Over tightened spokes

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531colin
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Re: Over tightened spokes

Post by 531colin »

jb wrote: 27 Jun 2022, 10:42pm Why anyone would expect the park tool or any similar make to be particularly accurate is a bit of a mystery, I expect mine to be reasonably consistent but you are best trying it on known good wheels to get a feeling of where the right tension lies. If I remove the spring from mine the nylon pointer bit binds slightly in the slot so that's going to throw it out for a start. Oiling it will also affect the reading.
They are accurate enough to be useful, unless you get a dud.
Otherwise, how was your "known good" wheel built? Or perhaps it came down from the mountain with Moses and the commandments? :wink:
The OP (Mr Tom) put his gauge on a known good wheel from a known builder, and the gauge gave insane readings....ie his gauge is a dud.

When I was building wheels at Spa I built some in the shop using one of the shop gauges, and some at home using my gauge. You couldn't tell the difference. I don't think we ever had less than 3 people building wheels in the shop or at home, and we all built to the same tension, using our own gauges at home and one of the shop gauges in the shop.
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531colin
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Re: Over tightened spokes

Post by 531colin »

We had one batch of dud spokes; they were so soft you couldn't build and stress-relieve a wheel, they failed elastically, ie soft failure, the wire stretched, "necked" and finally failed at the stretched point.

Out of curiosity, I built a rig to test spoke strength, using a lever to multiply my bodyweight. With "good" spokes I had to get to a few multiples of my bodyweight for the spoke to break, and that was quite exciting, bits flew all round my shed.

Image008 by 531colin, on Flickr

The failed ends are a particular shape, as above.

nobody should get anywhere near that sort of tension when trying to build a wheel
drossall
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Re: Over tightened spokes

Post by drossall »

531colin wrote: 28 Jun 2022, 10:22amnobody should get anywhere near that sort of tension when trying to build a wheel
:D :shock: :lol:
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531colin
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Re: Over tightened spokes

Post by 531colin »

Photo was taken in 2013, its all a bit hazy......
I made a test rig and rawlbolted it to the outside house wall, inside my lean-to shed.
I used a bit of scaffold tube as a lever to multiply my bodyweight, I probably made it to go to maybe 5 times my weight.
My 70Kg times 2 is 140 Kg, thats a sensible sort of amount, ballpark of the static tension on a driveside spoke.
4 times is 280Kg thats getting a bit exciting, thats a lot of force. I hung a bit of old carpet between me and the rig to protect me from flying bits of spoke, but it was a post-war house and I could easily crack the facing bricks by tightening up a Rawlbolt, and resin fixings weren't something i knew about at the time.....
i probably called it a day at about that sort of force.....but show me somebody who can get a spoke up to that sort of tension with a spoke key?
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interestedcp
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Re: Over tightened spokes

Post by interestedcp »

531colin wrote: 28 Jun 2022, 9:55am
drossall wrote: 27 Jun 2022, 11:47pm No comment on whether it's accurate, but I do remember, when reading up on things before building my first wheels, that one authority urged you not to look down the line of the spoke when building, just in case it should fail and shoot out of the wheel like an arrow into your eye. I've never forgotten that particular advice, whether or not it's a likely event :shock:
Written by somebody with an over-active imagination
Well, the advice stems from the very experienced wheel builder Jobst Brandt, who besides being an engineer, also wrote the seminal work "The Bicycle Wheel". He also simply advised putting on rim tape while tensioning to avoid the danger.

In the first edition of his book, he actually tested spokes, and found many of them rupturing under load. In his third edition, he repeated the tests, and found the spoke quality had improved significantly. He tested three DT Swiss 2.0mm db spokes with 3000 Newtons and they didn't break, though they stretched 6 mm.

So while the chance of a new, modern quality spoke from DT Swiss breaking when building wheels, likely is extremely low, putting on rim tape whenever one is tensioning spokes still is a good praxis. Occasionally aluminium nipples breaks, and when dealing with used equipment the spokes or nipples may be damaged to an extent that they may fail under tensioning. Also, apparently 2 mm nipples may get some engagement on thin 1.8 mm spoke thread, but disengage under high tension (no personal experience with that).

While the risk of damage is low, the worst case scenario can be really bad, and since the problem can be remedied by putting on rim tape when tensioning the spokes, there is no reason to take that low risk at all.
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531colin
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Re: Over tightened spokes

Post by 531colin »

3000 Newtons and a 6mm elongation.
I've never seen tensions or elongations like that in a wheel.....only on a test rig.
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interestedcp
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Re: Over tightened spokes

Post by interestedcp »

531colin wrote: 29 Jun 2022, 4:02pm 3000 Newtons and a 6mm elongation.
I've never seen tensions or elongations like that in a wheel.....only on a test rig.
Brandt had several points in testing spokes until they reached their elastic limit or started to break. First he could make some nice graphs showing the relationship between force applied and spoke stretch, but more importantly, he could demonstrate that spoke failure was strongly associated with metal fatigue, since the tested spokes reached their elastic limit far, far beyond what tension they would experience in a bicycle wheel; The swaged spokes always broke at their mid sections, and not their elbows nor in their threaded sections. There were no meaningful difference between spoke makes or spoke types either.
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rogerzilla
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Re: Over tightened spokes

Post by rogerzilla »

It would be quite hard to get 300N (almost 300kgf) through normal tensioning with the nipples remaining intact. IME they will round off (if you use a 3-sided spoke key) or, at a higher tension, actually split (with a 4-sided key) if they are ludicrously overtightened. The friction between the spoke and nipple just becomes too high and the nipples twist off, even if the threads hold.

