Originally Posted by
Russ Roth
Shape matters a lot and you sort of contradict yourself here. You mention no extra material is needed except the material needed to make the shape but that's still more material which would increase stiffness. Further, shape by itself changes stiffness. Take all the metal of an I-beam and lay it out flat and it won't handle anywhere near the load of the same metal in I-beam shape. It isn't just the amount of metal that matters but the shape as well.
The issue as I see it is the material used for the rim construction. Aluminum isn’t a stiff material. Adding additional aluminum doesn’t really do that much to increase the stiffness of the rim, especially if a similar wall thickness is used and the overall width of the rim is increased. Any aluminum rim…or even steel one or carbon for that matter…is going to deflect upwards when the weight of the system is at its highest over a particular section of rim. The spokes in that area are going to decrease tension and the spokes elsewhere are going to increase tension in response. This flexes the spoke heads and leads to fatigue.
Shape of the rim has little to do with this flexing of the rim. The difference in the shape just isn’t different enough to make that much difference.
Although I believe spoke metallurgy and design have changed the strength, so has similar changes in material and design of the rim. Older rims were easy to flex by hand vs newer rims in general.
Yes, there has been better design and metallurgy for the rim but the rims never really suffered that much from failure. Again, I’ve not experienced that many rim failures outside of a few worn out braking surfaces and some cracks. Rims don’t fail catastrophically like spokes do.
But spoke breakage isn't necessarily a reflection of wheel strength vs spoke strength, picking the right components for the job is important. But wheel strength is more the ability of the wheel to resist deformation and maintain its shape. Proper spoke tension is important to this point as is the rim material.
Everyone assumes that spoke tension is bad whenever this is discussed. Let’s assume that the spoke tension is “proper”. Spokes still break. Additionally, everyone starts from the assumption that a strong rim makes for a strong wheel. Why? What does the rim do that adds to wheel strength? If the rim were directly attached to the spokes like a wagon wheel or a car wheel, you’d have a valid argument. But the rim has no connection to the spokes. It is allowed to float on the spoke. The rim is
always undergoing deformation as long as the bike is under load. The spokes are what resist the rim’s natural tendency to fold up when either vertical or lateral pressure is put on the rim.
I agree that care should be taken when picking components for a wheel. Spokes
are on of the components of the wheel. But, all too often, spokes are an afterthought. Even here on the Bike Forums, there are endless questions about which hub to pick and which “strong” rim to pick but very few people even consider which spoke to use. If the problem is broken spokes, the solutions
is not a different rim.
The stronger the wheel as a unit, and the better the tension is maintained, the better every part of the system will hold up. Rolf as an example didn't use stronger spokes then what was available at the time they first came out and he attributed the strength to rim design and profile. Giving all credit to the spokes ignores the system and the biggest improvements there have been the rims.
And, there it is again. No one gives credit to spokes at all.
All of the credit goes to the rim strength. It’s misplaced credit.