The Science of Stopping on Your Motorcycle

Disc brakes can make or brake your ride

Modern motorcycles get faster and more powerful every year. But what good is a warp-speed missile if you can't stop it? Luckily, industrious engineering departments in the motorcycle biz have kept brake technology apace with the recent burst of horsepower increases.

The once ubiquitous, notoriously fade-prone drum units of yore are now a rarity on primary motorcycle brake systems, having been relegated to rear wheel duty where powerful stopping force isn’t as crucial. More reliable hydraulic discs handle the front braking chores on all but the smallest and least expensive bikes. But even though they’re a vital formula in the motorcycle equation, brakes, like tires, get very little respect for all the service they provide. Most of us take those essential pads, rotors and calipers for granted, so here’s a quick remedial disc course.

the science behind motorcycle brakes
Brakes can make or break your ride so it is important to understand the science behind the stopping power.Illustration by Jim Hatch

Brake Basics
Most disc brakes are operated hydraulically rather than mechanically, because it's easier to apply relatively low lever pressure to the high-pressure/low-movement action required at the caliper. Hydraulic systems make use of the fact that liquids are not compressible and utilize a specially formulated oil pushed through hydraulic hoses to displace caliper pistons, thus pressing the pads to the rotor. The three main components of a disc brake—the pad, the rotor/disc and the caliper—affect the degree and efficiency of a bike's stopping power.

Get Your Rotor Running
Brake rotors are made of cast iron or stainless steel, but stainless steel has fast become the material of choice on modern bikes. Iron rotors are still cheaper to manufacture and they dissipate heat faster, but they're also rust-prone and can be brittle. Stainless steel rotors, on the other hand, will probably outlast the bike they're attached to.

Pad It Out
The three basic types of disc brake pads are organic, semimetallic and sintered. When disc brakes first surfaced a couple of decades ago, all original equipment and aftermarket pads were called "organic" and made of asbestos fibers embedded in a binder, then stuck to a backing plate. When the toxicity of asbestos was uncovered, alternate materials were developed to take its place. Organic pads now most closely resemble the extinct asbestos pad and provide similar braking qualities.

On the Inside
Brake pads begin life as a potpourri of powdery elements blended to make up the pads' compound. The ingredients are then poured into a mold where they're cured, either by heat or a combination of heat and pressure. The resulting pad is then fused to a metal backing plate.

A conventional semimetallic pad has different-sized metal chips of varying components—usually softer alloys—embedded in an organic binder and bonded to the backing pad. These particles provide the desired friction and heat-carrying capacity of the pad.

Sintered metal pads are a complex mix of metallic powders combined with refractory material; the metal absorbs heat and the refractory elements provide friction. Friction modifiers and graphite are also in the mix. Modifiers allow the pad’s friction level to be finely tuned (so it doesn’t give a poor initial feel with a harsh final bite, for instance), and graphite minimizes rotor wear and helps eliminate noise.

These elements are mixed and pressed in a sintering process (the bonding and partial fusing of metal components under heat and pressure just below the melting point) to achieve form and thickness. Then the slug of sintered material is bonded to a backing plate. The process offers better alloy dispersion and consistent performance as the pad wears, and sintered pads can be better in moist conditions because they’re more porous—thus preventing water from collecting on the surface.

Is More Friction Better?
Many riders think a so-called highest-friction pad is the best, but brake pads are carefully engineered to balance instantaneous braking against pad reaction to cold brakes, high temperature or high speed. A sudden change in braking characteristics as you pull in the lever can be disastrous—and a high-friction pad can be too difficult to modulate on hard stops.

Insulate When You Radiate
All brakes create heat when they dissipate kinetic energy, and heat is hard on the calipers and brake fluid. Some brakes apply a layer of ceramic to the back side of the pad, between the backing plate and the pistons, to further insulate against the damage caused by high temperatures.

That's No Pig
When the pads squeal it's usually because they're resonating inside the caliper body, though occasionally the wail is caused by a glazed brake pad or imperfections on the disc. Using the correct balance of graphite in a sintered pad, the correct OEM "anti-squeal plate" or a high-temperature grease on the edges and back of the backing plate can usually fix this annoyance.

Tires and brake pads accomplish similar objectives—tires create friction against the road, and brakes create friction against steel or iron discs. So choosing replacement brake pads takes as much consideration as choosing the right tire. Do you ride hard or just cruise? Are you an all-season rider or a fair-weather weekender? Take these questions into account when you look for replacement parts, though most original equipment on today’s bikes is fine for all-around use. Remember, horsepower is nothing without restraint.