# Push/Pull spokes



## omgBikes (Dec 12, 2009)

I always use the terms "leading" and "trailing" spokes. I know some people say "push" and "pull" spokes. In terms of leading and trailing, which one in push and which one is push and which is pull? Thanks.


----------



## Mike T. (Feb 3, 2004)

Trailing are backwards radiating pulling spokes - on the drive side rear.


----------



## omgBikes (Dec 12, 2009)

Sorry, I don't know if my original question made sense. Are trailing spokes push or pull spokes?


----------



## rruff (Feb 28, 2006)

Trailing spokes are pulling while applying drive torque (rear wheel). I like the terms "push" and "pull" because many people don't realize that half the spokes transfer torque by increasing in tension (pulling) and the other half by decreasing (pushing). 

If you have disc brakes, then the leading spokes are pulling when the brakes are applied.


----------



## Mike T. (Feb 3, 2004)

omgBikes said:


> Sorry, I don't know if my original question made sense. Are trailing spokes push or pull spokes?


Of course your question made sense but I don't know how much more plainer I can be with my answer than what I posted - "*Trailing* are backwards radiating *pulling* spokes". (but that still only counts for DS rear)


----------



## omgBikes (Dec 12, 2009)

I don't know how I didn't get that from your first response. Sorry.

Thanks for clearing it up though guys. I assumed that's what it meant.


----------



## Zen Cyclery (Mar 10, 2009)

RSYSs have push spokes... They dont work so well.


----------



## Fixed (May 12, 2005)

*they only pull*

Standard spokes can only work by pulling, it's just that the trailing spokes pull harder then the leading spokes do. Just take an uninstalled spoke and trying pushing it -- it will bend with about 2 pounds of force. They might as well be made of nylon filament, if they had enough tensile strength. Maybe then it would be more apparent that they cannot work by pushing (compression). Looking at a wheel from the side, isn't it obvious which ones are trailing or "pulling"? See: http://sheldonbrown.com/wheelbuild.html#definitions


This is one of those debates that never seems to get resolved, though.


----------



## rruff (Feb 28, 2006)

It doesn't get resolved because you have made a fundamental error. The spokes in a bicycle wheel are pretensioned and they maintain some tension in all normal riding conditions. When applying torque the pulling spokes will increase in tension and the pushing spokes decrease. *Both* transfer torque and they do so in equal measure.


----------



## Mike T. (Feb 3, 2004)

Fixed said:


> Standard spokes can only work by pulling, it's just that the trailing spokes pull harder then the leading spokes do. Just take an uninstalled spoke and trying pushing it -- it will bend with about 2 pounds of force. They might as well be made of nylon filament, if they had enough tensile strength. Maybe then it would be more apparent that they cannot work by pushing (compression). Looking at a wheel from the side, isn't it obvious which ones are trailing or "pulling"?


I'll try to unleash poster *"meltingfeather"* on you from sister site MTBR.com. He's an engineer and is a champion of the "bikes don't hang from their wheels' spokes; they sit on them" school of thought. So maybe he will prove that even a "leading" spoke, under tension, will exert force until its tension equals zero.

If he takes my bait it will be interesting to sit back and watch it all unfold. We've covered this one a few times at MTBR.


----------



## Fixed (May 12, 2005)

*analogy?*



rruff said:


> It doesn't get resolved because you have made a fundamental error. The spokes in a bicycle wheel are pretensioned and they maintain some tension in all normal riding conditions. When applying torque the pulling spokes will increase in tension and the pushing spokes decrease. *Both* transfer torque and they do so in equal measure.


But they don't "push," right?

Think of a tug of war. There is one rope, with 5 people on each side. If each side is pulling on the rope equally, it is tensioned and stationary. However, if one side (stronger) starts pulling harder than the other (weaker), the rope is still tensioned, but then the rope moves off center. At that point, is the weaker side "pushing" on the rope?


----------



## ohdee (Nov 9, 2007)

Fixed said:


> But they don't "push," right?
> 
> Think of a tug of war. There is one rope, with 5 people on each side. If each side is pulling on the rope equally, it is tensioned and stationary. However, if one side (stronger) starts pulling harder than the other (weaker), the rope is still tensioned, but then the rope moves off center. At that point, is the weaker side "pushing" on the rope?


No, the weaker side is getting pulled. Take the same example, and this time the one side releases the tension (partially, or completely). What happens to the other side? They fall back. There's your push.


----------



## rruff (Feb 28, 2006)

Bad analogy... what is wrong with looking at how a wheel works?

