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Discussion Starter · #1 ·
My basic question is what causes spokes to break? I have about 4K miles on my wheels, Ultegra/mavic cxp21, Have never had any trouble except for minor truing issues. In the last 300 miles I have broken 2 spokes. Both rear drive side and broke at the hub. Is it time to rebuild the wheel with new spokes or just continue to replace them as they break? Or is this an indication of a larger problem? I'm 185lbs and not a particularly strong rider, but do a lot of long climbs with descents on rough pothole laden roads.

Thanks in Advance.
 

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fatigue

From what I've read, spokes break, at the bend where you describe, largely from fatigue. Fatigue is caused by repetative bending of the metal, loading and unloading, until it breaks. The best way to avoid this is to tighten the spokes enough so that they never unload and are always under high tension. Improperly tensioned spokes may be the cause.

You could just replace the broken ones and then tension the entire wheel. However, if it were me, I'd at least replace all the drive spokes at this point, having broken 2 already.

By the way, from what I know, the drive spokes are not broken merely because they are near the cogs and experience greater tension when pedaling. Rather, they are always under greater stress because the drive side spokes attach closer to the center line of the wheel, dish, so that they need greater tension to hold the rim in the center compared to the non-drive spokes. Therefore, every time you pedal, you are pulling on the drive spokes a little harder, as they are already under greater tension.

Doug
 

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Discussion Starter · #3 ·
Oops I meant non-drive side. But that makes more sense anyway. The non-drive side would be more suseptible to loading and unloading due to its lower tension. I think I'll just replace all the spokes since if it is fatigue the problem is not likely to go away.

Thanks.
 

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The Edge
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I had trouble with the Mavic CXP21 rim. No problems for the first 1000 miles, then suddenly I had to start carrying spokes with me to replace on longer rides. I would have someone really good rebuild the wheel. Riding is a lot more fun when you can focus on the trip and not the equipment failing you.
 

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If you're breaking spokes on the non drive side, it's because of too little tension. If you break then on the drive side, it's because of too much tension. One other possibility is bad spokes.
For myself, two broken spokes doesn't mean rebuild time. Four does.
 

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i'm no wheel guru but did have an experience that educated me on one (rare?) reason spokes break. if the spoke does not sit well in the hub eyelet/hole, it can cause that particular spoke to break over time. this happened to me with an american classic micro hub with Sapim CX-Ray spokes. the same spoke broke three times after which i contacted the builder and he sent me some more spokes and some washers to use when putting them on. this fixed the problem.

probably not what's happening with you but kind of interesting (IMHO).
 

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Chili hed & old bike fixr
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Ultegra hubs have spoke holes that are 2.6 mm in dia.( Actually this is true for all Shimano hubs except Dura-Ace) The proper size for a 2 mm spoke is 2.3 mm. This gives room for the spoke to move around in the hole causing the spoke to flex at the elbow. That in conjunction with the spoke tension in the wheel being too low causes the non-drive to fail prematurely. Of course the drive side must be tensioned higher and then the dish properly set with the non-drive side. These statements can be verified in "The Art of Wheelbuilding" by Gerd Schraner. He also suggests the use of thin brass washers under the head of the spoke to fill the slop in the spoke hole. Your results may vary.
 

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Discussion Starter · #10 ·
Interesting. That seems reasonable to me, a bit stupid on Shimano's part, but reasonable. But how does the washer help with the problem? I could see if it was a sleeve that slipped over the spoke it would work. Is it just that the brass deforms enough to basically be pulled into the hub hole?
 

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Chili hed & old bike fixr
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TimA said:
Interesting. That seems reasonable to me, a bit stupid on Shimano's part, but reasonable. But how does the washer help with the problem? I could see if it was a sleeve that slipped over the spoke it would work. Is it just that the brass deforms enough to basically be pulled into the hub hole?
Sorry for the delay, been busy. Yes the brass deforms nicely and fills in the slack. For best results , the wheel is built and lightly tensioned and then you set the spoke heads down with a soft punch and hammer. Another soloution is to use DT Alpine 3 spokes that are 2.3 mm on the elbow end. They are between the bb (Doh, that would be DB as in double butted) and straight gauge in weight.
 

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Actually, I vote for ONE broken spoke.

Kerry Irons said:
You rebuild the wheel when 3 spokes have broken. Not after only 2. And why wait for 4. Simple as that. :)
If somebody brings a wheel to me with one broken left side spoke, I retension the whole wheel. Here's why:

Spokes break for a reason. If all that you do is to replace the broken spoke and retrue the wheel, you have returned the wheel to exactly where it was before. That is to say, a wheel that is about to pop a spoke. Retension the wheel and make sure the spoke tension is as even as possible and you may solve the underlying problem and save yourself a complete rebuild later.
 

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My guess is...

It is difficult to assess when I don't see it but I would say you are running straight gauge spokes on the bike.... seeing that all other bases have been covered. If not I would have guessed that the wheel is improperly tensioned like the rest.

Generally non-drive side spokes should be double butted as they go through more strain (correct me if I'm wrong here).

I would change at 2. Well I did the last time, but 300 sounds like a really short distance for your spokes to be breaking because mine were straight gauge and they only started breaking at 10000 miles.

Regards,
Sean
 

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Mechanical engineer speaks:

Spokes generally break because they fatigue, and only rarely because they are stressed to failure. Fatigue happens due to repeated cyclical loading, and spokes usually fail at the bend, where the cyclical loads are the worst.
DB spokes reduce the loads there by increasing them at the thinner center part. Since the center stretches more, this results in less stresses at the bend of the spoke. This results in better durability, albiet with less overall strength.
Some people say that straight-gauge spokes make more sense. The wheel is stronger, and when spokes start to break, it's a good indication that the spokes/wheel has reached the end of it's reliable (if not useful) life. Others say that, no, your wheel will last longer more reliably with DB spokes, and if you want to make the whee stronger, just spec thicker spoke ends. Actually, a triple-butted spoke is going to be the most reliable, as long as the thickest part is at the bend.
 
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