And yes TJ.. They ARE in tension. The relief of tension is compression. Therefore the relief of tension has EVERYTHING to do with compressive strength.

If you look at it from an outside point of view, then who cares how tight the spokes are, it's all to do with how strong the wheels are in compression.

TandemJeremy
Edit -It is possible to build a wheel with string that has no ability to hold any sort of load in compression

Remember the tioga disc drive wheel anyone? Carbon fibre (or kevlar "string") no spokes as such, and it was under tension!

jesus.. you lot are hilarious.

spokes have a LOT of strength in compression, provided they are pre-tensioned.

bikewhisperer, on the contrary, You are hilarious and slightly confused I fear.

Answer me this. Bike stationary, rider on the seat. The spoke that is vertical from the bottom of the wheel to the hub. Is this in tension or compression?

Have you no answer for this simple question? compression my arse

Sorry, late post, I see you have aswered this with nonsence! A relife in tension is not compresion, it is simply less tension.

Right – so the spoke is in tension – so how can it be holding a compressive load if it is in tension? simple – it cannot. It does not have any compressive strength if it is in tension

Please – go and have a read up with an open mind – you have got yourself very confused

relief of tension is not compression. it is just less tension.

the rubber ban analogy is correct… with the exception that you are not compressing it to reduce the length only reducing the tension.

the spoke is in tension, when loaded the tension is reduced. it is not in compression. if it were it would poke out of the nipple end hole and shred your inner tube as it is unconstrained at that end.

If a static system is in a pre-tensioned state then any compressive force acting upon it can be considered as a compressive force upon it's components. In the case of a wheel the compression would be relieved as a reduction of tension in the lower spokes and as an increase in compression around the rim and transferred to the hub as an increase in tension in the upper spokes. The other forces cancel out and leave just a compression in the lower spokes.

I give up. You're all welcome to think what you might, but it's entirely down to the spoke tension. We're all arguing over the fact of agreement that tension is equivalent to a reduction in compression. Fine. Please make a special effort to include atmospheric pressure when you check the gauge to inflate your tyres the next time.

Under no load, every spoke is under tension and the system is under equillibrium.
Under say a vertical load applied at say the hub, then there will be an increase in tension in the upper spokes and a REDUCTION in tension in the lower spokes, but at no time will any spokes be under pure compressive loads only, if they were they would bend.
You cannot say that because there is a reduction in tension that the spoke is suddenly being compressed, it is not!

wow – 3 pages discussing how a wheel works ……

You cannot say that because there is a reduction in tension that the spoke is suddenly being compressed, it is not!

OK then.. So how come the lower spokes get shorter? I've said all along that the resultant force is compressive on the lower spokes. Under the equilibrium state they are in tension.

Just because something becomes shorter does not mean it is in compression. It's the elastic band analogy again.

It think you're getting confused by language. "compressing" means to shorten. Compressing something that is in tension does not make cause that something to be "in compression" unless it fully relieves the tension.

That's why tight wheels ate stronger – because the usually never lose tension when loaded. If they do ( become "in compression" ) then the spokes bend and the rim is likely to bend as it's unsupported.

OK then.. So how come the lower spokes get shorter?

you div. i think that sums up why you are getting it all wrong.

Yes.. The resultant force is a compression.. ie. a reduction in tension. They are both the same mathematically. If you think about it in terms of your bike frame and the ground it gets simpler… the resultant force between your dropouts and the ground is compressive.

That's why tight wheels ate stronger – because the usually never lose tension when loaded. If they do ( become "in compression" ) then the spokes bend and the rim is likely to bend as it's unsupported.

yes too. I said that.

The resultant force on the lower spokes IS compressive. Correct. The lower spokes however are still in tension because the compressive force doesn't exceed the tensile force.

I don't think you really understand what being in tension or being in compression means.

Take a spoke on its own. Stretch it and it's under tension. Compress it and beyond a minimal amount, it'll bend (because of the geometrey of the head and the fact that it's long and thin). In a wheel spokes are under tension. If not, they stop doing their job (since the nipple is free)

to put it another way, why don't we just build wheels completely loose if spokes take compressive loads?

no it isnt. reducing the tension in any object is not compressing it.

to compress it you must relieve ALL tension first. which you arent doing.

Final attempt.

I get fed up with you and put you on a rack 🙂

let's say you are normally 6 feet tall

I then apply a force which stretches you to 6'6". Ouch

someone else then decides that I'm being a bit harsh(!) and pushes against me and you shorten (compress) to 6'3" rather than your normal 6' height because i'm pulling harder than they're pushing against me.

By your rationale you're "under compression" because you've shortened to 6'3" but I'm pretty sure that if asked, you'd say that you were still being stretched ( under tension )

in fact come to think of it, we're not unlike spokes in compression and tension. Pull us ( the rack ) and we stay straight. Compress us and we bend/collapse.

now we're talking from the same page it's simple. And tracknico, you're wrong. If you're compressing a pre-tensioned component, then you're reducing the strain on it and therefore compressing it… It will be compressed until the compressive force is equal to the tensile force. After that it will bend. Most forces on a wheel aren't so harsh.