When the wheel is built the tension in the leading spokes exactly balances the tension in the trailing spokes. Torque is transferred from the hub to rim via the leading spokes losing tension and the trailing spokes gaining tension. *Relative to the static condition*, the leading spokes push and the trailing spokes pull.

Looking at your tug of war, imagine a rope that has a constant high tension pulling from both ends so that it is static (maybe a weight hanging from a pulley at both ends). You can move the rope by either pushing or pulling... both will work exactly the same.


----------



## ohdee (Nov 9, 2007)

Eh, just trying to dumb it down. Oh well.


----------



## ohdee (Nov 9, 2007)

Mike T. said:


> Trailing are backwards radiating pulling spokes - on the drive side rear.


So you're saying only the DS spokes transmit torque?


----------



## Mike T. (Feb 3, 2004)

ohdee said:


> So you're saying only the DS spokes transmit torque?


Not at all. But NDS rear spokes (on a disc brake equipped bike), the "pullers" are the forward radiating spokes when the brake is applied. Oh sure the backwards radiating ones transmit some drive torque.


----------



## rruff (Feb 28, 2006)

ohdee said:


> So you're saying only the DS spokes transmit torque?


Both sides will transmit torque if they are cross-laced, but it then gets complicated and depends on the stiffness of the hub body.


----------



## danl1 (Jul 23, 2005)

rruff said:


> Bad analogy... what is wrong with looking at how a wheel works?
> 
> When the wheel is built the tension in the leading spokes exactly balances the tension in the trailing spokes. Torque is transferred from the hub to rim via the leading spokes losing tension and the trailing spokes gaining tension. *Relative to the static condition*, the leading spokes push and the trailing spokes pull.
> 
> Looking at your tug of war, imagine a rope that has a constant high tension pulling from both ends so that it is static (maybe a weight hanging from a pulley at both ends). You can move the rope by either pushing or pulling... both will work exactly the same.


Seems to me that there's a useful distinction between 'pulling less hard' and 'pushing.'

Consider a weight hanging by two strings in a V. Pulling one rope will move the weight, though the tension in the other does not change. (actually, the 'pushing' rope in this case would _increase _tension, which is a challenge.) Conversely, releasing tension in one rope would move the weight, but no one that's ever stayed awake in a physics lecture would suggest that to be 'pushing' the weight with the rope. 'Permitting it to move' is more accurate language.

Another way to consider: This is only any argument at all because the materials are elastic. In an idealized model, tension remains unchanged as the torque transmits, and the argument disappears. That's instructive, because compressive forces are required for pushing, and there are none here. 'Pulling' and 'Pushing' might seem parts of a continuum, but are instead separate vectors. One needs to disappear before we can switch terms to the other. 

OP: For your question, this argument is all silly nonsense. As asked, the terms are simply colloquialisms. 'Leading' replaces 'pushing,' 'trailing' replaces 'pulling,' both wheels, both sides.Whether that's accurate to the physics is irrelevant.


----------



## Blue CheeseHead (Jul 14, 2008)

rruff said:


> Bad analogy... what is wrong with looking at how a wheel works?
> 
> When the wheel is built the tension in the leading spokes exactly balances the tension in the trailing spokes. Torque is transferred from the hub to rim via the leading spokes losing tension and the trailing spokes gaining tension. *Relative to the static condition*, the leading spokes push and the trailing spokes pull.
> 
> Looking at your tug of war, imagine a rope that has a constant high tension pulling from both ends so that it is static (maybe a weight hanging from a pulley at both ends). You can move the rope by either pushing or pulling... both will work exactly the same.


Dang, one of the first engineering rules I learned was "You cannot push on a rope".:mad2: 

Use of "push" here is a bit misleading. In this context it really means less "pull" or tension.

(You beat me to it Danl)


----------



## rruff (Feb 28, 2006)

danl1 said:


> Consider a weight hanging by two strings in a V.


The desire to come up with examples that do not simplify things and are not analogous is interesting. 



> Another way to consider: This is only any argument at all because the materials are elastic. In an idealized model, tension remains unchanged as the torque transmits, and the argument disappears.


Totally untrue... try putting a free body diagram together.


----------



## rruff (Feb 28, 2006)

Blue CheeseHead said:


> Dang, one of the first engineering rules I learned was "You cannot push on a rope".:mad2:


You can push on a rope that has been pretensioned...


----------



## Fixed (May 12, 2005)

*push/pull*



rruff said:


> You can push on a rope that has been pretensioned...


Been thinking about this. Would it be more meaningful to think of a tensioned wheel as one solid structure? Is it effectively similar to a disk wheel, where all parts of the wheel are contributing to whatever forces are being applied -- drive, braking, gravity, etc? 

