have to be very long. So on a vehicle that doesn't have space for long control arms, you have to go with a MacPherson strut. Some cars can get away with it (look at the unlimited Baja buggys) but any sort of production vehicle that has stuff like a motor or passenegers between the wheels, there's just not space.

In addition, MacPherson struts are superior when it comes to quick repairs. They'ree more simple and more rugged than a double a-arm setup. This means if the suspension fails in the middle of a stage, the driver and co-driver can get under the car and swap out the strut and finish the stage, whereas swapping a control arm, and associated springs and shocks is a much more difficult job in the field.

But when it comes to road racing, the double a-arm suspension is the way to go. By selecting the proper control arm length, you can design a geometry that allows the tire to have an explicit camber at a specific amount of travel. It removes one of the biggest compromises we have to make, which is static camber.

On a MacPherson strut, we have to pick the right amount of negative camber, so when we corner the tire ends up flat. What that means is that on the straights while accelerating and braking, we're actually not able to use all the tire, since it's cambered. With the a-arm setup, you can have the tire flat on the ground in the straights, and flat on the ground in the corners. Not to mention how much easier it is to run wide rubber on that a-arm setup.

So yeah, the MacPherson setup is "fundamentally" flawed when it comes to road racing... but it allows for a pretty good setup within the packaging limitations of a sedan type vehicle. Plus it has some advantages in the context of rally and off-road uses. So it's not like it's total crap, it just means there's a bit more work to be done to find the optimal setup for conditions.

[cody]

That makes way too much sense.

[AtomicLabMonkey]

Originally Posted by sperry

The reason you see MacPherson struts on rally cars (and stuff like monster trucks and off-road racers) is because they need a way to let the suspension travel long distances.

Imagine that Double A-Arm in a configuration that would allow say a foot of travel... the arms would have to be very long. So on a vehicle that doesn't have space for long control arms, you have to go with a MacPherson strut. Some cars can get away with it (look at the unlimited Baja buggys) but any sort of production vehicle that has stuff like a motor or passenegers between the wheels, there's just not space.

Overall good points, but the above doesn't necessarily ring true. A McStrut still has a lower control arm, so you have similar LCA length requirements if you're designing for a lot of travel.

[sperry]

Originally Posted by AtomicLabMonkey

Overall good points, but the above doesn't necessarily ring true. A McStrut still has a lower control arm, so you have similar LCA length requirements if you're designing for a lot of travel.

True, but it can attach under the vehicle, whereas the UCA has to bolt higher up, usually right where the motor is on a front-engine car.

[AtomicLabMonkey]

Originally Posted by sperry

True, but it can attach under the vehicle, whereas the UCA has to bolt higher up, usually right where the motor is on a front-engine car.

It all depends.. even LCA's have plenty of packaging constraints based on the engine & support frame, steering rack & mounts, etc. etc..

They make it work pretty well on trophy trucks.

[sperry]

Originally Posted by AtomicLabMonkey

It all depends.. even LCA's have plenty of packaging constraints based on the engine & support frame, steering rack & mounts, etc. etc..

They make it work pretty well on trophy trucks.

Trophy trucks don't need a low CG like a Rally car does.

Rally suspension is designed to sit very low in the suspension's throw at static ride height. It's actually very much like a road suspension at static through max compression. However, if the car becomes airborn, the suspension has a *ton* of droop combined with a very low fast rebound damper, which allows it to drop quickly to be ready to provide a soft landing when jumping. See pic.

Of course there's also the chance that due to homoligation rules, rally teams are simply required by the factory to use the "inferior" MacPherson struts because the factory can't afford to put double a-arm suspension on the production car... But, considering how many things are build into the street Impreza just to make it a better WRC car, I would bet if there really was a big advantage to the double a-arm, that's what would be on the street car so they could use it in the WRC.

[AtomicLabMonkey]

Originally Posted by sperry

Trophy trucks don't need a low CG like a Rally car does.

A low CG requirement only increases packaging conflicts with a very long LCA, due to the engine getting lower and lower.

Originally Posted by sperry

Of course there's also the chance that due to homoligation rules, rally teams are simply required by the factory to use the "inferior" MacPherson struts because the factory can't afford to put double a-arm suspension on the production car... But, considering how many things are build into the street Impreza just to make it a better WRC car, I would bet if there really was a big advantage to the double a-arm, that's what would be on the street car so they could use it in the WRC.

My guess would be that is indeed a rules requirement, and Subaru just doesn't use it in the production cars because of the increase in cost/unit over a McStrut. Even a $1 or $2 increase per car gets real expensive when you're selling hundreds of thousands of them every year...