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| If a chopper were a car, it would have a lot of positive caster. You can see when the wheel is turned it flops into what would be negative camber if this was a car! |
Lots of positive caster causes the wheels to flop to the side when turning, gaining negative camber in a turn, right when you need it the most, a good thing. Think of a parked chopper with the wheel flopped to the side. That’s an extreme example of negative camber gain with positive caster.
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| Kingpin Inclination Angle makes the spindle’s stub axle move in an upside down smile as the wheels are steered from one side to another. It causes the car to lift as the wheels are turned. The car naturally wants to stay at the top of the arc where the weight of the car isn’t being lifted. This is what causes good straight line stability and self steering. |
KPA increases stability by making the axle path travel in an upside down U shaped arc when viewed from the side during tuning. The apex of the arc is when the steering wheel is centered. When the wheel is turned, the steering actually lifts the entire front of the car, the lowest point being at the centered point. This makes the steering naturally want to stay centered in the direction of travel with the steering effort increasing the more you turn the wheel away from this center. This contributes greatly to steering feel and straight-line stability. It is possible to run too much KPA making steering impossibly stiff. KPA also causes the wheel to tilt outwards when the steering is turned increasing positive camber, which is not good for cornering traction.
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| A rally or rallycross car has a lot of the same requirements for the front end geometry as a drift car. They can use a lot of caster and KPA, usualy both having an equal angle. |
To counter this effect of KPA, the suspension designer balances KPA with positive caster, the positive caster canceling out KPA’s tendency to make the camber turn positive. In modern cars, there is usually quite a bit more positive caster than KPA so the steering geometry will generate more negative camber while turning. If you notice that certain cars renowned for straight line stability and good on center steering feel like Mercedes and BMW tilt there wheels inward when the steering is turned, it’s because those cars have a lot of positive Caster and KPA built into their steering geometry. The Nissan 240SX and 300ZX also exhibit those traits and have a lot of both in their geometry as well.
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| A road racing or time attack car will have the bias more toward positive caster and less toward KPA as camber gain under turn in is more important than self steering action. |
