Gears & Gadgets: The Most Annoying Sound, Part 1:  Disc Brakes

by David Cohen, Maryland National Capital Park Police Reserve Unit

You know the sound as soon as you hear it:  the sound of the brake rotor on your bike hitting the pads.   With each revolution of the wheel it hits again.   It’s like an audio version of the Chinese Water Torture.  There is no escaping it.   If you have mechanical disc brakes, you might be able to back off the adjusters to stop the scraping noise, but you’re left with having to pull the brake level almost clear to the handlebar to get any kind of braking power.   If you have hydraulic brakes, you just have to grin and bear it.  Well, maybe just bear it.  Fixing the scrape, though, is really not very difficult.

The most likely cause of the scraping is that the rotor has become out-of-true or is bent.  Before doing anything else, the first thing I do is take a Sharpie and number the spokes of the rotor.  Most rotors have eight spokes and you just number them 1 through 8, but clearly this method will work with any number of spokes.  If you prefer, you can use Roman numerals, I – VIII.  The reason for marking the rotor is to keep track of where the rotor is scraping, as it is very easy to lose track of the guilty section when you spin the wheel in an attempt to discover the source of the sound. 

Before truing the rotor, first check to make sure all the bolts that hold the rotor down are properly tightened and have equal torque.  Most rotors mounted on bicycles come from the factory with Loctite on the threads, and are usually pretty evenly torqued.  Stranger things have happened, though.  Most rotors use the star type bolt (a Torx bit), but you can use a smaller diameter Allen bit to check the bolt’s torque.  

Assuming all the bolts are tight and properly torqued, the next task is to check the true of the rotors.   While it is far easier to do this with the bike mounted on a repair stand, it can be done either with the help of a friend, or by yourself, with a bit of juggling.  Position yourself so you can see the rotor going through the brake caliper.  Spin the wheel slowly.  Be sure to keep your hands and fingers away from the rotors and calipers – they will eat your fingers, which is definitely not a comfortable feeling!  Odds are you will see a slight warping of the rotor when it makes contact with the caliper.  Remember those numbers you put on the brake rotor spokes?  Observe which number spoke corresponds to the brake rub. 

A disc brake rotor tool, like the Park Tool DT-2, makes this job a lot easier.  Basically, the tool is a piece of metal with two slots cut into it, one for larger corrections and one for finer corrections.  It is definitely a worthwhile investment, but if you don’t have one, a small adjustable wrench will work instead.  I use an antique “box” style adjustable wrench.  Fasten the wrench to the spoke you identified as causing the rub.  Here comes the tricky part:   very slowly apply pressure to the wrench, pushing it away if the outer caliper is being rubbed, or pulling it towards you if the inner caliper is being rubbed.  Keep in mind that you only need to move the rotor a millimeter or two to clear the calipers.  It is very easy to get over-enthusiastic and bend the rotor too far, which will cause it to rub the opposite caliper.  Remove the wrench and spin the wheel.  If the brake rub is gone, congratulations!  You may now continue with your ride.  If it still rubs, repeat the process, again taking care not to over-correct. 

You may find that even after correcting the rub, the rotor may eventually start rubbing again.  This is because metal has “memory”, and it might head back to where it was used to being.  Just repeat the process.   If your rub lasts for more than a third of the travel of the rotor, the problem could be that a caliper adjustment is needed, not a truing of the rotor.  That topic will be covered in future Gears and Gadgets column.

Of course, “The Most Annoying Sound” is not exclusive to disc brakes.  Bikes equipped with rim brakes can also fall victim to this ailment.  We will cover “The Most Annoying Sound for Rim Brakes” in the next Gears and Gadgets feature.

Until next time, keep those pedals cranking and be safe out there!

About the author:  David Cohen is a 12 year volunteer with the Maryland-National Capital Park Police.  When he isn’t riding or tinkering with bicycles, David can be found tinkering with vintage cars or World War II airplanes.  An avid historian, David enjoys researching and writing as well.   He can be reached at onyxsax@aol.com.

(c) 2019 IPMBA.  

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