Road Race Wheels Cerca 1986

[Mark McM]

Originally Posted by Kontact

You seem to be complaining about an extended analogy, but calling it a straw man. You should look up what a straw man is, because it is kind of an insulting term and it isn't what you mean.

Strawman: "straw man is a common form of argument and is an informal fallacy based on giving the impression of refuting an opponent's argument, while refuting an argument that was not presented by that opponent."

We weren't discussing how wagons deal with lateral loads. Thus your introduction of this topic was a straw man.

Originally Posted by Kontact

Pre-tensioned spokes don't have "high stiffness under compression" because they aren't compressed. That isn't happening, that isn't an engineering or physics concept and you are referring to it without any reference to actual science.

There are structures that use a mixture of pre-tensioned elements with compressed elements to become stronger, but spokes are only tensioned. They do all their work through their tensile strength and don't need to have any compressive stength, which is why even kevlar strands can serve as spokes, as in the Tiogo Tension Disc.

Pre-stressing of structures isn't just done to improve strength, it can also be done to improve stiffness. For example, by allowing what are normally tension-only members to contribute to stiffness when they are compressed (de-tensioned).

You seem to be hung up on artifically narrow definitions of words. "Compressive" and "compress" just mean making smaller/shorter. They don't have to refer to an absolute value. In this way, a spoke with an (absolute) pre-tension force can be subjected to a (relative) compression force. Since the wheel load shortens the spoke, it by definition applies a compressive load on the spoke (even if the spoke remains in net tension). One of the purposes of pre-stressed structures is that it allows us to change our reference planes

You also seem to keep dodging the fact that spokes can exhibit high stiffness in (relative) compression. I had hoped that the thought experiment I offered would illustrate this, but it appears not. So, here's an even simpler thought experiment, with a simple pre-stressed structure:

You have a 1 foot long hollow metal tube. The tube is fat enough that it can bear longitudinal loads in both tension and compression. The tube has a longitudinal stiffness of 10,000 lb/in. If you put the tube under 100 lb. of tension, the tube stretches 0.01" and if you put the tube under 100 lb. of compression the tube shortens by 0.01".

You also have a 1 foot straight pull spoke. Say that this spoke has a stiffness in tension of 10,000 lb/in, just like the tube. If you put the spoke under 100 lb. of tension, it stretches 0.01". But when it is in its free state, this spoke will buckle under compressive load. The spoke it its free state has a compressive stiffness of effectively zero.

Now we construct a pre-stressed structure with the tube and spoke. The spoke is placed inside and co-axial to the center of the tube, and anchored to plates covering the ends of the tube. The head of the spoke is anchored in a hole in the plate at one end of the tube, and threaded end of the spoke is screwed into a nipple anchored in a hole in the plate at the other end of the tube. The nipple is tightened until the spoke is loaded to a pre-tension of 200 lb. This is now a pre-stressed structure.

Put the combined tube/spoke structure under 100 lb. of tension - now how much does it stretch? The tube and the spoke are loaded in parallel, so the stiffnesses of the tube and the spoke combine. The total stiffness is 20,000 lb/in, so the structure now stretches only 0.005". Pretty straight forward.
But what if you put the combined tube/spoke structure under 100 lb. of compression? Since only the tube has a compressive stiffness when the structure is not assembled, does the combined structure only have a stiffness of 10,000 lb? No. The tube and the spoke structure still have a combined stiffness of 20,000 lb/in - just like it does in tension. The structure will compress only 0.005".

The question is: If the tube by itself only has a compressive stiffness of 10,000 lb/in, but the compressive stiffness of the pre-stressed structure is 20,000 lb/in, where did the extra compressive stiffness come from? The only answer can be the spoke provides that stiffness. And it does this even the load is acting to shorten (compress) the spoke.

So, there you go - a simple example showing a spoke that clearly exhibits a compressive stiffness. If you do not see this, than you many not have the analytic tools to understand a bicycle wheel. Some of the arguments here may be about symantics and reference planes, but this one is not.

Originally Posted by Kontact

You can't stand something on the top of a string, and there is no point in discussing anything as if you could.

There's a little trick that bike mechanics sometimes use to thread cable housings through internally routed frames. They first push a string through the frame, and then run the housing over the string. But how do you push a string through the frame? One way is to apply a vaccuum to the exit hole, and then feed the string in through the entry hole, until the string comes out through the exit hole. "But," you say, "you aren't pushing the string through the frame, you are pulling it with the vaccuum!" Wrong. A vaccuum doesn't provide a force - a vaccuum can't pull anything. The string isn't being pulled, it is being pushed.

But ... is it actually wrong to think of it as the string being pulled through the frame?

