Back wheel to avoid broken spokes?

[adscrim]

Because they never go out of tension. They have say, 100 units of tension to start with, and your load takes away 30. 70 remains. The nipples only lift the moment tension and load equals.

Think of a man and a child each hanging on a similar rope. The child's rope is not slack just because the child is lighter.

So it's a definition issue and the word 'stand' is used because the tension on the lower spokes is where the greatest change occurs - and we shouldn't use 'hang' because the upper spoke tension doesn't change to the same extent?

 

[Yellow Saddle]

So it's a definition issue and the word 'stand' is used because the tension on the lower spokes is where the greatest change occurs - and we shouldn't use 'hang' because the upper spoke tension doesn't change to the same extent?

Rephrase please. I don't understand.

 

[adscrim]

Rephrase please. I don't understand.

Under load there is a change in tension on the spokes in the 'load area'. They compress. There is no noticeable change in tension in the spokes above the 'load area'. They don't show any additional extension. Because of this we stay the hub is 'standing' on the lower spokes. However, the hub isn't really standing on the lower spokes as it wouldn't be able to remain up of the rest of the wheel structure was removed (think wooden wheels with actual columnar spokes bearing the load). In the same way the hub wouldn't be able to remain up if only the top spokes were in play - I assume the light weight rims we use would collapse. So the hubs neither hangs nor stands but utilises the whole wheel structure and wouldn't work without all the elements of that structure. Hence us having wheels without rigid spokes ie madfibre etc.

 

[Yellow Saddle]

Under load there is a change in tension on the spokes in the 'load area'. They compress. There is no noticeable change in tension in the spokes above the 'load area'. They don't show any additional extension. Because of this we stay the hub is 'standing' on the lower spokes. However, the hub isn't really standing on the lower spokes as it wouldn't be able to remain up of the rest of the wheel structure was removed (think wooden wheels with actual columnar spokes bearing the load). In the same way the hub wouldn't be able to remain up if only the top spokes were in play - I assume the light weight rims we use would collapse. So the hubs neither hangs nor stands but utilises the whole wheel structure and wouldn't work without all the elements of that structure. Hence us having wheels without rigid spokes ie madfibre etc.

OK, I get it. It's a statement, not a question. You are absolutely right in your description. You'll notice that I always use "stand" within quotations since there is no single word I can think of to to replace " So the hubs neither hangs nor stands but utilises the whole wheel structure and wouldn't work without all the elements of that structure."

I have explained that, and particularly the use of "stand" several times over a few years on here. It gets a bit tedious to repeat, hence my frequent requests that people just do a search. There is no FAQ on here.

 

[adscrim]

OK, I get it. It's a statement, not a question. You are absolutely right in your description. You'll notice that I always use "stand" within quotations since there is no single word I can think of to to replace " So the hubs neither hangs nor stands but utilises the whole wheel structure and wouldn't work without all the elements of that structure."

I have explained that, and particularly the use of "stand" several times over a few years on here. It gets a bit tedious to repeat, hence my frequent requests that people just do a search. There is no FAQ on here.

No, it was a question - genuinely interested. Or rather a badly phrased attempt to get my understanding confirmed.

Thanks

 

[Yellow Saddle]

No, it was a question - genuinely interested. Or rather a badly phrased attempt to get my understanding confirmed.

Thanks

No, it was a question - genuinely interested. Or rather a badly phrased attempt to get my understanding confirmed.

Thanks

You are right about lightweight rims that would just collapse. You can easily deflect a 700C road bike rim by 50mm just by pushing on it.
I have a thing about photographic wooden wagon wheels. They demonstrate the principle very well if you know what to look for. These wheels are constructed with tenons and mortises no (structural) glue. They are invariably painted. If the hub "hung" from the top spoke like one would intuitively think, then the paint would be cracked around the spokes, but it never is.

 

[Ciar]

I had the same issue recently on my commuter, just switched from backpack to pannier rack and bag and started popping spokes left right and centre.

in the end i went to my LBS and had him relace my original rim with some lovely DT Swiss spokes and so far so good!

 

[Profpointy]

Mmm, intrigued now.

Am I right that spokes are only ever in tension? And if so could you, in theory, use string instead of spokes or at least consider the wheel as if the spokes were string?

So, let's have a simplified 4 straight spoked wheel positiined with these spokes up down and horizontal and a static downward load on the hub

Cut the top spoke - what happens?

Instead, cut the bottom spoke, what would happen?

