by Barry Fraser
Disc brakes, the technology that has been a fixture on mountain bikes for years, is becoming much more common on road bikes. (Just look at the two road bikes reviewed in this issue.) This past spring, the UCI said that pro teams could test disc brakes in two races in August and September, with more testing to follow in 2016. The power, modulation and reliability of discs are so good, why shouldn’t the pros have access to the technology you can use right now? If you are a roadie who is new to the world of rotors, calipers and hydraulic fluid, here is what you need to know to take care of your disc-brake system.
Hydraulic fluid and bleeding
A disc-brake setup can rely on cables or hydraulic fluid to transfer the pulls of the levers into pressure on the rotors. Hydraulic brake systems are the focus on top road bikes. The fluid for this system will either be specialized mineral oil or an automotive brake fluid classified as either DOT 3 or DOT 4. Both of these fluids are incompressible, and thus transfer the force applied by the brake lever to the caliper efficiently. These fluids are not interchangeable as each system is designed to use one specifically. Brake fluid is designed to withstand heat and to handle moisture contamination. Because of the lightweight and compact nature of bicycle components, the minimal material means less surface area for the heat generated through braking to dissipate. As the components heat up, so does the fluid. If the fluid reaches its boiling point, it can break down and form gas bubbles. These gas bubble will compress and create a spongy feeling at the lever. In extreme cases of heat buildup (caused by dragging the brakes down a long descent, for example) or excessive moisture contamination, you may lose your ability to brake completely. The best way to avoid this failure is through routine maintenance and fluid replacement.
Bleeding is the common term given to the process of cycling out the old fluid and putting in fresh stuff. This process differs for each manufacturer, and sometimes across the model of disc brake. Each manufacturer also has a specific bleeding kit that includes fittings, tools and fluid. You should use only the specific kit provided by the manufacturer to protect the warranty and your safety. If you lose your hydraulic pressure, you lose your brakes, no matter how hard you squeeze the lever. On a properly maintained system, this is an unlikely scenario, which is why you should never cut corners or neglect your equipment.
Disc brakes pads are very different from their rim- brake counterparts. Disc-brake pads come in one of two materials, both of which are more dense than the rubber used with rim-brake pads. Disc-brake pads are usually organic-based resin components or metal. The majority of road pads will be resin-based, which are quieter and offer better modulation. Metallic pads are designed for highstress applications, such as downhill off-road conditions. Unless you are hitting triple-digit speeds descending the Alps, organic pads will get the job done.
When maintaining your ride, you must be very careful not to get any oil contamination on your disc rotors that can transfer to your brake pads. Any spray from
your drivetrain, or even your greasy fingerprints, can contaminate of your brake pads and greatly reduce their ability to stop. Unfortunately, if you do contaminate the pads, there is no guaranteed cleaning process. Replacing them is the safest option.
Disc brakes use a small diameter rotor as the braking surface. The material of the rotor has been cut away as much as possible to prevent buildup and reduce weight. They are also susceptible to bending if you lean something against them. When set up, the brake pads sit very close to the rotor. A slight bend in a rotor will cause brake rub and a telltale “shing” sound with every rotation.
Truing a rotor should be done with a specific tool designed for the job. You need something with a broad surface so you can make very slight adjustments. Using the wrong tool (a screwdriver, for example) can cause a permanent defect or knick in the braking surface. Watch your fingers when working with rotors: their gaps can act as guillotines on misplaced fingers.
Disc brakes work with much tighter tolerances than rim brakes. Even as the pads wear on the disc brakes, the braking surfaces effectively maintain the same distances from their discs. To get the best performance out of your system, calipers need to be centred perfectly over their discs. Before centring a caliper, start with a true disc in place. Next, loosen the two caliper binding bolts just enough so that the caliper can move freely. Squeeze the brake lever so the pads tighten and centre the caliper on the rotor. With continued pressure on the lever, snug the two binding bolts. You should end up with a perfectly centred caliper.
Soapy water works best for cleaning the brake system. Remove your wheels and the disc pads. Clean each component separately. Use a rag and an old toothbrush to clean the caliper, soapy water and fine steel wool for the rotor and a clean, dry rag for the pads. Be careful around the braking surface of the pads or the rotor: oil from your fingers or a contaminated rag can greatly reduce a component’s ability to stop the bike. Wearing a clean pair of nitrile gloves and having a dedicated scrub brush in your kit are the best ways to keep the wrong stuff from getting on pads and rotors.
Seating the pads
When you clean or replace disc-brake components, you have to seat the pads to maximize performance. You seat pads by making several high-speed stops on your bike. Find a paved surface so you minimize skidding. When you apply the brakes, try to mimic a car’s ABS system: apply a lot of force to the levers to make a hard stop, quickly release them, and then reapply immediately. You should be able to apply the brakes three to four times before stopping without skidding. This process scores the pads against the rotor surfaces ensuring the best contact with minimal squealing.