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Comment by denimnerd42 | Hacker News Reader

what do you mean it's not tension at the top? did you misspeak? bicycle spokes are solely under tensile forces. they can't support compressive forces at all. i'm a hobbyist wheel builder and a once upon a time professional bicycle mechanic during hs & college.

if you want to test this take nearly all the pretension out of your spokes and sit on your bike. feel which ones are taught and which ones are completely loose. or just go to walmart. those bikes hardly have any pretension in their wheels.

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This dialogue is reminiscent of rec.bycycles.tech arguments with Jobst, ca 1993.

A bike wheel is a linear elastic system, that can be thought of as a superposition of a uniformly set of tensioned spokes as one state, and a set of spokes in compression in the loaded zone (bottom of the wheel) as the other state. So long as the superposition of the two states obeys the limiting conditions (i.e. spokes in tension) they can be analysed separately.

The size of the loaded zone is related to the relative stiffness of the spokes (axial) and the rim (bending), and can be calculated using beam on elastic foundation methods. For typical rim/spoke combinations, this is approximately 4 spokes.

Outside of the loaded zone, spoke tensions essentially don’t change.

And thread. Thereby proving my point: Almost nobody knows how bicycle wheels actually work.

Well. I wish we could sit in front of a bicycle wheel and discuss it. Because I have a feeling we are shooting arrows at different targets. As a mountain biker I'm more interested in what happens when an extreme amount of load is applied to the wheel, not the model with assumptions applied. Definitely a difficult concept to discuss with only text. Anyways.. glad to have a good discussion with you about bicycle wheels. Don't find many people like you. :)

I think I would have been a mechanical engineer if I were born 20 years earlier or 20 years later. Software was just new and shiny enough and it let me build things with my mind, at a time when I believed I was clumsy (I actually have always had excellent fine motor skills, it's macro motor control I lagged behind in). One of my better friends in college was an ME. Learned all sorts of things about metal fatigue and oddly enough picosecond lasers from him.

I don't know if I found Lego or Lego found me, but I definitely think in terms of shapes. I was past my midlife crisis before I realized that I don't have a large working memory (smaller than average in fact) it's just that I've been doing mind palaces without pictures since I was very small. When I'm thinking of large computer systems I'm essentially thinking of them as physics problems.

I really should figure out space to have a bike again. I never rode when I lived in Seattle (Seattle drivers are nuts) but I don't live there anymore and I need to catch up on 20 years of tech.

I think you're both in violent agreement using different terms.

You're looking at the macro "It's all in tension" (superposition of two states) and hinkley is looking at the "bottom is a compressive change" (dynamic portion of the load).

What I'm not clear of is if you think that the upper spokes change tension between the unloaded case and the plain gravity load case (force on hub down, ground on rim up at the bottom), or if you expect the top half spokes to increase and the bottom half to decrease in tension. I think this is what hinkley thinks you think.

That was indeed my interpretation of that half of the conversation. That they were claiming that the axle is suspended (tension increases with downforce) by the spokes above the midline of the circle, which is what Brandt vehemently contended was false.

Nah they both understand it. One of them is just arguing to show off.

The real winner will be who comes up with application for LK-99 to improve bicycles

Electric bicycles baby.

Hey do you happen to recall, did Brandt prove this theory with a tensiometer or did I just hallucinate that?

Pretty sure he did, but I don't have r.b.t. archives. He was definitely a proponent of them, preferred a specific brand/style, and would have easily been able to do the experiment.

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.

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.

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.

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.

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.

They even make spokes that are rope

https://berdspokes.com/pages/technology

I wonder if those have less or more breakin issues compared to spokes. With a spoke you have to reset the angle of the bend to the shortest distance between the hub and the rim. But with cables they have to settle in along their entire length.

Doesn’t change the answer. They’re still compressing. They’re just pretensioned.

you sound highly confused, what is compressing? the rim? the spoke?

You need to reread Brandt.

There are two situations when you can push on a string. One is when it’s frozen, and the other when it’s tensioned. How do you unload a bow string? You push it off the notches.

bro. you're wrong. just build a bicycle wheel some time. it becomes very obvious how they work as you take it through the various stages of tensioning.

Why do you think someone would talk about Brandt’s work if they hadn’t used it?

I’ve built more than half a dozen bicycle wheels. My set, my spare that my brother road (into the ground - my first set and practically the only problematic ones, but he road over bumps without getting out of the saddle), a set my dad commissioned from me, and a pair that he had me build for a friend. All by age 17.

I then worked as a mechanic for two summers of college. I was never the fastest, but if we had a customer we could not afford to disappoint, I or the senior mechanic got the job because my repairs did not come back.

I saved three or four wheels that would have been scrap by unwinding the spokes halfway and building it back up again like a wheel build. Only added an extra ten or fifteen minutes but it works a charm. When a good customer comes in on Wednesday before an out of town bike ride you can’t afford to fuck it up. I think I only built a couple professionally, and usually singles. That’s a lot of labor and few will pay.

There's a lot more creep early on, which is compensated for by a staged tensioning over the course of a few days. They may require the spoke holes in the hub to be radiused, which can be a warranty issue between you and the hub manufacturer. Windup is controlled by a flat on the small bit of spoke used for threads.

They're ok (i.e., made it through Tour Divide with no issues), done well they're certainly better than badly done steel spokes, but it's not clear if the best builds are better than the best steel builds.

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