[Kontact]

Originally Posted by Mark McM

Strawman: "straw man is a common form of argument and is an informal fallacy based on giving the impression of refuting an opponent's argument, while refuting an argument that was not presented by that opponent."

We weren't discussing how wagons deal with lateral loads. Thus your introduction of this topic was a straw man.

Then I'm not sure why you posted this:

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.

Pre-stressing of structures isn't just done to improve strength, it can also be done to improve stiffness. For example, by allowing what are normally tension-only members to contribute to stiffness when they are compressed (de-tensioned).

Stiffness and strength, in basic engineering speak, are the same thing. A "strong" structure is one that resists deforming along whatever axis you're interested in.

You seem to be hung up on artifically narrow definitions of words. "Compressive" and "compress" just mean making smaller/shorter. They don't have to refer to an absolute value. In this way, a spoke with an (absolute) pre-tension force can be subjected to a (relative) compression force. Since the wheel load shortens the spoke, it by definition applies a compressive load on the spoke (even if the spoke remains in net tension). One of the purposes of pre-stressed structures is that it allows us to change our reference planes

It isn't relative. Materials have tensile strengths and compression strength and sheer strength. The terms are not interchangeable, or we wouldn't have materials that are strong in compression but weak in tension. Spokes have zero compression strength. This isn't a matter of how you look at things, it is a solidly understood engineering concept.

You also seem to keep dodging the fact that spokes can exhibit high stiffness in (relative) compression. I had hoped that the thought experiment I offered would illustrate this, but it appears not. So, here's an even simpler thought experiment, with a simple pre-stressed structure:

You have a 1 foot long hollow metal tube. The tube is fat enough that it can bear longitudinal loads in both tension and compression. The tube has a longitudinal stiffness of 10,000 lb/in. If you put the tube under 100 lb. of tension, the tube stretches 0.01" and if you put the tube under 100 lb. of compression the tube shortens by 0.01".

You also have a 1 foot straight pull spoke. Say that this spoke has a stiffness in tension of 10,000 lb/in, just like the tube. If you put the spoke under 100 lb. of tension, it stretches 0.01". But when it is in its free state, this spoke will buckle under compressive load. The spoke it its free state has a compressive stiffness of effectively zero.

Now we construct a pre-stressed structure with the tube and spoke. The spoke is placed inside and co-axial to the center of the tube, and anchored to plates covering the ends of the tube. The head of the spoke is anchored in a hole in the plate at one end of the tube, and threaded end of the spoke is screwed into a nipple anchored in a hole in the plate at the other end of the tube. The nipple is tightened until the spoke is loaded to a pre-tension of 200 lb. This is now a pre-stressed structure.

Put the combined tube/spoke structure under 100 lb. of tension - now how much does it stretch? The tube and the spoke are loaded in parallel, so the stiffnesses of the tube and the spoke combine. The total stiffness is 20,000 lb/in, so the structure now stretches only 0.005". Pretty straight forward.
But what if you put the combined tube/spoke structure under 100 lb. of compression? Since only the tube has a compressive stiffness when the structure is not assembled, does the combined structure only have a stiffness of 10,000 lb? No. The tube and the spoke structure still have a combined stiffness of 20,000 lb/in - just like it does in tension. The structure will compress only 0.005".

The question is: If the tube by itself only has a compressive stiffness of 10,000 lb/in, but the compressive stiffness of the pre-stressed structure is 20,000 lb/in, where did the extra compressive stiffness come from? The only answer can be the spoke provides that stiffness. And it does this even the load is acting to shorten (compress) the spoke.

So, there you go - a simple example showing a spoke that clearly exhibits a compressive stiffness. If you do not see this, than you many not have the analytic tools to understand a bicycle wheel. Some of the arguments here may be about symantics and reference planes, but this one is not.

What you've just explained above is why a rim that has compressive strength is made stronger by spokes with only tensile strength. And it is 100% true.

But when we flex the rim at the LAZ, the spokes directly above the LAZ are no longer adding their full tension to that structure, and if you are talking about those spokes, rather than all the spokes together, you are not talking about "compression". The compression is what the spokes pull the rim into, not what the spokes do to push on the hub. If you change the shape of the rim, the spokes in that spot aren't contributing to the wheel strength as much.

You've taken the concept of the wheel and tried to apply it to a single component of the wheel.

There's a little trick that bike mechanics sometimes use to thread cable housings through internally routed frames. They first push a string through the frame, and then run the housing over the string. But how do you push a string through the frame? One way is to apply a vaccuum to the exit hole, and then feed the string in through the entry hole, until the string comes out through the exit hole. "But," you say, "you aren't pushing the string through the frame, you are pulling it with the vaccuum!" Wrong. A vaccuum doesn't provide a force - a vaccuum can't pull anything. The string isn't being pulled, it is being pushed.