I was thinking of what would happen if you have a radial spoked drive wheel. While not a good idea, it could be done. When torque is applied, seems that every spoke would become a "pulling" spoke, and the only thing that prevents the wheel from winding up is the ability of the rim not to deform as more tension is applied -- but every spoke is contributing to retaining the rim's shape. In other words, it's like one solid structure. 

Does this make sense?


----------



## Fixed (May 12, 2005)

*something else*

Something else, I think we need to once and for all define our terms.

To me, "push" means a compressive force being applied, when there is zero or less then zero tension (engineers chime in, but keep it in lay terms). "Push" does not mean simply "tensioned, but less tension." I don't think anyone thinks of that as "pushing." On the other end, "pull" means applying tension, or greater tension that the static, pretensioned, state.


----------



## ohdee (Nov 9, 2007)

Fixed said:


> Been thinking about this. Would it be more meaningful to think of a tensioned wheel as one solid structure? Is it effectively similar to a disk wheel, where all parts of the wheel are contributing to whatever forces are being applied -- drive, braking, gravity, etc?
> 
> I was thinking of what would happen if you have a radial spoked drive wheel. While not a good idea, it could be done. When torque is applied, seems that every spoke would become a "pulling" spoke, and the only thing that prevents the wheel from winding up is the ability of the rim not to deform as more tension is applied -- but every spoke is contributing to retaining the rim's shape. In other words, it's like one solid structure.
> 
> Does this make sense?


A solid disc is no longer a tensioned wheel. Completely different structure. Yes, a radial spoked rear wheel will rotate (hub windup) until there is sufficient stiffness to rotate the wheel. 

Your definition.. "push means a compressive force being applied, when there is zero or less then zero tension." I don't know why you say when there is "zero or less than zero tesion" It's all about the net change. 

I design prestressed concrete bridges. Concrete doesn't behave well in tension, but thanks to post-tensioning, we can apply all kinds of external loads (incl tension) with no net tensile strain in the concrete. It's hard for me to explain this for a wire spoked wheel since there are moving parts... but it's really the same thing.


----------



## ohdee (Nov 9, 2007)

Mike T. said:


> I'll try to unleash poster *"meltingfeather"* on you from sister site MTBR.com. He's an engineer and is a champion of the "bikes don't hang from their wheels' spokes; they sit on them" school of thought. So maybe he will prove that even a "leading" spoke, under tension, will exert force until its tension equals zero.
> .


This is another good example. As long as everyone understands that bikes do in fact 'sit' on the lower spokes (which is true), and not hang from the upper ones (false), you will soon realize that tensioned spokes do take external compressive loads up to the point of zero tension. Forget the push/pull spokes for a minute and think about the simple static loading of a wheel.


----------



## Mike T. (Feb 3, 2004)

> Originally Posted by Mike T.
> I'll try to unleash poster "meltingfeather" on you from sister site MTBR.com. He's an engineer and is a champion of the "bikes don't hang from their wheels' spokes; they sit on them" school of thought. So maybe he will prove that even a "leading" spoke, under tension, will exert force until its tension equals zero.





ohdee said:


> This is another good example. As long as everyone understands that bikes do in fact 'sit' on the lower spokes (which is true), and not hang from the upper ones (false), you will soon realize that tensioned spokes do take external compressive loads up to the point of zero tension. Forget the push/pull spokes for a minute and think about the simple static loading of a wheel.


I rattled the cage of engineer, tensioned structures champion *meltingfeather* from MTBR.com but he declined my request to come here and give a mini lesson in engineering physics to those who (would probably) have their minds made up. I was disappointed but understood. I tried.


----------



## Blue CheeseHead (Jul 14, 2008)

rruff said:


> You can push on a rope that has been pretensioned...


The point of "you can't push on a rope" is that you cannot induce compression on a rope. 

The rope in your analogy does not move because it it being "pushed". It is moving because you are reducing a counter acting pulling force which allows the other pulling force (gravity) to cause motion. 

The slenderness ratio and connection methods of traditional spokes are such that spokes can transfer force only in tension (pulling), not compression (pushing).

Here is another analogy: Attach a rubber band between two bricks. Place the bricks on a rough suface. Pull one brick such that the rubber band has tension and set the brick down. Now push the brick toward the other brick. Does the entire system move? Did pushing on the brick, and thus the tensioned rope cause the system to move? I didn't think so.


----------



## ohdee (Nov 9, 2007)

Blue CheeseHead said:


> Here is another analogy: Attach a rubber band between two bricks. Place the bricks on a rough suface. Pull one brick such that the rubber band has tension and set the brick down. Now push the brick toward the other brick. Does the entire system move? Did pushing on the brick, and thus the tensioned rope cause the system to move? I didn't think so.


