How It Works: Hydraulic Disc Brakes

You know that the bike slows down when you squeeze the brake lever. The brakes convert the kinetic energy of a moving object to thermal energy (heat) as a result of friction.

Brake systems (although of the same hydraulic kind) have evolved tremendously over the years and can now slow a motorcycle faster than it can accelerate. Don’t believe us? How about these figures then: a BMW S 1000 XR can hit 160 kph from standstill in 6.1 seconds, in 151 metres. Yes, that’s fast! But it also brakes from 160 kph back to zero in 100 metres! And it’s not even a superbike.

The working principles of the hydraulic disc brake system is easy to understand:

  • Squeezing the brake lever will push the brake master cylinder’s piston, which in turn pushes brake fluid through the brake hose.
  • The brake fluid’s pressure now pushes against the brake caliper pistons.
  • The brake pads attached to the brake caliper pistons are pushed against the brake rotor (disc).
  • The pads squeeze the rotor which is attached solidly to the wheel, causing the wheel to lose speed.

Let’s take a look at the basic components of the hydraulic basic brake system (sans ABS):

Master cylinder

This part converts mechanical force from the lever to hydraulic pressure. The force you exert on the lever is called leverage ratio. The size of the master cylinder’s piston determines the amount of pressure in the system. It can exceed 1000 kPa.

Brake hose

A brake hose contains the brake fluid, enabling pressure to be transmitted from the master cylinder to the caliper pistons. Hoses are usually multilayered, with Teflon inner lining encapsulated by stainless steel or Kevlar or braided nylon, then wrapped in a protective jacket.

However, we’d have to point out that hoses that are not rubber do not contribute to higher braking forces. Instead, they contribute to more consistent braking as more pressure is applied, as they don’t expand as old rubber hoses do. Rubber hoses need to be replaced every four years.


The caliper consists of a number of pistons. It is here that the hydraulic pressure is amplified, as the pressure from the master cylinder is exerted uniformly over the much larger areas of the pistons. Brake pressure has to be multiplied, as the adult human hand can only produce an average of 0.4 kPa of maximum grip strength.

Brake pads

Brake pads are the components that generate friction with the brake disc, thus slow down the motorcycle. Brake pads consist of friction materials adhered to steel backing plates.

Brake disc

Rotors are typically made of stainless steel, with different amounts of iron and varying in size. Rotors are cross-drilled to assist in cooling and shedding water, debris and brake dust.

Brake fluid

The brake fluid is the medium through which the braking force is transmitted. Brake fluid needs to be non-compressible and has low viscosity for the ABS to work. Besides that, it needs to lubricate the master cylinder and piston seals, and most importantly, has a high boiling point.

The four grades are DOT 3, DOT 4, DOT 5 and DOT 5.1. DOT 3, 4 and 5.1 are glycol-based and can be mixed, while DOT 5 is silicone-based and is not mixable with other grades. Each grade corresponds to its boiling point.

That’s it for now. We shall continue with other aspects of motorcycle brakes and maintenance tips in the future.

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