Spoke Tension Meters

[Brucey]

Nice calibration job!

if the spoke is of reasonable quality it'll be ~250kgf to failure and my guess for the failure location is in the 1.8mm section but near to one of the butts.

Once you get into ~130kgf tension with your meter, each unit on the meter is worth about +20kgf. Since meters of this kind can't usually be read reproducibly to 1 unit, you could have a lot of scatter in your measurements even if the tensions in the DS spokes are in fact equal.

Having said that, I bet they aren't; it is very tempting to try and bully a rim to be round using spoke tension, but it is usually a better idea to allow a little out of roundness (eg near the rim joint) in a modern (deeper section) rim and have a more uniform spoke tension.

cheers

[iandusud]

Nice calibration job!

Having said that, I bet they aren't; it is very tempting to try and bully a rim to be round using spoke tension, but it is usually a better idea to allow a little out of roundness (eg near the rim joint) in a modern (deeper section) rim and have a more uniform spoke tension.

cheers

I'll second that.

[jb]

Some fine trials going on here. What would interest me more than spoke strength is how much force is required to pull a spoke out of the nipple with varying thread contact.
I.e. 3 or 4 threads in contact with each other should in theory give maximum joint strength, but the nipple is brass or aluminium so I guess a little more is needed. But what bugs me with wheels I have had built for me in the past is when the spokes protrude from the nipple leaving very little adjustment left and in some cases stick into the rim tape and have to be filed down (not so much these days with sunken rims).
When I build a wheel I like the finished spoke to be 2 threads below the bottom of the slot as there is already plenty of thread contact at this point, however the nipple diameter obviously drops below the head thus the strength is in the small bit of metal left on the square bit. I think it would be interesting to know what the breaking point is when the spoke is only in the narrow bit of the nipple and how few threads need to be in contact before the strength drops below what's practical for a good build.
I've no doubt this information is about somewhere but I haven't looked very hard yet.

[Des49]

Got round to calibrating it today, seemed to go OK ended up with a reasonable graph
even got an equation for it

Excellent work indeed. I like the equation!

My calibration jig relies on a 150kg digital scale, so I'm not going to test to spoke failure. But like the idea of producing a graph for each type of spoke. Though I tend to test each time I build a wheel with a spare spoke of the same type I am using, for the specific tension I want to achieve.

[Brucey]

Some fine trials going on here. What would interest me more than spoke strength is how much force is required to pull a spoke out of the nipple with varying thread contact.
I.e. 3 or 4 threads in contact with each other should in theory give maximum joint strength, but the nipple is brass or aluminium so I guess a little more is needed. But what bugs me with wheels I have had built for me in the past is when the spokes protrude from the nipple leaving very little adjustment left and in some cases stick into the rim tape and have to be filed down (not so much these days with sunken rims).

The fit between spokes and nipples isn't always consistent. The theoretical predictions of ~95-97% strength from three or four threads in contact are representative of standardised metric threaded fasteners and don't necessarily apply to spoke threads or indeed any fastener that is tightened more than once. In a typical nipple you get 16 to 18 full threads in engagement and the fit of the nipple on the spoke doesn't have to be very poor and/or the amount of adjustment under tension doesn't have to be much before you are best off with most of them.

When I build a wheel I like the finished spoke to be 2 threads below the bottom of the slot as there is already plenty of thread contact at this point, however the nipple diameter obviously drops below the head thus the strength is in the small bit of metal left on the square bit. I think it would be interesting to know what the breaking point is when the spoke is only in the narrow bit of the nipple and how few threads need to be in contact before the strength drops below what's practical for a good build.
I've no doubt this information is about somewhere but I haven't looked very hard yet.

IMHO two turns below the bottom of the nipple slot means the spoke is definitely too short. This is indeed inviting nipple failure. In some wheel builds (eg 13G spokes using converter nipples which are thin-walled anyway) nipple failure with short spokes is the most common spoke failure. if you screw up with a front wheel and the spoke end doesn't make it as far as the slot bottom probably you will get away with it (there is usually less corrosion going on and lower tension than a rear wheel DS) but it still isn't ideal. Deliberately aiming for the 'two turns below slot bottom' makes no sense whatsoever in my estimation; the reason is that if you have the slightest error in your calculations, the spokes will definitely be way too short at least half the time.

