Stop Tech | MC GARAGE

What You Didn’t Know About Brake Pads and Fluids

Photo: Milagro

As good as they are, stock brakes can often be better. And considering their life-saving responsibilities, there’s no shame in wanting the best. When your pads wear out and you replace both of them and the brake fluid—which you should do unless your bike’s service manual specifies replacing the fluid sooner—you have to look past the market-speak to get the real skinny if you’re going to get the improvements you want. Here’s how to tell what you’re getting and whether it’s really an improvement.

Virtually all modern streetbikes come with sintered metal pads, which are hard to beat for everyday street riding. They grip hard when they’re cold—which they almost always are in emergency stops—and they efficiently channel water away from the surface of the brake rotor in wet weather. Their high-temperature performance is usually as good as you’ll need at anything less than Lorenzo-versus-Marquez speeds.

Sintered metal pads start as a fine powder containing friction materials, a dash of lubricant, and ground-up magic braking beans. The recipe can be tweaked to provide more initial bite, more progressive feel, or better wet-weather performance in the final product. The powder is put into a mold, heated and compressed until it fuses into a solid block, then bonded to a steel backing plate.

Brake pads are rated for their friction coefficient on a scale of A to H; A is the rough equivalent of dragging a stick in the dirt to slow down your apple-crate go-kart, and H is the best you can get. Sintered metal pads achieve HH status, meaning both their cold-stop and hot-stop ranking is the highest. As with tires, a brake pad’s compound is tailored to the way it’s meant to be used, and although two different brands of pad may have the same rating, they can act quite differently on the motorcycle. One HH-rated pad might have more initial bite or could have a more progressive force/friction curve than another. Ratings alone don’t tell the whole story.

Organic and semi-metallic brake pads are made of different stuff than sintered metal pads and are typically softer and less grippy; most rate no higher than GG, a full step below sintered metal. They have their uses, though, such as on older bikes that didn’t come with sintered metal pads and whose brake rotors would wear faster with more aggressive pads. Some are made for dual-sport riding where gritty mud gets between the pad and rotor; the softer pad wears instead of the rotor. Organics don’t stop as well when cold, so they contain bits of metal such as copper or bronze that hold onto heat, keeping the pads warmer between stops. Poor cold stops are also common in some racing pads, which work much better hot than cold; their GH rating gives you that hint. Switching to an organic or semi-metallic pad can also make a sensitive rear brake a bit less so.

Brake fluid comes in two types. The most common is polyglycol-based, and is designated DOT3, DOT4, and DOT5.1. DOT4 is an upgrade of DOT3 and can––and should––be used to replace the older fluid. DOT5.1 differs from DOT3 and DOT4 in that it’s less viscous. It’s specified for ABS brakes because it cycles back and forth in the system more easily. Note that the other type of brake fluid, DOT5, is not a polyglycol, but a silicone fluid, and does not mix with them.

A characteristic of polyglycol fluid is that it’s hygroscopic, which means it absorbs moisture out of the air, sucking it through the fluid reservoir vents and past the caliper piston seals—just popping the cap on the bottle starts the contamination process. Each class of fluid has a “dry” and “wet” boiling point. Dry fluid in a sealed container has one boiling point; fluid with 3.7 percent moisture is considered wet and has a lower boiling point. It takes the fluid in the average streetbike about a year to degrade to where the wet boiling point is the one that matters. This happens sooner if you ride a lot in the rain or if you race the bike and subject the fluid to short periods of very high temperatures punctuated by long periods of sitting in the garage.

There are drawbacks to racing brake fluids just as there are with racing brake pads. The higher the dry boiling point, the more hygroscopic a fluid tends to be, so you need to change it more often. The reality is that on the street you’re extremely unlikely to ride hard enough to heat either the stock brake pads or brake fluid past their upper limits. Just keep an eye on pad wear—low fluid level in the reservoir’s sight glass is the tip-off—and change the fluid every two years to keep your brakes working at peak performance.

Quick Facts Sintered metal pads work better in wet weather because on a microscopic level their surface is granular. Water trapped between a flat pad and rotor initially has nowhere to go but through the slots or holes in the rotor itself. You apply the brake, not much happens, then the water is squeezed out and everything happens at once. Sintered pads allow the water to flow between the grains, giving another path of escape.