Tuesday, December 20, 2011

Braking Drift how-to

For a braking drift you're essentially keeping the front tires on the hairy edge of their traction limit and making the rears exceed theirs. The braking drift can be a very fast way to slide out a car that has initial oversteer on turn-in and understeer on sustained cornering. Setting up a car this way is easily done by reducing the effect of the frontal sway bar and lowering the dampening effect of the front shocks.

The technique is rather simple - you don't brake and turn at the same time, but rather brake hard, release, and while the weight is still at the front of the car you give steering input. Because of the physics of tire load and the characteristics of rubber, a tire is capable of doing more work when it is under a vertical load than if there is no load. This can be illustrated by trying to push a rubber eraser across a table. When the eraser has only its own weight as vertical load, it is quite easy to push around. However, if you were to push down on it with a finger and try to move it across the table it “pushes back” with a lot more force. This phenomenon would suggest that you can make a car handle better just by adding more weight, but this couldn’t be farther from the truth… When a car goes around a turn, the tire is asked to support the vertical load but also to support a lateral load as well. The relationship between added weight and a tire’s added lateral work capability is less than 1:1 – by adding more weight you are asking the tire to support more turning force than you are benefiting it. If it were possible to increase vertical load without increasing the lateral load, then it would be possible to reap the benefits of a tire’s increased capability to perform lateral work without having more weight to do lateral work for. This principle is the basis for how downforce increases grip - increasing the vertical load without increasing weight (lateral load) results in an increase in tire the traction capabilities (“lateral work”, often represented in the Traction Circle).

At the front:
Under braking, the vertical load on the front tires has increased, making it possible for the rubber to do more work, but the amount of weight that they are asked to redirect has not changed because the car still weighs the same. The work being done by the tire will be at the edge of the traction circle under heavy braking as it is, so if you kept braking and gave steering input you'd make the tire's load exceed the available traction. However, if you quickly release the braking force and quickly give steering input you might be able to utilize the temporarily enlarged traction circle (thanks to increased vertical load and therefore increased capacity for work) before the re-balancing of the weight causes the circle to return to normal size.

In short - if you try to brake and steer at the same time, the car will understeer due to frontal washout. If you brake and steer in quick succession, you will have increased load capacity and the car will turn in hard.

To the rear:
Now, with a lower vertical load (due to weight transfer forward) and the same lateral load on the rear tires the traction circle has, in effect, gotten smaller. It won't take much at this point to let the break traction back here. If the car is setup to do so, the rear may even break away on its own since you will still have a bit of braking force being asked of the tire in it's small traction circle (especially if there is excessive negative camber resulting in less contact patch before body roll takes effect). If the tires don't break away this easily you are then left with the options of E-brake, power-over, throttle-off, shift-lock or clutch kick to generate a higher load than the work is capable of doing.

Each technique will operate differently to move the work required of the tires to outside the boundaries of the temporarily smaller traction circle. E-brake, throttle-off, and shift-lock will serve to break traction by slowing the tire down while the power-over and clutch kick speed the rear wheels up (depending on how the kick is executed). Your best bet is to use a declarative method rather than an accelerative method, since by virtue of being a “Braking” drift (braking is the key word here, in case you can’t tell) the rears are being slowed already. At this point there should be a lateral load on both the front and rear tires (car is now post-turn in) with the fronts gripping and the rears sliding slightly. If you are too quick to apply power in the braking drift, you may cause the tire’s location on the traction circle to move the across the vertical axis and back into the center of the circle where it may grip again (also accelerating before the car is sideways will transfer more vertical load to the rear and give the rear tires more traction, causing understeer). You have to make sure that the car is effectively sideways before applying more power. When the rear tires have broken traction, the car is in the early stages of a braking drift.

From here, the line through the turn that the front wheels will take needs to be smaller in radius than that of the rear wheels, essentially meaning that the rear will have to be traveling slightly faster than the front. Controlling the angle of attack will be a matter of simply putting on the power and modulating the throttle so that the rear line is faster, but not so fast that it incites a spin. Power application should be done quickly but smoothly – too big of a sudden jolt of power and the tires will completely loose all grip and the car will spin, too slow and the rear may regain grip and you’ll loose the drift. The faster the rear goes the larger angle, and the slower they go the shallower the angle. The ability to control the front and rear end speeds is one aspect that gives a RWD-only car an advantage over a FWD (where the rears can only be slowed) and many AWDs (where the average driver cannot control the front and rear axles separately without special modifications).

I hope this helps clear things up - if not, you may want to consult a book like "going faster" or "high performance handling handbook".

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