If you haven’t bought a bicycle in a while, you might be surprised at the revolution in braking. Hydraulic disc brakes with calipers and pads entered the market around 1997, first appearing as aftermarket equipment on mountain bikes.
They’ve become steadily more common since then and are now standard equipment on high-end mountain bikes and appear on many mid-level mountain bikes, as well as some road bikes.
Given that about 100 million bicycles are manufactured worldwide each year (with about 20 million sold in the U.S.), it’s an intriguing market from a friction perspective.
Hayes Performance Systems was one of the first manufacturers to apply disc brake technology to bikes. Its Hayes Mag brake revolutionized the market and was the first widely used disc brake.
Hayes also makes brakes for motorcycles, snowmobiles, construction equipment, and many other types of vehicles.
John Thomas has spent more than a decade engineering bike brakes at Hayes and worked on automotive brakes for nine years at Honda. “The needs are very similar in that you’re converting energy from one form to another, kinetic to heat, just like any other brake,” he says.
Bike brakes are diminutive compared to other types of brakes, but they’re highly engineered, sophisticated products.
“We do all the typical tests that we carried over from motorcycles. We run a number of dyno tests, environmental tests, fatigue, life, vibration,” Thomas says. In addition, bike brakes are a highly regulated market, with a number of U.S. and European standards that must be tested for.
“One of the things that’s unique on a bike, we do a torque spikes test, which we do on motorcycles,” Thomas says, “but we also need to have these torque spikes in reverse” for trials riding, in which competitors literally hop their bikes from boulder to boulder, creating huge stress on the entire system and creating reverse torques.
Further proof that bike brakes are serious business: Hayes spent more than three years developing its most recent complete brake system, the Dominion A4. The company went through “a lot of iterations, a lot of field testing, involving a fully instrumented data-acquisition bike using professional riders.” The Dominion A4 was developed specifically for downhill riding, a subset of mountain biking in which riders can exceed 45 miles per hour.
And, like the world of automotive brakes, there’s constant pressure to reduce weight. “It’s human powered. Every gram is important. You’re trying to maximize the efficiency of the brake and do that energy conversion in as small a mass as you can.”
“NVH is quite difficult in the bicycle space,” Thomas says.
“You can’t optimize for one specific frame, and just, if you have one bad frequency, modify your masses until you get rid of that noise.”
One brake system needs to work with hundreds of different bikes, each with different frame stiffness, spoke tension, tires, and other factors. There’s also the matter of weight. A 30-pound mountain bike might be ridden by a person who weighs 150 pounds or 250 pounds, creating very different total masses that need to be stopped.
Another reason NVH is so challenging is that the vehicle itself isn’t making noise to mask the noise of the brake.
Typically Hayes offers two specific pad formulations for each of its brake systems, one fully sintered and one semi-metallic. The sintered pad works better in wet and extreme temperature conditions, whereas the semi-metallic is typically better overall. Sometimes the company also sells a third option, such as an aggressive racing brake pad.
Overall, the pad formulations are similar to those used in motorcycles, with a MU around .45 to .5.
Thomas explains that bike pads require a very quick burnish cycle. “Nobody wants to be out there pedaling up to speed and braking” over and over. “We need to be at a very high percentage of the final MU after about ten stops.”
Unique to Bikes
Thomas lists several areas where bike brakes are outliers in the brake world.
Because the brake system itself is so small, tolerances are much larger in relation to the brake. When you have “a +/- 5 thou tolerance and your part is 30 thou wide, that’s a big percentage of the width,” Thomas says. “So the tolerances are more difficult to deal with.”
Another difference: “Our retraction is huge. Our retraction tends to be about 12 thou per side.” In the world of bikes, “as soon as you release the brake the pads cannot contact the rotor” because the drag would be audible to the rider.
A Fluid Situation
“In the bicycle industry, brake fluid is a hot topic,” says Eric Schutt, Marketing Manager for Hayes.
“We have experience making brake systems using either DOT-based brake fluid or a mineral-oil based brake fluid,” he says, citing the company’s brake systems for lawn care vehicles, which do not use DOT because it can damage grass.
“One of the things that’s unique for Hayes, I believe, is that today we’re the only bicycle brake company that manufactures both a mineral-oil based system and DOT-based systems, and that’s to meet customer demands.”
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