You fail to consider the force of friction in this terrible example. Just because it doesn't 'move' doesn't mean there aren't changes in force going on. 

Just tell me this, how does a bicycle wheel distribute the weight of rider/bike to to the ground? If you understand that, then you'll see that you are completely wrong.


----------



## Fixed (May 12, 2005)

*nope*



ohdee said:


> This is another good example. As long as everyone understands that bikes do in fact 'sit' on the lower spokes (which is true), and not hang from the upper ones (false), you will soon realize that tensioned spokes do take external compressive loads up to the point of zero tension. Forget the push/pull spokes for a minute and think about the simple static loading of a wheel.


I don't think everyone understands that, but in fact, most disagree with that. I realize that this is a perpetual debate, but to me it seems nutty that a bike could "sit" on the lower spokes, rather than hang from the upper spokes, as it seems obvious that spokes have a great capacity for strength in tension, but almost zero in compression. I never have understood this position, despite years of hearing about it.


----------



## ohdee (Nov 9, 2007)

Fixed said:


> I don't think everyone understands that, but in fact, most disagree with that. I realize that this is a perpetual debate, but to me it seems nutty that a bike could "sit" on the lower spokes, rather than hang from the upper spokes, as it seems obvious that spokes have a great capacity for strength in tension, but almost zero in compression. I never have understood this position, despite years of hearing about it.


It's only a debate to the non-believers. It's really quite simple. A rim deflects (a very small amount) towards the hub (at the ground). This tiny deflection also affects the spokes in the same area, by shortening them (they are elongated by the spoke tension). This shortening results in less tension than before. The only time a wheel 'hangs' from the spokes is when the lower spokes are entirely detensioned. A highly undertensioned wheel would act this way since the lower spokes would go slack easily with weight applied.

Read Jobst Brandt's book. It's a great source for understanding how wheels distribute loads. If you have a tensiometer, you can even see it for yourself by having someone sit on a stationary bike and check the spoke tensions in the lower spokes. Check the before and after tension on the same spokes. Newton probably sounded like a nut too! :thumbsup:


----------



## Blue CheeseHead (Jul 14, 2008)

ohdee said:


> You fail to consider the force of friction in this terrible example. Just because it doesn't 'move' doesn't mean there aren't changes in force going on.
> 
> Just tell me this, how does a bicycle wheel distribute the weight of rider/bike to to the ground? If you understand that, then you'll see that you are completely wrong.


Sorry, the friction has little to do with it but resist the force vector that is wanting to pull the second brick in.

As a civil engineer I am pretty adept at understanding truss type structures as well as arches. A wheel is basically a combination of both. If you were to look at the force vectors of each element you would find that under normal loading conditons that spokes are in tension and the rim is in compression (due to the preloading of the tension in the spokes). 

Like it or not, a spoke contributes next to nothing in a compressive mode as 

a.) It is just too slender and would tend to bend before any substantal load is applied and 
b.) The nipple is not restrained from pushing out except by the rim tape and tube. (ie not much) 

A spoke has the least tensile load on it when it is at the bottom of the wheel. If you do not believe me, here is a study done at Duke. (See Figure 11) http://www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

This is further reinforced by http://www.johnforester.com/Articles/BicycleEng/Wheel.htm

No where in either article does it mention the hub being supported by "pushing" or compression in the spokes. I do think Forester's conclusion is a bit flawed due to his crude measuring devices. In the Duke study figure 11 shows in increased strain in most of the spokes, and a decreased strain in the bottom couple. Hmmm, off setting forces. 

A simple test of Forester's conclusion would be to load the wheel as he discribed and then cut the spokes he claims are "holding up by pulling down".


----------



## ohdee (Nov 9, 2007)

Hilarious. I'm a civil PE too with an MS in structures. Did you ever take a prestressed concrete class? 

All I'm saying is a leading spoke pushes the rim around in the same way the lower spokes support our weight. Call it what you want. It's real simple to me. 

And thanks for supporting my case that we're supported from the bottom spokes. 

