Does the hub/axle hang from or stand on the spokes ?

[Minion]

you unfortunately need an engineering degree,

The physics applicable to a bicycle wheel are covered in second year mechanics. It's not rocket science.

 

The FEA you mention make some ASSUMPTIONS, which does not apply in real world cycling. As they say, assumption is the mother of all FU's . For instance, they assume the hub is fixed in the x and y directions, which of course, it isn't.

Not quite. Invalid assumption is the mother of all F-ups. It is almost impossible to completely model any real world situation without making assumptions. Assumption is the root of all engineering.

 

The assumption that the hub is fixed in the x and y directions has no bearing on the overall result of the model. It is an arbitrary frame of reference. You could just as easily fix the rim and allow the hub to move freely. In fact, you could probably run that model to convergence by fixing only the y coordinate (i.e. to simulate the fact that the it is only really constrained vertically). It is, however, good modelling practice to make sure that all the degrees of freedom of a model are constrained. I guarantee that if you were to look at the result file, you would find that the reaction force in the x direction and the moment about the z direction will be zero or many orders of magnitude smaller than the force in the y direction or an other moment.

 

The reason they compare the model with real world measurements is to verify these assumptions. The fact that the modelled strains follow the same pattern as real world strains indicates that the model is on the right track. It is a useful illustrative tool that would, however, need to be refined to give accurate quantitative results.

 

Tellingly, though, the data that is included from actual, real-world tests, both on a test rig and on a bicycle, indicate that most of the load is carried in the lower spokes and almost none in the upper spokes.

Edited December 15, 2010 by Edam

[Guest agteros]

Tellingly, though, the data that is included from actual, real-world tests, both on a test rig and on a bicycle, indicate that most of the load is carried in the lower spokes and almost none in the upper spokes.

 

Which makes sense to a non-engineer as the wheel is part of a structure to prop the cyclist and bicycle up, pushing against gravity. it extends the fork and stays working against gravity to keep you off the ground. So, to me at least, it stands (pun.. not sorry) to reason that the bicycle/hub does not hang. Question to the hanger-ons is, what does the whole mechanism hang from, a sky hook?

[Topwine]

Which makes sense to a non-engineer as the wheel is part of a structure to prop the cyclist and bicycle up, pushing against gravity. it extends the fork and stays working against gravity to keep you off the ground. So, to me at least, it stands (pun.. not sorry) to reason that the bicycle/hub does not hang. Question to the hanger-ons is, what does the whole mechanism hang from, a sky hook?

It "hangs" from the rim, connected via the spokes.

 

To make it easier to understand, imagine a rim, a hub in the center and 2 spokes connected to the rim, one spoke to the North (N) and one to the south (S).

 

What do you think will happen if you apply the force (weight of cyclist and bicycle) at the hub directly downwards (south) to the earth ? Just think of this for a second or 2.

[Topwine]

 

Tellingly, though, the data that is included from actual, real-world tests, both on a test rig and on a bicycle, indicate that most of the load is carried in the lower spokes and almost none in the upper spokes.

 

In order to continue our debate, let us first define some points. What is your definition of (1)load being "carried" and (2) "almost none" ?

[The_Break]

Which makes sense to a non-engineer as the wheel is part of a structure to prop the cyclist and bicycle up, pushing against gravity. it extends the fork and stays working against gravity to keep you off the ground. So, to me at least, it stands (pun.. not sorry) to reason that the bicycle/hub does not hang. Question to the hanger-ons is, what does the whole mechanism hang from, a sky hook?

 

Hi Agteros

I really don't mean to insult you with this next comment as I understand this is not yor field and I would not argue with you in your work place, but these things will not make sense unless you have a technical background. There are numerous formula which you will need to understand, but to even start to explain them in detail will take a very, very long time here on the hub so when engineers explain it here, as I did at the start it is done in simplified form. Broken down to the very basics of physics as this is the starting point for all calculations.

 

Unless you can understand Newton, which you don't seem to, then it is gonna be hard to explain the rest.

 

I suggest what Topwine did as a good starting point. Imagine just two elastic spokes with a fixed rim and then try move the hub down. Think about what will happen in each spoke as you move it? What will happen when you release it. Then slowly start to add more and more spokes. You will see what happens. Then you just make the elastic less elastic and it starts to resemble a wheel.

 

Hope this helps? If you want we can go step for step till all is understood. But hope your maths is ok.

[Topwine]

The physics applicable to a bicycle wheel are covered in second year mechanics. It's not rocket science.

second year of mechanics in what field of study?

