Warped Rotors Explained
by Kevin O’Shaughnessy
I own a 2013 BMW F800GT with a little over 25K miles. Last riding season, I noticed a very slight pulsing in the front brakes that would come and go. This season, the condition has become much worse, so I believe the front discs are warped, though I have never used the brakes hard enough to overheat them. The calipers are Brembo, the front pads are original, and I have flushed fluids at regular intervals. Could pad material be a contributing factor to distortion in the discs? Should I replace them with aftermarket discs? —Al Mears
You could be right about the pads, but there is a possibility your rotors (discs) aren’t warped. Creating heat on a rotor is relative to the weight, speed and rate stopped. As any of these increases, temperature increases. There’s no way around that. Yellow sintered metals, such as bronze and brass tend to be long lasting, low abrasion (wear) and lower friction coefficient metals.
They provide linear knee point curves, which is the point where max braking effort declines at a peak temperature. These metals provide better feel before locking up. Silver metals, such as tin and aluminum, are both abrasive and have very high friction coefficients, steep knee points and are often used in part or whole as race pads. They lock up very easily. Street pads have variations and combinations of materials that vary in friction, abrasiveness and linear feel.
The goal on street is to provide a seamless transition with increased friction, unlike a pure race pad. Pads from 30 to 40 years ago contained more organics and less high friction metals. They didn’t have the same braking ability current models do, nor were the chassis and suspension able to handle the loads modern brakes offer.
The F800 is advanced in this respect, and can create some serious heat, while not feeling any different in performance. Modern rotors have larger displacement holes and thinner tolerances, to reduce weight and rotating mass, but may be more likely to overheat and warp. Warpage can happen with high temp cycles, such as getting super-hot after hard braking then rapidly cooling from water or cold air.
However, this isn’t a normal occurrence and indicates poor treatment. Color may not be an indicator of warping. Bluing of rotors is normal with certain pad and rotor materials. Straw yellow can be seen after 350 F and bluing occurs about 550 F. These are moderate temperatures for brake systems, as 400 F is common for casual street use, 1,000 F during track use and 2,700 F at the surface during stress testing. These colors typically remain at the surface. Light braking with abrasive pads often removes the layer, then recolors the rotor as the temperature increases.
Other than warpage, a subtle pulse when braking may be a buildup of pad material that displaces into the rotor vent holes. Wear on break pads is typically caused by the wiping motion that displaces metal from the leading edge of the pad to the trailing edge. As it displaces off the edge, it can attach to the rotor holes and begin to build a bump over time, which causes a pulse in the lever.
Replacing the rotors will fix the problem, but you can also use a rotor Flex-Hone to clean pad material off ($40 online). It’s essentially a flat ball hone. Using the abrasive brush will usually fix this issue, but be careful not to round out the holes or cause deflections in the surface. Keep moving in a flat circular pattern and don’t stay in one place too long.
This practice is used by racers that use high friction race pads, for similar reasons. Removing old material makes sure new pad material is applied evenly to the rotor. Dissimilar metals from other pads will negatively affect braking performance. Like metals adhere to each other more easily, increasing friction.
Brake pad break-in is not an issue on a stock bike that uses the same pad over again, but racers constantly change materials and require a fresh surface and break-in period. There are two ways a rotor should be checked for warpage: thickness and runout. Thickness is measured with a micrometer at several points around the rotor and at different depths from outside to inside. Look for “dishing,” where one part is thinner than another.
If any is detected, and it isn’t a lump of displaced pad material, replace the rotor. Runout is measured with a dial indicator perpendicular to the rotor.
I prefer attaching a vice grip type dial indicator with a locking snake armature to the axle shaft or axle pinch bolts. Use whatever point is available on the fork without marring the surface. I’ve made a small “L” shaped attachment that can be placed under the pinch bolts and held onto by the pliers. Clamping dial indicators can be found at Harbor Freight for $33 (item 63656, shown above). Set the indicator at a radius that doesn’t have a vent hole in the path and rotate the rotor slowly. You shouldn’t see much, if any deflection, but most bikes have spring loaded floating rotors, and side loading them with your hand could throw off measurement. Hold the tire if possible when rotating. If you see a pulse within a couple of millimeters, it could be debris or displaced pad material.
Most manufacturers recommend replacing the brake rotor if any warpage is found. If ever unsure, replace the rotors. Brakes are not something to take chances on.