The Neat Design that Jammed the Brake

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Brake blocks are mounted on the ends of the brake arms, typically by one of two methods.

In the plain mounting, the end of the brake arm is approximately parallel to the plane of the wheels. The brake-block holder has a bolt extending laterally that goes through a hole in the end of the brake arm and is secured by a nut and washer on the outer end. The hole is slotted so the brake block can be raised or lowered to match the rim of the wheel, and the holder itself can be rotated on its bolt to align with the rim. This is a sturdy and reliable mounting, used on the best of brakes. However, the brake block cannot be positioned so it is toe-in or toe-out with respect to the rim. Some claim that this is a defect, because, so they claim, brake blocks should be positioned in the toe-in position. (Toe-in is when the trailing end of the brake block, with respect to the movement of the rim, is closer to the rim than the leading end.)

Another design has a stud instead of a bolt protruding laterally from the brake-block holder. In this design, the mounting face of the brake arm is perpendicular to the plane of the wheels, and it carries a clamping bolt with a hole through it. The stud of the brake block is inserted through the hole in the bolt, and the bolt is drawn up tight with a nut. There is a washer on the bolt between the brake arm and the stud, and this washer is wedge-shaped. This combination allows all the adjustments of the plain type, plus, by rotating the washer before final tightening, adjustment of the toe-in. This washer is loaded purely in compression, first by tightening the nut, then by the braking force transmitted from the brake block. However, this design has a greater tendency to squeal during application of the brake, probably because of the greater flexibility of the mounting.

The designer of the Lee Chi brake attempted to produce the flexibility of the stud mounting with the cheapness and reliability of the plain mounting. To the plain mounting he added two washers between the brake-block holder and the brake arm, these two washers made in matching dished, spherical shapes. The hole in the brake arm was made larger, also. Then, the toe-in could be adjusted by slacking off the nut and moving the brake blocks while the two washers slipped at their spherical faces. When the nut was tightened, the brake block holder was held in postition. However, in this design the braking force developed by the brake block against the rim was resisted not by compression of the adjusting washer, as in the stud design, but only by the frictional force between the spherical faces of the two washers. If the retaining nut worked loose, as inevitably some would in service, under heavy braking the drag of the brake block rotated the brake-block holder from the parallel to the toe-out position. This created a very powerful toggle-action brake because the leading end of the brake block tightened against the rim. If the brake had been a hydraulic brake with relatively little friction in the operating mechanism, this might have merely pushed the brake further from the rim and, at the other end, opened the cyclist's fingers a bit. However, with the high friction of the standard cable brake, already under strong tension from heavy braking, the cable would not move back and the result was a very significant increase in braking force that appeared so suddenly that the rider was thrown over the handlebars before he could release the brake.

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