But ... is it actually wrong to think of it as the string being pulled through the frame?

In that case, you are carrying a string along in the flow of air, you aren't pushing or pulling it. It will travel through the tube without having any measurable strength at all. That isn't a useful example because the airflow isn't just acting on the ends of the strings but along its entire length.


I would be happy to buy you a beer, and hope no one is bothered by this debate. But a wheel works differently than the parts it is made of, and tension is not compression in reverse. Your examples are even better than mine for why a hub isn't sitting on the spokes below it.

[Hindmost]

Originally Posted by carpediemracing

Based on the clearance of the brakes (I had a set personally) those are really narrow wheels, probably 17-18mm rims with 18mm tires...

Mavic had the CX18 available at that time, supposedly for track use. I can't help but wonder that they didn't use some of their lighter rims, they are all dark anodized, and relabel them SSC.

[Mark McM]

Well, obviously at this point we're going to have to just agree to disagree. This will be my last post here, so I'll just add a few more comments before signing off.

Originally Posted by Kontact

Stiffness and strength, in basic engineering speak, are the same thing. A "strong" structure is one that resists deforming along whatever axis you're interested in.

Stiffness and strength are not at all the same thing. And we can not analyze statically indeterminent structures (like bicycle wheels) without knowing how the stiffnesses of the elements interact. If you knew anything about structural analysis, you'd know this, so I can only assume that you haven't done much structural analsys.

Originally Posted by Kontact

It isn't relative. Materials have tensile strengths and compression strength and sheer strength. The terms are not interchangeable, or we wouldn't have materials that are strong in compression but weak in tension. Spokes have zero compression strength. This isn't a matter of how you look at things, it is a solidly understood engineering concept.

I'm not disgreeing. But a well designed pre-stressed structure acts to keep all members within their loading modes where they are strongest, and keep them out of loading modes where they are weakest. For example, in a bicycle wheel, verticals loads will act so as to compress the bottom spokes. But these spokes are under a high pre-tension, so that when the compression from the vertical load is super-imposed on the bottom spokes, they remain in net tension.

Originally Posted by Kontact

What you've just explained above is why a rim that has compressive strength is made stronger by spokes with only tensile strength. And it is 100% true.

Yes, but I've explained more than that. As you say, when the wheel is loaded, the rim bends inwards. But how much does it bends inward? Which properties of which elements matter the most to determine this? If we don't know these answers, we can't know how the forces are re-balanced when the wheel is loaded, and therefore can't ensure that we are designing wheels with adequate strength and durability. What I've been trying (and apparently failing) to explain is why the stiffnesses of the (bottom) spokes matters so much in determining the re-balancing of the forces. (This goes back to statically indeterminant structure analysis mentioned above). Without knowing this, we can get a hint of how wheels react under load, but we can't get the whole story and will miss a big part of the picture.

Originally Posted by Kontact

In that case, you are carrying a string along in the flow of air, you aren't pushing or pulling it. It will travel through the tube without having any measurable strength at all. That isn't a useful example because the airflow isn't just acting on the ends of the strings but along its entire length.


I would be happy to buy you a beer, and hope no one is bothered by this debate. But a wheel works differently than the parts it is made of, and tension is not compression in reverse. Your examples are even better than mine for why a hub isn't sitting on the spokes below it.

Well, technically we are pushing the string (air can't really pull) - but as you say, we are pushing along the entire length (not just the ends). The point I was trying to make is that we often model "vaccuum" as providing a pulling force - just like we make models use centrifugal force, coriolis force and even gravity force, which science has shown are actually fictitious forces. But even if these forces don't really exist, it is very useful and generally not harmful to consider to model them as real. Likewise, while the bottom spokes of a properly functioning wheel are always in net tension, it is very useful and not harmful to analyze the forces superimposed on them from wheel loads as a compressive forces.

And yes, I'd be happy to have a beer with you.

[72gmc]

In ‘86 I was on shimano 600 hubs laced 32/3-cross to Matrix rims (new Trek 560). That year or early ‘87 I upgraded to tricolor Ultegra hubs, MA40 rims, and awesomely narrow avocet tires.

And I’ll have a beer, too.

[AngryScientist]

well this thread has taken an interesting side road.

mark and kontact - very good discussion! i think you are both discussing purely academic concepts with regard to wheel mechanics. i'm a dorky engineer also and can appreciate picking the details apart, and have learned a few things by reading through the discussion and links provided.

to the point - it will never happen - but it would be neat to see some pro level competitors go head to head on a real road race course with 1980's equipment pitted against 2017 tech and see just how much of an advantage the latest stuff has in a real race scenario.

good stuff here in this thread, and i applaud the participants for an open, fun discussion with lots of stuff on the table.