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First, this is a follow up to this: Why does my back wheel keep becoming untrue?

(The short version is I bought a used bike a few weeks ago, I broke a spoke on the back wheel, replaced the spoke, trued the wheel with a tensiometer, and found that it had become untrue again. The diagnosis was either the wheel was bent, or that the spokes were just settling and re-truing it would sort out the problem.)

I've been riding it over a week with an untrue back wheel (aka Kayak Mode). Today, I checked the wheel because the trouble had become a big issue again, and found I had two (!) broken spokes since replacing the first broken one a week or two ago.

So, why are my spokes snapping like spaghetti? I guess:

  1. Because I was riding without truing the wheel
  2. Because I'm riding too hard (i.e. coming off curbs)
  3. Some combination of the two, or something else.

Thanks!

Your pal, JKD

POSTSCRIPT July 28, 2011

First, thanks everybody.

I wanted to follow up for people who find this question later, and describe the rest of my experience with the wheel.

I kept breaking spokes after replacing them, and by the time I got to the shop again, I had broken five (!) more. They looked at it, I looked at it, and I ended up replacing the wheel. It rides fine now, and as far as I can tell, it was just an old wheel.

Kazah!

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5 Answers

Several possibilities:

  1. The wheel is poorly built -- over tensioned, improperly crossed, wrong side of the flange, etc.
  2. You're too heavy for the wheels.
  3. You're too hard on the wheels (for the ruggedness of the particular wheels).
  4. The spokes are corroded or otherwise reaching end of life.

Riding without truing the wheel doesn't help the situation, but wouldn't by itself cause a problem unless the tension was so far off that a few spokes were bearing the lion's share of the load.

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Riding a wheel improperly tensioned will indeed cause a wheel to fail far faster than is typical. Other than that, those are the main reasons for a wheel to fail. This problem is a damaged rim, based on info in his first question. –  zenbike Jun 26 '11 at 13:28
    
I'm leaning towards 1:random spoke tension, based on the wheel have been "fixed" without checking the spoke tension. –  Мסž Jun 26 '11 at 21:52
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I had the same issue with a set of wheels. I was breaking a spoke every week or two. My advice was that it was a cheaply built wheel with weak spokes. Whenever I replaced a spoke I marked it and I confirmed that it was always the original spokes that were snapping.

I upgraded to a new set of wheels and haven't had any trouble since. I'd suggest having a good hard look at the bike and either buying new wheels or upgrading to a better quality bike. Wheels are a pretty safe investment because you can transfer them to a new (similar) bike relatively simply.

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I have had a previously good wheel "go bad" after years of use (and perhaps 15-20K miles), requiring relacing. Spokes have a finite useful life. –  Daniel R Hicks Jun 26 '11 at 12:01
    
It doesn't have to be cheaply built or poor quality, but it will shorten that useful life if they are either or both. –  zenbike Jun 26 '11 at 13:26
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Generally, repeated issues with broken spokes indicates either damage to the rim, meaning that the metal hoop of the rim is physically bent while under no tension, or that the spokes are at the end of their fatigue life.

Any wheel has an expected use life, and usually, you will wear a track in the aluminum rim from braking forces before the fatigue life of the stainless steel spoke becomes an issue. However, if the the rim is damaged, as referenced in your previous question, then the tension of the wheel cannot be evenly consistent, because some spokes must be tighter than others in order to hold the rim true and straight. If our wheel is unevenly tensioned, it allows a far greater amount of movement of each spoke during the revolution of a wheel, and this stresses the spokes beyond their designed strength. And you break more spokes.

If the rim is not bent, the same issue applies, except it is smaller movement of the spokes over a far greater period of time which has pushed the spoke beyond its useful life.

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Spokes usually break from fatigue. Hitting things hard might dent the rim, or cause a spoke that is about to break from fatigue to fail, but it won't in itself fatigue rims.

If your a heavy guy and the wheels are light, that might cause the spokes to fail early.

Otherwise the chances are that the wheel just wasn't built very well. After all most cheap bike never actually go very far, so that fatigue life might not be the most pressing issue for the manufacturer.

Once one spoke in a wheel has broken from fatigue, the rest will follow shortly.

If the rim is not very worn and has no obvious dents, you may be able to get the wheel rebuilt by a good bike shop. Or you could by The Bicycle Wheel book by Jobst Brandt and it yourself.

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Just for future reference, I'll add this:

My son purchased a "Eurobike" of sorts a couple of months back and brought it on the week-long group bike tour we took last week. The bike is branded by a purportedly good outfit in California, and built (of course) in China. It is a 26-inch unsuspended hyprid, with a variable-speed NuVinci rear hub and a Shimano generator front hub. In general it appears to be a good quality bike.

Before the ride started it was discovered that one spoke was broken on the rear wheel (and a repair was effected), and during the last day of the ride a second spoke broke. Both spokes broke at the nipple.

On examination of the wheel it could be seen that the large-diameter hub and the 2-cross pattern caused the spokes to approach the rim at an angle substantially off from 90 degrees, causing quite obvious bending of the spoke where it enters the nipple. The tendency to break at this point was likely further abetted by undersized, poor quality spokes.

I am guessing that the wheel will need to be relaced, though we will see what the bike shop and manufacturer say after my son gets back to California. It's vaguely possible that the rim was drilled for the off-angle nipples but the wheel was built wrong, with the odd nipples in the even holes or some such. In any event, heavier gauge, better quality spokes are needed.

(Other than the spoke problem -- and an associated problem with the enclosed chain while servicing -- the bike performed admirably, handling some very substantial hills.)

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Out of interest, I've seen a couple of references to a "Eurobike". They're both in scare quotes, and I have no idea what it means. Any chance of an entry in the regional vocab or terminology wikis? –  Useless Jun 22 '12 at 12:43
    
@Useless - I'm not sure there's a rigorous definition, but to me the term means a hybrid/"city" bike with no/minimal suspension, internal geared hub, and enclosed drivetrain. Will generally have an integrated head/taillight system with hub generator, as this is apparently a requirement for such bikes in Europe. (I used quotes simply because the term is not well-defined, not with any intent to be derogatory.) –  Daniel R Hicks Jun 22 '12 at 15:35
    
The town bikes with a very upright seating position, which sound similar, generally get called "Dutch bikes" in the UK. Now I wonder what the Dutch call them. –  Useless Jun 23 '12 at 13:35
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