Scrub radius part 2.....

[Tony]

I been getting loads of PM's/ Emails lately regarding wheel off-sets and handling consequences so let's see if this helps.

 

 

For this exercise i've used an old school suspension configuration for simplicity but the results remain the same with modern configurations.

A larger scrub radius gives more kickback is because the tyre pivots on the ground not about the centre of the tyre contact patch, but rather around the point on the ground drawn between the ball joints. When you hit a bump though, the force acts through the centre of the tyre contact patch, so the further the centre of the patch is from the actual pivot point, the longer the lever arm trying to wrench the wheel around the pivot point.

To improve the kick back in earlier cars, the trick is to find wheels with a greater offset than stock. Higher offset wheels move the centre of the tyre contact patch further inboard, closer to the ball joint line reducing the scrub radius. But everything is a compromise. Higher offset wheels will tuck under the wing further and most people want a wider looking stance, not a narrower one. By playing with wider rims and offsets though, you can achieve a reduced scrub radius and still maintain the sidewall at the outside edge.

 

The king pin angle was created to reduce the scrub radius, but in so doing, it also created a secondary effect. The larger the king pin angle, the more quickly your wheels return to the straight ahead position on their own after a turn, similar to caster. The reason it does this is because turning the wheel in the steering axis (about the king pin inclination) actually lifts the nose of the car up slightly as well. The tendency is for the car's weight to return to the lowest position which in turn forces the wheel back to straight ahead.

 

Sometimes it helps to know where these terms came from. The Kingpin used to be an actual pin!.....not just an angle...