Automotive Race car suspension Class

What affect will it have running 2" less scrub radius on the front left compared to the right front?
I have been researching this and my understanding left scrub wedges the right rear on turn in and tightens up the car.
I have seen a lot of cars running a 2" spacer on the right front and and 1" less off set on the left front rim to keep the track width with in the rules and when I've asked them about it they've said they've done it to gain more left side weight. I was seeing it as a scrub radius gain as many of these newer stock cars run very little scrub radius.

Ranger Mike

Science Advisor
Gold Member
Read post 325 on page 13. Kingpin Inclination Angle (KIA) when installed is a compromise between scrub radius and weight jacking. The KAI and caster combine to weight jack the car in a turn. The amount of KIA will vary and you can change the KIA when you are building the race car. Caster can be dialed in at the track. In a left hand turn Positive caster will cause the left front corner to rise and add weight to that corner and the right rear corner. Adding positive caster to the left front will take out wedge or cross weight and loosen up the car. Most drivers run a split caster so the car will naturally turn left. Wheel spacers are a tuning trick I never much believed in as the car should have been built with maximum left side weight to be gin with. So if you build I right, you will have +1° left side caster and +3° right side caster (+4° caster for power steering) with the proper cross weight, left side weight camber build stagger etc.…
If the scrub radius is correct, the tire rotates left and right directly on it's contact area with the ground. It turns freely and doesn't fight the geometry of the steering or affect handling adversely.
If the scrub radius is not correct, the tire is being moved through an arc across the ground. It's harder to steer and very much fights the steering geometry.

Also, the greater the scrub radius, the more the bumps, pot holes and other pavement variations cause the wheel to be deflected from it’s desired path. The steering wheel will be rotated by the wheel deflection....kind of “the tail wagging the dog”.....
Also tends to increase loading on suspension components and wheel bearings, especially the distribution of weight between the large inner bearing and the smaller outer bearing. All in all, no good comes from too much scrub radius

Keeping the center of the tire as close as possible to the center of the king pin/ball joint center line, the happier the suspension will be and we have the least amount of scrubbing.

Positive scrub radius (KPI line or SAI line hits the ground inside the center of the contact patch) imparts a toe out force on the tires. This is typical of RWD cars. Negative scrub radius (KPI/SAI line hits the ground outside of the center of the contact patch) imparts a toe in force on the tires. This is typical of FWD cars. The selection of tire diameter and rim offset can radically change scrub radius. Sometimes changing it from + to - or - to +. All cars I have worked on have a little bit of scrub radius designed in and it helps with road feel. The reason for 'toe out on turns' is because the inner front wheel turns a tighter radius than the outer front during cornering. Without it the inner wheel will be dragged slightly sideways. If you draw a perpendicular line from the center each wheel of a during a turn, the lines should intersect at a common point. This is the point about which the car is rotating during a turn. Because of slip angle, dynamically that point is slightly ahead of its theoretical location. Slip angle is how far you have to turn the wheels compared to how far you would have to theoretically turn them to make a corner. The heavier the car, the faster you are going and the tighter the corner, the greater the slip angle.
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Thats it i didn't take into consideration the positive camber and my reduced KAI will reduce the left side scrub Radius
Starting to understand this Thanks again

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