 

 

Not quite. Invalid assumption is the mother of all F-ups. It is almost impossible to completely model any real world situation without making assumptions. Assumption is the root of all engineering.

Of course, I inferred invalid assumption, you should know that.

 

The assumption that the hub is fixed in the x and y directions has no bearing on the overall result of the model.

 

I don't agree; it makes all the difference, since you are evaluating a whole structure in its entirety , and not only parts of an structure. In this case however, you are correct that it has the same effect of showing magnitudes of results, but from an understanding point of view, of what is happening in real life, it is confusing and let people think it stand on the spokes, whereas it really hangs from the spokes in real life. It can be no other way.

[Minion]

In order to continue our debate, let us first define some points. What is your definition of (1)load being "carried" and (2) "almost none" ?

Using the picture of experimental data:

 

1.) Load being carried is the vertical load that must be transferred from the ground to the hub (e.g. the weight). Load being carried would be partly shown by a change in spoke tension. Changes in tension on spokes whose angle gives them a force component in line with the load will be carrying the load. Spokes at 90° and 270° would carry no load (they are perpendicular to the line of force). A shortening on a spoke in the lower half would contribute something to carrying the load. A lengthening spoke in the upper half would also be contributing to carrying the load.

 

2.) The plot shows that spokes at 180° (ground) experience 10x more strain (by magnitude) than those at 0° (top). The forces will differ by a similar amount. They experience about 5x more strain than spokes at 90° and 270°, but the forces in those spokes do not contribute to the load carrying (they do, however, contribute to maintaining the wheel's shape).

[Guest agteros]

Hi Agteros

I really don't mean to insult you with this next comment as I understand this is not yor field and I would not argue with you in your work place, but these things will not make sense unless you have a technical background. There are numerous formula which you will need to understand, but to even start to explain them in detail will take a very, very long time here on the hub so when engineers explain it here, as I did at the start it is done in simplified form. Broken down to the very basics of physics as this is the starting point for all calculations.

 

Unless you can understand Newton, which you don't seem to, then it is gonna be hard to explain the rest.

 

I suggest what Topwine did as a good starting point. Imagine just two elastic spokes with a fixed rim and then try move the hub down. Think about what will happen in each spoke as you move it? What will happen when you release it. Then slowly start to add more and more spokes. You will see what happens. Then you just make the elastic less elastic and it starts to resemble a wheel.

 

Hope this helps? If you want we can go step for step till all is understood. But hope your maths is ok.

I'm just putting something out there from a layman's perspective. Not taking offence, so long as there are facts, proper arguments preferably plain english, etc and no name calling.

 

In the end the whole thing is to keep the bike 'up', so using gravity and opposing forces, where does the opposite forces push against gravity through the wheel to keep the hub (and then forks + bike + rider) up?

[The_Break]

Using the picture of experimental data:

 

1.) Load being carried is the vertical load that must be transferred from the ground to the hub (e.g. the weight). Load being carried would be partly shown by a change in spoke tension. Changes in tension on spokes whose angle gives them a force component in line with the load will be carrying the load. Spokes at 90° and 270° would carry no load (they are perpendicular to the line of force). A shortening on a spoke in the lower half would contribute something to carrying the load. A lengthening spoke in the upper half would also be contributing to carrying the load.

 

2.) The plot shows that spokes at 180° (ground) experience 10x more strain (by magnitude) than those at 0° (top). The forces will differ by a similar amount. They experience about 5x more strain than spokes at 90° and 270°, but the forces in those spokes do not contribute to the load carrying (they do, however, contribute to maintaining the wheel's shape).

 

You cannot use that plot in your calculations. You are assuming the plot is correct. What we need to determine is how you get to the plot. The specific literature does not explain it all that well and to be honest, there is something bugging me about using strain in this way and relating it directly back to tension in the spokes, but I cannot put my finger on in right now cause strain was probably the subject I enjoyed least.

 

Let's stick to determining from 1st Principles if I may ask for such respect for our Profs who taught us that much.

[Topwine]

 

1.) Load being carried is the vertical load that must be transferred from the ground to the hub (e.g. the weight). Load being carried would be partly shown by a change in spoke tension. Changes in tension on spokes whose angle gives them a force component in line with the load will be carrying the load. Spokes at 90° and 270° would carry no load (they are perpendicular to the line of force). A shortening on a spoke in the lower half would contribute something to carrying the load. A lengthening spoke in the upper half would also be contributing to carrying the load.

