Trail braking is a driving and motorcycle riding technique where 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 driver fully releases brake pressure at any point between turn-in and the apex of the turn.
Motorcycling
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 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 enhance control, increase sight distance through the turn, and add evasive options for street riders.
Risks
There is risk with trail braking because excessive use of the front brake can result in a loss of grip as the tire's adhesion is split between braking and cornering forces. Effective trail braking requires finesse from the rider, which can be difficult to learn.
Variation in training
Motorcycle training curriculums vary in whether they introduce trail braking to beginning street riders, or focus on other skills at first and introduce the topic to intermediate riders later.
Guides such as the Motorcycle Safety Foundation Basic RiderCourse teach that the safest way for a beginning rider to approach a corner on a motorcycle is by doing all slowing, whether with brakes or engine, before the entrance of the turn, discouraging the use of any brakes while the motorcycle is leaned over. The reasons for not trail braking on the street, at least for beginners, include that the steep learning curve of trail braking makes it appropriate only for the race track. The benefit of learning trail braking to the street rider is that knowing and understanding how to slow while entering a corner gives a greater safety margin, particularly in blind, decreasing radius or downhill corners.
Freddie Spencer, founder of the Freddie Spencer's High Performance Riding School, as well as 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, and that trail braking gives the rider more control and significantly increases rider safety.
Spencer and Ienatsch say 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 said that, as soon as possible after initiating a turn, the rider should increase throttle smoothly and progressively throughout the turn. Spencer said that for every radius, motorcycle, and rider combination there is a maximum speed at which the turn can be navigated without exiting the road or suffering a low side crash. Code is saying that as long as this maximum speed is not exceeded, proper throttle control throughout the turn will result in higher corner exit speeds and faster lap times.