Bicycle Mechanics - how/why do spokes loosen?

yikes. I was about to commute the other day when I happened to check my rear wheel spokes and several were so loose I could move them significantly. I took it in to the LBS and they trued the wheel, but I'm wondering why and how this happens... it's not like I'm hitting a lot of potholes or something.

Unless the wheel has been damaged, the cause would be uneven spoke tension which would cause some to unwind as stress is put upon the wheel.

http://www.youtube.com/watch?v=2-wbr8_KI14

I should have known! dam gremlins.

Check the rim(s) for small cracks near the spoke holes.
Also uneven spoke tension can be a sign of a bent rim.
If the rim is in good condition the loose spokes may mean that they were never tight enough.

Spokes loosen because the wheel builder did a crappy job.

Unless the wheel has been damaged, the cause would be uneven spoke tension which would cause some to unwind as stress is put upon the wheel.

I don't see the mechanism whereby uneven spoke tension produces loose spokes. How does the uneveness occur in the first place? Wouldn't the wheel be out of true if the tension is uneven? If not out of true and the tension is uneven the true cause is a bent rim, and the uneven tension is merely an attempt to compensate for that. Same goes for blaming spoke breakage on uneven tension.

As for spokes loosening, the cause is almost always vibration. With time most spokes will loosen, but some factors seem to hasten the process. Narrow deep section rims seem to be spokeshakers more than others. Rough roads and pedal pounding, sprinting, hill climbing also seem to be factors.

p.s. For a prime example of what vibration can do: When I lived in CA I started on a long, long, long downhill coast off the San Bernadino mountains into the desert. I had climbed up the other side and everything was OK. A long section of road was gravel over tar and therefore very rough. By the time I started pedalling again I had lost one set of chainwheel bolts and two others were about to go!

As for spokes loosening, the cause is almost always vibration. With time most spokes will loosen, but some factors seem to hasten the process. Narrow deep section rims seem to be spokeshakers more than others. Rough roads and pedal pounding, sprinting, hill climbing also seem to be factors. Vibrations themselves is the final step in loosening the nipples, but the initial cause is too-low of tension. When you put load on the hub, the 4 spokes at the bottom lose tension equal to the load. So if you apply 100lbs load, each spoke lose about 25lbs each. This is caused by the rim flattening at the bottom and moving closer to the hub. On hard impacts, like hitting potholes or speed-bumps, you can actually load the wheel with 300-400 lbs load. If the spokes are at the loose-end of the tension-range, or even too-loose, they'll lose ALL tension. It's this untensioned state that allows the vibrations to untwist the nipples and loosen the spokes.

The solution to this is of course to tension the spokes towards the high-end of the tension-range. And to use a threadlocker compound like SpokePrep or blue Loctite.

I don't see the mechanism whereby uneven spoke tension produces loose spokes. How does the uneveness occur in the first place? Wouldn't the wheel be out of true if the tension is uneven? If not out of true and the tension is uneven the true cause is a bent rim, and the uneven tension is merely an attempt to compensate for that. Same goes for blaming spoke breakage on uneven tension.

It's pretty easy to build a wheel that has lots of uneven spoke tension and still run true. You can do it by overloading some sections of the rim and underloading adjacent sections. This works in your favor if you happen to break a spoke. You can easily adjust tension in surrounding spokes and end up with a nice straight albeit substantially weaker wheel.

As for mechanisms, if the spokes aren't stress properly around the hub, you'll end up with lots of uneven tension as the spoke conform to the hub bend. Slight cracks in the rim can lead to uneven tension. A rim that is failing can have uneven tension. I've had wheels that have cracked down the center of the inner wall that were perfectly normal looking on the outside. Another factor is wind-up of the spokes during building. As you layer on tension the spokes want to turn with the nipple. You need to hold the spoke while turning the nipple to notice it. As you ride, that wind-up is relaxed as the rim flexes towards the hub during the deformation of the rim on the road. If you've ever noticed a 'ping' or creaking sound on a new set of wheels, that is the spoke unwinding...and the spoke detensioning.


As for spokes loosening, the cause is almost always vibration. With time most spokes will loosen, but some factors seem to hasten the process. Narrow deep section rims seem to be spokeshakers more than others. Rough roads and pedal pounding, sprinting, hill climbing also seem to be factors.

I don't think that rough roads nor high torque situations have much to do with it. Mountain bike wheels a put through far more punishment than any road wheel ever is and they don't suffer any higher failure rate, in my experience, than do road wheels. Touring bike wheels tend to have more problems then other wheels because you are loading them with huge loads.

I suspect the LBSs send their guys out to loosen spokes when business is slow.

The OP took the wheel to the shop to get it trued, so it works.