Published: March 16, 2014; updated: June 24, 2014
Understanding and effecting weight transfer is crucial for driving fast.
I will only give a refresher of the theory here since most classrooms talk about the theory of weight transfer to some extent:
- Weight transfers forward under braking, rearward under acceleration and to the outside under cornering.
- Directions may be combined: a car which is braking and turning left has weight transfered to the front right corner.
- During weight transfer, some corners are loaded and some are unloaded. If a car is braking, the rear tires get unloaded by as much as the front tires get loaded. When weight moves diagonally, this relationship becomes more complicated.
Why do we care about weight transfer? Two reasons:
- Weight transfer changes the grip available to each tire. As we are trying to drive the car as fast as the available grip permits, changing grip level affects how fast we can go. To make full use of available grip and not to exceed available grip, we must know how much grip there is.
- We can create weight transfer to give additional grip to the tires that need it most, or alternatively to take grip away from tires that we want to lose traction. As the speed of the car is limited by available grip, adding grip will make the car faster. Taking grip away from tires is a more advanced technique that also can make the car faster, but indirectly. We'll talk more about this at the end.
Where is the weight?
You may have noticed in either street or track driving that the car turns better when it is not accelerating compared to when it is accelerating. The reason is how well the car turns is normally a function of how much grip the front tires have. When the car is accelerating, weight is shifted to the rear tires, the front tires get unloaded and therefore have less grip. As a result, the car tends to not turn as well under power.
Consider the case of accelerating out of a corner. A car which is cornering has the weight shifted to the outside. This means the inside tires are unloaded. As acceleration requires equal grip from left and right tires, there is less overall grip available for acceleration in a cornering car compared to a car that is driving straight. For this reason getting on throttle is balanced with unwinding steering - as steering is reduced, weight is returned to the inside tire and the car can accelerate more.
Timing Braking And Turning
Suppose now that we are looking at a typical corner following a straight. We brake for such a corner until the turn in point, then turn. You can see now that when the car is braking, weight is on the front tires, which incidentally are also the tires that define how much the car can turn. Of course, braking uses some of the grip, limiting the grip that can be used for cornering. If we could turn the car while the weight is on the front tires, but when they are no longer braking, the car would turn better compared to it coasting neutrally.
Luckily, this can in fact be done. Brake pads retract from the rotors much faster than weight moves in the car. By releasing the brake pedal and immediately turning the steering wheel, we initiate the turn with the weight still on the front tires but with the front tires no longer performing braking. Doing this permits the car to either turn more at the same speed or turn the same amount, trajectory-wise, at a higher speed.
Note that turning must be performed smoothly; an abrupt turn can exceed the grip available to the front tires with or without the weight transfer.
Trailbraking seeks to extend the time during which weight is over the front tires. During trailbraking there is an overlap between braking and cornering. As a result, the car cannot brake as hard as it would in a straight line, but it can corner better than it could neutrally. In many corners, such as Summit Point Main turn 1, the gain in lateral grip more than offsets for the reduced braking grip.
Timing Lifting And Turning
We established that an accelerating car does not turn as well as a neutral car. In some corners which are normally taken under power, lifting momentarily produces weight transfer forward which gives additional grip to the front tires.
Similar to how braking and turning can be timed to improve turn in, you can lift just before you turn in a corner taken under power to make the car turn better at corner entry. An example of a corner where this technique works well is Summit Point Main carousel section in moderately high powered or low grip cars.
Unlike timing braking and turning which takes advantage of a weight transfer already happening, lifting before a turn creates weight transfer where previously none existed.
Throttle steering page elaborates on rotating the car by creating weight transfers with throttle.
Creating Weight Transfer By Braking
If a car understeers on corner entry, brakes can be applied to move weight to the front tires. In this case we are not interested in slowing the car down - the car is capable of making the turn at the speed it is traveling. Brushing the brakes will move weight forward and permit the car to turn better at the same speed, or turn in at a higher speed and still fit in the pavement.
An example of a corner which benefits from weight transfer by braking is turn 4 entry at NJMP Lightning.
Rotating A Car By Reducing Rear Tire Grip
When a car initiates a turn, the rear tires must slip. In street driving, the slip is so small that the tire tread absorbs it completely. In track driving, intentionally slipping the rear tires makes the car rotate more. The fastest way through a corner is with the rear tires just short of losing grip completely - after all, the front tires are already turned in the right direction, only the rear tires are making the car travel straight.
Creating weight transfer away from the tire reduces the grip available to that tire, eventually making the tire lose traction and slip. Trailbraking, besides loading the front tires, also helps the car rotate by unloading the rear tires. When throttle steering, jabbing the throttle immediately after lifting can similarly overload and slip the rear tires to help the car rotate.