"...It sounds paradoxical, but when you are on your bicycle your weight is supported by the downward pull of the spokes on the hubs; it's just that the downward pull is less than it would have been had you not been on the bike, less than the original tension built into the wheel. "

"...From the spoke dimensions and the reduction in length the reduction in spoke tension is easily calculated. With two spokes at full reduction and two more at partial reduction the calculated change in force is so close to the 100 pounds applied weight that this looks like a pretty good explanation. Probably deflecting the rim at the bottom tends to increase its average diameter everywhere else, and thereby increase the tension in the other spokes, but the effect was not measurable on my indicator, which read to,0 002 inch. "


----------



## meltingfeather (Dec 16, 2009)

Mike T. said:


> I rattled the cage of engineer, tensioned structures champion *meltingfeather* from MTBR.com but he declined my request to come here and give a mini lesson in engineering physics to those who (would probably) have their minds made up. I was disappointed but understood. I tried.


well... i took mike's bait, but ohdee seems to have stated fairly well almost everything i would have. not to have my RBR registration for naught (or disappoint mike), i'll throw in a few bits.


----------



## meltingfeather (Dec 16, 2009)

Blue CheeseHead said:


> Sorry, the friction has little to do with it but resist the force vector that is wanting to pull the second brick in.


Friction is the reason the brick doesn't move. Hence, your example is irrelevant to this discussion. The fact that the force vectors change in magnitude, however, is not irrelevant.


Blue CheeseHead said:


> As a civil engineer I am pretty adept at understanding truss type structures as well as arches. A wheel is basically a combination of both. If you were to look at the force vectors of each element you would find that under normal loading conditons that spokes are in tension and the rim is in compression (due to the preloading of the tension in the spokes).


A bicycle wheel is _pretensioned_, unlike either an arch or a truss. Invoking your engineering education does not support your argument here, because you are discussing something to which your arguments don't apply. Even for engineers, certain aspects of analysis on pretensioned structures are very counterintuitive (_e.g._, a member in tension acting as a compressive member). What you should do is read a book on pretensioned structures. It is counterintuitive, but a basic tenet (and a big part if not all of the reason for existance) of pretensioned structures.


Blue CheeseHead said:


> Like it or not, a spoke contributes next to nothing in a compressive mode as
> 
> a.) It is just too slender and would tend to bend before any substantal load is applied and
> b.) The nipple is not restrained from pushing out except by the rim tape and tube. (ie not much)


Forget bike wheels for a moment and consider this example:
A pillar in a static condition is loaded with a compressive force at one end and the other end is fixed. The pillar strains (shortens, obviously) directly proportional to the load and the modulus of elasticity. No other observable forces act to support the load. You would say that the pillar is acting as a compression member and supporting the load (hopefully), and you would be correct.
Nothing in the example does not apply to spokes at the bottom of a wheel. Compressive load, strain (shortening), no other observable changes that act to support the load.


Blue CheeseHead said:


> A spoke has the least tensile load on it when it is at the bottom of the wheel. If you do not believe me, here is a study done at Duke. (See Figure 11) http://www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf


I think he (and any structural engineer) understands that, and further what the implication is: the change from the static condition is what supports the load. Pretensioning is kind of like shifting the zero point. The _change _is what matters, not the starting and ending points. I think that's where ohdee's following (and accurate) statment comes from. 


ohdee said:


> I don't know why you say when there is "zero or less than zero tesion" It's all about the net change.





Blue CheeseHead said:


> A simple test of Forester's conclusion would be to load the wheel as he discribed and then cut the spokes he claims are "holding up by pulling down".


Cutting spokes would alleviate stress in the wheel and it would no longer be a pretensioned structure. It would behave in accordance with your understanding, but it would not function.
I'm surprised that a civil engineer would challenge a structural in a discussion outside the realm of his expertise over a very basic part of pretensioned structural analysis. I hope you don't ever decide you can design a bridge.


----------



## meltingfeather (Dec 16, 2009)

Fixed said:


> I don't think everyone understands that, but in fact, most disagree with that. I realize that this is a perpetual debate, but to me it seems nutty that a bike could "sit" on the lower spokes, rather than hang from the upper spokes, as it seems obvious that spokes have a great capacity for strength in tension, but almost zero in compression. I never have understood this position, despite years of hearing about it.


Its being counterintuitive does not make it any less of a supportable, observable, and very useful fact.
If you really want to understand it, read up on pretensioned structures. The principle is common to most pretensioned structures.
Semantics may play a role here. Saying that a structural member (in this discussion, a spoke) acts as a compressive member, does not mean that it ever goes into actual compression. I think that's the hump for most people. It is just like the concrete at the bottom of bridge stringers: it never goes into tension due to the large compressive loads added by the tensioning cables. It does, however, act as if in tension for the purposes of structural analysis. A structural member that shortens under a compressive load (like a spoke), is a compressive member, regardless of the starting point. Since spokes can't support compressive loads on their own, they are laced into a wheel with a large amount of tension, so that they can strain and support the load without ever going into actual compression.


