[hinkley]

That book I linked has another name, “the wheel building bible”. Jobst Brandt earned an obituary in Bicycling magazine including quotes from his friend Tom Ritchey (one of the original mountain bike makers). Jobst was a bike fanatic and a mechanical engineer.

Bike spokes are not loose, they’re under substantial tension. Bolts, I just learned a couple weeks ago, work in the opposite way. A tightened bolt compresses the two pieces of metal together, and when you tug on them, the bolt doesn’t stretch more. The tension instead first cancels out some of the compressive force on the two pieces of metal, before the bolt ever feels more load.

Conversely, all the spokes on the wheel are under tension. When you put the wheel on a surface and push down, the compression cancels out some of the tension on the bottom of the wheel. Cancel out all of the tension, and the wheel turns into a potato chip if you don’t reload it exactly, perfectly on axis. IIRC, none of the prior models or theories for how a spoked wheel works could adequately explain how potato chipping happens. His does.

I used his book to build half a dozen wheels or so and the information it contained to fix many more.

[denimnerd42]

i own the book.. and also The Art of Wheelbuilding - Gerd Schraner

of course in a properly built wheel usually all the spoke are under tension...

i was just demonstrating the fact that the spokes on the upper half of the wheel are supporting the hub and are under greater tension than the bottom ones, the spokes on the bottom half of the wheel should remain in tension, but only through the fact that they are already under tension applied during the building of the wheel.

the fact that the wheel works by tension of the spokes becomes obviously apparent when you start to remove the pretension and then the spokes will feel loose on the bottom half. of course you'd never want to ride a wheel like that because it will quickly become out of true.. just like a walmart wheel.

[hinkley]

I think one of us needs to reread that book, because he emphatically denies that tension at the top of the wheel increases. It’s tension at the bottom that decreases.

> of course in a properly built wheel usually all the spoke are under tension...

No, a properly built wheel all of the spokes are always under enough tension you can bounce a penny off them. Always.

[denimnerd42]

no. you're assuming the rim has no deflection which is untrue. if you build a rim out of schedule 80 steel pipe then yea. but 300-400-500g rims on high performance bikes do not act like that. the spokes are constantly loading and unloading tension as they bash through rocks and over jumps. the point is that the pretension on the wheel needs to be high enough the spokes do not loosen too much under these forces. if they do in fact loosen too much the nipples will begin to loosen and unwind and the wheel will become out of balance.

[hinkley]

I’m not assuming the rim has no deflection. What makes you think that.

Anyway, this sums it up pretty well. Someone has a longer memory than I:

https://news.ycombinator.com/item?id=36891231

If you’re talking about twisted spokes unwinding, you don’t have to reach zero load for that to happen. You just need to reduce the load enough so the rotational force overcomes friction. Tension will also try to unwind a screw as well. But the thread pitch on spokes is very fine, which lessens that force. If you build spokes like wood screws we would have problems and that has nothing to do with reaching 0 newtons.

You can release a lot of those tensions by squeezing the spokes mid build. Just don’t wait until they’re too tight to do it. I had a pulse in my rear wheel that probably came from doing that wrong the first time. Unless it was a factory defect, I must have overtensioned and warped a brand new Mavic aero rim ever so slightly. Expensive lesson, but it could have been worse.