Do wheels hang from or stand on their spokes?

[mattc]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

But it is true. It's just a change in sign in equations.

Just as in electronics, a resistor can be used to *increase* the voltage.

I have to disagree here.
A voltage is a PURELY RELATIVE THING - so as you say, you can talk about +ve/-ve however you like.

But compression and tension are very different things. See what happens if you take a bare spoke and compress it along its length.

IMHO, talking about spokes "supporting by their compression" is using very unhelpful language that does NOT usefully describe the real physical world.

M
(This view may explain why I always struggle with electronics, despite finding quantum theory (fairly) intuitive).

[rogerzilla]

*not a tensiometer, which is a rather useless piece of apparatus when it comes to wheelbuilding.

Really?

Americans.  They don't know anything 

[TimO]

*not a tensiometer, which is a rather useless piece of apparatus when it comes to wheelbuilding.

Really?

They really can't spell.

Not to be confused with Extensometer or Tension meter....

[alexb]

Since you can build a wheel using string*, I think it's fair to say that the wheel hangs from the upper spokes.


*Not a very strong one, but good enough to prove the point.

[Tim Hall]

Presumably you can build a stronger one using those "emergency" kevlar spokes, wot are flexible like string, but much much more expensive.

[mrcharly-YHT]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

But it is true. It's just a change in sign in equations.

Just as in electronics, a resistor can be used to *increase* the voltage.

I have to disagree here.
A voltage is a PURELY RELATIVE THING - so as you say, you can talk about +ve/-ve however you like.

But compression and tension are very different things. See what happens if you take a bare spoke and compress it along its length.

IMHO, talking about spokes "supporting by their compression" is using very unhelpful language that does NOT usefully describe the real physical world.

M
(This view may explain why I always struggle with electronics, despite finding quantum theory (fairly) intuitive).

Both are forces. Apply compression to a string that is under 0 tension, and it will immediately deform. Apply compression to a string that is under tension and it will only deform if the compressive forces are greater than the tension forces.

Voltages aren't just relative, currents flow in a definite direction. Of course, sometimes it's convenient to say that positive charges flow in one direction, sometimes negative in the other.

[mattc]

Apply compression to a string that is under tension and it will only deform if the compressive forces are greater than the tension forces.

If the compressive forces are greater than the tension forces on an object, then it's under compression, NOT tension!

I can see we'll just have to agree to differ on this - I think we're speaking different dialects.

[dkahn400]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

It's simple algebra. Tension is negative compression. Reduce tension and you increase compression.

[dkahn400]

I'm really disappointed this thread has only grown to 3 pages by now. Tug of war has been mentioned briefly, so that's a point. No-one has mentioned moored hot-air ballons yet, or boats roped together but all pulling in different directions, so that's two points off.

Similarly no-one has yet mentioned pre-stressed concrete or cast aluminium moped wheels, but it's only a matter of time. Whoops, I've just named them all, so we're on our way.

[mattc]

It's simple algebra. Tension is negative compression. Reduce tension and you increase compression.

So I can compress something by letting go of it?

What next - resurrecting people by reducing their deadness quotient?

[mrcharly-YHT]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

It's simple algebra. Tension is negative compression. Reduce tension and you increase compression.

Thank you for putting this even more simply.

I think that part of the confusion is caused by using terms. In reality, there are numerous forces acting on a bicycle wheel. normally, they all resolve. When they don't, the wheel collapses. You can't really consider an single force in isolation, because that's not a bicycle wheel!

[Air Dancer]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

It's simple algebra. Tension is negative compression. Reduce tension and you increase compression.

Noway Hozay

Tension can only be reduced to zero. Compression forces then act on the spoke or strut. There was a person called Euler who came up with some principles about compression on struts and their buckling effects.

[Really Ancien]

The poor old rim seems to have been left out in all this chat. It is the rim which distributes the forces from the weight applied to the spokes. The degree to which the force on the spokes will vary is related to the stiffness of the rim. Deep section rims do this job very well, needing fewer spokes at the cost of poor ride. Sprint rims with 36 spokes would be the opposite extreme, they need their spokes to control the deformation of the rim, they are very comfortable. One effect is that any slight loosening of a spoke nipple on a deep section rim with a low spoke count will lead to that spoke rapidly slackening.

Damon.

[dkahn400]

In a properly built wheel the lower spokes support the load by a reduction in their tension. Reduction in tension is equivalent to compression.

I don't believe this is true either.  A reduction in tension does not mean the lower spokes are now supporting *any* weight via compression.  Just because two teams on a tightrope are pulling less forcefully...

It's simple algebra. Tension is negative compression. Reduce tension and you increase compression.

Noway Hozay

Tension can only be reduced to zero. Compression forces then act on the spoke or strut. There was a person called Euler who came up with some principles about compression on struts and their buckling effects.

In a bicycle wheel the tension of the lower spokes does not normally reduce to zero. A wheel with zero tension in the lower spokes is about to collapse. As long as they remain in tension the lowers spokes are able to take a compressive load. Reduction in tension = increase in tension.

[dkahn400]

The poor old rim seems to have been left out in all this chat. It is the rim which distributes the forces from the weight applied to the spokes. The degree to which the force on the spokes will vary is related to the stiffness of the rim. Deep section rims do this job very well, needing fewer spokes at the cost of poor ride. Sprint rims with 36 spokes would be the opposite extreme, they need their spokes to control the deformation of the rim, they are very comfortable. One effect is that any slight loosening of a spoke nipple on a deep section rim with a low spoke count will lead to that spoke rapidly slackening.

Damon.

