Understeer And Oversteer: Adding Vs Removing Grip
Published: June 24, 2014
When we drive on a road course, the car can understeer or oversteer. Both understeer and oversteer are generally useful; depending on the corner, one is desired over the other. In this essay I will discuss changing the handling of the car assuming you already know that you want either more understeer or more oversteer.
There are checklists of how changing or adjusting parts on a car affects its handling. For example, given adjustable shocks, stiffening the front shocks would make the car understeer more, softening the front shocks would make the car oversteer more, stiffening the rear shocks would make the car oversteer and softening the rear would make the car understeer. It is beneficial to also understand, in this example, that stiffening one end of a car takes grip away from that end, while softening one end of a car adds grip to that end.
In our example of shock settings, stiffer shocks would make the car "skate" over uneven pavement, whereas softer shocks would make the tires follow pavement imperfections better. Thus, softer shocks would generally provide more grip.
Why do race cars run stiff suspensions then? Stiff suspension reduces body roll in corners which permits more rapid direction changes. In other words, a car can be tossed into corners more aggressively, and transitions from turning left to turning right take less time. Suspension tuning is a complicated topic, but for the purposes of our present discussion we can assume that stiffer is faster until the car is so stiff that tires start to skate over the pavement.
Getting back to shock adjustment, suppose the car understeers too much, and we want to make it oversteer more. Our options for achieving this are softening the front shocks or stiffening the rear shocks. If we soften the front shocks, the front should grip more while the rear will grip the same. Thus the car should turn better but with more body roll in the corners. On tracks which consist of individual corners (straight - left turn - straight) softening the front should work great. On tracks with transitions (straight - left turn - right turn - straight) the car might start swinging too much in the transitions.
On the other hand, we can stiffen the rear shocks. The front should grip the same while the rear will grip less. If the car has large rear tires and has enough rear grip, this might work. Otherwise the car may have trouble putting the power down on corner exits - it might start powersliding instead - or the car might become spin-prone in the bumpier corners as the rear tires would lose grip abruptly there.
The right adjustment thus varies with the track and the car. For simplicity, some people recommend starting a car on full soft shocks; the only adjustment then is stiffening either the front or the rear depending on whether the car oversteers or understeers, respectively. With experience you may do both - stiffen the front and soften the rear, if the suspension is already in the ballpark of where you want it to be.
Consider trailbraking. Trailbraking transfers the weight forward, increasing the grip of the front tires and decreasing the grip of the rear tires at the same time. Generally speaking, gentle trailbraking meaningfully changes the front end grip while not removing enough rear end grip to cause problems. The reason for this is most cars are set up to understeer, some very much so, and a little trailbraking makes the car closer to neutral but not past neutral into oversteering territory.
With aggressive enough trailbraking (which normally means duration - holding the brake pedal longer while the car turns into the corner - rather than brake pedal pressure) weight transfer can get extreme enough that the rear end will lose grip, and eventually the car will spin. If you have ever witnessed a front wheel drive car spinning on a road course or at an autocross, this is typically the reason why.
On track cars that are close to neutral, the rear end may start to lose grip with moderate trailbraking. These cars often achieve the quickest time through a corner when they are trailbraked to the point where the rear tires just begin to slide, and the car is thus on the edge of spinning. At this point the most possible weight is transfered to the front tires, permitting the most lateral acceleration on the front, and the rear is ever so slightly sliding, thus helping with car rotation. Here taking the grip momentarily away from the rear yields the highest corner entry speed.
Finally, on a car with strong understeer the front may have so little grip that weight transfer forward, while increasing the amount of front grip, is insufficient to exceed the amount of rear grip. In this case aggressive trailbraking is performed up to the point when the rear of the car loses grip and starts to slide. Trailbraking is not done to increase the front grip, but rather to break the rear tires loose and drift the car around the turn.
Between adding grip and taking grip away, adding grip is generally faster, and is thus a better idea. A car which is drifting cannot go any faster, as it is limited by front tire grip level. If the same car had more front grip, it would be able to go faster. However, a car that drifts around a corner may be faster than a car that plows through the same corner at a lower speed.
Another concern with taking grip away, especially from the rear, is that the car becomes more spin-prone.