Even if you are working with (clumsy) 2mm spoke increments you would be better off with a +2mm longer spoke if you are otherwise finishing 2 turns below the bottom of the slot. The longer spoke will finish flush with the nipple top which is *perfect*.

Literally millions of wheels with single-walled rims have been built whereby spokes routinely needed to be cut or ground flush; this was a deliberate choice because the wheels are better than if the spokes are even 1mm (leave alone 2mm or BITD 1/16") shorter than this. With single-walled rims you should ideally be building with 10mm or 11mm nipples and the spoke doesn't need to be very short before you start seeing threads below the nipple. This looks worse than it is, for sure, but since it looks b.awful that isn't saying much.

Most matched spokes and nipples allow 1-2mm of spoke out the top of the nipple before 'crunching' ensues. I've taken the trouble to see what damage is caused by 'crunching' and the answer is that a good quality stainless steel spoke is not appreciably damaged by a brass nipple being crunched. It doesn't look pretty to have even (say) 3mm out of the top of the nipple (I certainly wouldn't be happy building wheels this way) but the net result is almost certainly still stronger than finishing two turns below slot bottom.

FWIW a crunched nipple may even be preferable to one that is not in some installations, depending on the nipple angles and the service conditions; the reason is that the bending stresses that might otherwise cause failure in the spoke, inside the nipple, are mitigated. The crunched nipple is a naturally intimate fit on the spoke and helps support the spoke where it is able to better withstand bending loads, in the unthreaded portion.

The only time a spoke poking out of the nipple top is likely to be a real problem is in some shallow double-walled rims; the spoke end can't easily be trimmed yet may still poke through the rim tape (which always sags into the holes in the inner wall of the rim) and this can cause punctures. if you build a wheel like this you feel a bit of twit having to get the rim corks out because you screwed up this way....

cheers

[Pebble]

Nice calibration job!

if the spoke is of reasonable quality it'll be ~250kgf to failure and my guess for the failure location is in the 1.8mm section but near to one of the butts.

Once you get into ~130kgf tension with your meter, each unit on the meter is worth about +20kgf. Since meters of this kind can't usually be read reproducibly to 1 unit, you could have a lot of scatter in your measurements even if the tensions in the DS spokes are in fact equal.

Having said that, I bet they aren't; it is very tempting to try and bully a rim to be round using spoke tension, but it is usually a better idea to allow a little out of roundness (eg near the rim joint) in a modern (deeper section) rim and have a more uniform spoke tension.

cheers

I'm thinking the nipple will fail, with nuts and bolts over tightening usually causes the thread to fail, or is that just down to the turning motion when too tight? Also feel the elbow could be a weak point

find out later.

[jb]

Thanks for that Brucey.

I've never had a wheel give problems with the nipples so it hasn't troubled me much if the spoke finishes up slightly below the slot. That's why it would interesting to see what the actual breaking point would be. Crunching up threads does have the advantage of them never coming loose - a problem on the slack side of heavily dished wheels but makes re-truing a pain and if spoke breakage does occur a long way from home its nice to be able to readjust the wheel on the hoof.
Having said that I don't disagree that the joint between spoke and nipple is strongest when flush with the top, just about how strong they need to be.
As an aside I've taken to drawing out wheel builds on cad & projecting the true spoke length. which allows you to take into account differing lengths from once side of the flange to the other and basically customise the length to your exact wheel with any variations in ERD or nipple size etc. from the norm. So my spoke ends tend to end up exactly where I expected them to be after selecting them from the sizes available.

None of this applies to them fancy wheels built with three and half spokes per side, I wouldn't touch them.

[Samuel D]

I'm thinking the nipple will fail, with nuts and bolts over tightening usually causes the thread to fail, or is that just down to the turning motion when too tight?

That hasn’t been my experience. The bolt usually snaps before the thread fails. I think that’s by design, since a damaged thread is invisible and might be put into service to fail in operation. If the bolt snaps (or at least torque drops in the plastic region), that’s a hint to even the most ham-fisted mechanic that something bad happened and needs to be rectified.

Spokes and nipples are a special case and I don’t know what fails first under load. The problem in wheels is usually fatigue.

I think your spoke will fail in the narrow section, but, looking at your set-up (still wondering how you’re measuring weight), the spoke appears to be bent at the end of the nipple. Maybe it will fail there instead. How far is the nipple screwed on?

That's why it would interesting to see what the actual breaking point would be.

