Above is the link to the full eleven page article in November’s Barnett Bicycle Institute disc brake technology or research article. Best brake wear article I’ve read. Rather long tech précis.

Bed-in or burn-in refers to properly breaking-in disc brakes. Shimano has specific bed-in recommendations for their equipment as do some other manufacturers. See end of blog.

Interesting technology to consider and much disc brake and pad testing comes from car racing engineers. Among their vast research Barnett purchased $40 and relied heavily on a science-research paper, “The Role of Transfer Layers on Friction Characteristics in the Sliding Interface between Friction Materials against Gray Iron Brake Disks” (Tribology Letters, Vol. 20, No. 2, October 2005), published by Springer Science+Business Media, Inc.

Conclusion first, to identify my bias on the disc brake bed-in subject after five years of tandem, and half-bike experience with disc brakes… and Barnett’s research. The vast majority of master age riders do not need to be overly concerned with bed-in for average mountain bike off road rides. Similar to aerodynamic road riding components, unless one can pedal their bike along smartly above 20 miles an hour, “aero make’a no diff”. The bed-in discussion really is academic. Barnett “… we’ve been doing considerable amounts of fascinating reading and testing… subject couldn’t be more complex.”

Complex because engineers need to measure, study, and conclude disc brake information from all of the following, weight, friction heat, weather temperature, friction time, gradient, speed, steel composition, and pad materials.

Previous thinking, brake pads are made with different phenolic resins to produce friction after a layer of pad resin has been transferred to the rotor. Latest thinking, this transfer layer may be critical for friction, but it consists of any number of things EXCEPT melted phenolic resin. Thermoset plastic cannot melt. It softens with heat and them becomes permanently hard. “When phenolic resin gets exposed to more heat than it can withstand, it breaks down into its chemical parts carbon ash, elemental gases, and hydrocarbon gases. Where did my break pad go?

Shimano’s “burn-in” procedure is described in S-Tec Video “HDB: Burn-In Procedure”. They recommend hard decelerations, but not full stops, “from a good speed”, while observing that braking power steadily increases with each stop. With fresh pads Barnett accelerated their Shimano test bike repeatedly to 14–15 mph, then decelerated hard to walking speed eight times, the recommended number. One should observe the gradual increase in stopping power that occurs with each repetition of the deceleration cycle and Barnett did experience this improvement in braking. The burn-in technique successfully replaced the rotor’s new manufacturing grinding lines with concentric wear lines, which means the rotor was fully conformed to the pad surfaces. Their test rider was 220 pounds, bike was 28 pounds with 160 mm rotors, and ambient temperature was 55 degrees.

Measurement complexities are pad composition, weight, surface friction size, time, and temperature. It takes more energy to slow heavy riders than their unmanly counterparts and longer friction time to wear down a larger rotor to the same degree. Softer resin compound pads conform fully to the rotor with as little as one hard deceleration. Harder metallic pad conformity takes more deceleration cycles, but metallic pads are more abrasive, so rotor conformity should develop more rapidly. Lower ambient temperatures cool the rotor more quickly, which reduces pad heating and slows down the rate of pad wear. Hmmmm.

Rotor color: Barnett observed their rotor braking track turned a brown/purple color, with the spider arms showing the brown color fading to a lighter yellow/brown discoloration extending to the spider arms. Steel goes through a series of predictable color changes that correspond to temperature achieved. In this case between 500 – 540 degrees F. Steel tempered reduces hardness and its resistance to abrasive wear. As important is the time the steel is held at the maximum temperature and the rate at which the steel cools. Brown/purple color indicates that some tempering has occurred. The rotor may only wear 10% faster, or the wear rate could be more dramatic. When the rotor wears out months later, no way exists to know if it wore out prematurely due to a temper change, or due to any number of other issues that increase rotor wear. Light yellow/brown color on the spider arms just inward from the brake track indicates a maximum temperature of 400 degrees F. Only very moderate tempering occurs at this temperature. It is normal and acceptable when yellow/brown tint appears on the rotor’s brake track.

Transfer layer: Can one tell it’s been properly established? No, and Barnett concludes the question is largely academic. Pad technology combines different materials, chemical binders, reinforcing fibers, abrasives, lubricants, and friction modifiers. Ingredients are mixed, compressed to a backing plate with pressure and heat to form the friction pad. Binders, such as the phenolic resin, glues the composition. Reinforcing fibers, nylon or Kevlar, gives structural integrity and the fibers resist pad crumbling. Abrasives, silica or zircon crystals, create friction against the rotor, a sandpaper affect. Lubricants, dry compounds of graphite powder, balance the abrasive’s friction, and friction modifiers, rubber powder, is a less aggressive abrasive component.

Friction wears pad material to dust debris. With intense braking heat some debris gets melded to the rotor as atomic, chemical, and electrostatic bonding. Barnett’s research show thickness 8 – 20 microns (.008mm to .020mm). That transfer layer might add to the slight rotor color change, as perhaps light reflection on the concentric wear lines. “… no conclusions about the presence or absence of the transfer layer can be based on visual evaluation. The transfer layer constantly builds up and breaks down. At low temperatures abrasives gouge the transfer layer, a desirable friction. Higher temperatures add debris dust to transfer layer to a certain point when the layer begins to soften and chemically break down, thinning the layer. Both too low and too high braking temperatures degrade the transfer layer. Test results show a temperature-range sweet spot for maintaining the transfer layer as narrow as 125 degrees F. Try to monitor this component on your next trail ride 🙂 In a single ride, the transfer layer may be created and eliminated multiple times.

Oh, one more complexity reducing brake efficiency is contamination such as skin oils, dirty mechanic hands, lube overspray, normal road detritus, car oils, dust… Clean rotors with alcohol. Barnett says the friction material on the pad is porous, so no effective method exists for cleaning contaminated pads…futility of cleaning pads. When cleaning fails to eliminate problems, pad replacement may be the only solution, but when the rotor is substantially worn new pads will wear out much faster. Pads and rotors should often be replaced as a set.

A different breaking in recommendation if interested.

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