How It Works: Brake Pads
Brake pads might seem like one of the simplest parts on your bike – they literally just use friction to slow either the rim of your wheel or a disc attached to it (as in a disc brake). But whether you’re dealing with disc or rim brakes, there are actually lots of choices to be made between durability, speed modulation and rim compatibility, all of which can make a huge difference in your ride.
The major challenge every brake pad has to overcome is heat dissipation. The friction of braking turns kinetic energy into thermal energy, resulting in a decrease in speed (which we usually feel) and an increase in temperature (which we usually don’t). A brake pad that can’t disperse the heat it generates will at the very least perform poorly, and in some cases can either heat a rim to the point of popping a tire, or warp a metal disc brake rotor, both dangerous situations.
A secondary challenge is the interaction between the brake pad and the braking surface. With different types of rims (carbon, aluminum, scandium, blends) and different types of pads (rubber, metal, sintered, organic), making sure both materials play nice not only ensures the integrity of the braking surface, but makes sure the brakes perform as desired. We caught up with industry experts to find out what makes brake pads work best, and what you should look for when replacing or upgrading yours.
With rim brakes, the temperatures generated during braking depend on the material of the rim. Christian Heuele of SwissStop explained that the brand’s rim brake pads for metal wheels are rated up to 180° Celsius, while pads for carbon wheels can sustain up to 320° Celsius.
A carbon-designed brake pad will work on an alloy rim, but since the rubber is harder and designed for higher temperatures, the braking will not be optimal. In contrast, a pad made for metal rims but used on carbon will deposit a lot of pad material on the rims as the pad disintegrates in the higher braking temperatures.
That’s why you need to remember to swap your brake pads when swapping aluminum rims for your carbon race wheels – forgetting to do so will result in that distinctive, earsplitting, screeching noise and generally poor braking performance.
There’s also a risk of damaging your rims. Nick Murdick, Shimano North America’s MTB Product Manager, told us that alloy brake pads can become embedded with flecks of metal which can damage carbon. Heule added that one should always check the owners’ manual in order to see which pads are approved by a rim manufacturer; some rims use a proprietary surface that only works with certain pads, so using others can sometimes void the warranty.
Disc brake pads open up another series of difficult choices and a new vocabulary. Essentially, the choice is between organic, or resin pads and sintered, or metallic pads. Heule told us that organic pads contain organic fibers bound together with heat resistant resins. Sintering, on the other hand, is a production process which Murdick described as “taking a container with the shape of the brake pad, filling it up with a [metallic] powder, laying a backing plate on top and heating and compressing it. Then a brake pad comes out!”
How do you choose? “Traditionally,” Murdick says, “metallic pads have offered less modulation and been seen as more noisy, whereas resin pads wear more quickly and can’t withstand higher temperatures.” On the positive side, resin pads are less expensive and more accurately replicate the feel of rim brakes, which is why they often come spec’ed on disc brake bikes. But metallic pads tend to last longer on muddy days – which notoriously can destroy resin pads in a single ride – and stay cooler than resin pads on long downhills.
Murdick assured us that most people we talk to who have tried both prefer a metal brake pad. He says, “Those really are the best choice for any time where heat is a worry.”
Murdick adds that the ‘howling’ often associated with disc brakes usually comes from contamination (like dirt, oil from your fingers, or other debris). Metal pads are better at resisting contamination, so [they] can be quieter in a sense that people care about.”
For larger riders, or those who like to use the brakes on long downhills to keep themselves at a comfortable speed, Murdick suggests a combination of larger rotors, pads with cooling fins (such as SwissStop’s exotherm2 or Shimano’s IceTech) and metallic pads. For those looking for consistent power, low noise, and great modulation, Heule suggests a resin type pad with a longer-lasting modern construction, such as the Swiss Stop RS.
Regardless of the type of pad, it is important to “bed in” disc brake pads correctly. “Bedding in is important because brake pads don’t grab a hold of stainless steel very well at all.” Murdick told us. What bedding in does is embed pad material on the rotor and create a surface that generates enough friction for good braking. Murdick suggests using a parking lot to do this as “any dirt could become embedded in the rotor with pad material”. Once you’ve found your parking lot, “you should be evenly and firmly applying pressure and not locking up the wheel. This will give an even amount of pad material on the rotor.“ Even once your pads are bedded in, Heule stressed that it is vital to keep grease, oil, and other liquids away from braking surfaces and to inspect both disc and rim pads frequently so that you don’t run out of stopping power halfway down a mountain and a long way from the nearest bike shop. Luckily, pads are affordable and small enough that we often carry a spare set on epic rides or weekends away.