Failure from ductile fracture, as in a test like Pebble’s, is a different thing from fatigue failure, which is the risk to the nipple when running short spokes in a wheel. So I’m not sure we’d learn much from the test. Would still be interesting.

[Brucey]

….As an aside I've taken to drawing out wheel builds on cad & projecting the true spoke length. which allows you to take into account differing lengths from once side of the flange to the other and basically customise the length to your exact wheel with any variations in ERD or nipple size etc. from the norm. So my spoke ends tend to end up exactly where I expected them to be after selecting them from the sizes available...

yebbut it may be all very good but you are still reliant on things such as

- the accuracy of your rim measurements
- the elastic deformations in rim and spoke
- the way the spoke fits and settles into the hub flange
- the roundness of the rim
- the way the rim joint has been made (sometimes the ERD is different locally here even if the rim is round)
etc etc.

In other words there are still lots of errors which can creep in.

I am happiest if I get the spoke ends to finish mid-slot. But they don't need to be very much shorter than this before I am unhappy, so shooting for flush with the nipple top isn't a bad idea.

FWIW the side-to-side of flange variations in spoke length only matter if you build 'skew' wheels (i.e. inside leading on one flange, inside trailing on the other). Otherwise (in symmetric wheels) it averages out. Arguably the only time you should be thinking of building 'skew' wheels is when you are building a rear wheel for a disc brake.

cheers

[Pebble]

That hasn’t been my experience. The bolt usually snaps before the thread fails. I think that’s by design, since a damaged thread is invisible and might be put into service to fail in operation. If the bolt snaps (or at least torque drops in the plastic region), that’s a hint to even the most ham-fisted mechanic that something bad happened and needs to be rectified.

Spokes and nipples are a special case and I don’t know what fails first under load. The problem in wheels is usually fatigue.

I think your spoke will fail in the narrow section, but, looking at your set-up (still wondering how you’re measuring weight), the spoke appears to be bent at the end of the nipple. Maybe it will fail there instead. How far is the nipple screwed on?

Bigger stuff I would agree, but have had problems with littler stuff (>M4) may be cut threads? spokes appear to be rolled

Weight was measured with scales that I believe to be fairly accurate (I like calibrating stuff) , I would say within 2%. Biggest inaccuracy is the tension meter itself, measure the same spoke in a few places and get +/- 0.5 each time. I'm treating readings as a rough guide rather than a precise figure.

May devise another method for calculating break point, bricks balanced on a beam under a lot of tension that is going to be released in a nano second could be a bit dodgy, don't want hit on the nogging with an air borne brick. Don't want to be another hospital stat of cyclist - head injury - no helmet


I agree under normal use a spoke won't fail in this way, fatigue fracture I suspect will be the usual cause. Not really sure why I am going to snap a few spokes - will blame it on lock-down madness.

[Samuel D]

Biggest inaccuracy is the tension meter itself, measure the same spoke in a few places and get +/- 0.5 each time.

I’ve wondered if a drop of oil at the contact points between tool and spoke, especially the outer two, would improve the reading consistency at the cost of impractical faff. My Wheel Fanatyk tensiometer has ball bearings there.

May devise another method for calculating break point, bricks balanced on a beam under a lot of tension that is going to be released in a nano second could be a bit dodgy, don't want hit on the nogging with an air borne brick.

Ha. The wood snapping would have already got the wind up me.

I agree under normal use a spoke won't fail in this way, fatigue fracture I suspect will be the usual cause. Not really sure why I am going to snap a few spokes - will blame it on lock-down madness.

Up with this sort of thing anyway! Wish I had the space for such experiments.

[531colin]

I can confirm that they go with a bang. I have only ever pulled spokes to (ductile) failure using a lever which multiplied my bodyweight. Safety specs. were worn. This is the ductile failure, I'm sure Brucey will have a name for the curious pattern. (poor camera in those days)
008 by 531colin, on Flickr

[Samuel D]

I'm sure Brucey will have a name for the curious pattern.

Not Brucey, but I’m fairly sure that’s the stocking stitch.

[Brucey]

cup and cone fracture, it is called, and is characteristic of ductile failure in tension



Not sure of the stitch in the cloth; wrong sort of 'materials science' I'm afraid...

cheers

[iandusud]

cup and cone fracture, it is called, and is characteristic of ductile failure in tension



Not sure of the stitch in the cloth; wrong sort of 'materials science' I'm afraid...

cheers

That's for a different thread