From your linked page

Negative values indicate a compression. This is a change from the unloaded state, so compression doesn't actually mean compression, it means reduction in tension.

So the spoke it not in physical compression, just under REDUCED tension. Nothing new there that we havent already covered!
NEXT!!!

Exactly bigyinn

That link just proves someone else has not understood either – is that a peer reviewed piece or just web waffle?

I suspect his analysis has assumed the nipple is fixed to the rim in the direction compression would be, which of course it isn't.

Ask yourself this: how does the rim exert a compressive force on the nipple when it's just loose in a hole? To paraphrase someone above: must be strong rim tape in your wheels.

From the link:

In the analysis I simply don't tell the computer to let anything buckle, so I can analyse a spoke with compression without worrying that it's really a spoke that started out with a tension and now has less tension

So the author admits the spokes are not in compression at any stage and that if they were they would buckle.

If the bottom spokes are in tension (as the author admits they always are) then they are pulling the hub in the same direction (roughly) as gravity. So the spokes above the hub must be maintaining equilibrium. Simple. Just because the bottom spokes may experience the biggest change in force, what does that prove? Only that the rim deflecting decreases the tension in the local spokes more than it increases it in the ones above.

The rim deflecting (I guess) is what causes straight pull spokes to un-hook. How would you "stand" on a straight pull spoke? It can push out from the hub or the rim so it can only work when in tension.

I never thought of thelondon eye like that – given me an interesting thought for the day, cheers.

05_five … i had that same thought re straightpull spokes when i was bored at 10 under the ben yesterday – only because i was running straight pull spokes – only restrained when hanging when at the bottom its free to move at both ends

Bikewhisperer: you, sir, are an idiot. That is all.

Great thread guys! My view is that the spokes are in tension – there is no actual compression, just less tension, as has been said before – pretty hard to call this tension really, that would be a bit like saying that when we jump in the air we are defying gravity.

My two cents worth thou is YES Spokes are in compression, but only in a cart wheel or the Mavic R-SYS system wheels, which don't work like normal spoke wheels:

http://autobus.cyclingnews.com/tech.php?id=tech/2007/news/06-20

And that should pretty much be the end of it i think.

Hmm bw any comments please?
The easiest way to prove this once an for all would be to lace a wheel loosely with say 8 spokes (for simplicity and exaggerating of the forces), leave them relatively loosely tensioned and then apply a load to the hub in the centre. I would expect to see some deformation of the lower spokes, or at the very least a REDUCTION IN TENSION in the lower ones.

Hmm bw any comments please?

I feel like I'm picking at this now but yes, just got home and it's still bothering me!… The analysis by Ian that I posted isn't complete and is only illustrative. He quotes this:
Loading is three upward point loads, 500N at very bottom of rim (centre of contact patch), 250N at each adjacent spoke/rim connection.
Doing a quick calculation with a Sapim leader spoke gives a middle section strength of 2872 N at a width of 1.8mm (the skinniest). So even considering that the spoke would (probably) only be loaded up to half it's strength (Park recomends around 550-1750 N in on it's tension meter calibration table) then it's reasonable to assume that in most normal situations it wouldn't be compressed beyond it's pre-tension. If it was then the nipple would separate from the rim, and the spoke would buckle once it contacted the rimtape. As rims really aren't that strong on their own when this would likely cause a flatspot.
Building a wheel loosely and with fewer spokes would only cause this event to happen at a lower force. Compression is not contraction. Compression is the cause and contraction is the result. This is even true in structures that have been pre-tensioned. It's just that the compression manifests itself as a reduction in tension.
And the whole difference in r-sys wheels is that they have spokes that have some strength in compression, not just in tension. That way they can stand up to compression with out such high pre-tension (as the article mentions) and mostly break as a result….

4 pages and not a single mention of "superposition". That being the principle which allows you to explain the fact that the wheel is a static tension structure with a superposed compression of the lower spokes. Of course the spokes are always actually in tension, but this mathematical principle helps an awful lot with the analysis as you can basically ignore the static tensions in the wheel when doing your calculations.

page three. here

This is a change from the unloaded state, so compression doesn't actually mean compression, it means reduction in tension.

bikewisperer – no matter how often you repeat it you simply are wrong – that quote is from your link

No spoke on a bicycle wheel is ever in compression at all. Ever.

