The Chainlink

Chainlink Tech Corner: Causes for Wheel Failure

By Scott Wilson

Wheels die for a lot of expect-able reasons: crashes, potholes, drunk college bros stomping wheels at the bike rack. But there are some other, more insidious ways a wheel can fail that you might not be aware of. Here, in pictures, are three of the top surprise ways a wheel can bite the dust.

1. Brake Track Wear:
We ask a lot out of rims. They have to hold the tire in place, support the weight of the bike and rider, stay round and rolly, and provide a braking surface for the pads to run on. It’s this last vocation, stopping, that gets them in trouble.

Look at this rim:

See how the knife touches the top and bottom but not the center? That’s because the brakes have dug out a valley. Brake pads act like sand paper, especially when they get old or imbedded with actual sand and road debris. Give them time and they’ll carve the aluminum right out of the rim.


It’s easy to see how close your rim is to dying. Most rims have a wear indicator, which usually looks like a black line or a round, sunken dot in the center of the brake surface. When the line or the dots disappear it’s time for a new rim. In my experience the wheel will act up when it gets weak and fall out of true more easily than usual.


You want to replace a rim as soon as you notice wear because it can explode fantastically from the inside out.


courtesy of the Internet


The reason why is that the inflated tube presses against the tire, holding it to the rim, but it also presses on the body of the rim at many times the outside atmospheric pressure. When the aluminum (or carbon or wood or steel) gets worn thin it won’t have the strength to hold that pressure inside, so, KaBoOm!


There isn’t much you can do to prevent rim wear other than cleaning the rims with a cloth rag every so often and replacing the brake pads regularly. This is especially important in the winter and early spring when there’s more road debris about to infect the rubber (or cork) pads. Also, check to make sure you don’t have one pad dragging on the rim while you ride.


Fun little tidbit: most rims, especially the fancy carbon ones, are only rated to about 125 psi, or 8.6 bar. So although Vittoria or some other company might have a tire that says “max 200 psi”, don’t go there unless you want to add another layer of excitement to your afternoon ride.


2. Fatigue:
With modern rim design and alloys you don’t see metal fatigue very often, but it can happen, especially on rims that don’t have very many spokes, like the old Bontrager Select wheels sold on mid-level Trek road bikes in the mid 2000’s.


What’s going on is that the spoke tension is too great for the aluminum (or whatever material) surrounding the spoke nipple, so it bulges or cracks, like trying to pick up a pizza by the center. Rim manufacturers specify a maximum spoke tension so this doesn’t happen, but wheel builders sometimes break the rules. The good news is that if a rim bulges or cracks and the spoke tension is within spec, the manufacturer will owe you a new wheel.  


Another spot to check for fatigue is the weld where the rim is joined, usually found underneath the brand label. Cheap rims have been known to come apart there.


3. Broken Spokes:

Spokes usually break at the bend where they go into the hub. This happens because at the atomic level the bonds that hold iron atoms of the spoke in a crystalline structure rearrange and become weaker after the thousands of stretch and release cycles they endure in the normal course of holding the wheel together while you ride. The spokes on each side of the wheel are set to exact, identical tension, so when one spoke suddenly breaks it affects the balance of all the other spokes; some spokes will suddenly become loose, others will get tight. Even when the spoke is replaced and the rims looks to be true the overall spoke balance can still be upset. This will cause another sudden spoke break, then another. The only way to stop the cycle is for the mechanic to release the tension on all the spokes, then gradually and evenly bring all the spokes back up to spec. If the mechanic does this and spokes are still breaking, which they sometimes do, then the wheel will need to be re-spoked. This usually only happens on old wheels, so if you’re going to pay $65-$75 to have it rebuilt, you might as well pay the extra $X for a new rim and peace of mind. 


Winter is hard on wheels, so for those of us who ride all year, now is the time to check our equipment to make sure it’s ready for the heavy riding season. Wheels don’t last forever, I’ve had rims go bad after just one year, but I’ve seen them last decades too. There are also about a hundred different ways a wheel can go bad at the hub, but we’ll save that for a later article. Check for the wear signs a few times a year and you should be free of any sudden disasters.

Scott Wilson is an MFA writing student at Columbia College as well as a seasoned professional bike mechanic. Scott’s “wrenching” experience includes bike shops, racing teams, and professional triathletes across the US. The aim of Scott’s technical articles is to explain in detail how bicycles and their individual components work...and in doing so, help you keep your own bikes running better and lasting longer.


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Comment by Jim Reho on March 31, 2015 at 10:16am

Thanks.  I'm learning a lot from these articles!

Comment by Scott Chillson on March 26, 2015 at 1:38pm

 "but the phrase is 'peace of mind.'" My bad. Thanks Dean. 

To Tominator, for most brakes you don't need to manually move the brake pads up and down because when you engage the brakes they trace a parabolic curve with the pad, centered around the pivot point for each caliper arm. Make sense? Here's a practical way to think about it: if you have linear pull arms pointing straight up from the brake post pivots and you squeeze the arms together at the top then the pads will meet at a lower point than their neutral because they each have to trace the curve of a circle. Post-mounted brakes, like linear pull and cantilevers, tend to move down the brake surface, towards the hub, as the rubber pads wear away. Caliper brakes move up towards the tire. Periodic adjustments are, of course, necessary to ensure that the pads are hitting the rim at a perpendicular angle and not sliding towards the hub or tire. 

Comment by Dean Bekken on March 26, 2015 at 12:56pm

Great story and well written, but the phrase is "peace of mind."

Comment by Will on March 26, 2015 at 12:47pm

I'd recommend keeping brake pads as far away from the tire as possible while keeping them on the braking surface. If you keep them too close, they can actually come in contact with the tire as they wear, which as you mentioned can cause a tire failure (maybe more of a burning than tearing?).

Comment by Tominator on March 26, 2015 at 11:41am

Great stuff. I wonder: would it help to move your brake pads up or down to keep the wear spot on the rim moving? Up, you have to be careful not to touch the side of the rubber tire or you'll tear a hole in it. Down though might be ok?

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