A Dangerous Combination of Front Fork and Self-Energizing Brake

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A self-energizing brake is one that uses the actual braking drag force to increase the force with which the brake is applied. The principle is well known in automotive engineering. Before the advent of power brakes, many cars, particularly heavy ones, were equipped with various forms of self-energizing brake in order to reduce the force required on the brake pedal. One simple type is the leading-shoe drum brake, in which the brake shoe leads its pivot point. That is, the pivot point is at the trailing end of the brake shoe, considered from the direction of motion of the brake drum. In this design, applying the brake causes the brake shoe to drag against the brake drum, which drag force then works to force the shoe against the drum with more force still, thus getting more braking from the initial application force, multiplying it by some factor. The troubles with self-energizing brakes are equally well known. They are very susceptible to changes in the coefficient of friction, which naturally changes the multiplication factor. If the brake gets hot on a descent, which lowers the coefficient of friction, the multiplication factor gets smaller and the brakes can barely stop the car, a condition known as heat fade. Conversely, if anything raises the coefficient of friction, such as escaped lubricant (which makes brake material sticky), the coefficient of friction rises and the brakes lock with the slightest application. These principles apply equally well to bicycle brakes that may be designed with self-energizing action, and to brakes that inadvertently have self-energizing action.

A cyclist asked a bicycle shop to upgrade his mountain bike by installing a particular brand of suspension fork and a particular brand of self-energizing brake. The bicycle shop complied with his request. After the cyclist had picked up the modified bicycle, he had cycled only a few blocks when he applied the front brake gently to stop beside his parked car and was immediately thrown over the handlebars as the brake locked. The self-energizing brake by itself was not dangerous, but the design of the front fork also produced a self-energizing effect on any brake that was mounted on it. What with new brake blocks, that are often a bit sticky, the combination of two self-energizing effects produced a brake that locked immediately.

The Scott Self-Energizing brake is a cantilever type in which each brake arm is mounted on a stud that is brazed to the front face of one of the blades of the front fork. In this design, the brake arm is allowed to move forward as the brake is applied, but is forced by a spiral groove to rotate simultaneously toward the rim of the wheel. This produces the additional force that forms the multiplying factor.

The particular RockShox suspension fork that was used has what looks like an economical and light design. Such forks must be straight to allow the telescoping action that provides the suspension action, but, like bicycle forks generally, the axle must be ahead of the steering axis to provide proper bike handling (rake is the technical name for the forward distance). In most such designs, this rake is provided by welding a plate on the front face of the fork, which plate has the slots for the front axle somewhat more than one inch in front of the fork blade. This particular RockShox design eliminated the additional plates on each side by slanting the fork blades relative to the steering column at the top of the fork, thus providing proper rake without the additional plates. However, like conventional forks, the studs for mounting the brake arms were welded perpendicularly to the blade, and therefore were tilted up relative to the normal position because the blade was tilted forward. This meant that the motion of the rim had a downward component relative to the brake-arm studs, and that therefore the braking force tened to drive the brake arms downward and together, thus multiplying the braking force. This self-energizing effect was small enough to be ignored when using conventional brakes, but when combined with a brake that had its own self-energizing effect the multiplied effect made it very sensitive to locking with small initial applications.The following calculation sheets quantify the effect.

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