Caster – This is the angle of tilt of a line drawn from the center of the top ball joint through to the center of the bottom ball joint, compared to vertical. Caster aids the wheels to straighten themselves out after cornering, however the more caster you have, the more that you may have to fight the steering entering a corner. If the top tilts toward the rear of the vehicle, then it has positive (+) caster. If it tilts forward toward the front, then it has negative (-) caster.
Figure 4 – Positive Caster
Caster Split – This is the difference between the caster of the front wheels. If the left front is set at +1 degree and the right front is set at +3 degrees, then your caster split is 2 degrees (Right front(+3) –Left front (+1) = 2).
Tread Width – This is the distance measured between the centerlines of the front tires, or the distance measured between the centerlines of the rear tires.
Tire Contact Patch – This is the actual area of the tire that is in contact with the track/road surface, or “footprint” of the tire. Things that affect the contact patch are inflation pressure, load, and load distribution on that tire. The load is affected by the suspension setup such as caster and toe settings.
Figure 5 – Tire Contact Patch
Under Steer – Also known as “Push” is when the front tires have to have more steering input than what should be needed to turn the corner. This is caused by the front tires losing traction and starting to slide. It gives the driver the feeling that the vehicle does not want to turn, or “pushes” straight ahead when the steering wheel is turned into the corner. Production cars typically are designed with under steer to benefit safety for the typically under-experienced driver. In addition to your suspension set up, this can also be caused by braking too hard entering a turn or entering a turn at too high of a speed.
Over Steer – Also known as “loose” is when the rear tires lose traction and start to slide to the outside of the corner. This is caused by the front tires having more traction than the rear tires, and gives the driver the feeling that the rear of the car wants to come around trying to beat the front end through the corner and make an attempt to get sideways. This can also be caused by the driver getting into the throttle too hard, and too early in the corner.
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