----------



## ohdee (Nov 9, 2007)

Mike T must of really rattled that cage. Welcome, meltingfeather, and thanks for cleaning up this mess.


----------



## Fixed (May 12, 2005)

*makes sense*



meltingfeather said:


> Its being counterintuitive does not make it any less of a supportable, observable, and very useful fact.
> If you really want to understand it, read up on pretensioned structures. The principle is common to most pretensioned structures.
> Semantics may play a role here. Saying that a structural member (in this discussion, a spoke) acts as a compressive member, does not mean that it ever goes into actual compression. I think that's the hump for most people. It is just like the concrete at the bottom of bridge stringers: it never goes into tension due to the large compressive loads added by the tensioning cables. It does, however, act as if in tension for the purposes of structural analysis. A structural member that shortens under a compressive load (like a spoke), is a compressive member, regardless of the starting point. Since spokes can't support compressive loads on their own, they are laced into a wheel with a large amount of tension, so that they can strain and support the load without ever going into actual compression.


To me, it makes perfect sense that the lower spokes are under less tension and the upper spokes under greater tension when the bike is loaded, and none of them go into actual compression (or protrude further outward at the nipple).

I don't know whether the dispute is semantics or substantive, but to me, under that scenario, I think it would be incorrect to conclude that the hub is standing on the lower spokes, as opposed to hanging from the upper spokes.


----------



## Mike T. (Feb 3, 2004)

meltingfeather said:


> well... i took mike's bait


I just knew you'd be on the edge of the chair, bouncing up & down going "I can't stand it any longer. I just gotta get in there and sort those ^&%$# fellows out."



> ...........not to have my RBR registration for naught (or disappoint mike), i'll throw in a few bits.


Stick around 'feather. You just might find yourself buying a roadie bike


----------



## rruff (Feb 28, 2006)

Fixed said:


> I don't know whether the dispute is semantics or substantive, but to me, under that scenario, I think it would be incorrect to conclude that the hub is standing on the lower spokes, as opposed to hanging from the upper spokes.


It is substantive... and you are wrong... 

In case anyone is still confused about this point, I'll concur that *nobody* is claiming that an untensioned spoke can support a significant compressive load. The point is that they are in static tension in a tensegrity structure, and therefore they can and do support compressive loads. 

Since I can't think of any new and creative ways to repeat what I've already written, I bid you adieu.


----------



## ergott (Feb 26, 2006)

OK

I hang a 120kg mass from a spoke. The spoke is tensioned. I can "push" the mass to the ground with the spoke until the spoke goes slack. Theoretically, as I move the spoke towards the ground, the mass will "weigh" less , but since there is still tension in the system, the mass will still move in the direction of the "pulling" load (gravity) until there is slack in the system (the floor). Elevators do this all day.

There. Now replace gravity with more spokes and you get the idea.

-Eric


----------



## Blue CheeseHead (Jul 14, 2008)

This certainly this was a spirited debate. I will yeild to those who have studied prestressed structures. Obviously, I have not. 

While this was all very interesting, it really did not adress the OP's question, which was pretty simple.


----------



## SwiftSolo (Jun 7, 2008)

rruff said:


> You can push on a rope that has been pretensioned...


Mate, if you put a tension gauge on those spokes you'd mostly find that all of the spokes remain under tesion, however, some less than others. I think the r-sys wheel was one of the leaders in attempting to put some spokes under compression (actually pushing). Most of us use the term "push" to indicate putting something under compression--not under less tension).


----------



## Fixed (May 12, 2005)

*ok*



rruff said:


> It is substantive... and you are wrong...
> 
> In case anyone is still confused about this point, I'll concur that *nobody* is claiming that an untensioned spoke can support a significant compressive load. The point is that they are in static tension in a tensegrity structure, and therefore they can and do support compressive loads.
> 
> Since I can't think of any new and creative ways to repeat what I've already written, I bid you adieu.


Just set aside for a moment the issue of whether the lower spokes can support a compressive load. Is it necessarily also true that the upper spokes do not support a load in tension? Does one exclude the other? When someone makes the statement, "the hub stands on the lower spokes, and does not hang from the upper spokes," it seems to me to be saying that.


----------



## Blue CheeseHead (Jul 14, 2008)

Fixed said:


> Just set aside for a moment the issue of whether the lower spokes can support a compressive load. Is it necessarily also true that the upper spokes do not support a load in tension? Does one exclude the other? When someone makes the statement, "the hub stands on the lower spokes, and does not hang from the upper spokes," it seems to me to be saying that.


