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

[The_Break]

The basic pattern of the results i.e. top spokes carry no load, bottom spokes carry almost all the load applies to all spokes. The first diagram shows results from 4 spokes from multiple positions. The actual values vary, but the overall pattern is consistent.

 

Now we need to be clear, you understand what the difference between strain and tension is right?

I.e. stress vs. strain.

 

As a hint, look at that document again on the wheel analysis where the used FEA to model. You will notice the strain varies along the length of individual spokes. If you saw a similar diagram showing Axial Stress (Stress = F/A) it would be consistant along the entire length.

Edited December 15, 2010 by The_Break

[Lounge Lizard]

Newton taught us this. For every action there is an equal and opposite reaction. I.e. if you press at the top of the wheel the bottom of the wheel is pushed back equally as hard by the ground. As the wheel's rim will also want to form a slightly ovalized shape as it is "squashed" it tends to want to pull the horizontal spokes as it tries to do this.

 

Tension in the spokes means you are trying to lengthen the spokes. This can be seen as the hub "pulling" the spoke in one direction and the rim/nipple "pulling" the spoke in the opposite direction. If you now push the rim/nipple toward the hub, you effectively "pull" less and thereby decrease the tension. Note: decrease the tension but not make it zero or negative. It is this constantly "positive tension" that keeps a wheel strong.

 

Now that is so amazing ! and I like the explanation of the rim wants to ovalize That makes sense.

How would the wheel collapse if weight is added Almost until it fails ?

I assume the 'ovalization' (probably not a real word?) will stress the more horizontal spokes until they snap.

If so the tension on the vertical (top and bottom) spokes will reduce to a point where there is none.

Is that not an indication that, if those top and bottom spokes were missing, a Stationery wheel actually hangs from the more horizontal spokes. (i.e. The wheel neither stands on the bottom nor hangs from the top ?

(but the missing spokes will be required for motion as they will become the horizontal spokes)

 

Furthermore - does the wheel really want to ovalize ? yes, if the weight is placed on TOP of the rim - like squashing a ball. But the weight is applied at the hub/axle/centre ?

 

Ok, thanks guys;

just seen the other newer post - Yes I was also thinking in terms of as straight (radial) spoke configuration - which appears to be more common lately. Not sure why, as in theory I think it will be weaker (?)

 

 

I understand the explanation better now - (i think) - the cross-laced spoke pattern will never have the top and bottom of the spoke vertical - so there will always be tension due to the 'ovalise' tendency of (at least the bottom half?) the wheel - therefore all lower spokes have their tension increased. (?)

 

Anyway, is that not the cause of all the confusion - the forces on a straight radial wheel are not exactly similar to a cross-laced spoke pattern.

 

Not sure I am any wiser - but I do feel better informed - Thanks Guys

Edited December 15, 2010 by Lounge Lizard

[The_Break]

Now that is so amazing ! and I like the explanation of the rim wants to ovalize That makes sense.

How would the wheel collapse if weight is added Almost until it fails ?

I assume the 'ovalization' (probably not a real word?) will stress the more horizontal spokes until they snap.

If so the tension on the vertical (top and bottom) spokes will reduce to a point where there is none.

Is that not an indication that, if those top and bottom spokes were missing, a Stationery wheel actually hangs from the more horizontal spokes. (i.e. The wheel neither stands on the bottom nor hangs from the top ?

(but the missing spokes will be required for motion as they will become the horizontal spokes)

 

Furthermore - does the wheel really want to ovalize ? yes, if the weight is placed on TOP of the rim - like squashing a ball. But the weight is applied at the hub/axle/centre ?

 

Don't forget, Newtons Law applies to all parts of the wheel. So if the hub is being "supported/pushed upward" by the bottom spokes it would need to do "pushed down" by the upper spokes in order that it does not accelerate off into space. So even though the wheel is loaded at its axle, it has an effective pull on the rim at the top.

 

If you simply apply this basic law of action and reaction to the entire wheel, bit by bit you will see what is happening.

[Minion]

Now we need to be clear, you understand what the difference between strain and tension is right?

I.e. stress vs. strain.

I would like to think so.

 

Strain is the change in length of the spoke due to the imposed load. In this case, it's actually the additional strain on top of the strain caused by the pre-tensioning.

 

In the case of a steel spoke loaded below its failure point, strain is directly proportional to stress (stress = Young's modulus x strain)

 

The spoke has a constant cross section at the measurement point. The change in tension is thus directly proportional to stress.

 

This means that change in tension is directly proportional to the measured strain. The shape of a graph showing change in tension will be exactly the same as that of the graph showing strain.

[The_Break]

I would like to think so.

 

Strain is the change in length of the spoke due to the imposed load. In this case, it's actually the additional strain on top of the strain caused by the pre-tensioning.

 

In the case of a steel spoke loaded below its failure point, strain is directly proportional to stress (stress = Young's modulus x strain)

 

The spoke has a constant cross section at the measurement point. The change in tension is thus directly proportional to stress.

 

This means that change in tension is directly proportional to the measured strain. The shape of a graph showing change in tension will be exactly the same as that of the graph showing strain.

 

Fair enough, I am glad you didn't just pull it off google. Seems you understand it well enough. But, there results do not fall in line with the Newton. They even say there is an anomoly they cannot account for in their calculations.

[Lounge Lizard]

Thank you fellows.

(sorry big cheese you lost me on your last post - getting too heavy for me)

[Minion]

just seen the other newer post - Yes I was also thinking in terms of as straight (radial) spoke configuration - which appears to be more common lately. Not sure why, as in theory I think it will be weaker (?)

