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Today while riding, my rear brakes locked up.

While investigating, the rear tire blew up causing part of the rim on one side to split off.

The tech at the shop said the brakes had worn through the rim.

Seems hard to believe that hard rubber can wear thru aluminum.

Is that common ?

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    More anecdata for you: yes It's a thing. I now consider rims a consumable. – Lamar Latrell Feb 27 at 19:13
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    Aluminium is pretty soft! It doesn't take long, for instance, for a tyre to wear through a frame if it's touching (perhaps because too-wide tyres are used or the rear axle is broken). – JoeK Feb 27 at 19:48
  • You've already had some great answers, so I won't give another one telling the same thing. However, since you mention "it's hard to believe that rubber can wear through aluminum". Well... believe it. In old tractors, there's rubber feet between the hood and the thick steel chassis. Due to years of vibrations, you can see that the rubber feet have started to wear a hole in the steel. – Opifex Feb 27 at 22:05
  • Thanks for all the good answers. Glad I have front disk brakes. – fixit7 Feb 27 at 22:43
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    @fixit7 front disk brake didn't save you because it was a disk. Instead, having a separate independent brake was what saved you. Disk rotors wear down over time as well, but they're not doing dual-duty holding back an air-pressurised tyre. Still, wearing through a rim shows you ride a lot, which is awesome ! – Criggie Feb 28 at 21:13
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Yes, that’s normal. The only other possible cause (apart from wear) would be very wide tires (relative to the rim width) at high pressure which can break the rim walls.

It’s actually amazing that rims usually last more than 10Mm (10,000km or 6,200 miles), considering all the abrasive dirt and dust. Sometimes small pebbles or aluminium shards even embed themselves in the brake pad rubber.

Make sure your brake pads wear through the rim’s braking surface evenly, not just in one narrow groove.

If your rims don’t have a wear indicator you can measure remaining rim wall thickness with special calipers. Alternatively you can compare rim width with deflated vs. inflated (to a high pressure) tire. If the rim bends outwards by a considerable amount (like >0.5mm) you should probably replace it.

Rear wheels usually wear faster than front wheels because there is more dirt on the rear wheel.

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    @Criggie I can assure you it's not common in Europe either, at least the parts I have seen. – Peter - Reinstate Monica Feb 27 at 11:57
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    It’s unusual but there is really no reason why it should be. Mega is just another SI prefix. People are totally fine with megabytes or micrometers. I’ve even seen tkm (thousand kilometers) instead of Mm which is ridiculous. – Michael Feb 27 at 12:04
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    Might be a good question there for English.SE - Why is Mm not a commonally used unit? Even space-scale distances are quoted in km, perhaps for relevancy. People know how far it is to walk/ride/drive one km. – Criggie Feb 27 at 16:31
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    @Criggie: Previously, Mega was used mainly in the world of electricity, like in MV or MW. Later only in the world of computers. Non-mathematicians probably have a problem understanding that 1Mm = 1000km ! ;) Don't ask for too much! – Carel Feb 27 at 19:34
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    European here. Yeah, "megametre" is legit but not normally used. After kilometers you just stick to them - 1000km; 1,000,000km, etc. The same for grams. A "megagram" would be legit, but everyone goes with "tonne" instead and don't go beyond that. – Vilx- Feb 28 at 12:26
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Rims can and do wear out, but they take a decent amount of time under normal conditions. Many rims have a wear indicator (e.g. a dot or a groove) to help tell you when they wear for wear due to rim brakes. You can also have failures long term if the rim gets damaged e.g. through hits and what not.

You're using friction to wear away not just the rubber in the rim, but that also wears away the rim material, and its ability to wear away the rim may go up through grit or other wear increases.

So, it's certainly possible that the tech's explanation is right, or some other type of rim weakness occurred.

It isn't a common occurrence since most people don't ride their bikes long enough to wear out the rims, and most people who do are cognizant enough of it through their own maintenance / shop maintenance that it is pre-empted.

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    It is surprising that parts of a bike typically considered non-maintenance show signs of wear when one rides a lot. I had an indentation in the cranks where the soles of my shoes would rub. Frames break. Handlebars break (not often, but freaky). And yes, rims break, but typically in a more benign fashion than with the OP: Typically a longitudinal crack appears that slowly lengthens. When it is long enough the rim, and specifically that side of it, starts to bend in a fashion that you cannot ignore. – Peter - Reinstate Monica Feb 27 at 14:50
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    Depends on how much you brake or whether you follow the principle that only cowards brake. But weather conditions also play a big part. Brakes & rims tend to wear faster in wet conditions. I frequently remove the wheels and brush the pads with one of these little brushes for suede shoes (brass bristles on one side, stiff plastic bristles opposite) They remove a lot of grit. – Carel Feb 27 at 19:40
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On top of the other answers, your brakes probably locked because the worn rim bulged out. Then they held the rim together until the shop freed them up.

Wear from a single large hard object embedded in the brake pad can be concentrated in a single line, and rather fast. This line of damage also concentrates the stress.

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You just discovered that rims wear out from braking.

To reduce this effect in the future, you can choose high quality brake pads that don't embed grit into them. The Kool Stop salmon colored ones have a different compound with iron oxide embedded in (that's where the slightly "rusty" salmon color comes from). A word of warning, though: the Kool Stop salmons in some setups have a tendency to squeal, and the squeal can be so annoying that you might prefer traditional brake pads instead. For example, on my touring bike, I use salmons only on rear (the rear brake for some reason doesn't have a tendency to squeal on this bike) and Shimano brake pads on the front (I tested salmons on the front and they squealed like hell).

I also recommend using rims that have a wear indicator. Some rims have a deep groove. However, the groove can act as a weakener in the rim (it is a stress riser), so those rims having a deep groove wear indicator usually have a lot of supporting material and structures behind the groove to prevent the stress riser from failing the rim. This extra material increases the weight of the rim. Also, if the wheel is very accurately radially trued, the groove can cause the brake pad to wear unevenly, so that the brake pads have a ridge matching the groove, thus touching even the bottom of the groove.

My favorite style of wear indicator is used in DT Swiss TK 540 rims -- the wear indicator there is a small partial hole in the rim brake track in few locations. When the partial holes disappear, you know it's time to replace the rim.

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