Fixed, while I have admitted my ignorance on prestressed structures, I am with you. It seems that while there is a net reduction in tension in the 4 or so spokes closest to where the wheel contracts the ground, there is a net increase in tension in the other 28 spokes. It seems logical that it is only this 6 oclock area of the the rim that significantly deflects as the only thing holding it's shape is the section of the rim and its arch. One might expect the spokes in that area, being elastic and under tension would want to pull the rim toward the hub (or visa versa). It would make sense that he remainder of the rim would not deflect because in addition to it's secton and it's arched segments it is being held in place by the spokes, which make very good tensile structures. But, alas, we are wrong.


----------



## meltingfeather (Dec 16, 2009)

Fixed said:


> Just set aside for a moment the issue of whether the lower spokes can support a compressive load. Is it necessarily also true that the upper spokes do not support a load in tension? Does one exclude the other? When someone makes the statement, "the hub stands on the lower spokes, and does not hang from the upper spokes," it seems to me to be saying that.


The easiest thing to do for you would be to get a tension meter and a friend and measure tension in the spokes of a wheel; first in mid-air and then on a bike that said friend is sitting on. What you will find is no measurable tension differences except in the spokes at the bottom. Do you understand what that means? For the spokes at the top to support the load by tension, the tension in them would have to go up... significantly. Does it? *NO*. The conclusion: the load applied to the wheel is supported by the spokes in which the tension changed, 'pushing' or 'less pulling' or whatever you want to call it so it fits with a vernacular that makes sense to you. :thumbsup:


----------



## meltingfeather (Dec 16, 2009)

Mike T. said:


> I just knew you'd be on the edge of the chair, bouncing up & down going "I can't stand it any longer. I just gotta get in there and sort those ^&%$# fellows out."


dangit, mike. you've got me wired. :blush2: 


Mike T. said:


> Stick around 'feather. You just might find yourself buying a roadie bike


what's that?


----------



## meltingfeather (Dec 16, 2009)

omgBikes said:


> I always use the terms "leading" and "trailing" spokes. I know some people say "push" and "pull" spokes. In terms of leading and trailing, which one in push and which one is push and which is pull? Thanks.


leading = pushing
trailing = pulling


----------



## Mike T. (Feb 3, 2004)

meltingfeather said:


> what's that?


It's one of those arse-up, head-down bikes.


----------



## Fixed (May 12, 2005)

*would love to try*



meltingfeather said:


> The easiest thing to do for you would be to get a tension meter and a friend and measure tension in the spokes of a wheel; first in mid-air and then on a bike that said friend is sitting on. What you will find is no measurable tension differences except in the spokes at the bottom. Do you understand what that means? For the spokes at the top to support the load by tension, the tension in them would have to go up... significantly. Does it? *NO*. The conclusion: the load applied to the wheel is supported by the spokes in which the tension changed, 'pushing' or 'less pulling' or whatever you want to call it so it fits with a vernacular that makes sense to you. :thumbsup:


I have a Park tensiometer, but from what I understand, if you have a 32 spoke wheel, the 4 lowest spokes see a decrease in tension, and the remaining 28 spokes share whatever potential increase in tension there may be. I'm afraid that my tensiometer would not have enough resolution to show the potential increase in tension across 28 spokes, compared to the decrease in 4 spokes. In other words, if they are balance, the increase could be 7 times lower per spoke compared to the lower spokes. 

Nonetheless, from the example you gave, could the conclusion be that the rim is only deforming at the bottom, which causes the lower, adjacent, spokes to see less tension. I still don't see how that means the lower spokes are "supporting" the load.


----------



## ohdee (Nov 9, 2007)

The other 28 spokes are not sharing a load (aside from the initial tension). They are keeping the wheel together, as they do in the static case. There is no change in tension with these spokes when the wheel is loaded radially. It's the bottom 4 (or so) only that absorb the load. If I apply a 100 lb load at the axel, the sum of the change in tension of the *lower 4 spokes only* will be exactly equal to 100lbs. It's been proven through experiments and finite element computer models.


----------



## rruff (Feb 28, 2006)

Fixed said:


> Nonetheless, from the example you gave, could the conclusion be that the rim is only deforming at the bottom, which causes the lower, adjacent, spokes to see less tension. I still don't see how that means the lower spokes are "supporting" the load.


Ever make a free body diagram? It is something we did in 1st year mechanics, in which you take the known forces on an object that is not accelerating, and determine the rest of them based on a) vector sum of all forces must be equal to zero, and b) sum of all torque equal to zero. 

Now look at the hub that has all these "vector" forces (spokes) on it and also has a downward force applied via the fork. The sum of all vertical components of spoke tension *relative to the unloaded state* will exactly match the vertical force applied by the fork. Since the only spokes that have much of a contribution to this are the lower ones, then they are supporting the load.