 

I think you confuse load being carried versus load being supported. The bottom few (4 or so ) spokes show a change in tension from positive to negative, which indicates they go from being in tension to being in (carrying) compression. But we know spokes can not support compression forces, they tend to buckle, as shown in the video of Hannes.

 

I also disagree with you that the spokes at 90 and 270 would carry no load. They all tend to carry pretty equal tensile loads, except the bottom 4 or so, which carry compression loads. Go and see that video, it shows the distribution of forces and deformation of the wheel quite clearly.

 

2.) The plot shows that spokes at 180° (ground) experience 10x more strain (by magnitude) than those at 0° (top). The forces will differ by a similar amount. They experience about 5x more strain than spokes at 90° and 270°, but the forces in those spokes do not contribute to the load carrying (they do, however, contribute to maintaining the wheel's shape).

 

It's different forces in play here at the bottom. There is now bending moments because of the buckling and deformation.

[fastbike]

Wow, this stuff is realy like reinventing the wheel.

[Topwine]

You cannot use that plot in your calculations. You are assuming the plot is correct. What we need to determine is how you get to the plot. The specific literature does not explain it all that well and to be honest, there is something bugging me about using strain in this way and relating it directly back to tension in the spokes, but I cannot put my finger on in right now cause strain was probably the subject I enjoyed least.

 

 

I think this is it. From both literature they assume the spokes as analogous to being a disc...

 

"The test results are compared with an analysis that considers the spokes as a disk that can carry force in one direction only."

[The_Break]

I'm just putting something out there from a layman's perspective. Not taking offence, so long as there are facts, proper arguments preferably plain english, etc and no name calling.

 

In the end the whole thing is to keep the bike 'up', so using gravity and opposing forces, where does the opposite forces push against gravity through the wheel to keep the hub (and then forks + bike + rider) up?

 

The hub needs to be balanced from a force perspective.

 

Assume tension in top spoke is X pulling upward. Assume tension in bottom spoke is Y pulling downward. Assume hub has an external mass of M acting downward on it with a resulting force of Mxg or W. To balance the hub and get a resulting force of 0 on it the equation is

 

Force = 0 = X - Y - W

Thus X = Y + W

 

Thus you see that although the equation is statically indeterminate (doesn't have enough info to solve it using basic maths) it does show that the upper spoke actually increase its tension.

 

The force that holds the bike up is the equal and opposite that the hub exerts on the frame. In other words if we look at the external forces being applied to the hub by the spokes

 

Sum of forces = X - Y = W and hence it is effectively the spokes holding the hub up which holds the frame up.

 

Now you can see why I do not agree with using strain and that diagram cause the tension in the top is larger as originally stated. That diagram "indicates" that there is no force in the uper spokes which is nonsense.

 

What is more, in order that the wheel spokes are never loose, it is a basic requirement that W will always to smaller than Y. This means that both the upper and lower spokes always remain in tension and thus the wheel neither truly hangs or is supported by the upper or lower part of the wheel, but rather suspended by tension in all spokes that fluctuate in a signus graph as originally stated.

[Topwine]

I'm just putting something out there from a layman's perspective. Not taking offence, so long as there are facts, proper arguments preferably plain english, etc and no name calling.

 

In the end the whole thing is to keep the bike 'up', so using gravity and opposing forces, where does the opposite forces push against gravity through the wheel to keep the hub (and then forks + bike + rider) up?

You can even make it more easier to understand. Imagine, there was only 1 spoke to the north connected to the hub and the rim. The opposing force would come from the earth first, transferred to the rim at the bottom, this will tend to lift the rim up, but it is balanced by the weight of the rider and the bicycle which hangs from the top of the rim via the hub connected by the 1 spoke. More simpler than this I cannot, and leave it to the "professor" , because now this "whiner" is going to bed

[The_Break]

I just read that report again. That graph is not a strain graph for the strain inside the spokes. It is a strain graph for the strain inside the rim structure itself. It therefore has no bearing on this argument at all and can therefore be dismissed. That is why it never made sense to me before and also why it never satisfied Newton's Third Law.

 

So, I think having said that, the only explanation we still have is the orinigal one.

 

Good night and may The Force (yes the one in the upper spokes) be with you!

Edited December 15, 2010 by The_Break

[Guest Big H]

They are right.....The hub needs to be balanced from a force perspective. ( copied from above)

.... Or else Tweedle Dweep and Tweedle Dumb will rule

Edited December 15, 2010 by Big H