To get 300N, the spoke was probably loaded with weights or by using a tensile test rig.
drossall
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Re: Over tightened spokes

Post by drossall »

interestedcp wrote: 29 Jun 2022, 2:44pmWell, the advice stems from the very experienced wheel builder Jobst Brandt, who besides being an engineer, also wrote the seminal work "The Bicycle Wheel". He also simply advised putting on rim tape while tensioning to avoid the danger.
That would account for it. I read that book when building my first wheels, and I still have it.
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interestedcp
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Re: Over tightened spokes

Post by interestedcp »

rogerzilla wrote: 29 Jun 2022, 9:38pm To get 300N, the spoke was probably loaded with weights or by using a tensile test rig.
Most likely. While he doesn't go into detail on his test rig in 3. ed., one can infer from his other writing that he thought it was basically impossible to reach that kind of tension or even getting close to it, just by turning a spoke key.

He does however mention in on of his Usenet posts, that the combined stress of tensioning spokes and spoke twisting(torque), may break spokes so they fly out of the eyelet. So twisted spokes may break far below the static load of 3000 Newtons (roughly 300 kgf) required to destroy them in a test rig.
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531colin
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Re: Over tightened spokes

Post by 531colin »

interestedcp wrote: 30 Jun 2022, 12:27am ...........the combined stress of tensioning spokes and spoke twisting(torque), may break spokes so they fly out of the eyelet. So twisted spokes may break far below the static load of 3000 Newtons (roughly 300 kgf) required to destroy them in a test rig.
All I can say is its never happened to me. As I mentioned before, we had a batch of spokes which were too weak to build and stress relieve a wheel. They failed either during building or stress-relieving, but it was a very gentle failure....nothing flew about the room except bad language.
During the course of repairing a wheel that has had the odd spoke fatigue and fail, you will occasionally break a spoke where the nipple is corroded. Such spokes may already be cracked and waiting to fail but you can't know that. Again I have never had a spoke turn into a projectile.
drossall
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Re: Over tightened spokes

Post by drossall »

If you had, then you probably wouldn't be here writing this :lol:

This has been a lengthy sub-thread over what I think we all agree is an unlikely scenario. If I had to write a risk assessment for wheel building (please, no!), it probably wouldn't be at the top - even though it's hard work thinking of bigger risks in the build itself. But it's too late now, and 40 years of habit is still going to have me avoiding looking down the line of a spoke on the jig, even if you prove that it could never happen :lol:

Even though, these days, I wear glasses :D
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531colin
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Re: Over tightened spokes

Post by 531colin »

Initially I was confused.....now I am completely bewildered.

Tom sent me his tension gauge, and I checked it against mine on a couple of wheels I have knocking about....pretty ordinary stuff, rear aiming at 120/80kgf, front aiming at 100.
The two gauges agree as closely as it is reasonable to expect. For example if you put one gauge on the same spoke a number of times, I expect no more spread of readings than one either side of the mode....(the most common reading.)....that was the sort of level of agreement between the 2 gauges. So, what on earth is going on? ....any bright ideas?

Here we are, 2 Park gauges. Mine is a bit older, dirtier, and showing more signs of wear. Its also got my name scratched on it, and it reads "USA" after "Park Tool". They look pretty similar mechanically, and I put a vernier on the wire of the springs, they were both 2mm.

ImageIMG_5615 by 531colin, on Flickr

This is Tom's conversion chart...

ImageIMG_5616 by 531colin, on Flickr

and this is mine....

ImageIMG_5617 by 531colin, on Flickr

I know which i find easier to read!

The sharp-eyed among you will notice that there is a bit of variation between the 2 charts....
for example, if you were aiming at 100kgf on a 1.8mm round steel spoke...
a deflection of 21 on Tom's chart will give you 105kgf
a deflection of 21 on my chart will give you 99kgf
So a little different, but not enough to spoil a wheel.

The numbers Tom gives for deflection/gauge/tension indicate (to me at least!) he is reading the right column of the chart. (Sorry, Tom, I had to check!)

I don't see how there can be much difference between 2 people using the same gauge, but for the record....
I hold the gauge in my (right) hand so its level and I can see the numbers....I squeeze the handles together and put it on the spoke, and release the handles fairly quickly. As above, if I read the same spoke multiple times I get pretty close agreement on the deflection.

So, as above.....anybody got any idea what is going on here? It beats me!
slowster
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Re: Over tightened spokes

Post by slowster »

Does using the meter closer to the nipple make a difference, e.g. including a non-butted part of the spoke, and if so how much?
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531colin
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Re: Over tightened spokes

Post by 531colin »

slowster wrote: 1 Jul 2022, 6:18pm Does using the meter closer to the nipple make a difference, e.g. including a non-butted part of the spoke, and if so how much?
Just tried it with some DT 1.8/2mm butted spokes. I got them 1 deflection division "stiffer" if I put one "leg" of the gauge on the 2mm bit next to the nipple.
Quite surprisingly reproducible, as the positioning is necessarily a bit variable....its a smaller target to aim for than the 1.8mm middle bit!
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