You are just completely muddled up as can be seen by anyone who reads your posts.

Compression is the cause and contraction is the result

From your post above – do you actually understand what the words mean?

Edit – that guy in the link is suffering from the same confusion you are.

The spokes that are below the hub are still in tension when the bike is loaded – not compression. they have been compressed but by less than the tension that is on them. As they remain in tension they are not carrying any compressive load – the bike is not standing on them – its hanging from the top spokes.

You cannot have a load standing on something that is in tension. its simple impossible

No spoke on a bicycle wheel is ever in compression at all. Ever.

So what I just said about R-sys wheels is wrong then, or for that matter any carbon spoked wheel?

Compression is the cause and contraction is the result

From your post above – do you actually understand what the words mean?
Erm, yes. one is a verb and the other a noun.

Ok any wire spoked wheel – solid wagon wheel type is the opposite – they stand on teh lower spokes. wire spoked wheels hang from the upper spokes – clearly obviously and simplky

If it is in tension there is no weight on it. simples

You clearly dont understand what compression and contraction mean if you think one causes the other.

TJ I hate to bring this to you but in a wire spoked wheel or a wooden spoked wheel the forces are the same. In a wooden spoked wheel the rim is heated to red hot before being applied to the wheel. This allows it's tension to hold the wheel together after it's cooled and bound itself to the wheel. There is negative pre-tension involved, or in this case pre-compression.
Oh whoops.. there's an equivalence.
In the case of the spoked wagon wheel any increase in pressure below increases the pressure on the lower spokes and supports the hub. This is in spite of the fact that there is a force acting upon the hub from each of its spokes, restoring it to it's central position. The pre-compressive force cancels out and all that is left is the restoring force from the ground through the rim to the hub.
Now lets talk about the pre-tensioned hub.
You can guess the rest.. the tension cancels out and all that is left is the (resultant) compression on the lower spokes.
I've bracketed the (resultant) compression as I don't want it to cause you a problem.
Saying that one of the above examples would be able to support tension and not compression is not a valid answer. Both are true.

To explain the basics in simple terms, when the rim is in compression, it releases some tension from the spokes at the bottom of the wheel at that point in time. This constant flux of higher and lower tension in the spokes creates a fatigue cycle that culminates in metal faitigue at either the bend or the nipple as these are the points of the spoke in contact with other metal surfaces that will abrasively wear the spoke or nipple creating stress raisers in these areas. Spokes tend to break at these points for this reason.
The only other stress raiser that can affect a wheel's performance apart from crash damage is a chain derailling into the spokes from improperly adjusted or damaged mechs. This causes stress raisers at the butting point of the spoke during contact. The spokes will then inevitably fail at these points in due course.

The reason it gets explained as the spoke being compressed is that it's easier for the customer to comprehend.

Somebody once tried to tell me that World Cup DH rider's wheels last longer because they ride smoother. This was not the case with Gee Atherton during his World Championship year, as he told me he can get get through four rims in a race weekend. But the wheels I built for a privateer WC competitor lasted all season with no issues. and he used them for a few video shoots post season before he sold his bike with the original wheels on it.

The thing that helps that the most is un-bending the spokes at the nuckle-head when you build them. ie. Press the bends with your thumb when you build them, or hit them with a soft hammer. Getting a straight line on the outside spokes means that they will not flex on each wheel turn, and therefore won't fatigue.

page three. here

Yeah, but that's not actually in the thread itself.

The reason it gets explained as the spoke being compressed is that it's easier for the customer to comprehend.

On the contrary – this thread demonstrates just how hard it is for ordinary people to get this concept. What it does make easier is the analysis for those folks who can get their heads around the idea that when you talk about a compression you mean a superposed compression on a pre-stressed structure.

To those suggesting there isn't a compression because of the pre-tension, how do you think you analyse a bridge made from pre-stressed concrete? Do you consider the material as a uniform whole, and use standard bending equations to work out the compression in the top of the structure and the tension in the bottom? Or do you make life all difficult for yourself by arguing that concrete can't actually take a tensile force, hence the bottom must always actually be in compression due to the pre-stressing?

bikewhisperer – Member

TJ I hate to bring this to you but in a wire spoked wheel or a wooden spoked wheel the forces are the same. .

Wrong – they are completely opposite.

You really are confused.

