Road Bike, Cycling Forums banner

1 - 7 of 7 Posts

·
Registered
Joined
·
2,227 Posts
Discussion Starter #1
I had a problem with a wheel the other day and was wondering exactly what happened--the wheel was just some chap thing that came with a mid range TT bike, called EXA "A Class" or somesuch. I was using the wheel on the trainer and it made a noise like a spoke breaking and started wobbling. When I looked at it, I found one spoke was quite loose, but none were broken. I was able to retighten the spoke OK and the wheel is round enough for my purposes, no more problems.

My question, as this has happened to me before over the years--what was the mechanism that caused that spoke to spontaneously lose about 5 turns of tension? Did the nipple actually spin or did the threads "strip"? If it is the later, why can I tighten then back up in a way that suggests they are not damaged? Did the spoke deform under tension and become narrower and allow the threads to slip for a second then return to its normal diameter?

Any insights would help, like I say I don't really care about the wheel, just want to understand what happens.
 

·
Chili hed & old bike fixr
Joined
·
1,126 Posts
I think that you should look closely for cracks in the rim that are at the spoke holes. Also examine for deformation of the flange of the hub.
 

·
Registered
Joined
·
21,507 Posts
Word choice

The key question is the difference between spontaneously and instantaneously. You describe what sounds like instantaneously - it happened very quickly. If this was the case, then curlybike's suggestion is a good one - look for a spoke pulling through the rim. If you just suddenly realized that things were out of whack, then it could have been building for a while, and that points to a wheel without enough tension. The mechanism is that every time the wheel goes around, the spoke is allowed to go completely slack and so the nipple can turn just a bit. Repeat this over and over (256 times per minute at 20 mph) and you will find that the spoke gets loose pretty fast. If the wheel has enough tension, the spoke will never go slack and this will not happen.
 

·
Registered
Joined
·
2,227 Posts
Discussion Starter #4
that might be the case, I can't see anything that looks like an obvious failure in the rim or hub. It failed in a way that felt and sounded just like a broken spoke, so it was pretty instant.

I wonder how they make these crummy OEM wheels on midrange bike? They all see to copy the low spoke count of the high end wheels with cheeseball components that make for flexy and un reliable wheels. I can't imagine a use for this rear wheel other than the trainer.
 

·
Registered
Joined
·
1,194 Posts
I had a mavic cosmos wheel that had a deeper profile rim. Inside the rim were aluminum "nipple beds" that conformed to the inner shape of the rim. One cracked in half as I rode away from my house one day. It was similar to what you described, I thought I'd broken a spoke. I probably could have tightened the nipple a few turns so it sat against the inside of the rim and used the wheel, except for the 2 broken bits rattling around in there. Check and see that your wheel doesn't use a similar design, I doubt that it does but you never know.
 

·
Arrogant roadie.....
Joined
·
4,232 Posts
If it actually "spun" 5 times, then you have an extremely poorly built wheel.

If, OTOH, it's as loose as a 5-turn backoff in tension, most likely your rim is broken.
 

·
Banned
Joined
·
6,360 Posts
Nips pulling through the rim?

Only time I've had something similar, I was able to true the wheel two or three times, and it would work for a day or two, then suddenly go limp again. Turned out the nipples were pulling through the rim, but the cracks were so small I didn't notice them at first. Finally I sat and stared at the thing awhile and realized there were little bulges surrounding several holes. It was a machine-built wheel and could have been overtensioned, or it might have been that in those days I weighed more than a Pontiac Grand Am.
 
1 - 7 of 7 Posts
Top