Road Race Wheels Cerca 1986

[Mark McM]

This discussion is getting a bit far afield of the original conversation, so I'll just add a few comments.

Originally Posted by Kontact

The bottom spokes have a change in stress/strain because of the unloading/loading cycles, not because they are ever compressed

This is where you are going astray. 'Compress' means to make shorter. The bottom spokes are clearly being compressed - direct measurements have shown that the bottom spokes get shorter when a wheel is loaded. This is a fact that you can't be denied, but that you haven't fully addressed. As long as the spokes never completely lose tension, they are just as rigid as they are lengthened or shortened from their static (pre-tensed) state.

Originally Posted by Kontact

If you built a wheel and put an enormous load on it without letting it roll, those 2 or 3 spokes on the bottom would simply dangle there as the rest of the spokes snapped under tension.

Not quite - as has been demonstrated many times (often by poorly landed jumps), a rim will usually buckle before the (top) spokes snap. In normal use the rim is kept from buckling by the support of the bottom spokes. As explained earlier, a wheel that has lost support of its bottom spokes is not a functioning wheel, and a non-functionling wheel is not instructive in how a functioning wheel works.

Originally Posted by Kontact

I think you've just fooled yourself into thinking that since the top spokes aren't doing all the work, something opposite than must be true. But what is really happening is that the work is evenly distributed to the evenly round shape of the rim, except where the rim's roundness has been mechanically interrupted by the small contact point of the ground.

By your arguments, I see that you both the lack the tools to analyze the mechanics of the wheel, nor have done any research on previous engineering analyses on wheel mechanics. I've already provided a few links, I can provide some more if you are actually interested.

As far as having "fooled myself", then you must think that is the case with all the other engineers who have studied the wheel, and come to the same conclusions that a wheel "stands" on its bottom spokes. And there are many engineers on that list.

[Kontact]

Originally Posted by Mark McM

This is in correct, and we don't need to look very far from the wheel to find another example of flexible members becoming rigid in pre-stressed structures.

A bicycle tire is made of a flexible fabric casing - it has tensile strength, but no compressive strength. Air is a gas - it doesn't have a distinct shape or volume, let alone any (tensile strength). But when you fill the tire with air under high pressure, the tire becomes rigid, able to exert forces in multiple directions. How can this be? It, like the wheel, is a pre-stressed structure, and the tire casing becomes rigid and able to exert forces in directions it can't when it is not pre-stressed. Likewise, the spokes in a pre-tensioned wheel can exert forces in direction that they normally can't when they are not in a pre-tensioned wheel.

And further consider - how does the tire transfer load to the rim, to keep the rim off the ground? The rim is not supported by pneumatic pressure - air pressure pushes with the same force in all directions, so the air pressure on the top is pushing the rim down with as much force as the air pressure on the bottom is pushing the rim up. Therefore, it must the tire casing fabric that is supporting the wheel. But how does a flexible fabric push upward on the rim?

There are more mechanical actions involved in tires (and wheels) than your simple viewpoint considers.

I don't think saying that the hub is supported by all the spokes is "simple", but it is more complicated than comparing spokes to a fluid. The simple fact of the matter is the spokes are in fixed locations, the rim has a lot of compressive strength but is flexible, and that (unlike a tire), you can unload a specific part of the structure because the spokes in question are fixed in space.

Regardless, spokes work under tension, gas under compression. If you want to point to a compressive structure, consider a table top that sits loosely on four columns as legs. If the floor under one of those columns droops a little, the compression comes off that leg. Does this pull the table top in that corner the way you claim a spoke pushes? No, the compression just decreases in that leg and the other 3 legs do the work of holding the rigid table top up.

What I'm talking about is a localized phenomenon that happens solely because the rim isn't perfectly rigid and can distort with its contact with the ground. Hit the ground hard enough and it will actually become flat. Same forces. There are no spokes pressing out that can prevent that.

[Kontact]

Originally Posted by Jobst Brandt

You can test this by plucking a front wheel spoke (near the nipple) before and after you put weight on the handlebars. You'll find that
the only spokes to change are those about the tire contact patch on
the floor. These spokes are compressed and lose tension. If the load
is great enough, they will become slack and the wheel can collapse
sideways. In any case, a wheel can only bear loads that do not
consistently slacken the preload.

http://yarchive.net/bike/wheel_stresses.html

[Mark McM]

Originally Posted by Kontact

http://yarchive.net/bike/wheel_stresses.html

Exactly. Including this remark also in the linked posting:

That leaves it up to the observer to come to
terms with the description that "the wheel stands on the bottom
spokes".

[Kontact]

Originally Posted by Mark McM

Exactly. Including this remark also in the linked posting:

And I'm saying that you haven't "come to terms with" it, since you said this:

At any time, only a few spokes are actually supporting the external load - these spokes are the spokes closest to the ground contact point.

