Trail braking and load transfer help a vehicle with turn-in

Rally driving takes one look at understeer and then sprays gravel in its face. Photo: Shutterstock.
Rally driving takes one look at understeer and then sprays gravel in its face. Photo: Shutterstock.

If you're ever chasing a lap time (be it in a real car or in a sim platform), then one thing that you need to master is trail braking.

To give it a simple description, it's the method of easing off the brake pedal as you enter the corner (instead of doing all the braking in the approach to the corner and letting go of the pedal completely).

It allows you to brake just a tad later, because you're able to continue reducing the speed all the way up to your tightest part of the turn.

Much more than that though, it also allows drivers to take advantage of the load transfer effect (which you may remember gives the front a bit more grip under braking, the rear a bit more grip under acceleration, and the outside a bit more grip in cornering) all the way into the corner.

The principle applies to all forms of trail braking, but the actual technique will differ substantially between cars and surfaces.

On a circuit in a car with lots of downforce, there's more grip available at higher speeds. This is because your aero package is carving through more air and therefore generating more downforce, which is how it helps to push the tyres down and give them more grip.

So, as the speed comes off there's less downforce, and you'll need to ease off the pedal just to avoid locking them up (or activating the ABS if the vehicle has it) even in a straight line.

Again on a circuit, whether the car has lots of downforce or none at all, we also have to allow for the side-to-side load transfer taking some weight off the inside front as we begin to turn in. When drivers don't ease off enough they cause the inside front tyre to stop turning, which is the most common way they get a so-called flat spot (a patch of excessive wear that will make the wheel vibrate).

On a sealed surface, you may also take this load transfer effect to the extreme level on a particularly hot lap and deliberately have the rear slide just a tad so as to help point the car where it needs to go. This is particularly the case in a vehicle that has understeer (whether it's been set up that way deliberately or not). Keep doing this to rear tyres though, and they'll overheat, lose grip and wear prematurely, but you don't care about those factors in qualifying or an in-lap.

So that's one of the ways tarmac drivers manipulate the balance of a car and save a little time. However, rally driving takes one look at understeer and then unceremoniously sprays gravel in its face.

Simply due to the surface being loose (whether it's gravel or snow, or even a worn and dusty tarmac closed public road) the understeer is dramatically more severe and so the solution is dramatically more obvious. But it's the same solution.

The main purpose of sliding in rallying is exactly as above, to turn the body of the car more in the approach to a corner, thus counteracting the effect of understeer which would be to run wide on the exit (or in some cases for rallying, not making it around that super-tight corner at all).

However, and this is important, while at first glance rallying just seems to be a lot of controlled slipping and sliding about, it's important to make the distinction as to when and where the sliding is desirable, and then where and when it isn't helpful (at least not nearly as much).

Getting sideways on the approach is deliberate, but sideways on the way out is incidental, and too much is actually undesirable because you'll be unable to accelerate as effectively. The goal is to point the car at the exit of the corner and accelerate as early as possible, just as it is for the slow-in fast-out method on a circuit.

Additionally, in most (or all) braking zones for tight turns rally drivers will also take advantage of the pendulum effect of turning the opposite way first, so the back really swings around, commonly referred to as the Scandinavian flick. Another thing worth noting is this flick does not normally require the use of the handbrake if the approach speed is high enough. The load transfer is sufficient to unload the rears and turn the understeer of a slippery surface into oversteer.

Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.

This story Getting the car slowed and turned on various surfaces first appeared on The Canberra Times.