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I've seen a few failures like this lately from touring cycling groups, and I'm wondering what causes this kind of rim failure (if it can be generalized).

failed rim

poster on group reported shortly after this that there were two other (three total) spots looking like this. Looks very distinct from the side blowouts caused by brake wear.

Is it just overtightened spokes?

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    One word: Stress. The strength of the rim was exceeded by the force placed on it by the spoke. Of course, there are a dozen possible reasons for this, both terms of weakening the rim or increasing the spoke force. It may have been steady or transient.
    – Daniel R Hicks
    May 5, 2018 at 2:04
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    That rim is buggered. Stop riding on it immediately.
    – Criggie ♦
    May 5, 2018 at 4:54
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    If its a touring bike, I'm guessing its carrying a load. Could be the rider needs a higher spoke-count wheel. 36, 40, 44 and 48 spoke wheels are available. Look at "tandem-specific" ones too - they're built for increased loads.
    – Criggie ♦
    May 5, 2018 at 5:05
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    Poor quality rim. Possibly spoke tension too low so that the spokes work too much and cause metal fatigue.
    – Carel
    May 5, 2018 at 7:47
  • Not my rim. Just one of many I've seen posted to touring groups over the past months by folks wondering if they can keep riding. (Of course that answer's no.) Just hoping to avoid the same mistake myself (if it is a mistake).
    – WPNoviceCoder
    May 7, 2018 at 2:08

2 Answers 2

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If there are several spokes showing this failure, it's not because of a single impact.

I think it's safe to say that the rim was simply overloaded and has suffered metal fatigue from stress cycles imposed by the differing forces on the spoke as the wheel rotates.

Can't say if the failure is because of overloading beyond what the wheel was designed for, or poor design or a manufacturing problem.

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IMO it is not over-tightened spokes. I've had something similar, where a rim started cracking radially away from the hole. Adjacent spoke holes showed similar but less damage.

My guess is that it was from a large impact with a road fixture, where the afflicted spoke was at the top of the wheel opposite the impact, so when my weight hit the saddle and the rim was forced up and back by the edge of the surprise-kerb.

So ask the rider if they've hit any big potholes or other hard impacts, and check the opposite side of the rim for dings or dimples. The tyre probably won't show any damage.

That rim is shot and needs replacement. It is not safe to ride.

You could buy a new rim and reuse the hub/spokes. It is always good to build a wheel at least once in your life. Then get it final-trued by a LBS.

Contrary to wagon wheels, spoked wheels are "hangers" under tension, not "compression" spokes to carry the load. A static bike wheel is distributing all of the bike's weight on the top few spokes. Not sure what the distribution is, but the top one is carrying the single most weight, with the horizontal spokes carrying none.

If the wheel is rolling, the dynamic situation will confuse things even more, but the weight of the bike+rider is still hung not held-up.

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    In reality, the tension works differently. Under load all spokes have nearly same tension, expect a few at the bottom which are looser. If you do the math, it turns out the result is same as wagon wheel spokes with constant tension added. Read "The bicycle wheel" by Jobst Brandt or for example williamscycling.com/assets/images/product%20tech/… online for more detail.
    – ojs
    May 5, 2018 at 8:25
  • @ojs yep sounds right, and then Sudden Hard Impact. I wonder if OP's wheel has damage on adjacent spoke holes too. Photo shows a possible crack on the next spoke down.
    – Criggie ♦
    May 5, 2018 at 9:55
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    Like normal load, impacts loosen a few spokes at the point of impact and very slightly increase the tension everywhere else. This crack is in the direction of spoke tension and would be either because of fatigue or sudden increase in tension. The later doesn't happen.
    – ojs
    May 5, 2018 at 10:16

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