Spoke Compression

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Here's a quick one while I wait for the UPS man to show up with some hubs for our next round of builds.

People often think of spokes purely in tension. A spoke's tensile strength is relevant and important, but a spoke's compression characteristics are probably more important. Spokes almost never break because their tensile limit was exceeded. That happens when you crash and someone rolls over your wheel and spokes, or some such thing, but when a rear non-drive side spoke fails (most common spoke failure), that's not what's happened. Those spokes almost always fail at the hub, and they fail through a lack of tension, not an excess of it. 

To illustrate this, I have people imagine that they are holding a paper clip. Grab a friend and play tug of war with that paper clip until you break it. Good luck with that. Now, take the paper clip and fold the wire back and forth a few times. It breaks easily. A very similar cyclic loading is what's happened when a rear non-drive spoke fails. The spoke doesn't have enough tension so it goes from slack to tight as the wheel rolls, and that cycling weakens the wire and breaks the spoke. 

The straight gauge spoke is trying to poke a hole in my palm

More heavily butted spokes (spokes that have thinner middle sections relative to their ends) resist this cyclic loading better than less heavily butted spokes. As the butted spoke goes slack, the mid-section of the spoke is able to bend and "soak up" that compression. The less butted or straight gauge spoke doesn't bend as readily (or at all) in the middle, so it sends that compressive stress to the only place it can go - the end of the spoke. 

The picture above shows how I illustrate this for people when I have spokes at hand. The CX Ray and Laser give almost no resistance before their mid sections kick out. The D-Light gives some resistance. The straight gauge just hurts your hand. We use straight gauge spokes for tuning our spoke thread machine - we've never built a wheel with straight gauge spokes and we won't do it. 

For spokes that are unlikely to ever go into compression, like the drive side spokes on a rear or the disc side spokes on a front disc wheel, this doesn't come into play. This makes the D-Light a great choice there, because they're still nice and light, but offer some extra insurance against long term "creep" of the spoke, which results in loss of tension over long periods of time. Young's modulus, tensile strength, impressive physics-sounding words go here. CX Rays actually gain some stiffness from the working that they go through in their journey from being a Laser to becoming a CX Ray, so they resist that creep a bit better than Lasers. 

That's all. 

 


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  • James Streeter on

    From the picture, it looks like the butted spoke elastically buckles in compression reducing its ability to carry load. Perhaps, this results in load being redistributed to the opposite spoke(s) in tension and the adjacent spoke(s) in compression.

  • Dave on

    Précis!

  • Joe Clement on

    Almost like stress reversals, huh?

  • dave on

    I should add that I think that would be murder on the rims and hubs, too. But an interesting concept. We'll have to try it sometime.

  • Dave on

    The thing is that it's not carrying load when it's in compression. All spokes have already given up on being in tension at this point. it's possible that a tension-neutral wheel could be built and work, using inelastic spokes on which the wheel could stand. I think Mavic R-sys work that way, and I'm not entirely sure that some of the Industry Nine aluminum spoke wheels don't work this way (never gave it very much thought really) but logistically they have to be straight pull spokes and there are a lot of other compromises (aluminum spokes unless you want manhole cover weight, hideous aerodynamics, etc)


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