BRAKING POINTS | PART THREE
EVERYTHING YOU NEED TO KNOW
ABOUT BRAKE ROTORS AND PADS
PRESENTED BY TRP
How can you get the best out of your brakes this season? While you might have some of the most powerful brakes on the market, not having the right contact between the pads and rotor can make your brakes go from being able to stop a freight train to not being able to stop at all. You shouldn’t have to be a pro to ride perfectly tuned brakes, which is why we are going to share some of our braking expertise from years of research and development with our professional athletes on World Cup tracks around the world by covering everything you need to know about disc rotors and pads in this TRP technical column.
Brake rotors are an essential component in the hydraulic equation as they provide the key point of leverage to decelerate the wheels. However, not all rotors are equal as they can differ in their manufacturing process, the cutout design, and the diameter, all of which can completely change how your brakes feel and how they end up performing.
Brake rotors start out as plates of hardened stainless steel which are then cut into those familiar circular shapes using a plasma cutter, water jet, or hydraulic press. From there, each rotor is given a centerless circular grind to sand off any burs from the cutting process. This circular grind not only flattens the surface of the rotor to ensure a consistent braking track, but it also gives the rotor a raw circular finish which allows for pad material to be transferred to the rotor during the bedding process (which we will get into later). These circular grooves cannot be cut too deep as that can cause the uneven addition of pad material onto the rotor, kind of like the way wind causes ripples in a sand dune, which can lead to shuttering and vibrations as you brake. And they cannot be cut too shallow either or there will be no frictional surface for the pads to bite into and thus no stopping power. Therefore, the manufacturing process of a rotor and the surface grind it receives is a crucial aspect of the rotor being able to hold consistent friction and operate as a lever to stop motion.
Not only does the surface grind impact a rotors performance, but the cutout design on the braking track can completely change the way a rotor operates in terms of power and cooling. For optimal performance of a brake (looking at stopping power and modulation ability only) a disc with no cutouts would actually be best as that would allow for full contact of the friction surface between the pad material and the rotor. Same as a race car, the optimal grip is from a slick tire on smooth asphalt. However, because a bicycle is not a race car and we don’t live in a perfect world, the rotor needs to deal with the issues of cooling and debris removal, along with providing ample stopping power. The cutouts are spaced so an opening passes the pad at some point during the rotation and allows for gas exchange to occur which dissipates the heat buildup along with providing a path for debris to be pulled out. If you remove too much surface area on the rotor to achieve a greater cooling ability, more surface friction between the pad and rotor is lost causing the overall reduction of deceleration. With our thicker 2.3mm rotors, cooling is not a big goal of the cutouts because the thickness allows for extra heat-capacity leaving the cutouts to be designed for maximum surface area to the pad and consistency during high intensity usage. Therefore, optimal performance of a rotor is a compromise between power and cooling, so you’ll have to decide which is more important to suit your riding style when shopping for your next set of rotors.
Another important aspect in tuning a braking system to fit your riding style is the diameter of the rotors you choose to run. With all things being equal, a larger rotor is basically a bigger lever to stop motion, AKA: more stopping power. With the hub being the center of lever rotation, say you want to run a 223 mm rotor over a 203 mm rotor, you are essentially grabbing the rotation with a 10 mm longer lever (measuring the radius rather than diameter). Therefore, a larger lever equals a larger mechanical advantage in stopping the rotation so if you are finding that your current setup is not doing the job in terms of delivering enough stopping power or modulation, experiment with increasing the size of your rotors until you feel comfortable with the leverage it provides. We recommend experimenting with rotors first as they can have a larger impact on how much power and heat management a brake can offer. Once you have selected your rotors, it is time to turn an eye towards different pad compounds and the more finite tuning options they can offer.
Pads are always going to be the cheapest and most replaceable part off any disc brake system and yet, they are the most integral part in the stopping equation as they apply the necessary friction to decelerate momentum. Made from a powdered mixture of metallic flakes and organic resin in a formula guarded closer than Coca-Cola’s, they are pressed into shape and affixed to a metal backing through high pressure bonding. Brake pads can be found in many different mixtures ranging from full resin to full metallic, offering the rider different friction coefficients and operating temperatures that can change the way a brake feels and reacts during actuation and at different points on the trail.
Heat sensors placed on the caliper help identify maximum operating temperatures, a very helpful data point for our laboratory engineers.
Photo: Nicolas Brizin, Commencal 100% Race Bike
When testing pads and potential mixtures with our athletes and engineers, the main aspect we focus on is heat because this is what directly influences the friction coefficient of the pads. The three temperature ranges that define the performance characteristics of a pad are the ramp-up, the sweet spot, and the falloff. During the ramp-up phase, the pad material makes its first contact with the rotor and the temperature will start to increase from the applied friction. As the temperature increases, it will hit an optimal range called the sweet spot which can vary in activation temperature from compound to compound allowing some to hit the sweet spot much quicker than other. This is where the surface of pad becomes almost sticky as it presses into the rotor giving a larger friction coefficient and more bite. When the temperature of the pad rises above this optimal point, you reach the falloff phase where the pad gets too hot, the material starts to melt (glaze), and the bite begins to diminish. Think of these three phases in terms of a sticking a piece of gum to the underside of your desk in high school. Fresh out of the wrapper it will be pretty hard to get that gum to stick. However, let it sit in the sun or give it a chew to apply some heat and you’ll likely find that it becomes much more pliable and stickier to surfaces. This is the same idea with pads. Apply the right amount of heat to activate the optimal friction coefficient of the compound and you achieve its full stopping power.
This can be demonstrated from internal testing with our DH-R EVO caliper, Performance Resin Pads, and 203/2.3 mm R1 Rotor. As you can see, after each cycle with the lever load staying at a constant 80 N, the braking power increases in accordance with the increasing temperature.