----------



## meltingfeather (Dec 16, 2009)

Fixed said:


> I have a Park tensiometer, but from what I understand, if you have a 32 spoke wheel, the 4 lowest spokes see a decrease in tension, and the remaining 28 spokes share whatever potential increase in tension there may be. I'm afraid that my tensiometer would not have enough resolution to show the potential increase in tension across 28 spokes, compared to the decrease in 4 spokes. In other words, if they are balance, the increase could be 7 times lower per spoke compared to the lower spokes.
> 
> Nonetheless, from the example you gave, could the conclusion be that the rim is only deforming at the bottom, which causes the lower, adjacent, spokes to see less tension. I still don't see how that means the lower spokes are "supporting" the load.


Sorry, but you are not going to, without understanding the basics, disprove a fact by trying to apply colloquial reasoning.
The fact that you, "don't see" it does not logically lead to the conclusion that there is some other explanation for something that is already well understood. What does follow logically is that if you want to remedy your lack of understanding of the system, learning is the answer.
Here's a start.
The relevant sentence (my emphasis):
"*Tests show* that the bottom spokes carry virtually all the load by compressive forces, which reduce the tensile prestress set up in the spokes when the wheel was made."
The reason for my emphasis is to show that not only is the conclusion easily arrived at by applying structural analysis, it can be (and has been) easily measured, as I suggested. You would get the same result from your Park TM-1 as from a more sophisticated device.


----------



## Fixed (May 12, 2005)

*another question*



meltingfeather said:


> "*Tests show* that the bottom spokes carry virtually all the load by compressive forces, which reduce the tensile prestress set up in the spokes when the wheel was made."


I'm not so much debating as truly trying to understand.

I'm curious whether the change in tension of the lower spokes is caused purely by the load, or is it from deformation of the rim (assuming a low profile aluminum rim)? Do you see the same change in a wheel built with a deep section rim, like a Zipp 808, which I'd think would not deform at all under normal loads?


----------



## rruff (Feb 28, 2006)

Fixed said:


> I'm curious whether the change in tension of the lower spokes is caused purely by the load, or is it from deformation of the rim (assuming a low profile aluminum rim)? Do you see the same change in a wheel built with a deep section rim, like a Zipp 808, which I'd think would not deform at all under normal loads?


The stiffness and construction of the rim will certainly make a difference... but that isn't what we are discussing is it? We are discussing whether or not pretensioned spokes can support a compression load.

If you'd like to know whether or not the lower spokes *must* support most of the load in *all* wheels, then the answer is no... they don't. I think you will find that they do in an 808 though.


----------



## meltingfeather (Dec 16, 2009)

Fixed said:


> I'm not so much debating as truly trying to understand.
> 
> I'm curious whether the change in tension of the lower spokes is caused purely by the load, or is it from deformation of the rim (assuming a low profile aluminum rim)? Do you see the same change in a wheel built with a deep section rim, like a Zipp 808, which I'd think would not deform at all under normal loads?


That's great. It sets you apart from most, who want to endlessly 'debate' by saying over and over that spokes in tension can not support compressive load, but do not (IME) really want to understand.
To answer your curiosity, it is all related. The load causes deformation of the rim, which reduces tension in the spokes, which creates the balancing net force that supports the load. The gemoetries and mechanical properties of the components (including the shape and stiffness of the rim) affect the way the forces are distributed and the resulting deformations. There is nothing that "does not deform at all" under load. The results are qualitatively the same for all wire-spoked wheels.


----------



## Blue CheeseHead (Jul 14, 2008)

meltingfeather said:


> That's great. It sets you apart from most, who want to endlessly 'debate' by saying over and over that spokes in tension can not support compressive load, but do not (IME) really want to understand.
> To answer your curiosity, it is all related. The load causes deformation of the rim, which reduces tension in the spokes, which creates the balancing net force that supports the load. The gemoetries and mechanical properties of the components (including the shape and stiffness of the rim) affect the way the forces are distributed and the resulting deformations. There is nothing that "does not deform at all" under load. The results are qualitatively the same for all wire-spoked wheels.


I agree with that completely.:thumbsup: 


Would you be surprised if I told you I recently took a personality test that says I can be stubborn? I didn't think so.


----------



## Mike T. (Feb 3, 2004)

Blue CheeseHead said:


> Would you be surprised if I told you I recently took a personality test that says I can be stubborn?


You're lucky. Some of us have been classified as terminally stoopid. We're just lucky that we're receptive to a well-presented theory here & there. (Mike mischievously rattling 'feather's cage again)


----------

