IRS Caster Settings

[V8Ranger]

How much caster is generally designed into the rear suspension of a SLA design? I looked through a few suspension books and found very little information on road racing cars and no information on off road cars.

Thanks,
Matt

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[btshannon]

Caster is primarily a steering angle and not measured at the rear end. Caster is the rearward (or forward in some cases) tilt of the steering axis in reference to vertical line through the wheel's centerline.

Perhaps you were looking for some info. about IRS camber?

[btshannon]

Think about the front forks on a bicycle, if they were rotated 180 degrees, the bike is very difficult to ride (positve caster). Caster settings affect the car's stability.

[ntsqd]

Caster is used when the wheels are allowed to steer. The angle of the steering axis in the fore-to-aft plane means that for the wheels to turn the inside tire must actually raise that corner of the vehicle. You can see this by holding your hand fingers flat and slightly tilted from vertical. "pivot' it around the center of your little finger and notice how the base of your thumb moves up in one direction and down in the other. Since a left turn requires that the left side be raised and a right turn requires that the right side be raised; and Gravity Sucks, you end up with a vehicle that wants to go straight. To put it another way, both tires are trying to turn away from their corner in order to lower that corner (each is trying to toe-in), but the steering linkage won't (shouldn't) allow that so the 'compromise' is that the vehicle wants to go straight.

In the rear there is very little need for this angle unless you've deliberately intro'd rear steering like some late model cars. You might be able to exploit it to help counteract torque thrust induced toe-out, but that's about it.

[tedmales]

so a car with no caster will push instead of turn?

[ntsqd]

Just the opposite. Castor induces push, to a degree, since it creates an inate tendency for the car to want to go straight in a hands off the steering wheel condition.
Each front tire is trying to lower it's corner by turning. The left tire has to turn right to do this. The right tire has to turn left. The steering linkage won't allow independent movement so the 'compromise'' reached is straight ahead since that is the lowest each tire can go w/o raising the opposite corner.

An analogy would the bike mentioned above. Ride a Cruiser no-handed. They have a lot of 'castor' which makes this easy to do and not wobble all over the place. Try riding a BMX bike no-handed. They have very little 'castor' and are harder to make go straight. Takes a lot more effort to do this on a BMX bike than on a Cruiser.