Tuesday, September 23rd, 2014

TRAIL BRAKING

Trail braking is an advanced driving technique that all racers must eventually master, like ‘driving the line’ and ‘heel-n-toe shifting’. Trail braking is when the brakes are continued to be applied beyond the corner turn-in point and gradually released up to the point of apex, as opposed to the normally taught practice of releasing the brakes before starting the turn-in. This practice is used for creating weight transfer towards the front tires, thus increasing their traction, reducing understeer, and aids in steering the car more effectively. It also allows for a  more accurate corner entry speed adjustment. Once a driver has mastered trail braking, it will help to enter corners at higher speeds.

The alternative is: do all of your braking in a straight line, release the brakes entirely, then turn in. The trouble with this technique is that when you release the brakes, weight and traction will be removed from the front tires just when you need them to be loaded enough to help turn the car into the corner.  So the car may have the tendency to understeer away from the corner. This is typical behavior for street cars (engineered with built in understeer) that have been adapted for racing. Also if you do not trail brake, there is a period (from release of braking to apex) when spare grip is available. Overlaying your braking and turning inputs means that you can use the entire grip available in this section of the corner.

At first it’s recommended to brake lightly into the intial turn-in, and then as your skills increase, continue brake application deeper into the corner. This skill will allow you to delay braking on the straight-away even further.  The best corners to first practice in are slow and sharp corners, where it is both easier and most beneficial. With further practice you can use trail braking with your left foot in medium-speed corners while continuing to use the right foot to cover the accelerator. Then once you finish braking, move to neutral throttle, and then to progressive acceleration just before the apex. Neutral (or balanced) throttle is when the driver feathers or covers the throttle, applying just enough power to keep the car at a constant speed, not accelerating nor decelerating.

TRAIL BRAKING BENEFITS:

  • helps you turn the car into a corner better by maintaining the transfer of weight onto the front tires, giving them more traction, and helping compensate for any understeering tendency the car may already have
  • braking deeper in to the turn, therefore being able to brake later on the straight-away and maintaining maximum speed longer

PITFALLS:

  •  requires additional finesse from the driver at a very busy point on the racetrack, possibly exceeding the driver’s skill level
  • excessive use of the front brake can result in a loss of grip as the front tire’s adhesion is split between braking and cornering forces, actually causing more understeer
  • due to an exaggerated weight transfer to the front, the rear tires may lose grip, and result in mild oversteer
  • if the brake bias is set to nearly neutral (instead of the usual front-bias), heavy braking may cause the rear wheels to lock, effectively causing the vehicle to spin
  • excessive braking – you may simply slow down too much

 

The Grip Circle

The maximum grip level that a tire can give is depicted by the outer circumfrence of the Grip Circle. A tire can give max grip while cornering, braking, accelerating, or a partial combination of these.  Trailing the brakes gives you the opportunity of using 100% of the tire’s grip from the braking point on towards the apex of the corner. So as your cornering traction requirements increase towards the apex you must reduce the braking inputs so as not to exceed the tire’s limits.

Trail Braking

Diagrams borrowed from Scott Mansell’s article on trail braking.
Advanced tip: a purpose-built race car will probably oversteer if you don’t trail brake as they are designed expecting the driver to use this technique. If you turn into a corner with your feet off both brake and throttle, the front tires will have all their traction budget available for turning while the back wheels will be doing some (engine) braking. Net result: oversteer. Application of the brakes settles down the oversteer by substituting a proportionately balanced loss of steering traction since the brakes are biased towards the front. In this scenario, you actually use the brake pressure to control the rate at which the car rotates into the corner.
 

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