All the spokes are supporting the external load, with the exception of the spokes that have lost their tension over the contact patch. That's what Brandt said in the quote, and it is the opposite of what I quoted you as saying.

Brandt is describing the wheel as a whole and the way you would measure the forces in them. Once you zero out preload, the forces in the wheel vary exactly the same way a wagon wheel would. But you have taken that observation and used it to come up with the odd conclusion that the only spokes supporting the hub are below it. That's your idea, and it is wrong.

[Mark McM]

Originally Posted by Kontact

All the spokes are supporting the external load, with the exception of the spokes that have lost their tension over the contact patch. That's what Brandt said in the quote, and it is the opposite of what I quoted you as saying.

Brandt is describing the wheel as a whole and the way you would measure the forces in them. Once you zero out preload, the forces in the wheel vary exactly the same way a wagon wheel would. But you have taken that observation and used it to come up with the odd conclusion that the only spokes supporting the hub are below it. That's your idea, and it is wrong.

Firstly, I don't think you are understanding what Brandt has said. Secondly, I think you reading too much into some of my statements.

I think you need to re-read the section of his book (linked previously) titled "Theory of the Spoked Wheel". You are saying that all the spokes bear the load EXCEPT the bottom few spokes. Brandt is saying that loads are borne PRIMARILY by the bottom few spokes. Here is a quote from the book, which I have bolded a few portions that support my point:

A wheel with wire spokes works the same as one with wooden spokes except that the built-in force in its spokes is different. In a wooden-spoked wheel, force is transmitted from the ground to the hub by compressing the bottom spoke. This spoke becomes shorter as it furnishes the upward force to the hub. As in a wooden-spoked wheel, the bottom spokes of a wire wheel become shorter under load, but instead of gaining in compression, they lose tension. With the same load, the net change in force is the same for both wheels. The algebraic sum of negative and positive forces (compression and tension) is the same.

That the bottom spokes support the wheel need not be taken on faith. An experiment will show that only a few spokes at the bottom of the wheel are affected by a vertical load. The relative tension of a spoke can be found by plucking it like a guitar string. The pitch of a spoke, just as the pitch of a guitar string, increases with more tension and decreases with less tension.

So, the bicycle wheel is like a wagon wheel, and a wagon wheel supports loads ONLY with the bottom spokes. The bicycle wheel is the same, the difference being that the bicycle wheel requires that the bottom spokes have a pre-tension greater than their compression load. And that pre-tension is achieved by the static pre-tension of ALL the spokes. In other words, the job of the bottom spokes is to bear the load, the job of the rest of the spokes is to keep the bottom spokes under net tension. All the spokes are required, but they do different things (which is exactly how every other pre-stressed structure works). I don't see anything I've said contradicting Brandt.

[Kontact]

Originally Posted by Mark McM

Firstly, I don't think you are understanding what Brandt has said. Secondly, I think you reading too much into some of my statements.

I think you need to re-read the section of his book (linked previously) titled "Theory of the Spoked Wheel". You are saying that all the spokes bear the load EXCEPT the bottom few spokes. Brandt is saying that loads are borne PRIMARILY by the bottom few spokes. Here is a quote from the book, which I have bolded a few portions that support my point:




So, the bicycle wheel is like a wagon wheel, and a wagon wheel supports loads ONLY with the bottom spokes. The bicycle wheel is the same, the difference being that the bicycle wheel requires that the bottom spokes have a pre-tension greater than their compression load. And that pre-tension is achieved by the static pre-tension of ALL the spokes. In other words, the job of the bottom spokes is to bear the load, the job of the rest of the spokes is to keep the bottom spokes under net tension. All the spokes are required, but they do different things (which is exactly how every other pre-stressed structure works). I don't see anything I've said contradicting Brandt.

I think your misunderstanding is because you think a wagon wheel is fully supported by the bottom spokes. It isn't - without the compressive support of the rest of the spokes the rim, spoke and hub would not be able to maintain their relationship under any sort of side load. Picture a table with skinny legs and no bracing - nudge the table and the legs just fold.

Because you think a wagon wheel is just balanced on whatever spoke is on the bottom, you think that's an accurate model of what a wheel is. But a wagon wheel is a balance compression structure, while a bicycle wheel is a balanced tension structure. In both cases the structural integrity of the rim comes from it being pushed out or pulled in by spokes.

In other words, you couldn't have wagon wheel where the extendable spokes only went to full length at the bottom of the rotation. The rim would collapse.

The bottom spokes are "taking a load" but that isn't the same as supporting the hub. They are under net tension, and "the load" is a decrease in tension to those spokes.

Brandt was making about point about net forces, not telling you that you can push a string uphill.

