The roll center is the imaginary point around which your car rolls in corners. Adjust it with shims.
The roll center's height relative to the CG determines how the car rolls and transfers weight.
The roll center is an imaginary point where lateral forces act. Your car's center of gravity (CG) is where the mass actually sits. The distance between them = the lever arm that causes roll.
High roll center (close to CG): Less leverage on roll, car resists roll more, feels more stable but less progressive.
Low roll center (far from CG): More leverage on roll, car rolls more, feels more progressive but 'tippy'.
Key: RC height is set by arm pivot points. You move it by shimming inner or outer pivot positions.
Most competitive setups use a relatively high front RC (for stability and clean initial turn) and a slightly lower rear RC (for progressive weight transfer on exit). This gives 'steer in, stable out' character.
Thin spacers under pivot balls that move the roll center up or down. Small amount = big effect.
Shims are thin washers placed under inner or outer pivot balls. They change the pivot point height, which moves the roll center.
Inner pivot shims (under arm mount):
More shims = higher pivot = higher roll center = less roll = more stable
Fewer shims = lower pivot = lower roll center = more roll = more progression
Outer pivot shims (at upright): Less common, but can fine-tune roll center location and camber curve simultaneously.
Even 0.25mm of shim changes roll center by ~0.5mm at the tire. Use digital calipers to measure shim stack accurately.
John's X4 setup used specific shim values: +3mm front inner upper, +2mm rear outer upper, etc. These are per-setup fine-tuning on top of base geometry.
Front and rear don't have to match. Different roll centers front vs rear changes how the car transitions.
You can set different roll center heights front and rear. The RELATIONSHIP determines transition behavior.
Front RC higher than rear: Front rolls less, rear rolls more → car rotates more in corners (looser exit). Common for tight tracks.
Front RC lower than rear: Front rolls more, rear rolls less → car resists rotation (understeer exit). Common for fast tracks.
Equal front/rear RC: Neutral roll — car rotates consistently through corners. Can feel either stable or neutral depending on driving style.
If unsure, start with slightly higher front RC (add 0.25-0.5mm inner front pivot shims). This makes turn-in crisper. Then adjust rear to taste.
How much the front end resists diving under braking. Set by arm angle relative to the ground.
Anti-dive is the percentage of weight transfer that the suspension geometry absorbs (vs all going to the springs). It's set by the angle of the arms relative to the ground when viewed from the side.
If arms slope UP toward the front (front pivot higher than rear pivot), you get anti-dive. The geometry resists the front compressing under braking.
Typical values: 2–5% anti-dive in touring cars. More = more stable under braking. Less = front dives more, loads front tires.
Change inner pivot HEIGHT difference front-to-rear. Raise front inner pivot higher → more anti-dive. Lower it → less anti-dive. You can also use shims under rear inner pivots (raising rear inner = effectively more anti-dive).
The rear's version of anti-dive. How much the rear resists squatting under power.
Anti-squat is like anti-dive but for the rear. When you hit the throttle, weight transfers to the rear. Anti-squat determines how much of that weight goes to the suspension geometry vs the rear springs.
Typical values: 30-80% anti-squat in touring cars. More = rear squats less, pulls cleaner. Less = rear squats more, can generate more rear grip initially.
How it's set: Rear arm angle (slope) relative to ground. More slope = more anti-squat. Adjusted via inner pivot height with shims.
Too little anti-squat + modern tires = traction roll. The rear squats so much the inside rear wheel lifts and the car rolls OVER. Dangerous.
If you see the car roll inward on corner exit, INCREASE anti-squat immediately.