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B1. ROLL CENTER
A “roll center” is a
theoretical point around
which the chassis rolls,
and is determined by the
design of the suspension.
Front and rear suspensions
normally have different
roll centers. The “roll
axis” is the imaginary line
between the front and rear
roll centers. In general, a
higher roll center on one
end of the car creates
more weight transfer at
that end. The amount that a chassis rolls depends on the position of the roll
axis relative to the car’s center-of-gravity (CG). A lower roll center generally
creates more grip due to the reduction in weight transfer. A lower roll center
also creates less sideway tire scrub.
Roll-centers have an immediate effect on a car’s handling, whereas anti-roll
bars, shocks and springs require the car to roll before they produce an effect.
ROLL CENTER BASICS
Here are some basic facts about center-of-gravity (CG) and roll center (RC):
• Center-of-gravity (CG) is the point on the car on which all forces are
directed.
• Roll center (RC) is the point around which the car rolls.
• Each end of the car (front and rear) has its own roll center.
• RC and CG are (ideally) in the middle (left-right middle) of the car.
• RC is usually below the CG in cars.
ROLL CENTER IN ACTION
When the car is cornering, centrifugal force is applied to the car’s CG which
pushes the car to the outside of the corner. This causes the CG to rotate
around the RC to the outside of the corner. Since the RC is below the CG, this
causes the car to roll to the OUTSIDE of the corner.
• When the RC is far away from CG (lower RC), when the car corners the
CG has more leverage on the RC, so the car will roll more. In that case,
the anti-roll bars and springs would have more effect on the car’s
handling.
• When the RC is closer to CG (higher RC), when the car corners the CG
has less leverage on the RC, so the car will roll less. In that case, the
anti-roll bars and springs would have less effect on the car’s handling.
• If the RC is in the same position as the CG, when the car corners the CG
has no leverage on the RC, so the car would not roll at all. In that case,
the anti-roll bars and springs would have no effect on the car’s handling.
EFFECTS OF ROLL CENTER ADJUSTMENT
Front Roll Center
(Mostly affects on-throttle steering during mid-corner and corner exit)
Lower
• Increases on-throttle steering
• Decreases car’s responsiveness
• Decreases weight transfer at front of car, therefore increases grip
• Increases chassis roll
• Better on smooth, high-traction tracks with long fast corners