artemidorus said:
This is an interesting topic. Dave, I respect your experience but a number of local wheel builders have suggested non-butted spokes for heavier riders. Certainly, but obviously anecdotally, my first adequate rear wheel (of 4) on my road bike has been one built with non-butted spokes. I say adequate in that it does not ping even when pedalling hard out of the saddle, unlike the three previous wheels that all ultimately failed. This could be, of course, because I had it built by Sydney's best wheelbuilder (by reputation).
What do others think? Does anyone know of any data?
DT says:
"Why are spokes thinner in the middle, espcially on more expensive bikes?
Such butted or reduced spokes increase the life of the wheel. Spokes with a smaller diameter in the middle are not only lighter and more aerodynamic, they most importantly have significantly better elastic properties than conventional straight spokes. They react to extreme overloads in a similar manner as the butted bolts used in machine construction. If the wheel is subject to radial impact, the affected spokes absorb the resulting overload like a spring, protecting the hub."
Sheldon Brown says:
"Double-butted spokes do more than save weight. The thick ends make them as strong in the highly-stressed areas as straight-gauge spokes of the same thickness, but the thinner middle sections make the spokes effectively more elastic. This allows them to stretch (temporarily) more than thicker spokes.
As a result, when the wheel is subjected to sharp localized stresses, the most heavily stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke hole."
Sapim says:
"Sapim butted spokes have two major benefits: less weight and more strength! Technological advances using cold forging (SAPIM Forging Technology) 'stretch' the spoke, retaining the linear molecular structure of the material, thereby increasing the spoke strength at the middle by at least 48%. As an added bonus, the thin middle section of the light Laser and Race spokes also provides better shock absorption. "
Jobst Brandt says:
"Butted spokes are more elastic than unbutted spokes. Under load they resist lossening better than straight spokes. Their resilience helps the rim distribute loads over more spokes and reduces peak stress changes. Butted spokes are lighter without giving up any strength....
Straight spokes greater stiffness reduces elastic interactions among spokes which, in turn, subjects them to higher stress by concentrating loads over fewer spokes."
I see 4 advantages of straight spokes:
1. cheaper
2. stiffer (relative to same wheel built with butted spokes)
3. when one breaks the wheel will go slightly less untrue due to the lack of interaction of and relative stiffness of remaining spokes.
4. Easier to build with due to smaller elastic interaction amongst spokes.
The disadvantages outweigh the advantages, in my book, because the wheel built with straight spokes is less durable. The increase in durability with butted spokes more than makes up for the additional costs. Rims and hubs are dynamically involved too. Since the elasticity of spokes is only one portion of the overall wheel, rims and hubs built with double butted spokes benefit from the additional elasticity compared to straight spokes.