Road Race Wheels Cerca 1986

[oldpotatoe]

Originally Posted by Kontact

It's Stien not stien

It's weird, not wierd..get it?

[Hindmost]

Originally Posted by El Chaba

...For regular road race duty, it is the "regular" SSC and not the Pave model...

Thanks for the suggestions. Do you know how the regular SSC's differ from say, the GP4's? The literature usually refers to the Paris-Roubaix model.

[54ny77]

For the mountain goats, GEL 280 w/lovely Vittoria CX sew ups, all on Campy Super Record hubs.

Mmmmmm.....had me a set of those at one point ages ago, oh man were they nice.

Can't remember the lacing, probably 2x front and 3x drive side rear.

Good grief I was a lot lighter then....no way on earth I could even think about using that kind of setup today!

[shovelhd]

Originally Posted by 54ny77

For the mountain goats, GEL 280 w/lovely Vittoria CX sew ups, all on Campy Super Record hubs.

Same here, except I rode Clement silks supplied by the team. These were for smooth roads only (I was 134 pounds). For rough roads, I opted for the same thing with GL330's.

[Mark McM]

Originally Posted by Kontact

I can't see how this could be true. The hub isn't sitting on top of few spokes, it is dangling from the ones above it. If anything, the spokes connected to where the rim touches the ground have the least load on them because the pressure from the ground decreases the pre-tension on those spokes.

The advantage of pre-stressed structures is that it allows structural members to bear loads in ways that they ordinarily would not be able to. For example, standard concrete can't normally bear much bending load, but pre-stressed concrete can, allowing long slender concrete bridge spans to be built.

In the case of the wheels, it can easily be shown that the loads on the wheel are not transferred to the top spokes. If the loads were transferred to the top spokes, you'd expect that the tension in the top spokes would increase when weight was applied to the wheel - but in fact, the tension in the top spokes stays the same under load. Instead, the only spokes with significant changes are the bottom spokes, which bear the load under compression (de-tensioning).

The reason that the loads are not transferred to the top of the wheel has everything to do with the relative stiffnesses of the spokes vs. the rim. The rim is under bending, and so flexes easily under load. But the spokes are very stiff in tension/compression (they retain their compression stiffness as long as they aren't completely de-tensioned). This means that the rim will bend inward under load, and only the bottom spokes will take all the load, with little of the load being transferred else where around the wheel.

This mechanism for how tension spoke wheels bear loads has been known for quite some time. It was first presented in the book, "The Bicycle Wheel" by Jobst Brandt, and been reconfirmed many times. Engineers who studied, modeled and measured the response of wire spoked wheels under load have all reached the same conclusion. Here's a quote from a recent interview on BikeRadar with Keith Bontrager (of Bontrager wheels) where he discusses this:

KB: 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. It’s hard to get your head around. It’s a pre-stressed structure, which behaves in ways which aren’t obvious.

[Mark McM]

Originally Posted by bikinchris

More crosses builds a stronger wheel, because the spokes lean on each other so to speak.

This has often been hypothesized, but it hasn't actually been demonstrated in practice. In fact, there has been research showing that spoke crossing actually makes little difference in wheel strength/durability. In this study by Henri Gavin, several otherwise identical wheels but with different numbers of spoke crossings were tested by both static measurement, and by applying strain gauges to spokes and then riding the wheels to measure actual spoke stresses. He found that the number of spoke crossings had little affect on spoke loads. (By the way, Henri Gavin's measurements of wheels under actual riding loads also confirmed the conclusion previously mentioned, that wheels "stand" on their lower spokes.)

[54ny77]

Yup. Them or GP4s for crits "just in case."

What were those red mavic rims? They were the super light things. Very pricey in their day also.

Originally Posted by shovelhd

Same here, except I rode Clement silks supplied by the team. These were for smooth roads only (I was 134 pounds). For rough roads, I opted for the same thing with GL330's.

[wallymann]

Originally Posted by Mark McM

...This means that the rim will bend inward under load, and only the bottom spokes will take all the load...

doesnt this really mean that the bottom spokes are *unloaded*?

you just said that spokes cannot take much compression, so how can they take any load?

if the rim bends inwards, the bottom spokes yield, meaning that load is taken up by the rest of the spokes that stay in tensions...yes?

[Kontact]

Originally Posted by oldpotatoe

It's weird, not wierd..get it?

Not really.

[Mark McM]

Originally Posted by wallymann

doesnt this really mean that the bottom spokes are *unloaded*?

That's getting into symantics. It is the bottom spokes that have a stress response to external loads. It is the bottom spokes that can suffer from fatigue (and breakage). Therefore, it is correct to say that the bottom spokes are bearing the load.

Originally Posted by wallymann

you just said that spokes cannot take much compression, so how can they take any load?

As bikinchris said, as long as the bottom spokes have adequate static tension and never completely de-tension, they can bear compressive loading. This, afterall, is the entire purpose of pre-tensioning the spokes - a wheel with inadequate static tension can not bear much load.

Originally Posted by wallymann

if the rim bends inwards, the bottom spokes yield, meaning that load is taken up by the rest of the spokes that stay in tensions...yes?

When we are talking about rim and spoke deflections, keep in mind that the stiffnesses are very high, so the deflections are very small. A typical wheel has a stiffness of about 20,000 lb/in, so under normal loading, the deflection is measured in thousands of an inch.

Also keep in mind that there are typically 3 - 5 spokes in the LAZ (Load Affected Zone) at the bottom of the wheel. If they are each tensioned to 100 kgf, it could take several hundred kgf of load to completey de-tension the spokes.

