Spokes shearing

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I've been using my CK7 for my commute now for about 2 months and I've just had a spoke shear at the hub end in the rear wheel (Shimano R500). Before I got rid of my old bike (Falcon explorer), equipped with bog-ordinary and cheap-as-chips Rigida 700Cs I had a spate of rear wheel-spokes shearing (3 in about a week). This I put down to fatigue, exposure to salt over the winter and the wheels not being top-quality. Now I've had one go again and I'm (a) concerned and (B) peeved since I thought the better brand would last longer.
One of the roads on my commute route has a rather broken surface, but it's more like the top layer of aggregate has come loose and left it rather pocked than wholesale breakup and potholes. Would this be enought to cause me problems? For info, I currently weigh 15 stone and am 6ft 4 in tall.

 

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I've been using my CK7 for my commute now for about 2 months and I've just had a spoke shear at the hub end in the rear wheel (Shimano R500). Before I got rid of my old bike (Falcon explorer), equipped with bog-ordinary and cheap-as-chips Rigida 700Cs I had a spate of rear wheel-spokes shearing (3 in about a week). This I put down to fatigue, exposure to salt over the winter and the wheels not being top-quality. Now I've had one go again and I'm (a) concerned and (

peeved since I thought the better brand would last longer.
One of the roads on my commute route has a rather broken surface, but it's more like the top layer of aggregate has come loose and left it rather pocked than wholesale breakup and potholes. Would this be enought to cause me problems? For info, I currently weigh 15 stone and am 6ft 4 in tall.

Had another one shear this morning, rear wheel again and I think sprocket side, which is the same as last time. Are Shimano R500's prone to this or am I just unlucky?

 

[amaferanga]

Had another one shear this morning, rear wheel again and I think sprocket side, which is the same as last time. Are Shimano R500's prone to this or am I just unlucky?

Once a couple of spokes go its time for a rebuild. You could use the existing spokes, but you should detension all the spokes and retension. Any decent LBS will do this for you if you're not sure how to.

 

[marzjennings]

Could be uneven tension in the spokes and more an issue with the original build than the parts. Where did you get the wheels? Were they built by your LBS? When was the last time you checked the tension in every spoke?
If they're new wheels they should have been checked out after their first 100miles.

I'm about the same size as you and I have to really careful with spoke wear and tension and as such they get a quick check every weekend. I'll turn the bike over, spin the wheels to check to see if they're still true, flex every pair of spokes and then pluck every spoke to make sure their pitch is about equal.

 

[amaferanga]

R500 are factory built wheels, but even with a handbuilt wheel there shouldn't be any need to check them after 100 miles. Any competent wheel builder should stress-relieve their wheels before using/selling them and make sure there's no spoke wind-up. If you have to check wheels after 100 miles use then its likely you got the wheels from a crap wheel builder.

Likewise for checking wheels regularly - if they're built properly in the first place then there should be no need to check them unless you put the rim out of true or pop a spoke. If the tension is changing in the spokes over time and needing regularly adjusting then the wheel wasn't built properly in the first place.

 

[accountantpete]

Probably naff spokes unless the the spoke holes in the hub are badly worn - they can develop a sharp edge which cuts into the spoke.

 

[RecordAceFromNew]

Perhaps counter-intuitively, most spoke breakage is due to too little rather than too much tension, when they can go slack at the 6 o'clock position under load. This is a particularly common phenomenon with factory built wheels.

Nearly all spokes break because of fatique (indicated by breaking cleanly as if they can join seamlessly again, vs elongated, jagged breakage if they failed because they were pulled apart). Also counter-intuitively, all else being equal butted, thinner spokes have longer fatique life because they are more flexible and therefore can shed/share loading better. Jobst Brandt's book The Bicycle Wheel describes the subject brilliantly.

 

[PpPete]

The spokes that are not tight enough in the first place still go tight as they go to the 12 o'clock position. It's the repeated tension & relax that causes the fatigue failure.

Shimano R500 20 spoke front/24 spoke rear ?

Each spoke is very heavily loaded as it goes over the top...at least half as much again as a 36 spoke wheel, and then relaxes, and again for every revolution. Add in the poor road surface and bingo!

As RecordAce says... double butted spokes have better fatigue life.
If it's only 2 months old it's surely a warranty claim... but how about a quiet word with the LBS, would they rebuild the wheels for you if you paid for better spokes?

 

[screenman]

Just tell all your mates that you are putting out so much power that you keep snapping spokes.

Just pop it into a decent LBS and pass the problem and a few quid onto them.

 

[Tynan]

24 spokes?

I've nearer 15st than 14 and I have 36 spokes on the rear