Brake functions

What are brakes and what are they for

Hydraulic brakes consist of two mutually connected pistons with brake fluid used as a medium for pressure transfer. One of the pistons with a small diameter and a relatively long stroke is controlled by means of a hand-operated lever or of a pedal; a brake yoke with the other piston characterized by a large diameter and a relatively short stroke is placed at the opposite end of the interconnection hose. The forces acting onto the pistons are inversely proportional to the surface areas of the pistons; this means that the larger is the piston diameter in the brake valve the lower force is needed to push the brake lever. The small piston diameter in the brake valve and, thereby, the shift magnitude of the brake plates is a few tenths of a millimetre. There is no force that could separate the brake plates from each other after the pressure release, which requires a low run-out of the brake wheel face. The brake wheels are manufactured either as solid pieces or in the form of steel rims riveted to low-play aluminium hubs. Brake wheels with a more significant run-out make the variable braking force to be perceptible as “kicking” in the brake lever.

In the brake system, pressure is transferred by the brake fluid that must be able of withstanding high temperatures caused by friction between the brake lining and the brake wheel. The temperature resistance of the brake fluid is a very important parameter due to small air bubbles expanding in a micro-droplet of water at boiling temperatures with two possible consequences. The first of these consequences occurring at moderate brake lever depressions, when the way from the lever piston to the storage tank is closed, is a quick expansion of these air bubbles causing an increase in the thrust applied on the brake pistons resulting in intensive braking up to wheel blocking the other consisting in an increase in the brake fluid temperature causing air bubbles to be given off somewhere in the brake valve area pushing excessive brake fluid out of the hose into the tank. Now there is not only brake fluid but also air with a considerably higher compressibility than the fluid present in the hose between the brake lever and the brake valve. Under these circumstances, a single brake lever depression up to the handlebars need not result in adequate pressure rise CAUSING a brake system failure. The braking effect may be increased by a quick repeated depressing the brake lever, though, but not to the same extent as with correctly functioning not aerated brakes.

The brake fluid is a hygroscopic medium able of absorbing air humidity decreasing its boiling point. This is the reason why brake fluid used in vehicles should be exchanged in approximately two years’ intervals using fresh products supplied in new airtight packages for re-filling.