effect of spoke type on wheel stiffness

Discuss light weight issues concerning road bikes & parts.
Oswald
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by Oswald

ergott wrote:When you put lateral force on a wheel it pulls on the spokes more. It doesn't try to bend them. Spokes could be made out of string and have no resistance to bending. They would still work the same.


I think you are correct: http://www.spinergy.com/catalog/tech_sp ... ure_v2.php

by Weenie


youngs_modulus
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by youngs_modulus

Calnago, the phrase "cross sectional area" in this context implies a cutting plane normal to the major axis of the spoke. In the world of engineering, there's no debate about this question.

Ergott and WMW have covered your stiffness question quite effectively, so I'm just adding my voice to the chorus. Adding cross-sectional area to spokes increases the stiffness of the spokes in direct proportion to the increase in area. It doesn't help that Sapim makes some strange claims about strength and stiffness when comparing Lasers to CX-Rays. Since CX-Rays are just flattened Lasers, they can't be either stronger or stiffer than Lasers. (Cold working isn't a factor here; the drawing process alone cold-works butted spokes as much as they *can* be cold-worked). Sapim's marketing claims make no sense to mechanical engineers, but they're pretty effective at confusing consumers.

And adding more tension to an already-tensioned wheel does not make it stiffer. For example, if your spokes are at 100 kgf, and you crank the tension up to 125 kgf, your wheel will not be measurably stiffer. Both Damon Rinard and Jobst Brandt have answered this question quite well with both theory and experiment.

In truth, though, experiment is not required. It's a trivial question from an engineering perspective, even if it's counterintuitive to non-engineers. It's analogous to someone at the dawn of iron ships asking whether you can build a ship that floats from a material, iron, that is heavier than water. The answer is obvious to anyone with an engineering or science background--no physicist would have needed an experiment to convince himself that a steel ship could float--but non-scientists might want to see a floating steel ship before they fully buy the idea. That's not unreasonable, I guess.

Cheers,

Jason

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WMW
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by WMW

youngs_modulus wrote:And adding more tension to an already-tensioned wheel does not make it stiffer.


There are some cases where the opposite is true. If the tension is high enough that the rim is near buckling, then *reducing* tension can make it stiffer... since it will no longer begin to buckle when stressed.

It is also good to understand that if the problem is that the rim is hitting the brake pads, then it is unlikely this is because the wheel structure from hub to rim is flexing too much... because the rim on the other side the force application is actually moving in the opposite direction you'd expect.

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Calnago
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by Calnago

Thanks all. Yes, finally my questions over this whole subject have been answered quite nicely. And thank you Jason, for explaining that Sapim's claims about extra strength and all are nothing more than marketing spin. That did confuse me, as you say.

I have learned something valuable in this thread. Thanks again.

I still like shiny sliver round spokes however. They reflect the sun better and make the wheels glisten. :D To me, that is of much more value than the marginal aero schmero gains that may be had from flattening an otherwise perfectly nice round spoke.

Cal
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bm0p700f
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by bm0p700f

The stiffness of a spoke when bending depends on it length, youngs modulus and in area of moment of intertia. This last factor is dependent on the shape of the beam (spoke) as much as it's area. given the rim moves when a wheel flex the spoke has to bend if only a little bit. However Ergot assertion that the spoke is pulled is probably true also as the amount of bend is so small.

A Cx-ray and a laser spoke have the same cross sectional area. When I have build the same wheel with both spoke I struggle to find any difference between them. The aero benefit of the Cx-ray is very small (november cycles did a back to back test) too leading me to wonder is there actually any point in them?

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WMW
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by WMW

bm0p700f wrote:The aero benefit of the Cx-ray is very small (november cycles did a back to back test) too leading me to wonder is there actually any point in them?


Did November publish their results?

They never do a direct comparison, but Tour tests a lot of wheels in the tunnel of all kinds, and based on their findings I'd say it's pretty significant. Definitely not something you'd want to throw away if you care about aero.
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Causidicus
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by Causidicus

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Last edited by Causidicus on Wed Jun 18, 2014 1:33 am, edited 1 time in total.

youngs_modulus
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by youngs_modulus

Calnago wrote:Thanks all. Yes, finally my questions over this whole subject have been answered quite nicely. [snip]

I have learned something valuable in this thread. Thanks again.

I still like shiny sliver round spokes however. They reflect the sun better and make the wheels glisten. :D To me, that is of much more value than the marginal aero schmero gains that may be had from flattening an otherwise perfectly nice round spoke.

Cal


I'm glad we were able to help. There's nothing wrong with round silver spokes. I'll agree with others that there are likely some non-trivial aero benefits to CX-Rays as compared to Lasers. I understand this on the road, but I'm baffled by the people who happily pay $3/spoke for aero spokes on mountain bike wheels. To each his own, I guess.

Cheers,

Jason

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by bm0p700f

yes they did on aeroweenies! http://www.novemberbicycles.com/blog/20 ... -rays.html

Any links to that tour tests WMW. I am still on the fence about how significant the aero benefit is. I want to be real but...

Sapim also claim on there website that increasing tension increase wheel stiffness :hmm: add that to the strengh claims and you wonder about there fatigue claims also (well I don't Cx-ray's are not more fatigue resistant than lasers)

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