The wheels are one-piece cast alloy components with a rim configuration suitable for the tubeless tyres fitted to all models. Note that not only should the tyres always be of the specified size and speed and/or load rating for the wheel to which they are fitted, but that they should also always be of the same make and type from the selection approved by BMW. Do not fit any make or type of tyre that is not approved by BMW; the BMW importer or an authorised BMW dealer will be able to provide details of the current list.
The front brakes are of the hydraulically-operated twin disc type. The application of the handlebar lever moves the piston in the Magura master cylinder assembly producing an equivalent increase in pressure transmitted by the hydraulic fluid to all parts of the system. The Brembo brake caliper bodies are bolted rigidly to each fork lower leg and contain a separate aluminium alloy piston in each body half. The pistons are arranged facing towards each other on each side of each disc, so that the increase in pressure, magnified by the difference in piston sizes, causes each piston to move towards the disc, pressing the brake pad friction material against the disc surface. Given that none of the components are stuck with dirt or corrosion, exactly the same clamping force should be applied to each disc to slow the wheel. Each piston is surrounded by a fluid seal and a smaller dust seal to exclude any dirt or moisture. As the piston moves outwards the fluid seal distorts and, on release of brake pressure, springs back into shape, thus retracting the piston with it to prevent brake drag. As the pad friction material wears, the piston moves through the fluid seal to compensate tor this wear, more fluid entering the system from the master cylinder reservoir to increase the system's capacity accordingly.
The rear brake fitted to K75S and to all 100 models uses a Brembo-built master cylinder/fluid reservoir assembly operated by the rear brake pedal and mounted on the rear of the right-hand footrest plate. It acts on a single brake disc bolted to the final drive crownwheel flange via a Brembo caliper, in all other respects it is similar to the front brake system. Note that while early 1984 100 models were fitted with drilled rear brake discs, all later models are fitted with undrilled components.
Other 75 models are fitted with a rod-operated drum rear brake in which two shoes, with friction material bonded to their outer surfaces, are forced outwards against the inside of a drum, mounted in the rear wheel casting, by the rotation of a camshaft. On release of pedal pressure, the shoes are pulled clear of the drum by heavy return springs. Since only one cam is fitted, the brake is only of the single leading shoe type.
Next Section: 2 - Front wheel: removal and refitting