I'd suggest you would get away with cutting the bottom spoke, but not the top one.

Thus wouldn't you be hanging from the top spoke.

I may be wrong, but can't see a goof

 

[the snail]

The upper spokes don't receive extra tension. The only spoke where tension change are those in the load affected zone.

That's not strictly true. The load is supported by reduction in tension in the load affected zone, but there is an increase in tension in the other spokes, small, and presumably insignificant in terms of fatigue though.

 

[Yellow Saddle]

That's not strictly true. The load is supported by reduction in tension in the load affected zone, but there is an increase in tension in the other spokes, small, and presumably insignificant in terms of fatigue though.

Yes, it is not strictly true but I always hesitate to introduce it early on. People find the "stand on the bottom spoke" concept difficult to digest as it is. The tension in spokes outside the load affected zones do change every so slightly with each cycle and the reason for that is the slight change in overall rim shape when it gets compressed (flattened at the bottom). As you say, the significance is small because the change is very small. In the one and only published FEA I can find, the change seems to be in the order of 0.1 % of the change at the bottom spokes. This change doesn't relate the "hang from the top spokes" theory though.

 

[Yellow Saddle]

Mmm, intrigued now.

Am I right that spokes are only ever in tension? And if so could you, in theory, use string instead of spokes or at least consider the wheel as if the spokes were string?

So, let's have a simplified 4 straight spoked wheel positiined with these spokes up down and horizontal and a static downward load on the hub

Cut the top spoke - what happens?

Instead, cut the bottom spoke, what would happen?

I'd suggest you would get away with cutting the bottom spoke, but not the top one.

Thus wouldn't you be hanging from the top spoke.

I may be wrong, but can't see a goof

Yes, the spokes are always in tension unless something catastrophic happens.
Yes, string would work (and they do, if you look at Spinnergy wheels with Kevlar spokes). The strings do have to have good tensile strength though, otherwise the scenario becomes silly.
I don't like the 4-spoke model because it is not realistic. You will have to now define the stiffness of the rim, whether or not the spokes can take load in deflection and whether or not the spokes are rigidly pinned to the hub and rim. Don't go there, it'll not help your understanding.

If it helps, hold a wheel in your hands by holding the axle vertically. Now push on the tyre, towards the hub, with your other hand. Note between which two points the forces act. Note how no other point on the wheel is involved. It is easier to visualise the effect like this than on a vertical wheel.

 

[the snail]

The surprising point to me is that the maximum tension on a loaded wheel is at the edges of load affected zone rather than in the upper part of the wheel, which makes sense as the rim is straightened out so that it is closer to the hub at the bottom and further away at either side. Sort of like a flat tyre which bulges out next to the contact point.

 

[Profpointy]

Yes, the spokes are always in tension unless something catastrophic happens.
Yes, string would work (and they do, if you look at Spinnergy wheels with Kevlar spokes). The strings do have to have good tensile strength though, otherwise the scenario becomes silly.
I don't like the 4-spoke model because it is not realistic. You will have to now define the stiffness of the rim, whether or not the spokes can take load in deflection and whether or not the spokes are rigidly pinned to the hub and rim. Don't go there, it'll not help your understanding.

If it helps, hold a wheel in your hands by holding the axle vertically. Now push on the tyre, towards the hub, with your other hand. Note between which two points the forces act. Note how no other point on the wheel is involved. It is easier to visualise the effect like this than on a vertical wheel.

I think I get what you're saying. Thanks.

Still don't like the "standing on the spoke" language when it can't ever be in compression, but I think I get your point, and maybe even agree

edit : I have Brandt's book somewhere. Might give it another go. Didn't find it easy to follow 30 odd years ago when I last looked at it.

 

[Yellow Saddle]

The surprising point to me is that the maximum tension on a loaded wheel is at the edges of load affected zone rather than in the upper part of the wheel, which makes sense as the rim is straightened out so that it is closer to the hub at the bottom and further away at either side. Sort of like a flat tyre which bulges out next to the contact point.

Yes. Those two spokes straddling the centre spoke move outwards from centre.

 

[Yellow Saddle]

I think I get what you're saying. Thanks.

Still don't like the "standing on the spoke" language when it can't ever be in compression, but I think I get your point, and maybe even agree

edit : I have Brandt's book somewhere. Might give it another go. Didn't find it easy to follow 30 odd years ago when I last looked at it.

Give it another go. It is brilliant and very, very elegantly written.
The man says a 1000 words in just ten, yet retains clarity.