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Double wishbone and kingpin angle

Tech Articles

Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

Hi guys, the video on this topic I feel was a little brief when it came to something I’ve notice change the biggest feel of the car!

Wider wheels as you say increasing scrub radius seemed to increase the feeling of “tramlining” in the McPherson strut M3 I drive, the feeling of diving into camber changes in the track.

The car I will be working on most is my skyline r32 that has double wishbone suspension. It would seem to me like by changing my camber will in turn alter the kingpin angle and scrub radius? Or am I wrong in thinking this.

Is there more to the scrub radius with regards to toe changes lock to lock or will it remain the same but with the only side effect being heavier feel (RWD Applications)

Thanks and so far the info gains is great

Hi Sam, good point - Maybe we do need to expand on this topic a little further. The reason I left it relatively brief is that for the most part on production cars the KPI is what it is and we can't adjust it independently. You're right that adjusting the camber will impact the KPI and to a degree we can consider that a necessary knock on effect from getting the camber where we need it to be. Provided the caster angle is greater than the KPI, the net effect when we turn into a corner is still camber gain which is what we want. Increasing the KPI also has the effect of improving straight line stability which is desirable.

While changing the camber angle will impact the KPI, this will have minimal effect on the scrub radius. The reason is that the scrub radius is defined by the relationship between the wheel centreline and the king pin inclination. If we draw a vertical line through the wheel and through the upper and lower ball joints, the scrub radius is the distance between where these two lines intersect with the ground. Now if we rotate the hub a few degrees the relationship of these two lines doesn't change - We've simply tipped both of them over by the same amount, remembering that the hub will rotate around the balljoint.

I've attached a quick picture that hopefully will help explain what I'm talking about here. The image uses a macpherson strut but the principle remains the same with a double wishbone suspension system.

That diagram makes sense thanks!

Min my head adjusting the camber angle brought the line closer to the centre of the tyre but the vertical line through the wheel also moves further away at the same time so this is where my logic was wrong in my head!

Thanks!