Ray Thomas is a Portland bike lawyer.
The hill down from Washington Park past the Multnomah Athletic Club to Lincoln High School in Portland is steep and crowded with cars. Our Monday and Thursday noon time work out rides follow this final descent into the city.
Riding together in a group that changes shape as we descend, usually talking, we ride our brakes to keep from accelerating too quickly as we coast down the hill. At the bottom is a light that we never seem to make. As we talked and waited for the green, we were all surprised by the explosion that signaled the failure of my front rim. Looking down, I saw a thin band of metal that had been blown out and away from what used to be my front rim. My bike was not rideable, so as I walked across the bridge over I-405 I had several minutes to remember the last two times this had happened.
The first, occurred about 10 years ago after my trusty green touring bike and I had just made it to the top of Portland’s West Hills and the rear rim exploded, the result of brake pad wear. I was practically stopped and by pure luck did not crash, but it does not take a great deal of imagination to picture what would happen when a rim explodes at speed. The suddenly flat tire would make the bicycle practically uncontrollable, and the rim breakage would likely prevent the wheel from making a full revolution through the brake pads, stopping the bike immediately and causing a complete loss of control.
The second occurred in 1997 when another rear rim failed, after about eight years of riding. After my last explosion experience, I wrote an article about it for Oregon Cycling (“Law of Friction,” June 1997). This most recent blast has stayed in my mind so vividly that this article is the result.
After I got to a bike shop, close inspection of my front wheel revealed that in the ten years that I have owned this bicycle the brake pads had worn the rim so much that it lost its structural integrity, popping out under the force of the air pressure of the inner tube.
After the wheel was replaced, I talked about the incident with Randall, the bike mechanic. He took a straight edge and held it up against the most worn part of the rim; a thin crescent shaped opening revealed itself when held up to the light, a concave surface demonstrating wear from brake pads. Examination of the wear surface with a fingernail revealed slight longitudinal gouges in the braking surface of the wheel, probably caused by grit picked up by the brake pads.
Later, I cut the now useless rim with a hacksaw. The cross-section clearly showed the thinness of the rim wall. An engineer friend explained that the section where the rim failed was not defective metal, but instead showed classic signs of metal fatigue and failure. Since all but the cheapest wheels are made with aluminum, the metal on the wheel had fractured at the point where it was worn thinnest from years of brake pad wear. The torn metal had an almost crystallized appearance, rough to the touch. The point of failure was still surprisingly thick, about a millimeter, which surprised me as I expected a paper-thin place at the point of failure. My front wheel was an accident that was a long time in the making.
In each of my three wheel failures the equipment was almost ten years old. Years of year round riding had caused road grime, brake pad and aluminum rim dust to get rubbed under great pressure by the brake pads into the side of my wheel. Brakes rely upon friction to work and considerably more rim wear occurs than most people realize. Most of us are accustomed to periodically replacing our brake pads but many riders fail to consider that the rim is also part of the brake system, and that it gets ground down every time the brakes are applied.
One of the biggest advances in bike technology is the improvement of brake performance. The old-fashioned center and side pull brake mechanisms often required a strong pull to slow the bike down, while many of the newer versions are capable of locking the rear tire or making the bike do a front wheelie with just a single finger’s pull of the brake lever. High mileage wheels that have not been crashed or otherwise slammed into oblivion must be checked by the careful rider or bike mechanic every time a flat is repaired or the brake pads replaced.
Several high-end wheel sets have hardened sections or ceramic coating designed to increase strength and lower wear. While this may be a good improvement, it is still necessary to examine wheels for wear, particularly when a high-speed catastrophic rim failure is the possible result.
One would think that mountain bikes would be particularly prone to brake pad induced wheel failure. Off road riding conditions cause greatly increased brake and rim wear. I can recall going through a set of brake shoes in a single weekend of gritty trail riding. Perhaps the lower air pressure in a mountain bike tire (35-65 psi) as opposed to a road bike tire (80-125 psi or more) accounts for the fact that I have never managed to blow up a mountain bike wheel.
If you have several seasons on your favorite wheels, particularly if you ride year round or in dirty conditions, be sure to check your wheels or have them checked. There does not appear to be a handy ratio for brake pad versus rim replacement frequency. It is not recommended that heavy steel wheels be purchased in order to increase rim life because steel is so hard that brake performance suffers greatly in the wet. Instead, buy a nice set of light aluminum rims. The weight of the wheels has a great impact on how light a bike feels and rides. Keep an eye on brake pad and rim wear and enjoy many miles of good riding on your new wheels.