[Mark McM]

Originally Posted by Kontact

I think your misunderstanding is because you think a wagon wheel is fully supported by the bottom spokes. It isn't - without the compressive support of the rest of the spokes the rim, spoke and hub would not be able to maintain their relationship under any sort of side load. Picture a table with skinny legs and no bracing - nudge the table and the legs just fold.

Because you think a wagon wheel is just balanced on whatever spoke is on the bottom, you think that's an accurate model of what a wheel is. But a wagon wheel is a balance compression structure, while a bicycle wheel is a balanced tension structure. In both cases the structural integrity of the rim comes from it being pushed out or pulled in by spokes.

In other words, you couldn't have wagon wheel where the extendable spokes only went to full length at the bottom of the rotation. The rim would collapse.

The bottom spokes are "taking a load" but that isn't the same as supporting the hub. They are under net tension, and "the load" is a decrease in tension to those spokes.

Brandt was making about point about net forces, not telling you that you can push a string uphill.

It's not clear to me what point you are trying to make here. This appears to be a strawman, to divert attention from your failing arguments about how wheels support vertical loads. Are you going to claim that bicycle wheels "hang from their top spokes" when under lateral loading, too?

[Kontact]

Originally Posted by Mark McM

It's not clear to me what point you are trying to make here. This appears to be a strawman, to divert attention from your failing arguments about how wheels support vertical loads. Are you going to claim that bicycle wheels "hang from their top spokes" when under lateral loading, too?

If you are going to claim a strawman, point to it. I don't know what you are talking about.

I have disagreed with your assertion that the bottom spokes "support the load". The load of the rider's weight is supported by how the rims and hub are connected by tensioned spokes. The rim is kept from collapsing as much from vertical spokes but horizontal ones that force it to remain round. Under load, the hub is net "hanging" from the upper 180° of tensioned spokes just as it would be supported by the lower 180° of compressive spokes in a wagon wheel. Brandt's point is that this doesn't matter because the distributed pre-tension or pre-compression is so much higher than the actual load that nothing measurably changes - the rim remains round and the spokes don't get longer or shorter.

The exception is the spokes over the contact patch that are actually changing length. On compressive spokes they are compressed shorter, and on tensioned spokes they are allowed to relax shorter. And that happens not because of the direction of force (up and down), but because of the mechanical fact that a circular rim touches the flat earth at a tangent, creating a point stress on the rim. That point stress takes the rim out of round and affects the spokes directly above it.

Mount bike the on rollers and all of a sudden your spokes that "take the load" aren't the ones directly under the hub, but the two sets between the hub and each roller. And it is only happening because those are the two points distorting the roundness of the rim. Put a wheel in a cradle with the same curve as the rim and you wouldn't detect any load at all.

"The load" Brandt is talking about is just the mechanical measured change in spoke compression, not a useful description of how hub fails to fall into the rim (how the rider is supported). The rider is supported by distributed tension throughout the wheel, and not by a single section of spokes pushing up. Especially not spokes in capable of gross compression.

[Mark McM]

Originally Posted by Kontact

If you are going to claim a strawman, point to it. I don't know what you are talking about.

I have disagreed with your assertion that the bottom spokes "support the load". The load is supported by how the rims and hub are connected by tensioned spokes. The rim is kept from collapsing as much from vertical spokes but horizontal ones that force it to remain round. Under load, the hub is "hanging" from upper 180° of tensioned spokes just as it would be supported by the lower 180° of compressive spokes in a wagon wheel. Brandts point is that this doesn't matter because the distributed pre-tension or pre-compression is so much higher than the actual load that nothing changes - the rim remains round and the spokes don't get longer or shorter.

The exception is the spokes over the contact patch that are actually changing length. On compressive spokes they are compressed shorter, and on tensioned spokes they are allowed to relax shorter. And that happens not because of the direction of force (up and down), but because of the mechanical fact that a circular rim touches the flat earth at a tangent, creating a point stress on the rim. That point stress takes the rim out of round and affects the spokes directly above it.

Mount bike on rollers and all of a sudden your spokes that "take the load" aren't the ones directly under the hub, but the two sets over each roller. And it is only happening because those are the two points distorting the roundness of the rim. Put a wheel in a cradle with the same curve as the rim and you wouldn't detect any load at all.

"The load" is just the mechanical measured change in spoke compression, not a useful description of how hub fails to fall into the rim.

So, that wagon wheel question was just a strawman then?

I think my argument can be summed up by these lines from the Keith Bontrager interview:

If you looked at the change in tension when a load is applied to a wheel, the tension in all the spokes away from the contact-patch area doesn’t change. The weight is not hanging from the spokes. The tension is _reduced_ across the contact patch; the wheel is standing on those spokes. The change in tension is the same as the compressive load.

