Spoke Count

[barq]

There's so much wheel mythology that it is probably second only to the chain cleaning/lubing threads. Black spokes versus stainless steel anyone?

Anyway I think the idea of spokes being under compression is confusing because one thinks of them bending or pushing back through the eyelet, through the rim tape... Here's how I understand it based on Brandt and Schraner:

All the spokes start off under fairly equal tension*. When the wheel is loaded the rim deforms ever-so-slightly at the point of contact with the ground. The spokes beneath the hub are now under less tension than they were before. In normal use this compressive force will not exceed the existing spoke tension, consequently the spokes aren't entirely unloaded and the wheel retains its strength. The greater the spoke tension the more compressive force the spokes can accommodate before going slack. This is one reason why strong wheels need tight spokes.

[* Obviously there will be significant drive-side / non-drive-side differences in dished wheels.]

 

[NickM]

So if building rear wheels half radial (X3 drive side, radial non-drive side) it might be better to use double butted spokes on the non-drive side and plain guage on the drive side?

 

[02GF74]

If the weight of bike and rider hangs from the top spoke, what is the top spoke hanging from? The rim? What is keeping that part of the rim which is bearing the weight in place? The other spokes, or at least those which are not in compression?

It is the strength of the rim. Take a decent quality rim and stand on it - it should retain it shape pretty well. The non vertical spokes will help to keep the rim in shape.

Regarding the rim flattening at the contact point with the gound - is this conjecture or has it been measured? i.e. how do we know it is not the spokes stretching? ( I guess it should be posible to calcuylate the elongation of a spoke). The air filled tyre is between the rim and gound so is not the load sread out by the rim.

and before anyone asks, black spokes are definitely faster.

 

[Smokin Joe]

Points of information:


There is no way that a normal spoke can ever do anything but tension 'cos the nipple will just push through the hole in the rim in compression.

.

Well put Martin.

The definive answer to anyone who thinks that spokes bear any compression whatsoever.

 

[hubgearfreak]

So if building rear wheels half radial (X3 drive side, radial non-drive side) it might be better to use double butted spokes on the non-drive side and plain guage on the drive side?

i don't really know why you'd want to do that?

but as most people have said, spokes break at the elbow, so using unbutted ones would offer no advantage

maybe the trick is to use a transmission system that is more symmetrical
ie. doesn't have the drive side under much greater tension, but that's a whole different dabate

Materials, especially aluminium, are way better than they were 30 years ago Factory built (low spoke count) wheels have much stronger rims 'cos they can now.

agreed...but a 36 spoker with better modern rims will be substantially stronger still, for very little weight penalty

Low spoke count wheels will be unusable if a spoke breaks. STI/Ergos aren't repairable on the road either but this doesn't stop us using them (or indexed gears even).

the thing about ergos is that the advantages seem very real, unlike low spoke wheels whose advantage is just a couple of ounces weight and when they go, it's just ping and game over without warning

 

[barq]

Regarding the rim flattening at the contact point with the gound - is this conjecture or has it been measured? i.e. how do we know it is not the spokes stretching? ( I guess it should be posible to calcuylate the elongation of a spoke). The air filled tyre is between the rim and gound so is not the load sread out by the rim.

The deflection of the rim against the ground has been measured (skip to the end of the linked article for a helpful diagram).

You can also measure the compressive force acting through the pre-tensioned spokes by using a tensiometer. The spokes under the hub are under less tension (i.e. a compressive force is acting) when the wheel is loaded. Take the load off the wheel and they get tighter again.

 

[Smokin Joe]

The deflection of the rim against the ground has been measured (skip to the end of the linked article for a helpful diagram).

You can also measure the compressive force acting through the pre-tensioned spokes by using a tensiometer. The spokes under the hub are under less tension (i.e. a compressive force is acting) when the wheel is loaded. Take the load off the wheel and they get tighter again.

That's nothing to do with compression, because as Martin C pointed out there is nothing for the spoke to be compressed against. What you are measuring is the difference between the spoke being under tension and being relaxed.

 

[NickM]

i don't really know why you'd want to do that?

Because Sheldon thinks the idea has merit:

...half radial rear wheels can be substantially more durable than conventional ones, in cases where the wheel is highly dished. The high amount of dishing called for to make room for more and more sprockets has caused an increase in spoke breakage on the left side of rear wheels. This is caused by metal fatigue.

