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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals
Is it just the track control arms that need shimming up to get the correct roll centre on most cars?
Roll center is an endless void of more and more complex definitions.
Luckly, its often easy to change. Some form of spacer between the knuckle and lower control arms ball joint is what I most commonly see. Raising the ball joint also changes it but would most likely be for lifted vehicles, and probably much more difficult to accomplish. You can change the inner pivot mounting point up or down, or the same for the upper control arm(top hat for McPhearson).
Getting the correct roll center is significantly harder. If you're keeping things fairly simple and just lowering the vehicle, raise the roll center towards its original location, and it shouldn't cause any major issues. In a moderately incorrect sense, roll center could be thought of as a sway bar, raise the roll center for a thicker sway bar, lower it for a smaller one if you want to use it as a tuning tool.
Roll center, or instant center(what you use to find your roll center) is directly linked to camber change as the wheel travels. You'll likely need to change your alignment to account for the more aggressive camber curve(if raising the roll center for a lowered car)
Sorry for all the vagueness, every cars different and might prove the exception. I can give you a more complicated definition if needed.
As Robbie said, it is a rather complex subject.
Assuming you mean a strut type suspension, raising the inner pivot point and/or lowering the strut location point to get the same angle as the original 'should' work, but moving the inner may require the bump steer to be corrected - might need it anyway, though!
I'd suggest picking up a good book on suspension design and modification and reading it carefully - it will be well worth the cast of purchase!
Thanks guys i will look into it, and yes for a Mcphearson strut