The Ultimate Guide to Suspension and Handling: Part 9 It’s All in the Geometry – The Roll Center

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Believe it or not, a drift car is perhaps one of the cars most sensitive to roll center location.

The biggest effects that roll center location has on a car’s handling have to do with how the car responds to steering input in a corner, how much the car rolls over in a corner and how friendly the car’s balance and mechanical grip at the limit are.

Here is a good picture I stole showing what a roll couple is and how it affects body roll.

The distance between the roll center and the center of gravity is called the roll couple.  The CG location for each end of the car can be located by jacking the car up a known distance from side to side while it is on corner scales, and observing the change in corner weights. Then the data is fed into an equation to give you the coordinates of the CG. Since it’s fairly safe to say that most people don’t have a precisely flat surface and expensive corner scales, it’s usually safe to estimate theat the CG for the front suspension is around crankshaft height. In the rear, it’s usually at the floor of the trunk. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn about the roll center.

Think of a moment arm as a lever arm where centrifugal force works to lean the car over.

The longer the roll couple, the more leverage centrifugal force has on on the suspension through the center of gravity and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple and high roll angles does not have that much effect on overall weight transfer but it will increase dynamic weight transfer which can make a car twitchy and harder to control at the limit of traction.

Although intuitively, it seems like a lot of body roll would cause a huge amount of weight transfer, it is not the case.  Body roll negatively affects vehicle response to driver input by slowing it.  The dynamic weight transfer associated with the roll can make the car twitchy at the limits of adhesion where there is little forgiveness for control overshoots.  This is one of the reasons why you want to limit roll.

If you draw a free body diagram, you can see that the CG shift laterally under roll is very small, even with this super exaggerated diagram with tons of roll.  Typically the weight transfer due to roll is under two percent, even in a car that rolls a lot.  You are not gaining much by limiting roll.  Lowering the CG and or increasing track width are much more effective means of reducing weight transfer.