I'll chime in with some wheel building theory. Note that I am not any form of engineer or describe myself as an expert, I have merely read some of the books and read some forums like rec.bicycles.tech for a long time where people like Jobst Brandt and Sheldon Brown would contribute. I have also built many wheels for myself, family, friends and customers (when I worked in various bike shops) and have found the following points to hold true.
- Spokes lose tension temporarily when they reach the bottom of the wheel when they are being ridden.
- The fewer spokes there are the more tension each spoke will lose at the bottom
- The more flexible the rim is the more tension each spoke lose at the bottom, lighter and shallower rims tend to be more flexible
- Higher tyre pressure will reduce the tension in all the spokes
- Higher loads (weight) will reduce the tension in the spokes at the bottom
- Pedalling action will reduce the tension in the 'pushing' spokes
- Impacts will reduce the tension (presumably at the bottom).
- Spokes that lose all tension cause problems such as the nipples unwinding and the spoke fatiguing and failing particularly at the elbow (and less commonly at the start of the threads)
- Nipples unwinding can be solved by thread lockers such as loctite, linseed oil etc, or by increasing the tension in the spokes
- Fatigue failures can be dramatically reduced by stress relieving the spokes but for some reason many wheel builders don't do this and few factory build wheels are
- Highly tensioned spokes tend to result in wheels that are less prone to require re-truing.
- Wheels built with highly tensioned spokes are more likely to have fatigue failures in the form of cracking around the spoke holes.
- Wheels built with highly tensioned spokes are more prone to 'potato chip' failures
- Spokes are almost never broken from too much tension.
As you can see some of these factors will combine together this explains why the most common spoke breakage is at the elbow of a pushing spoke. But the weight the wheel carries is almost certainly the largest factor. This explains why the most powerful sprinters still use low spoke count wheels, whereas 40 and 48 spoke wheels are common on touring bikes and tandems. Historically many bikes (British at least) were built with 40 spoke rear and 32 spoke front wheels to reflect the different loads. Somewhere along the way someone figured out that it was cheaper to use the same drillings for the rims and suddenly 36 spoke F&R wheels were standard even for high performance wheels. Then someone got the idea that fewer spokes were lighter and that most high performance wheels were used by people who could get away with 32 spokes. This was in the days of low profile sub 300g rims. Then came the deeper section rims, I suspect more for strength than anything else but they were also marketed as being more aero. This allowed builders to use fewer spokes again but come with a weight penalty. Sheldon said that having the same spoking F&R means that you either have a front wheel that is unnecessarily strong (and therefore heavy) or you have a rear that is not strong enough.
Velocity Deep Vs are renowned as being strong rims, but they weigh in a 520g or so. If I were to build a set of wheels with these I could probably get away with 24R, 20F spoking even though I weigh 110kg at the moment
Although I'm not particularly hard on wheels. In fact my race wheels have carbon rims that are lighter but deeper than this.
toolonglegs wrote:My Aksium lasted about 1000kms before it totally destroyed itself... Cracks on every spoke hole... Not the greatest wheel for a big guy. But this was 2006 so they may have improved.
I like Mavic wheels ( live 10 km's from the factory )... But I never keep them after the 2 year warranty is up!.
If I were to build a set of wheels for TLL for example they would end up being heavier and have more spokes than he would choose to use so it is no surprise that he goes through wheels at a steady rate. The next best solution would be to tension them up higher and hope that he breaks them through some non-warranty mis-adventure before the rims cracked through fatigue.
bardygrub wrote:Before the service i had a slight buckle in the front wheel, which i reported and they trued the wheel. After about three rides i noticed the wheel was buckled again, checked the tension of each spoke and found one compleately loose.
Wheels that have low spoke counts are more susceptible to having the nipples unscrew due to the de-tensioning at the bottom of the wheel. Once this process starts a spoke loses even more tension which is what bardygrub found. The solution is either a threadlocker to stop it unscrewing, or more tension overall. (or wheels that have either more spokes or heavier rims)
rkelsen wrote:Your tyre pressure is too high.
Increased tyre pressure does contribute to the unscrewing of the nipples by reducing the tension in the wheel overall, and possibly also by not letting the tyre cushion impacts as well. Some of rkelsen's other wheel woes sound also sound like either a lack of tension and a failure to stress relieve the spokes, the tyre pressure contributed to the lower tension. The commuter wheel also sounds as if it was afflicted by a bad batch of spokes. Shogun Metro SE's were famous for this in the early 90's and Sapim had trouble with a batch recently (2007?) too.
Part of wheelbuilding is science and you can plug in the numbers and say that for a given weight and rim you need x number of spokes in each wheel at x tension, part if it is art because some riders are just harder on wheels than others of similar weight/height/power. It is easy to overbuild wheels to make them reliable, but to put 48 spoke Deep Vs under a 45kg climber will not give them good performance. This is where the good builders (like TWE) really add value to extract maximum performance at adequate strength/reliability.
Thanks for reading such a long diatribe.