Tech Tip: Xray X4 Roll Center, Camber Gain, Axle Height

In this Xray Tech Tip video, Alexander Hagberg shows us how to adjust the roll center on an X4.

Lower Arms
He starts by explaining how to adjust the roll center via shimming the lower arms

Here's how I remember the basic effect of adjusting roll center via the lower arms:

• Add shims = raises the roll center (moves the RC closer to the CG) = less roll = less side bite, faster rections
• Remove shims = lowers the roll center (moves the RC further from the CG) = more roll = more side bite, easier to drive

Upper Arms
He then explains how to adjust roll center and camber gain via shimming the upper arms.  I found the upper arm shimming advice that Alex gives to be the opposite of what I expected.  This is due to Camber Gain.  Alex says:

• Adding shims to the inside of the upper arms (or lowering the outside) lowers the roll center (moves the RC further from the CG).  Normally this would add side bite but due the the reduced camber gain, actually reduces side bite and will free up the car
• Removing shims to the inside of the upper arms (or adding to the outside) raises the roll center (moves the RC closer to the CG) which normally  would free up the car but, due to increased camber gain, adds side bite to an X4

Axle Height
Finally, Alex talks about axle height adjustments
Add shims = takes away grip, free up the car, more corner speed
Remove shims = more traction, more grip

Remember to adjust your droop number.  Example, if you add a 0.5mm shim, you need to remove 0.5mm from your droop number (4.6 would now be 4.1)