Race cars and roll centers, what does it all mean?

One adjustment we use to affect the cornering characteristics of any car, but especially race cars and for the purpose of this article road racing cars, whew. Is the roll centers of the front and rear suspensions. For this discussion we will use a typical US production based sedan with a double control arm front suspension and a live axle at the rear. This will be perfect for describing the affect roll centers may have on the car's dynamics without having to get into a lengthy discussion on calculating the roll center's locations.

The roll center is the point about which a vehicle will lean during cornering, the front and rear of the vehicle both have a roll center and they are determined by suspension geometry. At the rear, in our example, we will use a lateral bar to locate the solid axle. This is commonly called a track, or Panhard bar. It attaches to the chassis on one end and the axle on the other. The elusive roll center is located approximately in the center, left to right, of the bar. This is the point at which the vehicle will roll when encountering cornering forces.

To get an idea of how this can affect cornering characteristics we need to look at how moving the roll center closer to, or further away from that end of the vehicle's center of mass affects the vehicle's tendency to roll.

Take a stick and mount it to a base of some kind which restrained that end of it. The other end has a weight on it and is free to move. Now take that rig and mount it to a turntable with the stick vertical and the weight at the top. When you turn on the turntable, the weight will obviously be pushed towards the outside. Now think of the weight as the sprung mass of the car and the base of the stick as the roll center.

You can see that if the distance between the base and the weight is increased, like lowering the roll center, it will be easier for the same acceleration to move the weight further. If you shorten the length of the stick, like raising the roll center, there is less of a deflection of the weight.

Now this isn't a real model of what is going on, but it is illustrative enough for this example. So as you raise the roll center, it moves that point closer to the center of the mass of the rear of the car. This will make the car lean less.

This adjustment will change the loading on the rear of the car and affect how much load transfers across the pair of tires that on end of the vehicle, but it won't change the total weight transfer. The only way you can affect actual weight transfer is to change the center of gravity height, the track width or the cornering speed.. And we will take that on in our next article.

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