But as to your original question - yes if the bottom spokes completey de-tensioned, the load will be transferred to other spokes - but at this point, the wheel has been overloaded and has failed. When the bottom spokes become completely un-tensioned, there will be no lateral support of the rim at the bottom, and wheel will become unstable, and easily flex sideways. As this is not a normally functioning wheel, this is not a valid case to model spoke loading.

So, if you want to make sure your wheels "hang from the top spokes", just build them with no static tension - but don't be surprised if the wheels become unrideable. For me, I'll take my wheels with high static tension, so that they "stand on the bottom spokes".

[Kontact]

Originally Posted by Mark McM

The advantage of pre-stressed structures is that it allows structural members to bear loads in ways that they ordinarily would not be able to. For example, standard concrete can't normally bear much bending load, but pre-stressed concrete can, allowing long slender concrete bridge spans to be built.

In the case of the wheels, it can easily be shown that the loads on the wheel are not transferred to the top spokes. If the loads were transferred to the top spokes, you'd expect that the tension in the top spokes would increase when weight was applied to the wheel - but in fact, the tension in the top spokes stays the same under load. Instead, the only spokes with significant changes are the bottom spokes, which bear the load under compression (de-tensioning).

The reason that the loads are not transferred to the top of the wheel has everything to do with the relative stiffnesses of the spokes vs. the rim. The rim is under bending, and so flexes easily under load. But the spokes are very stiff in tension/compression (they retain their compression stiffness as long as they aren't completely de-tensioned). This means that the rim will bend inward under load, and only the bottom spokes will take all the load, with little of the load being transferred else where around the wheel.

This mechanism for how tension spoke wheels bear loads has been known for quite some time. It was first presented in the book, "The Bicycle Wheel" by Jobst Brandt, and been reconfirmed many times. Engineers who studied, modeled and measured the response of wire spoked wheels under load have all reached the same conclusion. Here's a quote from a recent interview on BikeRadar with Keith Bontrager (of Bontrager wheels) where he discusses this:

I didn't mean that the hub was hanging just from the 3 spokes directly above it, but that the hub was suspended and was not supported from the spokes between it and the ground. This is what you said:

Originally Posted by Mark McM

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

That makes no sense.

Originally Posted by Mark McM

As bikinchris said, as long as the bottom spokes have adequate static tension and never completely de-tension, they can bear compressive loading. This, afterall, is the entire purpose of pre-tensioning the spokes - a wheel with inadequate static tension can not bear much load.

You appear to be saying that plum bob string could resist forces pushing up from below if the bob was so heavy to be putting preload on the string. In reality, the spoke and the string have no real rigidity - the only thing supporting the flexible spoke from below is the rim strip. It can't be a compressive structural member if it isn't supported in that direction and has no rigidity.

Under no circumstances do the spokes push out against the rim.

[Mark McM]

Originally Posted by Kontact

That makes no sense.

Please present your arguments against it.

Do you refute that if a spoke is under static tension, that it behaves elastically the same in both tension and compression, as long as it is not completely de-tensioned?

Do you refute that only the bottom spokes have any meaningful changes in stress/strain when the wheel is loaded?

Do you refute that the top spokes in a wheel have no significant change in stress/strain when the wheel is loaded?

If you can not refute any of the above, how can you claim that a wheel load is not supported by the bottom spokes?

(I suspect that you will have a hard time refuting any of these, as they have all been independently verified many times.)

I'll grant that the mechanics of pre-stressed structures are not obvious at first. But if you learn and understand the principles of Superposition of Forces and Statically Indeterminant Structures, you'll understand how the bottom spokes of a tension spoke wheel can (and must) bear the loads.

[Kontact]

Originally Posted by Mark McM

Please present your arguments against it.

Do you refute that if a spoke is under static tension, that it behaves elastically the same in both tension and compression, as long as it is not completely de-tensioned?

Do you refute that only the bottom spokes have any meaningful changes in stress/strain when the wheel is loaded?

Do you refute that the top spokes in a wheel have no significant change in stress/strain when the wheel is loaded?

If you can not refute any of the above, how can you claim that a wheel load is not supported by the bottom spokes?

(I suspect that you will have a hard time refuting any of these, as they have all been independently verified many times.)

I'll grant that the mechanics of pre-stressed structures are not obvious at first. But if you learn and understand the principles of Superposition of Forces and Statically Indeterminant Structures, you'll understand how the bottom spokes of a tension spoke wheel can (and must) bear the loads.

The hub is held in space by all the spokes that have tension on it. Mechanically, the only spokes that don't have the same tension are those pointing straight at the ground, because the rim is being pushed out of round across a short section that is directly against the ground. When that happens those spokes fully fully participate in the the suspension of the hub in space.

That's all that is happening.


The bottom spokes have a change in stress/strain because of the unloading/loading cycles, not because they are ever compressed. 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. In other words, they aren't under near the load as the other 29 spokes, and only come under great load when the wheel rotates and the rigid structure of the rim is no longer compressed by the contact patch.

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.

[Hindmost]

This thread has been a remarkable flashback for me. BITD I lived a 20 minute bike ride from the Old Wheelsmith. Ric and Jon could help you with any question regarding purchase of rims and spokes. Almost anything desired was in stock. They would show you the correct spoke lengths as recorded in their notebook.

On occasion Jobst Brandt would roll in, he lived 5 blocks away, and he would offer an opinion.

[Mark McM]

Originally Posted by Kontact

You appear to be saying that plum bob string could resist forces pushing up from below if the bob was so heavy to be putting preload on the string. In reality, the spoke and the string have no real rigidity - the only thing supporting the flexible spoke from below is the rim strip. It can't be a compressive structural member if it isn't supported in that direction and has no rigidity.

Under no circumstances do the spokes push out against the rim.

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.