Old 07-30-15, 08:17 AM
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Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

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Originally Posted by habilis View Post
@FBinNY: From reading many of your posts, I'm sure you know exactly what you are talking about. Otherwise, I wouldn't ask that you please elaborate on these two statements:

1. "wheel stiffness depends almost entirely on the total amount of steel, namely the cross section X the no. of spokes."


In Statement 1, you suggest that rim profile counts for little. I was hoping that my deep-V rims at least gave me some strength for their extra weight. (I recently hit a bump in the road and suffered a snake-bite puncture but no deformation of the rim. Could a flexier rim have saved the tube? Just speculating.)

In Statement 2, you suggest that I could loosen the tension on all (or even only some) spokes and it would have no effect on wheel stiffness. If I understand you correctly, this runs counter to everything I've read about wheel building. What am I missing?
Before continuing with this post, read about Young's Modulus, which describes the basic and fundamental relationship between tension and deflection (stretch).

Within the working range the amount a spoke will stretch with added load, ie. your weight, is the same whether the tension starts at 10okgf and goes to 120, or starts at 70 and goes to 90 is exactly the same. So, it's not the initial tension that matters, only the change. So applying the principles of Young/s Modulus, it's apparent that since the elastic properties of the materials can't change, the easiest way improve the stiffness of a given tension structure, ie. a bicycle wheel is to add material (steel) so either fatter spokes or more spokes or both.

There are other things that improve stiffness, for example changing the spoke's bracing angle, but this isn't as easy as it seems at first glance. For example we could increase flange separation (best option), but we're constrained by space available. Or we can increase flange size, but since the spokes are tangent or near tangent, flange size has little effect because spokes are pulling from near the center-line anyway. So it's back to more steel, or as I sometimes write more "spoke". There's no magic here.

Rim stiffness does make a difference, but it's not as great as people would like to believe. Even the stiffest rim is like taffy compared to the stiffness the spokes provide. The rim's main contribution, besides being a tire mounting surface is as a compression ring to hold the spokes under tension. But some added rim stiffness does help because it spreads the loads among more spokes, especially in lateral deflections. Consider truss bridge construction. As spans get longer the depth of the trusses is increased, and likewise deeper or wider rims will be stiffer and spread the load bringing more spokes into play, and thereby improving stiffness somewhat.

The key to understanding stiffness in wheels isn't to think about what it's like statically, but what will happen as it deflects under loads, keeping in mind that it's a system of elements working as a whole, not acting independently of each other.
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Last edited by FBinNY; 07-30-15 at 09:02 AM.
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