The Motorcycle Brake System: How It Works

Your motorcycle brakes have a tough job. We riders expect a firm pull on the lever to shed our speed in less time and distance than it took the engine to build it. What's amazing is that it does. Take BMW's S1000XR for example. From a standstill it accelerates to 100 mph in 6.1 seconds and 497 feet. Yet it slows to a complete stop in just five seconds and in less than two-thirds the distance. That impressive performance is due to the power and resilience of the modern hydraulic brake system, which is comprised of the following components.

MASTER CYLINDER

Whether operated by your right hand or your right foot, this converts mechanical force into hydraulic pressure via a lever acting on a piston that presses on a fluid. The force with which you pull the lever, the lever’s leverage ratio, and the size of the master-cylinder piston determine the amount of pressure imparted on the system, which may exceed 150 psi.

BRAKE LINES

These multilayer hoses transmit pressure from the master cylinder to the calipers and typically utilize a Teflon inner lining surrounded by a braided-nylon, Kevlar, or stainless-steel reinforcing layer that’s then wrapped in a protective sheathing. The stronger the hoses the better, since any expansion under pressure will reduce braking force and feel. Because rubber hoses lose their strength over time, most manufacturers recommend replacing them every four years.

BRAKE CALIPER

The brake caliper is where the multiplying power of hydraulics comes into play. Since pressure applied to a confined liquid is transmitted undiminished and with equal force to all surfaces within the system, the pressure from the master cylinder is exerted uniformly on the much larger area of the caliper pistons, increasing the force many fold.

BRAKE PADS

If any part of the brake system deserves sympathy, it’s the pads. Their job is to convert your bike’s kinetic energy into heat by way of friction. And we want them to do it quietly and repeatedly and without getting too hot. It’s not uncommon for the brakes on an aggressively ridden streetbike to heat up to 350 degrees Fahrenheit, while the pads and discs on a racebike may exceed 600 degrees. For an explanation of the main types of brake pad material, see the “Pick Your Flavor” sidebar.

BRAKE DISC (OR DRUM)

Transferring the brake pads’ resistance to motion to the wheels and on to the tires’ contact patches are the brake discs or drums. Discs are typically made from stainless steel, while drums are lined with cast iron. Both materials provide a durable, heat-resistant surface for the brake pads to press against. Modern brake discs are relieved, and or drilled, to assist in cooling and to help shed water and debris.

BRAKE FLUID

This is the elixir that allows it all to happen. Brake fluid is responsible for transmitting force from the brake lever to the back of the brake pads. It needs to be non-compressible to effectively transmit pressure, have low viscosity to be compatible with ABS components, have good lubricity for master-cylinder and caliper pistons seals, offer corrosion resistance, and also have a very high boiling point.

Brake fluid is available in four grades: DOT 3, 4, 5, and 5.1. DOT 3, 4, and 5.1 fluids are glycol based and miscible, while DOT 5 fluid is silicone based and can’t be mixed with any other type of fluid. Glycol-based fluids are hydrophilic and will attract and absorb moisture out of the air. DOT 5 fluid is hydrophobic, but due to the repeated heating and cooling cycles and the imperfect sealing of master cylinders and calipers, all fluids will eventually ingest some quantity of water. The difference is that glycol-based fluids will pull moisture out of the air on their own while DOT 5 will not, meaning DOT 5 has a much longer service life.

If you’re thinking DOT 5 sounds really appealing right now, think again. Expense, as well as the fluid’s compressibility and viscosity, makes DOT 5 unsuitable for everyday use. So why does it exist? It was created for the military to use in vehicles that will be parked for years at a time. Harley-Davidson used DOT 5 until a decade ago but specifies DOT 4 now.

A fluid’s classification has little to do with its chemical makeup. Rather, it’s based on the fluid’s boiling point. The Department of Transportation sets minimums for each grade’s “dry” and “wet” boiling point, with the former state completely free of moisture and the latter containing 3.7 percent water as is common after a year or so of regular use. DOT 3 fluid has the lowest minimum dry boiling temperature at 401 degrees (284 degrees wet), while DOT 5.1 has the highest at 518 degrees (356 degrees wet).

As the temperature ratings above suggest, any water content in the brake fluid will reduce its boiling point. Boiling brake fluid will make your brake lever feel spongy and braking force will be diminished. This condition is known as brake fade or, more specifically, fluid fade. Replacing your brake fluid regularly (most manufacturers say every two years) will help ensure your brakes always perform their best. See page 62 to learn how.

What's the Difference? What's the Benefit?
Nearly all modern sportbikes have radial-mount brake calipers, and the general consensus is that they're better. But why? First, the difference between radial-mount brakes and regular (or axial-mount) brakes: Radial brakes are secured via bolts that are parallel with the brake disc (see below) as compared to bolts that are perpendicular to the disc face (see photo at right) as on traditional brakes. Radial mounting typically offers better caliper-to-disc alignment and is significantly more rigid than axial mounting. A more rigid setup yields better pad-to-disc contact for improved power and, more significantly, better feel at the lever.

HH-rated pads are the strongest brake pads one can buy, but do you have any idea what those two Hs stand for? The letters indicate the friction rating and specifically pertain to the puck’s coefficient of friction, with the first letter quantifying the CoF at a normal operating temperature and the second letter listing the pad’s CoF at an extremely high temperature of 650 degrees Fahrenheit. G and H ratings are common for motorcycle brakes, with a G-rating offering a CoF between 0.45 and 0.55. H is the highest rating available and corresponds to a CoF of 0.55 or greater.

The Brake-Pad Material Breakdown
Brake pad choice is as varied as the options at your local ice-cream shop. Here are the three most popular flavors.

ORGANIC These pads are devoid of metal content (and asbestos) and instead blend rubber, glass, or Kevlar materials into a heat-resistant binding resin. Organic pads are the softest and thus the quietest option available and are exceptionally easy on rotors. They're also easy on your wallet. Downsides include shorter service life (compared to semi-metallic or sintered pads) and fading with aggressive use. Organic pads are most commonly found on older (pre-1990s) bikes, smaller modern bikes, and for rear applications on some larger modern machines.

SEMI-METALLIC These pads incorporate some metallic material (usually 20 to 40 percent by volume) into the friction material to increase the pads' friction level and durability and to improve fade-resistance under heavy use. These pads were invented as cars and bikes became faster, requiring stronger, more resilient brake systems. Semi-metallic pads are still fairly quiet, don't score rotors as much as all-metal sintered pads, and are a popular option for a variety of bikes.

SINTERED The name refers to the production process, which uses extreme heat and pressure to cement powdered metal (usually bronze) to the backing plate. Sintered brakes have a very high coefficient of friction and excellent heat transfer, making them ideal for aggressive use and racing. The downsides are price, increased rotor wear, and the fact that they usually require some heat to perform their best.