Roll Bars

I've been confused about how roll/antiroll bars work for years, but have finally figured it out...I think!

The bar connects the left and right wheels, but is not perfectly rigid which still allows the two wheels to move independently. The stiffer the bar, the less independent they are. As the body rolls, the outside tire goes into bump while the inside tire goes into droop. The roll bar resists the difference in the two wheel's suspension position; hence, it also limits body roll and is how it got its name.

The bar has no effect unless the body is rolling so it is not active during straight line acceleration or deceleration. In effect, a roll bar is merely an auxillary spring that only acts during roll. It can be used as an adjunct to the springs during cornering which precludes having to use stiffer springs. Stiffer springs aren't normally desired as that can result in less tire grip on rough road surfaces.

A bar has three effects:

 1. It limits body roll. This is a good thing as increased body roll increases suspension movement and can result in tire camber changes.

 2. It increases roll resistance at that end of the car versus the other end. If we increase the front end roll resistance by using a stiffer bar, more sprung weight is transferred to the front end than before.

 3. It increases sprung weight on the outside tire and decreases it on the inside tire. This has the adverse effect of decreasing the total grip on that axle because of the tires' sensitivity to load.

These last two effects are used to adjust the car's understeer/oversteer balance.

What has always confused me is the effect a bar has on the vertical forces/loads between the two tires on the same axle. Many sources are wrong as they state that the bar transfers load from the outside to the inside tire. This is simply not correct!

As the body rolls and the outside wheel goes further into bump, the bar attempts to raise the inside wheel into bump as well. This has the effect of leveling the body. If the bar were perfectly rigid, both wheels would go into the same amount of bump and the body would remain level.

As far as the vertical tire forces go, the bar increases the load on the outside tire while removing the same amount of load from the inside tire. It acts as a load transfer route in addition to the springs. If the bar is rigid enough, it can take all the load off the inside tire and that tire will come off the ground! This can be seen in some race cars which may lift the inside tire during hard cornering.

This sounds good at first as more weight/load on the outside tire increases its cornering force, but because of the tires' sensitivity to load, the total grip available on that axle from the two tires decreases. So the stiffer the bar, the less overall grip that axle can provide.

The above explanation was for real world roll bars. However, GPL works differently. GPL decreases the load on the outside tire and adds it to the inside tire. This is exactly opposite of what real world bars do so GPL doesn't correctly model the effect of roll bars on the tire forces. GPL's roll bars do limit roll as they decrease the difference between forces on the two tires on the same axle. This also decreases the suspension position on the outside tire and increases it on the inside tire. As a result, camber and the camber thrust effect is decreased on the outside tire and increased on the inside tire. This also should result in the tire temperature difference being decreased. However, the roll bars don't directly affect any load transfer between the front and rear axles.