Trail braking is a technique whereby the brakes are used beyond the entrance to a turn (turn-in), and then gradually released (trailed off).
Depending on a number of factors, the rider fully releases brake pressure at any point between the turn-in point towards the apex.
In applying this technique, motorcycle riders approach turns applying front brakes to reduce speed. As they enter the turn, they slowly ease off the brakes, gradually decreasing or trailing off the brakes as motorcycle lean increases. This is done for several reasons.
First, it gives more traction because the downward force on the front tire is increased by load transfer.
Second, as the brakes are applied and the weight shifts forward, the forks are compressed. The compression of the forks changes the motorcycle’s steering geometry, decreasing stability in a way that makes the motorcycle more apt to lean and more quickly change direction.
Third, decreasing speed decreases the motorcycle’s cornering radius. Conversely, accelerating while turning increases the motorcycle’s cornering radius.
Fourth, trailing off the brakes while entering blind or tight corners allows the rider to slow if something unexpected blocks the rider’s path. Because the motorcycle is already on the brakes and the front tire is getting additional traction from already slowing, the rider can slow even more with very little risk, depending on surface conditions. However, applying the brakes after the motorcycle is already leaned over can be exceedingly risky, depending on surface conditions and lean angle. (this is where the throttling is an important combination for exiting the turn, it also increase stability as applying the throttle gives you more grip feel on the rear)
Traditionally, trail braking is done exclusively with the front brake even though trailing the rear brake will effectively slow the motorcycle, also decreasing the turning radius. If the motorcycle is leaned over, forces from the front brake and the deceleration causes the motorcycle to yaw (lean), while the use of the rear brake generates a torque that tends to align (straighten) and stabilize the motorcycle.
The rider’s ability to correctly choose his turn in, apex, and exit points reduces or eliminates the need for prolonged trailing of the brakes into turns. This technique is commonly used when racing but can enhance control, increase sight distance through the turn in the track and on the road, which adds evasive options for street riders.