spoke tension for 26" Sputnik rims

[Samuel D]

Interesting thread. Should spoke tension be adjusted for the weight of the rider? For example, I’m pretty light – if I told Spa that, would they build a wheel with slightly lower tension to reduce the risk of a rim cracking?

Or is it sufficient/better to accommodate rider weight by choosing an appropriate rim and spoke count in the first place?

[Brucey]

Interesting thread. Should spoke tension be adjusted for the weight of the rider? For example, I’m pretty light – if I told Spa that, would they build a wheel with slightly lower tension to reduce the risk of a rim cracking?

Or is it sufficient/better to accommodate rider weight by choosing an appropriate rim and spoke count in the first place?

I think that some wheels are built with high spoke tension because the most likely outcome is that the wheel will be reliable in the short term (almost regardless of loading), even if the rim is at greater risk of cracking in the long run. A useful improvement in the chances of avoiding cracking is if the wheels don't see winter road salt, or are protected in some way from it. I often use waxoyl to protect the aluminium around the spoke eyelets; if applied to a hot rim it runs around each eyelet and protects it quite well.

The minimum spoke tension is usually set by the risk of nipples unscrewing in use. However if threadlock is used on the nipples, the spoke tension could be set so low that the rim is at risk of fatigue in bending. Whenever the spokes are at risk of running completely slack in use (even momentarily) the spoke is likely to be moving around in the hub and wearing the hole in the hub. Under these conditions even a wheel with threadlock on the nipples can appear to lose tension.

Speccing wheels is not an exact science and there is nearly always more than one solution that will work OK. Modern wheels tend to use stiffer rims and fewer spokes (and in terms of outright speed this may even be best) but from the feel of the bike when you are riding it, I quite like rims that are less stiff, built with more, skinnier, spokes, even though this is presently a deeply unfashionable way to build a wheelset. These wheels often feel more responsive because the rim itself can be made lighter. The wheel can be slightly more flexible too, and although the deflections should be very small, they may also contribute in some small way to the feel of the wheel in use.

cheers

[Brucey]

...Old man whingeing mode "on".....
Seems to me this Forum was a different place back then, with proper discussion, and an absence of people slagging each other off in interminable wrangles about the merits or otherwise of the latest spangly bits..........mode "off", but may return...

I don't disagree. However I am worried that slagging/wrangling may itself be regarded as a spangly new thing, itself to be argued/wrangled about....

cheers

[NUKe]

Brucey and Colin have forgotton more about wheel building than I will ever know. and I bow to their years of experience.

However as the question was from a novice I still say the more important part of wheel building is the roundness and the true. you can't build a wheel with a tensiometer, the more important tool is the trueing stand. If you build by setting the tension only , you get a pringle. I know I tried it when I first aquired a Parktool gauge. Dont get me wrong its a great tool, but is useful only as part of the armoury

work slowly and methodically and you eventaully get a nice round wheel. I dare say Colin and Brucey can do the things blindfold, from years of practice. I personally still go through all the emotions of Oh ***** this is never going to work through to ellation and immense satisfaction.

some good videos on youtube which can help And sheldon browns description of lacing is quite good. dont forget to destress the spokes

[NUKe]

forgotten the start of the thread sorry to have said Novice mercalia
but my comments still stand for a novice

[531colin]

Interesting thread. Should spoke tension be adjusted for the weight of the rider? For example, I’m pretty light – if I told Spa that, would they build a wheel with slightly lower tension to reduce the risk of a rim cracking?

Or is it sufficient/better to accommodate rider weight by choosing an appropriate rim and spoke count in the first place?

I would say there is not much risk of the rim cracking anyway, unless there is a design/manufacturing fault in the (batch of) rims, or you specify a rim thats too light for the purpose. (discuss....?)
If its touring wheels you are after, then there is very little benefit to be gained by specifying a light wheel.....much better to use a regular wheel and be happy with a margin of safety, and a light tyre for ease or speed.....better than a light wheel and a Marathon plus tyre....and I have seen that done.
If you are looking for a wheel for racing (or pretend racing) then that is outside my experience, but reduced-spoke wheels with twice as many driveside spokes as NDS spokes get good reports.

Brandt's wheel book was first printed in 1981, so it pre-dates modern deep-section rims. He reckoned a 50kg load on the axle would produce a maximum vertical deflection of 0.15mm at the rim, so I think wheel flex will make a minor contribution to ride comfort.

