Art & Science: Brakes, Slipper Clutches, And Intake Trickery

Practically every current sportbike has a slipper clutch or some device that limits engine braking on deceleration. Reduced engine braking, or back-torque as it is sometimes called, gives the rider more control under braking, allowing for deeper corner entries and, ideally, quicker lap times on the racetrack.

There are two main factors that turn a four-stroke internal-combustion engine into a brake when the throttle is closed. One is friction; the valve train requires a considerable amount of power, and-at high rpm especially-friction is a major source of engine braking. The second factor is the vacuum created in the intake tracts when the throttle butterflies are closed. While it may seem more evident that the engine's compression causes braking-and, indeed, many refer to the subject at hand as compression braking-that is not necessarily the case. Immediately following the compression stroke in a four-stroke engine is the power stroke, during which any air or gaseous mixture pressurized by the compression stroke is allowed to expand. In other words, the compression and power strokes cancel each other out as far as engine braking is concerned.

During the intake stroke, however, the descending piston must pull against closed throttle plates, creating a vacuum in the cylinder that requires significant force to overcome. Transmitted through the drivetrain, that force acts as a brake. Under normal circumstances, the motorcycle's rear wheel stays on the ground and engine braking can in fact aid the whole braking process. Things get tricky, as they usually do, under more extreme situations. During heavy braking, the rear wheel can skim the ground or leave the pavement entirely for brief periods of time. Chances are the motor is spinning at high rpm as well, creating a lot of engine braking. Should the rapidly decelerating wheel momentarily touch down or encounter a bump, the sudden braking force acts to compress the rear suspension, pulling the wheel off the ground. Of course, that removes the braking force from the equation, so the suspension tries to push the wheel back down onto the ground. The cycle repeats and can easily go out of control with the wheel hopping at a furious rate and a visible distance off the ground.

Sportbike development over the last couple of decades has not only increased the amount of engine braking typically generated, but also increased the susceptibility of the chassis to that back-torque. Ever-higher rev ceilings increase friction. Higher compression ratios create more of a vacuum in the intake system. Stronger brakes and lighter chassis mean the rear wheel is more likely to be off the ground during aggressive braking. And better-handling bikes with grippier tires can be braked deeper into corners. As always, improvements in one area call for changes in others.

In the early '80s, Honda used Sprague, or one-way, clutches on the Shadow and Interceptor models in an effort to reduce rear-wheel hop. This arrangement separated the clutch into two, with half the plates acting as a normal clutch. The other half worked through a one-way bearing, such that under deceleration the load was transferred through only a few plates. At high revs or under the abrupt shock of a sloppy downshift, the half-clutch would slip, reducing braking at the rear wheel.

Later versions of back-torque limiting clutches used on Suzuki's TL models, for example, incorporated a two-piece cam set as part of the inner hub with ramps machined into each piece. On acceleration, the clutch acted as normal. On deceleration, inertia caused the two pieces of the cam to separate due to the ramped design, lifting the outer hub and allowing the clutch to slip. Most current designs utilize a similar concept, with ramps forcing the clutch open on deceleration.

While a slipper clutch reduces the effects of engine braking, another solution is to go right to the source and decrease or eliminate the vacuum created by a closed throttle, in turn reducing the amount of braking force created. This can be accomplished by using a setup similar to the automatic fast-idle circuit utilized on many automobiles and motorcycles that consists of an air bypass in the throttle bodies. Allowing air past the butterflies has the same effect as opening the throttle, raising the idle; typically, a feedback circuit controls a servomotor that allows a set amount of air past the butterfly to keep the idle at the desired speed. A similar arrangement can be used to decrease the vacuum in the intake tract on deceleration, reducing the amount of engine braking. MV Agusta uses just such a setup (dubbed Engine Brake System) on the literbike versions of the F4, as does Honda on the '07 CBR600RR. Called the IACV (Intake Air Control Valve), the system goes a step further than on the MV and also works at small throttle openings to smooth the off/on throttle transition.

While slipper clutches and intake bypass circuits both reduce engine braking, a slipper clutch has a further advantage: Whereas an idle-control setup reduces engine braking at the source, the engine itself is still spinning at an rpm matched to the rear wheel. Under severe braking and downshifting, a slipper clutch lets the engine rev lower than it otherwise would, helping to prevent over-revving when the rear wheel tries to spin the engine past redline.