|The balance between positive caster, KPI and scrub radius is particularly critical in high powered FWD and AWD cars as all of these factors can really affect torque steer.
If some of this is good, more positive caster and KPA are not necessarily better. The same moment that creates the self-aligning torque can contribute towards torque steer in a FWD or AWD car. This is why FWD and AWD cars often have less positive caster and KPA in their geometry. Excessive caster and/or KPA can result in understeer because tipping the tire on edge and lifting it extremely can cause the corner weight on the outside tire to drastically increase, artificially increasing weight transfer. Extremely wide tires exaggerate this effect. Sometimes when running much wider wheels and tires on certain cars like a 300ZX, you want to actually reduce the positive caster.
|The Dog II runs 6 degrees of positive caster, a large amount for a FWD car. It doesn’t have much torque or front scrub and the tilt of the caster actually moves the tie rods to a position with less bump steer on this particular car.
In severe cases it can result in too much negative camber is the case of excessive positive caster or too much positive camber in the case of excessive KPI. You can see this in some drift, dirt or rally cars as contact patch is sacrificed to get stronger and faster self steer action. Too much positive caster and or KPI can also result in more bump steer as it changes the relationship of the tie rod ends to the ball joint and inner lower control arm pivot.
|The RWD Berk BMW has a lot of positive caster in its suspension geometry like all BMW’s. Road racing cars tend to have a few more degrees positive caster than KPI while drift cars tend to have about an equal amount. This is because road racers need the negative camber gain while drifters need strong self steer.
Scrub radius comes into play in the game of balancing positive caster and KPI. To review; the scrub radius is the distance between the centerline of the tires contact patch and the point where the steering axis line (or KPA line extended or Dave Point) hits the ground. The scrub radius is a moment arm where the tire’s contact patch can give feedback to the steering.
|Here is your review of scrub radius. Hellaflush advocates take notice. This stuff matters if you care about performance. With careful tuning you can have a functional Hellaflush car. If you increase your KPA you can reduce your scrub even with a wheel with more offset. Increasing KPA moves the Dave point toward the center of the tire. It should be balanced with more positive caster though.
It is good to have a little scrub radius as it helps develop steering feel, the feeling of the steering wheel getting light as the front tires lose grip and firmer with more grip. It also helps improve the self steering effect. For a RWD car you generally want to run between ¾ of an inch to as much as 3”. In AWD and FWD cars, you want to try to limit the scrub radius to less than 2” because it increases the effects of torque steer.
|Powerful AWD cars like the GST Impreza L must also be aware of the implications of running too much front caster just like a powerful FWD car.