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Trail braking is a motorcycle riding and driving technique where the brakes are used beyond the entrance to a turn and are gradually released up to the point of apex.
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In a broader scope, trailing off the braking pressure either while straight line braking or, as above, after turn in has begun, allows for a less abrupt and more accurate final corner entry speed adjustment. Some corner entries, such as decreasing radius turns, are more adapted to the leaned over trail braking technique. In turns where a quicker steering action is more applicable, trailing the brake while turning in is unnecessary.
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 motorcycles 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 motorcycles 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.
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 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 and add more 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.
Controversy
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 performing all of the slowing before the entrance of the turn, discouraging the use of any brakes while the motorcycle is leaned over. The argument against trail braking on the street, at least for beginners, is 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 now defunct 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, both argue that trail braking should be used in nearly every corner as a means to help the motorcycle change direction, stating that trail braking gives the rider more control and significantly increases rider safety.
Spencer and Ienatsch agree with the physics of angular acceleration and note that the slower any vehicle is going, the tighter the radius of the corner it can navigate. This is seemingly opposed to Keith Code's writing that, as soon as possible after initiating a turn, the rider should get on the gas smoothly and progressively throughout the turn. Spencer points out 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.
Driving
In four wheel vehicles trail braking pertains to using the brakes past the corner entrance (as opposed to the normally taught practice of releasing the brakes before starting the turn). This practice is used for creating weight transfer towards the front tires, thus increasing their traction and reducing understeer. It works best in light vehicles that have their brake bias to the front.
In order to be properly performed, the driver must have excellent sense of the vehicle's behavior and be able to keep the braking effort within very tight limits. Excessive braking effort may result in the vehicle heavily understeering, 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.
Once a driver has mastered trail braking, it can help enter the corners at higher speeds, or avoid an accident if the driver has entered a corner at a speed exceeding the vehicle's (or driver's) capabilities.
Racing
A drift-inducing technique called "the brake drift" is used in racing, involving a series of light rear brake trail-braking pulses (usually 2 or 3), followed by a momentary full-force rear braking and sharp releasing of the rear brakes. Mastering continuous trail braking as used under road conditions is a prerequisite for learning brake drifting. This is one of the most used drifting techniques in rally racing because - if done properly - allows the driver to enter and exit the corner with full throttle.
Depending upon cornering situations, techniques like trail braking can be used to maintain more speed upon entry of a corner, and attaining more grip while turning into the corner, and has an effect on apex selection. In this technique, brake pressure is applied slightly later than usual upon deceleration, and is maintained during steering input, sometimes all the way to the apex. The action of braking causes a weight transfer in the vehicle, shifting more weight from the rear of the car forward to the front tires, increasing the normal force on them and in turn increasing the amount of traction the front (steering) wheels have. Because of the characteristics of weight transfer, this technique causes weight to be shifted away from the rear of the car, resulting in lower rear traction, and can be used to induce oversteer in some cases.
Physics of trail braking
As compared to the traditional circular entry, trail braking reduces the time for the requisite direction change needed to reach the driver's chosen apex. With the traditional corner entry where all deceleration is completed prior to steering input, the vehicle carries a constant speed and radius to the apex. The combined peak tire forces act on the car perpendicular to its direction of travel to accelerate it toward its current turn center. The vehicle takes a circular path to the apex at a constant speed.
In comparison, trail braking splits up the peak force the tire is able to generate partially toward braking (longitudinal force acting on a tangent to the direction of travel,) and partially toward steering (lateral force acting perpendicular to direction of travel.) See the traction circle for more info. The combined vector force acting on the vehicle's center of gravity accelerates it in a more rearward direction as shown in the illustration. This causes the vehicle to travel on a Euler spiral shaped path of constant radius and speed reduction.