|
When you brake, inertia acts on the centre of gravity (CoG) of the car. Since the CoG is above the roll center, the CoG tries to rotate forward and 'over' the roll center. The practical upshot is that weight gets transferred forward, from the rear wheels to the front ones. The practical upshot of that is that the front wheels grip a lot better than the rear wheels while braking.
To stop a car most efficiently, we need to apply a braking force to each wheel in proportion to the weight on that wheel. At rest, this will be in proportion to the static weight distribution of the car. However, when decelerating, weight shifts forward so proportionately more braking force needs to be applied to the front wheels than the rear.
How much forward brake bias you need changes dynamically, and depends largely on the height of the CoG (and the wheelbase, total weight, front and rear weights, and the coefficient of friction). For example, if you raise the ride height, the CoG goes up, more weight gets transferred forward under braking (via leverage), and so you need more forward brake bias. Another example: if your fuel tank were in the front of the car, then as it emptied your CoG would transfer slowly backward, creating backward weight transfer and so requiring less forward bias. Final example: if you're racing on snow (or any surface with a very low coefficient of friction) then the brake bias will need to be very close to the front/rear weight distribution of the car.
You always want to set the bias so that the front wheels will lock 'before' the rear wheels. Of the two evils, a locked front wheel is by far the less; locking a rear is usually followed by the rear of the car trying to overtake the front, based on the destabilizing side force that the still-rolling front tires provide. If the fronts lock up first, steering control will be lost and the car will go straight or slide down the camber of the road.
How much 'before' depends on the track, and the driving style. If you haven't got the hang of braking yet then it is safer to set the bias well forward, so that the fronts lock well before the rears. As you get better at feeling the brake, then move the bias more to the rear.
The front brake bias will vary from driver to driver and from car to car. It doesn't seem to need to be changed for each track. With the 'old style' differentials (e.g. 85/30+3) I generally prefered 54-55%; with the more realistic differentials (e.g. 45/85+1) it is much easier to steer the car into the corners using the brake and so I tend to use 52-53%.
When determining the figure to use for your combination of driving style and car, you may find it helpful to use the replay facility to see which wheel locks first, as it's sometimes hard to be sure from the cockpit. Generally, start with a bias too far to the rear (say, 50%) and move it forward by 1% increments, firstly past the point where the rear wheel(s) lock up first, and then past the point where you can't steer while applying moderate braking effort.
After using the 'which wheel locks first method' to set the gross brake bias, you can then fine-tune it to slightly alter the corner entry handling. Cars with slightly too much front brake bias will tend to understeer at corner entry while cars with slightly too much rear bias will tend to oversteer.
| « Tire pressure | Final drive ratios » |
|
|
|