Threshold braking

Threshold braking or limit braking is a driving technique most commonly used in motor racing, but also practised in road vehicles to slow a vehicle at the optimum rate using the brakes. This is mostly used in vehicles without an anti-lock braking system fitted, which automatically regulates brake pressure when locked wheels are detected.

The technique involves the driver controlling the brake pedal (or lever) pressure to maximise the braking force developed by the tires. The optimal amount of braking force is developed at the point when the wheel just begins to slip. Braking beyond this point causes the tire to slide and the friction adhesion between the tire and driving surface is reduced. The aim of threshold braking is to keep the amount of tire slip at the optimal amount, the value that produces the maximum frictional, and thus braking force.

When wheels are slipping significantly (kinetic friction), the amount of friction available for braking is typically substantially less than when the wheels are not slipping (static friction), thereby reducing the braking force. Peak friction occurs between the static and dynamic endpoints, and this is the point that threshold braking tries to maintain.

Because available friction at a given moment depends on many factors including road surface material, temperature, tire rubber compound and wear, threshold braking is nearly impossible to consistently achieve during normal driving. In vehicles not equipped with ABS the following technique can be used to approximate threshold braking:

* Press the brake pedal about half way in, as if you were braking normally.
* Smoothly increase brake pedal pressure until wheels lock up. On all but the highest grip surfaces the front wheels will lock first by design.
* Reduce brake pedal pressure by a small amount, enough for the locked up wheels to regain traction and start rotating again.
* If more braking is desired, increase brake pedal pressure until the wheels lock up, then release pressure, and so on.

Anti-lock braking system performs these operations many times per second in rapid succession. A human driver can perform one lock-release cycle per second with practice.

In a situation where a car needs to turn or swerve to avoid a collision but is traveling too fast the above technique allows the driver to retain steering control of the car while rapidly decelerating. As the brakes are released in each lock-release cycle, the driver evaluates whether the car is moving sufficiently slow to turn safely. Then he either turns the steering wheel to steer the car or brakes more in a straight line. Simply standing on the brake pedal would lock up the front wheels with no possibility of changing the car's direction.

ee also

*Cadence braking
*Anti-lock brakes
*Friction