 

I understand the explanation better now - (i think) - the cross-laced spoke pattern will never have the top and bottom of the spoke vertical - so there will always be tension due to the 'ovalise' tendency of (at least the bottom half?) the wheel - therefore all lower spokes have their tension increased. (?)

I can't find any experimental results for radial laced wheels, but the FE modelling I linked includes a simple model with radial spokes. The forces in the simple, radial, wheel were almost indistinguishable from those in the cross-spoked model. I think it's thus reasonable to assume that a radial-laced wheel will carry loads in real life in a similar way to the cross-spoked wheel i.e. almost all load carried in the bottom spokes, almost none carried by the top.

[Guest agteros]

A properly built wheel neither hangs nor stands but rather 'suspends' in a vacuum as it were

 

I'm with you BigS

Hubs are actually anti gravational devices, and spokes are the anti-matter artifacts of the first rotation of the whole system...

[The_Break]

I can't find any experimental results for radial laced wheels, but the FE modelling I linked includes a simple model with radial spokes. The forces in the simple, radial, wheel were almost indistinguishable from those in the cross-spoked model. I think it's thus reasonable to assume that a radial-laced wheel will carry loads in real life in a similar way to the cross-spoked wheel i.e. almost all load carried in the bottom spokes, almost none carried by the top.

 

I just looked at that figure again. It seems they may have forgotten to apply a tensile force to the theoretical model. Check the values and the colour increments used.

 

The problem I have with some of the experiement using a disc assumption is that it does not account for the tension in the spokes. A ridig disc will show stress/strain in the lower part of the wheel like this, but once again it is in my opinion not a good model if you relate it to Newton. I know I said this a lot, but one equation here is not making sense.

[The_Break]

I'm with you BigS

Hubs are actually anti gravational devices, and spokes are the anti-matter artifacts of the first rotation of the whole system...

 

Oh no, I hate string theory and now you are forcing me to do the calcs on this one.

 

I sure hope that my wheels do not bend space time cause then they are for sure too heavy.

[Minion]

Fair enough, I am glad you didn't just pull it off google. Seems you understand it well enough. But, there results do not fall in line with the Newton. They even say there is an anomoly they cannot account for in their calculations.

The anomaly mentioned in the FE report relates to the overall magnitude of the results from the model compared to the physical measurements. Their model showed strains that were 10x smaller than those from the measurements. The overall pattern was, however, the same as the measured results.

 

Their results do fall in line with Newton. The hub is being pulled outwards in all directions by the pre-tensioned spokes. The rim is also being pulled in in all directions.

 

If a force is applied to the rim, that part of the rim actually flattens locally, rather than the whole rim ovalising. This releases some of the pull in the bottom spokes; they aren't pulling up as much on the rim, but this is balanced by the ground pushing up on the rim.

 

Now look at the hub. The bottom spokes have had some of their pull released. The upper spokes (which still have their original pull) now want to pull the hub upwards. Putting weight on the axle then balances the loss in pull from the lower spokes.

[Minion]

I just looked at that figure again. It seems they may have forgotten to apply a tensile force to the theoretical model. Check the values and the colour increments used.

 

The problem I have with some of the experiement using a disc assumption is that it does not account for the tension in the spokes. A ridig disc will show stress/strain in the lower part of the wheel like this, but once again it is in my opinion not a good model if you relate it to Newton. I know I said this a lot, but one equation here is not making sense.

He using the superposition principal. Provided the magnitude of the negative model 'tensions' never exceeds the magnitude of the actual pre-tensions, it's a valid way of modelling it. The actual tensions would then be found by simply adding the tensions in the model to the pre-tension values.

If the negative value were greater than the pretension, it would imply that the spoke would no longer contact the rim (real tension goes to zero) and a different, more complex model would be needed.

 

The model is really showing change in strain relative to the pretension state rather than absolute strain relative to the stress-free state.

[Minion]

(sorry big cheese you lost me on your last post - getting too heavy for me)

I was attempting to impress your avatar with big words

[Guest Big H]

Oom dit lyk my JB het hierso 'n 'ally' hier in die_breek

 

Gelukkig weet Johan waarvan hy praat..... Maar dit lyk my asof die wielnaaf se nuwe slim jannie klaar 'n nuwe tysis geskryf het deur so baie te Google....... Maar al daai slimgeit help jou net mooi boggerol as jy nie weet hoe om jou eie wiele te bou nie

[Guest agteros]

Oh no, I hate string theory and now you are forcing me to do the calcs on this one.

 

I sure hope that my wheels do not bend space time cause then they are for sure too heavy.

 

I will not mind a pair of wheel like that. Just active (sit on bike?) and the goal posts (and hills) are sucked TOWARDS you. In no time at all you'll be finished, and this all without breaking out a sweat. Only problem I can think of is to get the timing computer (and results) ... All might be for naught with a set of wheels like this

[Topwine]

Gelukkig weet Johan waarvan hy praat..... Maar dit lyk my asof die wielnaaf se nuwe slim jannie klaar 'n nuwe tysis geskryf het deur so baie te Google....... Maar al daai slimgeit help jou net mooi boggerol as jy nie weet hoe om jou eie wiele te bou nie

 

Gelukkig weet ek hoe om my eie wiele te bou... en ek verskil met jou, Johan weet nie waarvan hy praat nie. Is dit nou 'n persoonlike opmerking?, nee, dit is slegs 'n feit.