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

,

Cars that have to accelerate out of low speed turns and mostly operate at lower speeds, especially ones with high power like Justin Pawlak’s Mustang can benefit from careful consideration of their roll centers.

Non aero dependent hill climb cars like Rhys Millen’s Hyundai can also benefit from careful placement of roll centers.

The converse is true for a short roll couple.  If you have short roll couples, engineers say that you have a lot of geometric anti roll which has the same effect as stiffening the suspension, it increases weight transfer to the outside tire.  This can increase over or understeer depending on what end of the car you do it to.

Roll center placement all goes out the window on an aero dependent cars like the Nemo Evo due to the super high spring rates that cars like this must run.

The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. Although lowering the center of gravity and increasing the track width are the two most effective ways to reduce weight transfer, over lowering increases the roll couple and dynamic weight transfer.

This visually shows what happens to the roll moment on an over lowered car.  For cars that are extremely low, like some race and drift cars, the roll center must be corrected for this reason.  Check out the difference in roll center height vs CG height in lowered and non lowered examples.

This can cancel any steady state weight transfer advantage that lowering the center of gravity can have. The huge roll couple created by over-lowering will require an overly stiff suspension to control body movement. And when your suspension is too stiff it won’t absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground nnd you can’t drive fast if your tires aren’t on the ground.  This is called tire shock by us engineers.

Really high roll centers cause a jacking moment that can result in the car transferring so much weight that it can actually flip over.  This image is a visual representation of how that could happen.

A high roll center can cause the tires to jack and tuck under when cornering hard.  This jacking is very dangerous and is the reason why old VW bugs and pre 1964 Corvairs with swing axles had a disturbing tendency to flip. On most cars the ideal location for the roll center is 2-4 inches above the ground for the front suspension and 4-10 inches above ground for the rear suspension with the rear roll center higher than the front. This is so the car will transfer more weight on to the front of the car due to an increase in geometric anti roll giving a more predictable tendency to understeer at the absolute limit.  Most purpose built racecars are like this.