stress relieving new wheels myths?

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mercalia
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stress relieving new wheels myths?

Postby mercalia » 22 Feb 2016, 1:13am

is the metallurgical reason to do it really a myth? or is it really to get the spokes properly seated in the hubs and rim, and remove any twists? is there real evidence for the former reason?

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531colin
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Re: stress relieving new wheels myths?

Postby 531colin » 22 Feb 2016, 8:20am

http://www.sheldonbrown.com/brandt/stress-relieving.html

Short form......everything that happens to spokes (forming the wire, bending the elbow, thread rolling, wheel-building) builds stresses onto some of the steel. These built-in stresses shorten the fatigue life of the spoke. Loading the spoke briefly after the wheel is built will take (some of) the stressed areas to yield point, and plastic deformation (bending) will remove (some) built-in stress.

Brucey
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Re: stress relieving new wheels myths?

Postby Brucey » 22 Feb 2016, 8:47am

mercalia wrote:is the metallurgical reason to do it really a myth?....


no.
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CREPELLO
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Re: stress relieving new wheels myths?

Postby CREPELLO » 22 Feb 2016, 9:16am

mercalia wrote:is the metallurgical reason to do it really a myth?

Can you explain why you think it is a myth?

pwa
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Re: stress relieving new wheels myths?

Postby pwa » 22 Feb 2016, 9:21am

Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? If so, I have always assumed that it is to get the heads of the spokes properly bedded in so that final truing will be enough to ensure the wheel remains true when ridden. The notion that there may be further benefits is new to me.

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Re: stress relieving new wheels myths?

Postby Brucey » 22 Feb 2016, 9:43am

pwa wrote:Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? If so, I have always assumed that it is to get the heads of the spokes properly bedded in so that final truing will be enough to ensure the wheel remains true when ridden.....


just that...? -the job is a lot less than half done.

Now supposing you had some spokes tightened to over 200kg it would be obvious that they would be at risk of breakage, would it not?

In simple terms the reason for this is that the material in the spoke is too close to yield, and once service stresses are superimposed failure is likely, probably via fatigue cracking.

In a normal wheel, think of it this way; all the normal spoke forming and wheelbuilding operations leave local regions of high stress in the spokes. These local stresses are far higher than those which would be developed if you had >200kg spoke tension, so just at that point in the spoke failure initiation is just as likely.

Once a spoke starts to crack then it is doomed to fail (sooner rather than later) and leaving all the spokes with local high stresses is a 'Russian Roulette' job; sooner or later more of them will start to crack.

cheers
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531colin
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Re: stress relieving new wheels myths?

Postby 531colin » 22 Feb 2016, 12:38pm

pwa wrote:Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? ...............


No. Stress relieving is described by Brandt in the article I linked earlier. The process you describe doesn't load individual spokes enough (or reliably enough) to take their pre-existing areas of stress to yield point.

pwa
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Re: stress relieving new wheels myths?

Postby pwa » 22 Feb 2016, 12:44pm

531colin wrote:
pwa wrote:Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? ...............


No. Stress relieving is described by Brandt in the article I linked earlier. The process you describe doesn't load individual spokes enough (or reliably enough) to take their pre-existing areas of stress to yield point.


Ta. I'll investigate.

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Gattonero
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Re: stress relieving new wheels myths?

Postby Gattonero » 22 Feb 2016, 2:21pm

Brucey wrote:
pwa wrote:Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? If so, I have always assumed that it is to get the heads of the spokes properly bedded in so that final truing will be enough to ensure the wheel remains true when ridden.....


just that...? -the job is a lot less than half done.

Now supposing you had some spokes tightened to over 200kg it would be obvious that they would be at risk of breakage, would it not?

In simple terms the reason for this is that the material in the spoke is too close to yield, and once service stresses are superimposed failure is likely, probably via fatigue cracking.

