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How to Adjust and Maintain Disc Brakes

By Joe Wilson

Disc brakes ushered in a new era for cycling, but the learning curve was a bit steep for many shop mechanics and consumers. This led to an initial stigma due to a basic misunderstanding of how the brakes were serviced and how they functioned. Initially, hydraulic brakes suffered the most because of the perceived bleeding difficulties; I know many people who prefer mechanical brakes over hydraulics because of a bad experience that could have been prevented. Over the last few years, disc brake refinements and the average consumer knowledge has increased. Now, instead of customers leaving their bikes for brake service, we are consistently selling sets of pads and bleed kits.

Today, discs are on many of the bicycles sold, save for road and cyclocross bikes, where it isn’t so much of a performance advantage as it is a weight penalty — or where rule compliance is a necessity. But, with the recent Union Cycliste Internationale (UCI) rule change, allowing bikes to have them, I imagine they will begin to show up at more races. Disc brakes are a good thing, they work very well in a variety of conditions and are available with features that make them simple to adjust and fine tune without the aid of a mechanic. The key is setting them up correctly, being able to identify when they aren’t functioning well, and what can be done to remedy the situation.

Hydraulic vs. Mechanical

There are two types of disc brakes currently available: hydraulic, which requires brake fluid or mineral oil — depending on the manufacturer — to actuate the pistons in the caliper, in contrast to the mechanical type that use standard brake cables and housing. The majority of hydraulics will have two opposing pistons that approach the rotor evenly as the lever is pulled. Most mechanicals have a single dynamic piston controlled by the brake cable/lever. One notable exception, IRD manufactures the Dual Banger mechanical brake that activates two opposing pistons simultaneously, but I have not seen it used extensively.

Photo by Seattle Bike Supply

Caliper Mounting Standards

The fitting method to the frame and fork varies. The most common style in use is the post to International Standard (I.S.) adapter. With these adapters the frame and fork have two holes drilled in an I.S. tab, located 51mm apart, center-to-center. The bolts that are inboard of the tab and the brake caliper then mount to the post portion, which has two threaded holes located 74mm apart. The majority of currently available suspension forks, and some frames, now come with caliper post mounts, eliminating the need for separate adapters — unless a larger rotor is desired. Some brake sets utilize I.S. mount calipers bolted directly to the I.S. mount on the frame. When choosing the brake rotor/adapter combinations for your bike you must be sure that the adapter matches the rotor size you will be using, otherwise the brake pads will not have enough contact with the rotor or the wheel will not fit properly.

Beginning Brake Setup

When beginning hydraulic disc brake installation, mount the levers to the handlebar, route the hose/housing on the inside of the left fork leg and affix the hose at the fork’s crown with a zip tie. Rear brake hose guides will usually be present, which indicate proper routing. Once again, use zip ties to secure the line. For older bikes with cable stops, you will either need to use zip ties or specific adapters that bolt to the cable stops and secure the lines — Shimano calls these disc brake support guides. If your hydraulic brake lines are too long, shortening them is a simple task and most brake manufacturers provide olives and barbs/inserts with new brake sets to help with this task. Refer to your user manual or the respective tech area of the brake manufacturers website for further instruction.

For mechanical brakes, cut the housing to proper length utilizing cable stops/guides. Once the housing is cut, locate the ends in the cable stops, use the correct ferrules and install and secure the cables. If using full-length cable housing, install it and then secure it to the frame with zip ties or the aforementioned brake support guides.

Rotor Installation and Standards

At this point the rotors should be installed. If you are using two different rotor diameters, be sure to mount them to the correct wheels. To my knowledge there are two common methods of mounting rotors in production, one is the six-bolt standard, the second is Shimano’s center-lock. The first type distributes six T-25, or torx bolts, evenly along a 44mm bolt circle diameter (BCD). The Shimano type uses a spline interface and a lock-ring to secure the rotor; in most cases a hyperglide lock-ring tool inserts into the lock-ring to either tighten or loosen it. Some Shimano “Saint” rotors require a tool specific to that lock-ring. For Shimano system wheels utilizing thru-axle fork designs, an external bottom bracket installation tool is used to turn the lock-ring. The Park BBT-9 is a suitable tool for this task. If using rotor bolts, be sure that there is some sort of locking compound on the threads, if none is present, use blue Loctite or an equivalent. Also, most rotors are directional so check to make sure they are oriented properly — I recommend using rotors that the brake was designed to work with. If using another manufacturers rotor, check that the width and diameter is the same.

Photo by Seattle Bike Supply

Another situation that can create an issue is using six-bolt rotors with center-lock wheels. In this case you will need a center-lock adapter, which can usually be purchased for around 20 dollars. It should also be noted that companies other than Shimano produce rotors and hubs that utilize the center-lock system, i.e. SRAM and DT Swiss. Be sure to tighten the lock-ring or rotor bolts to the manufacturers own specifications. Variations to the six-bolt or center-lock standard pattern includes Rohloff hubs, which use a four-bolt system with a 65mm BCD.

Pad Check & Wheel Mounting

Warning!!

A hydraulic brake should not be actuated without a pad spacer or until the wheels/rotors and calipers are mounted.

Before beginning caliper mounting be sure that the pads are in proper position. For hydraulic brakes, remove the pads and press the pistons into the caliper using a non-metallic object such as a plastic tire lever, then reinstall them. For mechanical brakes, back off the pad adjusters. In either case if the pads are excessively worn, replace them.

When the pads are in position, mount the wheels. Be sure that each wheel is set in the dropouts and that the quick releases or thru axles are properly installed.

If pad replacement is necessary you must break the pads in before going on a ride. A good method is to take the bike to an open area and then get up to speed and make a few lengthy stops. This process seats the pads to the rotors, allowing for proper brake performance. Pad break-in should be performed after the final adjustment is completed and in some cases small tweaks may be necessary.

[Ed. Note: This is part one of this article. Part two, coming up in our winter issue, will address brake caliper adjustments, rotor truing and trouble shooting.]

Joe Wilson is a professional bike mechanic at River City Bicycles in Portland, OR. He is also an event mechanic for Best Buddies Int’l. He may be reached at josephwilson81@gmail.com