Damon is of course correct. In a wheel with an infinitely stiff rim the load would hang from the top spokes.

[Wendy]

In a complete wheel the tension of the upper spokes does not increase when the wheel is loaded at the hub, rather the tension in the lower spokes decreases.

This is the bit I have difficulty in believing, and why I think you're wrong.

[LittleWheelsandBig]

In a complete wheel the tension of the upper spokes does not increase when the wheel is loaded at the hub, rather the tension in the lower spokes decreases.

This is the bit I have difficulty in believing, and why I think you're wrong.

Try the spoke note experiment higher up the thread and you may change your belief.

[TimO]

I'm really disappointed this thread has only grown to 3 pages by now. Tug of war has been mentioned briefly, so that's a point. No-one has mentioned moored hot-air ballons yet, or boats roped together but all pulling in different directions, so that's two points off.

Similarly no-one has yet mentioned pre-stressed concrete or cast aluminium moped wheels, but it's only a matter of time. Whoops, I've just named them all, so we're on our way.

Are these the requirements for Godwin's Law as applied to technical bicycle forums/newsgroups?  At this point we should probably invite Jobst Brandt to the forum, and watch the sparks fly!  Flame wars about how wheels work, and whether hubs hang from spokes etc are more or less traditional in bike forums.

[dkahn400]

In a complete wheel the tension of the upper spokes does not increase when the wheel is loaded at the hub, rather the tension in the lower spokes decreases.

This is the bit I have difficulty in believing, and why I think you're wrong.

It's one of the few bits there's no real argument over. It's a proven fact. If this is all it will take to convince you try the ping-the-spokes test. When you load the bike up the note in the top spokes does not change; that in the lower spokes drops appreciably.

If it helps here's a description of a finite element analysis.

[Wendy]

Here's quite a well-written explanation, IMO:

http://hea-www.harvard.edu/~fine/opinions/bikewheel.html

TimO, you are aware that Jobst claims flints don't cause UDEs, in spite of many telling him otherwise.  Ahahahahahaha!

[Wendy]

Prestressed concrete is not about the tendon holding any compressive load, it's about the tension in the tendon preventing the concrete from experiencing tensile load.  Same goes for a preloaded church spire, for example.

You might try to argue that the tension in the lower spokes prevents it from experiencing compression, but a reduction in tension is still more tension than a slack state, and still pulls downwards, only less so.  It's the difference in tension between the above spokes and the below spokes that supports the load, not any compression.

[Ian H]

Tension can only be reduced to zero. Compression forces then act on the spoke or strut. There was a person called Euler who came up with some principles about compression on struts and their buckling effects.

A spoke can only act in compression, however poorly, if it is fixed in position to resist compressive loads. At the rim it isn't.

[dkahn400]

I'm really disappointed this thread has only grown to 3 pages by now. Tug of war has been mentioned briefly, so that's a point. No-one has mentioned moored hot-air ballons yet, or boats roped together but all pulling in different directions, so that's two points off.

Similarly no-one has yet mentioned pre-stressed concrete or cast aluminium moped wheels, but it's only a matter of time. Whoops, I've just named them all, so we're on our way.

Are these the requirements for Godwin's Law as applied to technical bicycle forums/newsgroups?  At this point we should probably invite Jobst Brandt to the forum, and watch the sparks fly!  Flame wars about how wheels work, and whether hubs hang from spokes etc are more or less traditional in bike forums.

Generally it comes down to a semantic problem about what exactly the words "hang" and "stand" mean. The point about the moped wheel is that it is impossible to tell whether the solid spokes are in tension or compression. It doesn't matter as the behaviour of the wheel under load is the same in either case. With such a wheel people usually have no trouble agreeing that the lower spokes take a compressive load as the rim deforms although those spokes may actually remain in net tension.

The lower spokes of a bicycle wheel are in tension and remain in tension when the hub is loaded. The degree of tension is reduced and the spokes also shorten as the rim flattens out at the contact patch. Reduction in tension and shortening = compression. Further, if you regard the spokes in isolation the bottom ones are exerting a downward force on the hub, and the top ones an upward force. When the hub is loaded the upward force of the top spokes does not change but the downward force of the lower ones reduces to match the increased load. Again it's simple algebra: reduction in downward force = increase in upward force. This is what I mean when I say that the wheel stands on the lower spokes.

Wire spokes in isolation are unable to take a compressive load, and as fitted into a bicycle wheel are unable to push upwards on the hub. However, when the structure is regarded as a whole a load is taken through deformation of the rim and compression of the lower spokes. Obviously the upper spokes play their part by providing the tension necessary for the structure to behave as it does, but (nearly) all the action takes place in the lower half of the wheel.

[Wendy]

The problem with semantics, and the way you present the argument, is that a reduction in tension is not actual compression, though I do see your argument for calling it an increase in compression in pure mathematical terms.  It's still tension compared to the slack state.

Cast moped wheels are probably best considered more like wooden wagon wheels with wooden spokes. If I recall correctly, wagon wheels are constructed with the spokes in compression, in which case you'd be justified in saying the lower spokes support the hub.

[dkahn400]

Here's quite a well-written explanation, IMO:

Hubs hang from the rim!

He seems to agree with what I said earlier, and which you thought was wrong. Most of what he says is correct, but where I take issue is here. He says we have to regard the wheel as a structure and the role of all the spokes is important. I agree with that. He then says that we therefore cannot regard the wheel as standing on the lower spokes. I disagree with that but accept it as he has explained what he means by "standing". But he then maintains that the hubs therefore hang from the rim. By his previous argument this is the wrong conclusion as the role of all the spokes is equally important.