Its also worth re-stating that spokes are working in the straight-line phase of their load vs. extension curve (Young's modulus), so that the same (additional) load will produce the same (additional) elongation.....so a "tight" wheel is just as stiff as a "slack" wheel, unless the wheel is so slack that some spokes are under no tension at all at times. Thin spokes are "stretchier" than thick ones, but I wouldn't know where to start to quantify the difference.

[mercalia]

well I made some sound files for those who have good ears

rear drive side spokes

http://vocaroo.com/i/s0dQjbZwXdh9


rear nds spokes

http://vocaroo.com/i/s0gvxlX8hNmj

all twangs a different spoke, tensions seem very consistant

really puzzled by all this. I wonder whether to take the gauge back, I did test the spokes again, read the instructions and it didnt say anything about jiggling the handle or exerting extra pressure downwards to over come any friction and let it settle (to a lower value ) ( which it does a bit. 2-3 points down)

I might add the wheels I have seem nicely made & very consistant in the tensions and have stood up well to the battering of my weight over London pock marked and crippled roads and also off road

[Brucey]

I would say there is not much risk of the rim cracking anyway, unless there is a design/manufacturing fault in the (batch of) rims, or you specify a rim thats too light for the purpose. (discuss....?)

unfortunately SCC doesn't work like that. Cracks can develop in parts that are far below their yield stress. Fatigue can contribute as well as corrosion. Many rims yield slightly around the spoke holes even at 120-130kgf (if you see silver anodising round the spoke holes change and go slightly milky during the build, this is what is happening).

Brandt's wheel book was first printed in 1981, so it pre-dates modern deep-section rims. He reckoned a 50kg load on the axle would produce a maximum vertical deflection of 0.15mm at the rim, so I think wheel flex will make a minor contribution to ride comfort.

Certainly, but when you go over a big bump the force could be x4 that, momentarily, and so could the deflection in the wheel. You would certainly notice 0.6mm play in the bearings, or extra flex in the fork, etc etc so I think that Brandt's analysis underplays the role wheel stiffness might play in ride feel.

Its also worth re-stating that spokes are working in the straight-line phase of their load vs. extension curve (Young's modulus), so that the same (additional) load will produce the same (additional) elongation.....so a "tight" wheel is just as stiff as a "slack" wheel, unless the wheel is so slack that some spokes are under no tension at all at times. Thin spokes are "stretchier" than thick ones, but I wouldn't know where to start to quantify the difference.

But that assumes that the rim stiffness plays no part in the in the stiffness of the wheel. Brandt's original analysis assumed that the rim had no stiffness (IIRC he modelled it as a series of pin-jointed sections, which was arguably OK for a shallow rim of the period), but real rims do have stiffness and in order for the wheel to deflect radially (or laterally) the rim has to bend elastically. The tension in the build thus affects the size of the flat region at the bottom of the wheel (radial loads). The rim can be thought of as deflecting like a set of restrained cantilever beams, and the length of the beams (size of flat) ought to make a difference to the resultant stiffness. It ought to have been easy enough to carry out a deflection test using wheels with different tensions and rim stiffnesses, but maybe Brandt didn't do that.

cheers

[531colin]

I didn't mean the mechanism of rim cracking, but the incidence.
As far as I'm concerned, cracking round the eyelets is a rare or very rare mode of rim failure, unless there is a design/manufacture problem with a (batch of) rims like http://forum.ctc.org.uk/viewtopic.php?f=5&t=66655, or the rim is specified too light for the intended use.

A 200kg load on a wheel might well deflect the rim by 0.6mm. Its still small beer compared to the tyre deflection.

Why does the build tension affect the size of the flattened zone at the bottom of the rim? (Brandt's load affected zone, LAZ)
The build tension doesn't alter the stiffness of the rim, and if the spokes are in the linear bit of the Young's modulus curve, the tension on the spokes doesn't alter their elasticity either.

[Brucey]

if the spoke tension is too high it just weights the probabilities in favour of cracking. It can happen because a rim is badly made but if the spoke tension is high enough and you use the wheel in the winter it'll happen anyway even with 'good' rims. Cracking is the thing that effectively limits spoke tension in very many rims these days. Many Mavic rims will crack in winter service if you exceed a certain tension and so will many (most) other rims too.

I've recently seen several aero-rimmed Bontrager wheelsets with cracked rims. I've seen enough now to see a pattern; the cracked spoke hole is almost invariably near the rim joint. I think that the rims are not perfectly round and there is at least one spoke near the rim joint that gets overtightened in a bid to make the wheel rounder. The overtightened spokes are the ones where the rim cracks first.