In a wagon wheel the rime is in tension and the spokes compression, in a wire spoked bicycle wheel the rim in in compression and the spokes in tension

seriously bikewisperer – please have calm think about this – you are very confused.

I ask you again. In a wirespoked bicycle wheel you have agreed that all spokes are in tension at all times – more on the upper less on the lower once a weight is aplied to the hub – so how can there be a weigh standing on the lower spoke if it is in tension as everyone is agreed it is? Answer – it cannot.

Carbon fibre is a product that has the properties of strong rope in it's raw form.
How do you think you could engineer this product to handle compressive loads?
The only way to work carbon ATM is to impregnate the weave with resins to improve it's suitability for use in certain applications.
All of these applications so far, require the fibres to remain IN TENSION.

e.g. the nose cone and gearbox protection elements of F1 cars are fabricated in CF but for only one reason. Under compressive loads they break bown in a uniform manner that controls the deceleration. And it's because of this property they're used. i.e. they break, but we can predict how they react.
Not an attractive property for a pair of wheels.
So CF wheels work in the same way as all other wheels. There are fibres in tension, but you may not be able to see how or where they're used.

aracer, if you've got a customer in a bike shop with a broken wheel and you start spouting shit about bridges, they tend to lose interest and go elsewhere telling everyone how much of a complete cock you are.
K.I.S.S.
Explain it in terms they understand, and they tend to stay and listen.
And if they like the product, buy.

How often do you explain to a customer in a bike shop how a spoked wheel works? If we're discussing it outside the context of talking to a customer in a bike shop, don't you think it makes sense to use standard engineering practices to analyse what's going on (ignoring of course that bicycles defy all normal rules of physics)?

All of these applications so far, require the fibres to remain IN TENSION.

Wrong. CF is actually just as strong in compression as in tension – provided you stop the fibres from buckling, which is what the resin does. There are an awful lot of places CF is used in compression on a bicycle – eg seat tube, seat stays, top tube on a carbon frame, carbon seatpost. Oh and also the spokes in R-Sys wheels. It's not the resin taking the compressive load in these cases, it's the fibres.

I'm sure nosecones on F1 cars are made from carbon composite for the same reason as lots of other bits – strength to weight ratio.

aracer – I have not understond your point. Are you in the "hanging from the top spokes" or "standing on the bottom" view?

Every weekend when they come in with broken spokes on their "factory built" Specialized.
To explain why their £2000 bike has just let them down.
They don't give a shit about engineering principles, they just want to know how much it'll cost to get them back on the mountain.

As a professonal wheelbuilder, I do understand what's going on.
But customers in shops don't care IME. They just want it fixed.
I don't tell you how to build bridges, don't tell me how to build wheels.

They don't give a shit about engineering principles, they just want to know how much it'll cost to get them back on the mountain.

Exactly, which is why you don't go into the explanation of how they work (whatever you might think the answer to that is).

As a professonal wheelbuilder, I do understand what's going on.

But then you don't actually need to know to build good wheels. I wouldn't dream of telling you how to do your job, any more than I'd tell the bloke actually building the bridge how to do his. You actually seem to have a very good understanding of how wheels do work (not so good on how carbon composite does). However if you're trying to analyse wheels (eg to determine when and how they'll break, or the optimum use of components), then it does make sense to use the pre-stressed and superposition model. Your description is more fundamentally accurate, but leads to the "hanging from the top spokes" idea, which then results in the fallacy that the load is supported by the tension in the top spokes increasing.

Are you in the "hanging from the top spokes" or "standing on the bottom" view?

I was trying hard to avoid that part of the argument, but if forced I'd describe it as a pre-stressed structure with superposed forces equivalent to standing on the bottom spokes. The whole problem you seem to have is with people saying the bottom spokes are in compression – when they're quite clearly not – without adding the necessary explanation about this compression being superposed on the pre-tension, resulting in a net (lower) tension. Meanwhile you keep repeating "more on the upper less on the lower once a weight is aplied to the hub" when in fact the tension increase on the upper spokes is insignificant.

i never realised people could be so dumb.

to maximise their ability to distribute stress, spokes must be in tension. ditto carbon fibres, or indeed any resin-bound composite material.

it's engineering at its most basic. jesus!

ro, **** off.
This thread is pretty interesting and doesnt need your help to turn it into a slanging match. Either add something to the discussion or (preferably) p!ss off!