A spoked wheel relies on having all of the spokes in constant tension. A highly dished rear wheel starts with very light tension on the left side spokes. The torque of hard pedaling combined with cyclical weight loading can cause the left side "leading" spokes to occasionally go completely slack momentarily.

Repeated cycles of tension and slackness cause these spokes to fatigue at the bends, and ultimately break.

With half-radial spoking, the amount of dish is very slightly less to begin with if you run the radial spokes up along the inside of its flange ("heads out.") In addition, since there are no "leading" spokes, no amount of torque on the hub can reduce the tension on any of the spokes. In fact, if you have an old wheel that has been breaking left side spokes, "half rebuilding" the wheel into a half radial will solve the problem once and for all.

I've just built my first half-radial wheel. I know it doesn't matter... but it looks rather nice

 

[Chris James]

That's nothing to do with compression, because as Martin C pointed out there is nothing for the spoke to be compressed against. What you are measuring is the difference between the spoke being under tension and being relaxed.

Blimey, everybody is right. The spokes are under NETT tension, as result of resolving two contradictory forces - the tension applied when building the wheel and the (lower) compressive force applied by the weight of the bike and rider.

 

[hubgearfreak]

but it looks rather nice

that's the the best reason. you can ignore all the debates on compression, tension, spoke pattern, whatever. if it looks good to it's owner & builder, then that's the main thing.

also, i've found that once you start building wheels, there's no stopping your new found skill. i have 36 spokes crows foot on my fixed for no better reason than to be different.

it had the added benefit of my lesser favoured LBS (the one that stocks many spokes) laughing at me saying that it couldn't be done when i asked for 48 of one length and 24 of another

 

[hubgearfreak]

pics of crows foot (not mine) here

http://www.terminalvelocity.demon.co.uk/WheelBuild/

 

[gkerr4]

wow - i've got to say - they look great!

 

[barq]

Blimey, everybody is right. The spokes are under NETT tension, as result of resolving two contradictory forces - the tension applied when building the wheel and the (lower) compressive force applied by the weight of the bike and rider.

Yep, I agree with that. Well put Chris, you've expressed it much more clearly than I had managed.

As for crows foot spoking, I've seen it before and it catches the eye. It just looks subtly different even to non-wheel nerds.

 

[robbarker]

Jobst et al look at the forces exerted by the bottom spoke. It's got the load pushing down at the top, and the reaction force from the ground pushing up at the bottom. The spoke is thus being compressed, and is supporting the load on the hub. The top spoke is in exactly the same state as it was in before the hub was loaded, and is therefore nothing to do with situation.

You've missed out the crucial point.

The load does act through the hub and pushes down on the bottom spoke (and, indeed, on all the spokes on the lower half of the wheel, to a lesser extent the farther from vertical they are at any given instant)

To say that the spoke is being compressed is not true. The reason a spoked wheel works is because the tension in that bottom spoke is always greater than the load being applied to it. It never goes into compression (which would cause it to collapse) but remains in tension, albeit less tension than if no load were being applied.

In this sense, the load on the wheel is supported by the reduction in tension of the lower spokes.

That's why wheels built with more tension in the spokes are stronger. The retain more tension under load, so there's less chance of the spoke head flexing, and hence less chance of the elbow becoming fatigued.

A wheel with more spokes is stronger since the reduction in tension under load per spoke is less.

Low spoke count wheels work becasue they use strong rims and spokes which allow more tension to be employed. This is fine until one does break, which leads to the wheel going far more dramatically out of true than would be the case with a 36 spoke wheel, for example.

To my mind the only adavntage of low spoke count, bladed spoke wheels with flash shallow section rims is a cosmetic one. Having said that, we all like nice looking bikes and that is a perfectly legitimate reason to use them.

 

[heretic]

This can only be understood through the theory of prestressed wires, it's similar to reinforced concrete beams acting in tension, which is supposedly impossible, but isn't if you get my drift.
Compression is negative tension. The loading reduces the tension in the bottom spokes, ie they are under compression. The wheel stands on the bottom spokes, there is no measurable effect on any other part of the wheel. BUT this is only possible because all the other spokes are under tension