Mathematically tension wheels seeing radial loads are bit like a section through a cylindrical pressure vessel, where the pressure inside the vessel can vary (spoke tension) and the wall of the vessel can have variable bending stiffness.

At one extreme you can have something like a balloon (low bending stiffness walls, relatively high pressure) and at the other extreme you could have (say) a near-empty gas cylinder (low pressure, high bending stiffness in the walls). Under external loads, the latter will deform into some kind of gross oval before it develops a flat spot whereas the former will always give a flat spot.

Rims of different stiffnesses (and even the same rim with wildly different spoke tensions) will do something similarly different and the result is that the apparent stiffness of the wheel can in fact vary with spoke tension.

If you had a wheel with a rim of infinite stiffness (or at least high in relation to the spoke tension forces) it would stay round under axle load and the spokes in the upper half of the wheel would increase in tension in a way that would exactly mirror the loss in the tension in the lower half.

cheers

[531colin]

So your Bontrager rims are another example of a poorly designed/manufactured (batch of) rims, like the DH19 rims of a few years ago, which had a sudden spate of cracking. Modern Mavic rims all seem to be light for their width/intended use, even compared to the Mavic rims of years ago. I haven't used Mavics in a while, any Mavic I had in the past the brake pads wore through, and thats the complaint I recall reading on here about Open Pro....I don't recall reading about a spate of cracked Mavics.

I think you are saying that spoke tension will in effect "brace" the rim so that the built rim is stiffer vertically than the unbuilt rim, and higher spoke tension will "brace" the rim more than low tension. If thats the case, it would have to be independent of the Young's modulus of the spoke? I can't imagine the magnitude of such an effect, are we talking of 0.6mm difference in deflection?..

[Brucey]

So your Bontrager rims are another example of a poorly designed/manufactured (batch of) rims, like the DH19 rims of a few years ago, which had a sudden spate of cracking. Modern Mavic rims all seem to be light for their width/intended use, even compared to the Mavic rims of years ago. I haven't used Mavics in a while, any Mavic I had in the past the brake pads wore through, and thats the complaint I recall reading on here about Open Pro....I don't recall reading about a spate of cracked Mavics.

the Bonty rims that have cracked have been the exception, and the reason is almost certainly exceptional spoke tension. I'd argue that it is bad wheelbuilding more than bad rims, because these rims don't fail if the spoke tension is kept to a sensible level. No rims are perfect, but here some twit has chased the last 0.25mm of radial run-out and has used excessive spoke tension to get it.

Round where I am I don't think there are many rims that can't be made to crack if you give them enough spoke tension and enough road salt. I see cracked rims of all kinds, all the time, just some more than others.

I think you are saying that spoke tension will in effect "brace" the rim so that the built rim is stiffer vertically than the unbuilt rim, and higher spoke tension will "brace" the rim more than low tension. If thats the case, it would have to be independent of the Young's modulus of the spoke? I can't imagine the magnitude of such an effect, are we talking of 0.6mm difference in deflection?..

Kind of; what I'm saying is that if the rim deforms easily to give a flat shape, that redistributes the spoke tension most efficiently. If the rim is stiffer, (in relation to the spoke tension) it is different to that. As to the extent of the effect, that is going to vary with the wheel. I don't think it will be large in terms of displacement but that doesn't mean that you won't feel it; it'll be one of several things that add up to give the ride feel.

I don't think it is a coincidence that in both touring and 'racing' spheres, there is presently interest in using wider tyres. More people are using stiffer (disc ready) forks and stiffer (deep section) rims.

cheers

[Dave855]

I'm in the final stages of building a rear Sputnik wheel (sapim race non drive side and sapim strong drive side) and struggling with the spoke tension. I have the Park tool but according to the chart the deflection readings translate differently to those described in earlier posts. I currently have deflection readings of 24 drive side which translates as 148kg. Is this far too much? None drive side is only 17 which translates as 81kg. Dish is correct.

Earlier posts suggest I should be looking for 25 drive side and 19 non drive side. But this would equate to 167kg drive side on my chart!

[531colin]

The bits of the chart you have to read are........
2mm round steel spoke for a "strong"....its 2mm for most of its length
1.8mm round steel for a "race" .....the thinner middle bit is 1.8mm

[Brucey]

OK so you have

Sapim strong -which are 14g / 2.0mm where you measure them and
Sapim Race - which are 1.8mm (according to sapim) in the centre section.

http://www.parktool.com/assets/doc/product/TM-1_Table_3-21-14.pdf

'24' corresponds to 107 kg on 2.0mm

and

'17' corresponds to 70kg on 1.8mm

cheers