Motorcycle riders apply front brakes to reduce speed on the approach to a turn. As they enter the turn, they slowly ease off the brakes, gradually trailing off brake pressure as lean angle increases. This technique increases traction because the downward force on the front tire is increased by load transfer. When the brakes are applied and weight shifts forward, the forks compress, changing the motorcycle's steering geometry in a way that makes the motorcycle more apt to lean and more quickly change direction. Decreasing speed also decreases the motorcycle's cornering radius, while accelerating while turning increases it. Trailing off the brakes while entering blind or tight corners allows the rider to slow if something unexpected blocks their path. Applying the brakes after the motorcycle is already leaned over, however, can be risky depending on surface conditions and lean angle.
Traditionally, trail braking is done exclusively with the front brake, even though trailing the rear brake will also effectively slow the motorcycle and decrease the turning radius. The rider's ability to correctly choose their 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 also enhance control, increase sight distance through the turn, and add evasive options for street riders.
Excessive use of the front brake can result in a loss of grip because the tire's adhesion is split between braking and cornering forces. Effective trail braking requires finesse, which can be difficult to learn.
Motorcycle training curricula vary in whether they introduce trail braking to beginning street riders or defer it to intermediate riders. The Motorcycle Safety Foundation Basic RiderCourse teaches that the safest approach for a beginner is to complete all slowing before the entrance of the turn, discouraging use of any brakes while the motorcycle is leaned over. The steep learning curve of trail braking is cited as making the technique appropriate primarily for the race track.
Freddie Spencer, founder of Freddie Spencer's High Performance Riding School, and Nick Ienatsch, author of the 2003 book Sport Riding Techniques and chief instructor of Yamaha Champions Riding School, say trail braking should be used in nearly every corner as a means to help the motorcycle change direction, giving the rider more control and significantly increasing safety. Spencer and Ienatsch argue that the physics of angular acceleration mean that the slower any vehicle is going, the tighter the radius of the corner it can navigate. Instructor Keith Code argues that as soon as possible after initiating a turn, the rider should increase throttle smoothly and progressively throughout the turn, and that as long as the maximum safe speed is not exceeded, proper throttle control will result in higher corner exit speeds and faster lap times.
In four-wheeled vehicles, trail braking means using the brakes past the corner entrance rather than releasing them before starting the turn. It creates weight transfer to the front tires, increasing their traction and reducing understeer. The technique works best in light vehicles with brake bias to the front.
The driver must have an excellent sense of vehicle behavior and must keep braking effort within very tight limits. Excessive braking effort may result in heavy understeer, or — if the brake bias is set to nearly neutral — in the rear wheels locking, effectively causing the vehicle to spin as in a handbrake turn.
There are two reasons for trail braking: keeping load on the front tires so the car rotates better into the corner, and maximizing tire traction through the corner. A side benefit — though not a primary reason to use it — is that it often allows the driver to begin braking later, since braking ends later. Trail braking is not used in every corner. In very fast turns, the driver wants to be squeezing back on the throttle at approximately the time of turn-in, since this helps the car's balance and overall grip. As a general rule, the slower and tighter the turn, the more trail braking helps rotate the car; the faster and more sweeping the turn, the less it is used.
A drift-inducing technique called the brake drift is used in racing, involving a series of light rear brake trail-braking pulses — usually two or three — followed by a momentary full-force rear braking and sharp release of the rear brakes. Mastering continuous trail braking is a prerequisite for learning brake drifting. Brake drifting is one of the most used drifting techniques in rally racing because, if done correctly, it allows the driver to enter and exit the corner with full throttle.
Trail braking can be used to maintain more speed upon corner entry, attain more grip while turning in, and influence apex selection. Brake pressure is applied slightly later than usual upon deceleration and maintained during steering input, sometimes all the way to the apex. Braking causes a weight transfer that shifts more mass from the rear of the car forward onto the front tires, increasing normal force and traction at the front. This simultaneously reduces rear traction and can be used to induce oversteer.
Compared to a traditional circular corner entry — where all deceleration is completed before steering input — trail braking reduces the time needed for the direction change required to reach the chosen apex. In the traditional method, the vehicle carries a constant speed and radius to the apex, with peak tire forces acting perpendicular to the direction of travel to accelerate the car toward its turn center.
Trail braking splits the peak force the tire can generate partially toward braking (a longitudinal force acting tangentially to the direction of travel) and partially toward steering (a lateral force acting perpendicular to the direction of travel), a relationship described by the traction circle. The combined vector force acting on the vehicle's center of gravity accelerates it in a more rearward direction, causing the vehicle to travel on an Euler spiral-shaped path of decreasing radius and reducing speed.
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