Does it matter if a wheel is 2mm 'not round'?

[Yellow Saddle]

Some people seem to think that local changes in spoke tension cause changes elsewhere in the wheel. It doesn't work like that. The typical notion is that if you look at a wheel head-on and consider it as a clockface, any change made at 12 o'clock (say tightening the spokes) will have an effect at 6 o'clock, the spot directly opposite. Novices struggle with this but it is wrong and misleading to suggest it.

A local change remains local. If there is a bump at 12 'clock and you tighten the spokes at 12 o'clock to flatten the bump, only that region changes. Nothing else is affected.
When teaching people to build wheels I often came across this problem and one way I tried to instill the concept is to get them to look at a car's wheel on a parked car. The car's weight causes the tyre to bulge at the bottom only. It doesn't cause some sort of stretch at the opposite end. The implication of this is that the hub always remains the centre of the wheel and any force acting on the wheel only acts between that spot on the periphery and, the hub itself. This is analogous to spoke tension. The car's weight bulging the tyre at the bottom is no different from spoke tension pulling the rim inwards.

 

[Venod]

The car's weight causes the tyre to bulge at the bottom only

You can compress air, you can't compress the metal in a wheel rim to make it smaller.

 

[Yellow Saddle]

You can compress air, you can't compress the metal in a wheel rim to make it smaller.

You are mistaken. Metal compresses and stretches. This is the very fundamental of engineering and is expressed in Young's modulus. Look it up.

A fully tensioned bicycle wheel with say 32 spokes, has 3.2 metric tons of pressure squeezing the rim inwards and it shrinks by a millimeter or so in diameter. Simultaneously, the spokes stretch by about 1mm each. And because the spokes are made from a Hookean material, each bit of extra tension or compression creates a linear quantity of deformation. Besides, if metal does not compress, where do you propose the bumps go when a wheel is trued?

 

[swee'pea99]

Thanks very much for all the responses. I shall keep note of yellow saddle's detailed instructions, which make sense to me and definitely sound worth having a go with at some point. Having said which, I decided in the end not to go for the wheel - or, rather, to go in only at a stupidly low price to allow for professional trueing if it proved necessary, and I got out-bid. (I've been spending a fair bit on hi-fi & records lately, and there are dark forces to be placated...)

Thanks again.

 

[Venod]

Metal compresses and stretches.

Yes but it has limits, you can't continually keep tightening spokes to shrink the rim.

if the car tyre had been filled with water, do you think it would still flatten at the bottom without movement elswhere.

 

[Yellow Saddle]

Yes but it has limits, you can't continually keep tightening spokes to shrink the rim.

Who said it doesn't have limits? All bicycle wheel rims are shrunk as they stand there. No-one tries to shrink the rim right down to the hub. Actually, I'm not sure what your point is. I think you are trying to squirm out of your now disproven perception that solids don't compress.

if the car tyre had been filled with water, do you think it would still flatten at the bottom without movement elswhere.

Yes, but obviously with less change, up to the point where the water's compressibility exceeds the tyre casing to resist the increasing pressure. However, that scenario is not perfectly analogous to a wire-spoked wheel and it is not the one I used. Nor did I fill said imaginary wheel with concrete, in case that'll be your next move.

 

[Venod]

I think you are trying to squirm out of your now disproven perception that solids don't compress.

I am not trying to squirm out of anything, I am just pointing out that a an air filled tyre is a bad analogy,

Yes, but obviously with less change, up to the point where the water's compressibility exceeds the tyre casing to resist the increasing pressure

You cant compress water.

You also said

There is only one rule: Don't loosen spokes

This can't always be achieved, if the spokes won't tighten anymore how would you remove the bunp ?

 

[Yellow Saddle]

I am not trying to squirm out of anything, I am just pointing out that a an air filled tyre is a bad analogy,

Well, saying that my analogy is bad is different from saying that you can't compress the metal in a rim to make it smaller. The latter is true. The former is debatable and is only introduced now. Keep it out of this post or suggest alternative analogies which I may or may not adopt depending on their aptness.

You cant compress water.

Yes you can. You can also compress steel, rubber, uranium and unobtanium. I've already explained that.

The old adage that you cannot compress water comes from blowing gaskets on engine cyclinder heads. Water leaks into the compression chamber, the piston pushes against it, the water has nowhere to go and boom goes the gasket. Your father/teacher told you this happens because water is not compressible. He should have said "water is not as compressible as gas."

This can't always be achieved, if the spokes won't tighten anymore how would you remove the bunp ?

Why can't this be achieved? All the other spokes in the wheel reached the tension required to pull the rim in, why won't the ones under the hump?

But, context is everything. The OP had a wheel with a hump that we have to assume was smaller than the part of the wheel without the hump. In that context (and many others not applicable here), you never slacken spokes. Why?
1) Reason I stated above
2) Because it is a used wheel and during use, a wheel never spontaneously tightens a series of adjacent spokes and forgets to tighten the last three so that you are left with a hump. In fact, a wheel never tightens spokes by itself. Nipples, if they move, always move out (loosen). Therefore you always tighten spokes on a used wheel with a radial or lateral bump.

However, all this is a red herring. You started off by thinking that metal cannot compress, now I am giving a mini lesson on wheelbuilding.

Metal can compress
Water can compress
And just for interest sake, ice turns into water when compressed.

 

[Venod]

Therefore you always tighten spokes on a used wheel with a radial or lateral bump

Another statement. I have to disagree with.

I was recently knocked of resulting in a out of true rear wheel, tightening the spokes wouldn't remove the out of true, the only way was to completely delace the wheel, check and manipulate the rim so it sat on a flat surface without any gaps under then rebuild it.

 

[Yellow Saddle]

Another statement. I have to disagree with.

I was recently knocked of resulting in a out of true rear wheel, tightening the spokes wouldn't remove the out of true, the only way was to completely delace the wheel, check and manipulate the rim so it sat on a flat surface without any gaps under then rebuild it.

Like I said, context is everything and the context has now changed.

OK, let me try this.

Do you think that the collision spontaneously tightened some spokes so that the wheel went out of true?

Answer only yes or no, I don't want another tangent spoiling the line of argument. Once I have that answer, I'll proceed.

 

[Venod]

Yes,

but a one word answer isn't enough, it tightened the spokes by increasing the distance from the hub to rim, therefore increasing the tension in the spokes on one side and decreasing on the other side, because of this increased tension the spokes wouldn't tighten any more.

The op doesn't know if the wheel is out of true from loosening spokes or a collision.

 

[Ajax Bay]

Bet you wish you'd gone to the JK now @Afnug

 

[Venod]

Bet you wish you'd gone to the JK now @Afnug

It would have been far more enjoyable, even with my lack of fitness at the moment

 

[Yellow Saddle]

Yes,

but a one word answer isn't enough, it tightened the spokes by increasing the distace from the hub to rim, therefore increasing the tension in the spokes on one side and decreasing on the other side, because of this increased tension the spokes wouldn't tighten any more.

The op doesn't know if the wheel is out of true from loosening spokes or a collision.

Help me understand this. Does your model above allow a hub to move position within the circle of the rim?

 

[Venod]

Help me understand this. Does your model above allow a hub to move position within the circle of the rim?

Don't know what you mean, the out of true was mostly lateral. thus increasing the length from hub spoke hole to rim spoke hole on one side and shortening it on the other.