In a normal wheel, think of it this way; all the normal spoke forming and wheelbuilding operations leave local regions of high stress in the spokes. These local stresses are far higher than those which would be developed if you had >200kg spoke tension, so just at that point in the spoke failure initiation is just as likely.

Once a spoke starts to crack then it is doomed to fail (sooner rather than later) and leaving all the spokes with local high stresses is a 'Russian Roulette' job; sooner or later more of them will start to crack.

cheers


Spokes are not tensioned at 200kg, the hub or rim would explode before that.
130-140kgf is the maximun I know for low-count spoke wheels, on the rear drive side. On normal wheels, 100-110 average is already a good tension.

Pre-stress has to be always done, and it also shows the builder how good the job hads been done. A wheel that is well build, will "crack" very little once stressed.

When being tensioned, the spokes are subject to torsion, and they need to find their "seat" in the hub flange, and so the nipples in the rim.
Some builders do have hydraulic presses, to stress the wheel at a force just under the spoke tension. There's no need to go mad, it needs a uniform, gentle but firm press.
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

Freddie
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Re: stress relieving new wheels myths?

Postby Freddie » 22 Feb 2016, 2:39pm

He didn't say they were tensioned that much, he was posing a hypothetical situation by way of comparison.

Brucey
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Re: stress relieving new wheels myths?

Postby Brucey » 22 Feb 2016, 6:52pm

Freddie wrote:He didn't say they were tensioned that much, he was posing a hypothetical situation by way of comparison.


precisely.
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Gattonero
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Re: stress relieving new wheels myths?

Postby Gattonero » 22 Feb 2016, 7:36pm

Freddie wrote:He didn't say they were tensioned that much, he was posing a hypothetical situation by way of comparison.


It's a red herring.
Keep the example to the real world: at 200kg the rim, whether has eyelets or not, will break.
Such figure has nothing to do with the "stress relieving", as far as I can see from the OP?
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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andrew_s
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Re: stress relieving new wheels myths?

Postby andrew_s » 22 Feb 2016, 7:39pm

pwa wrote:Is "stress relieving" the process of laying the newly made wheel flat on the ground and pressing down on the rim, all the way around its circumference, on both sides? If so, I have always assumed that it is to get the heads of the spokes properly bedded in so that final truing will be enough to ensure the wheel remains true when ridden. The notion that there may be further benefits is new to me.

Laying the wheel flat and pressing down is done allow the spokes to untwist. If you don't, the spokes will untwist, making pinging noises, when you start to ride on the wheel.The rim will also go slightly out of true.

fatboy
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Re: stress relieving new wheels myths?

Postby fatboy » 22 Feb 2016, 7:40pm

531colin wrote:http://www.sheldonbrown.com/brandt/stress-relieving.html

Short form......everything that happens to spokes (forming the wire, bending the elbow, thread rolling, wheel-building) builds stresses onto some of the steel. These built-in stresses shorten the fatigue life of the spoke. Loading the spoke briefly after the wheel is built will take (some of) the stressed areas to yield point, and plastic deformation (bending) will remove (some) built-in stress.


What's the recommended way to stress relieve a wheel. I've always gone for the hub on the ground and press the rim but I gather that's not the best/enough. Brandt's article tells you what not to do rather than what to do.
"Marriage is a wonderful invention; but then again so is the bicycle puncture repair kit." - Billy Connolly

Brucey
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Re: stress relieving new wheels myths?

Postby Brucey » 22 Feb 2016, 7:45pm

Gattonero wrote:
It's a red herring.


It absolutely isn't.

My point is that if you leave your wheel un-stress-relieved, the peak stresses in the spokes are locally much higher than any average you could ever achieve in reality, and they are very likely to break as a consequence.

Brandt's book describes a safe and effective method of stress relief, which involves grasping and squeezing (hard) groups of spoke crossings. The only really bad thing about this method is that it makes your hands sore, so wear gloves.

cheers
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