[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.