Whether you want to say that the bottom spokes support the load, or that the bottom spokes merely pull down on the hub less (while the rest of the spokes actually support the load), may sound like just a symantic argument, but I say it is not, and here's why: Focusing on the bottom spokes is a better way to design and build good wheels.

Firstly, this view is better at keeping one mindful that one of the most important factor in making a strong a durable wheel is making sure that the bottom spokes remain in net tension.

Secondly, it reminds one that the wheel will only durable if there are sufficient number of spokes in the LAZ, which means matching the number (and type) of spokes to the rim radial stiffness.

Thirdly, disregarding crashes and other extreme events, spokes fail under fatigue, which is result of a high number of loading cycles - and these loading cycles occur as spokes enter and leave the LAZ at the bottom of the wheel as the wheel rotates.

The viewpoint that wheels are supported by a few spokes at the bottom ("the wheel stands on the bottom spokes") is far more useful in regard to engineering and building wheels than the "wheels hang from all the spokes" viewpoint. The bottom of the wheel is where all the important action is, and concentrating elsewhere is just a distraction.

[Kontact]

Originally Posted by Mark McM

So, that wagon wheel question was just a strawman then?

I think my argument can be summed up by these lines from the Keith Bontrager interview:



Whether you want to say that the bottom spokes support the load, or that the bottom spokes merely pull down on the hub less (while the rest of the spokes actually support the load), may sound like just a symantic argument, but I say it is not, and here's why: Focusing on the bottom spokes is a better way to design and build good wheels.

Firstly, this view is better at keeping one mindful that one of the most important factor in making a strong a durable wheel is making sure that the bottom spokes remain in net tension.

Secondly, it reminds one that the wheel will only durable if there are sufficient number of spokes in the LAZ, which means matching the number (and type) of spokes to the rim radial stiffness.

Thirdly, disregarding crashes and other extreme events, spokes fail under fatigue, which is result of a high number of loading cycles - and these loading cycles occur as spokes enter and leave the LAZ at the bottom of the wheel as the wheel rotates.

The viewpoint that wheels are supported by a few spokes at the bottom ("the wheel stands on the bottom spokes") is far more useful in regard to engineering and building wheels than the "wheels hang from all the spokes" viewpoint. The bottom of the wheel is where all the important action is, and concentrating elsewhere is just a distraction.

I still don't know what argument you consider "straw man".

As far as all this goes,
"Whether you want to say that the bottom spokes support the load, or that the bottom spokes merely pull down on the hub less (while the rest of the spokes actually support the load), may sound like just a symantic argument, but I say it is not, and here's why: Focusing on the bottom spokes is a better way to design and build good wheels. "
it isn't semantic. You appear to be arguing that a flawed physics model is better if it makes engineering easier. And maybe it does make it easier to design a wheel, but that doesn't mean it should be a substitute for reality.


Wheels support weight like a suspension bridge rolled up in a circle. If wind lifts a section of the bridge frequently, that will unload those cables. We wouldn't argue that those cables are supporting the lifted roadway, so it doesn't make sense to say the LAZ spokes are supporting the hub.

What the LAZ spokes are doing is going through stress cycles, because the tension keeps coming on and off. If you rolled a wagon and bike wheel until spoke failure, the wagon spokes would fail as they came into the LAZ and compressed, and the bike wheel spokes would fail as the spokes came out of LAZ as they re-tensioned. That might not be important to an engineer trying to do a job, but it is completely different physically.

Additionally, if you took both kinds of wheels and loaded them statically until failure, the wagon spoke on the bottom would compress until it shattered, while the bike spokes along the top will break first. And if you eliminate the LAZ compression by using a rim that can't change shape, the same thing will happen as the net tension just increases on the top spokes.


So while I fully appreciate that bike wheels are designed around the transient tensions at the LAZ and the damage those work cycles cause, it doesn't make a suspension wheel into a compression wheel in reality. It is okay to acknowledge reality even if reality isn't particularly useful to a wheel builder or rim designer.

[bikinchris]

Years ago, I had started talking to people standing around at the mechanic stand at a multi day ride. The conversation turned to wheels and I mentioned that wheels actually stand on the spokes and it blew everyone's mind. After getting a little flack, I just walked away.

[Kontact]

It sounds like counter-steering being how a bike is steered. People talk about it like it with surprise, but it isn't really accurate either.

[wallymann]

fwiw...this FEA suggests that moderate loads (when the bottom/vertical spokes retain some tension) are carried by increased tension in the spokes just fore-aft of bottom/vertical spokes.



watch the video to see how the loads change around the wheel as the overall weight borne is increased.

https://www.youtube.com/watch?v=w4kz0k4AdI4

[oldpotatoe]

winter already??

Yikes..