Clubber… Yes I understand what you mean. Overall the simplest way to describe the system is with the spokes being under compression. Any physics lecturer would be twatting you with a cricket bat if you didn't use the above…

Right. Which is why wheels hang from the top spokes…

A reduction in tension is not compression until such time that the value of compression exceeds the value of tension.
So if a spoke already has a tensile force exerted upon it, to physically COMPRESS the spoke you would need to exceed the tensile force already being applied in the opposite direction. Reduction of tension is not compression until this happens.

I admire your tenacity clubber.

agree with bigyinn. he is not wrong. and neither am i.

your not compressing a pre-tensioned component. the component is currently under tension. it is in tension. releasing this tension is not compression.

that is fact. saying im wrong doesnt make it so.

It's about time we turned this discussion to tyres…so if I squidge a 30 PSI tyre with my thumb, which is the best tyre for compressing under tension???

tracknicko – Member

agree with bigyinn. he is not wrong. and neither am i.
You Gordon Brown in disguise?? 😆

bikewhisperer – Member
So lets go with the "my gang's bigger than your gang" logic and say you won. hubs hang from the upper spokes and tension has nothing to do with compressive strength.

Or how about "everyone including me is against you"?

Anyway if you had a wheel with one spoke. With the bike static and the spoke at the top of the wheel, would you say the wheel is in compression?

Compression is net forces acting in the direction opposite to tension. Until you've completely unloaded the tension you're not in compression. Spokes don't work in compression at all.

If a crazy man argues with himself alone in a locked room, is he still right if no one else can hear him?

bigyinn – Member

If a crazy man argues with himself alone in a locked room, is he still right if no one else can hear him?

Of course I am right I always am! Why does this jacket have funny long sleeves with buckles?

I started this thread with an open mind as years back I read part of a book about this exact subject, but couldn't remember the answer.

Bikewhisperer has completely failed to bring me round to his way of thinking, and I'm convinced everyone else is correct!

My engineering design lecturer used this example to highlight how singple things may look ie you expect the lower spokes to be holding up hub but infact hub hangs …… apprently according to bike whisperer he was wrong …. Musta been a thick git ..

Also funny story from my time as shop monkey … Had a boy come in a complain about one of my wheel builds which were locally held in high regard. comes in shooting his mouth off about shit wheel building and the like As he picked it up friday and returned on monday

Look at the wheel and find it with a monster v into the rim – not just a flat spot – like he had dropped 10ft onto a kerb with a flat tire ….all spokes still nicely tensioned except the damaged area ….. Asked him what he hit …said nowt ….

Unfortunantly for him it was quite spectacular damage and i found the details documented on a downhill forum ….basically flat tire and cased a jump on the fort william motorway ……the look on his face was priceless when i asked about the jump and the flat tire 🙂

once again the stw dickheads are out in force, (must be an early finish) now i claim to no nothing about the physics of wheels and how they work.but it seems to me that Mr Bikewhisperer has got his knickers in a twist and wont let it lie.you sir are a buffoon of the highest order.oh and wrong as well.

Read and learn …..as some one once said knowledge is power

When a bicycle wheel is built, the spokes all start out loose, then they are gradually made tighter and tighter. When complete, every spoke pulls the hub towards the rim, but all the spokes are in balance, so the hub and the rim stay put. This process is occasionally referred to as pre-tensioning, because you are putting tension in the spokes, even before they wheel has to support any load.

It's hard to visualize (I suggest looking at a bicycle wheel), but every spoke is pulling on the hub simoultaneously, in all directions. The tension in every spoke is (very close to) identical. What's interesting is what happens to the wheel when a load is applied (when you get on the bike). If you measure the tension in all the spokes, only those spokes in the bottom of the wheel change tension significantly – the tension decreases. In other words, the bottom spokes become more loose, all the other spokes remain unchanged.

In mathematical terms it is possible to describe the bottom spokes as being in compression. They have less tension than they had before, so if you count the starting (pre-tensioned) state as the zero state, you have put them "in compression". The reason they can support this compression is that the spoke has been pre-tensioned.

The "compression" of the bicycle spokes is really a mathematical fiction. They are compressing only in that there is less tension than there was before. Relative to the starting (pre-tensioned) state, they have compressed, but relative to the totally slack state, the spokes are still in tension. In other words the lower spokes which are described as being in "compression" are still pulling downwards on the hub. Clearly, pulling downwards can not have the effect of holding up.

Only the upper spokes are actually pulling upwards on the hub. the hub hangs from the upper spokes. but Oddly this does not contradict the following statement, that the lower spokes play the most dynamic role in supporting the load.

im bored now, so im going out for that nice experience we call a bike ride.

Yep, all teh stw dickheads are now out!

It's hard to visualize (I suggest looking at a bicycle wheel),

thanks, v helpful! 😀

blimey… went our for a couple of hours and then this…
I promise you folks I'm not arguing something that is impossible, just an equivalent method… Did you know for instance that centrifugal force DOES exist if you talk about it from a rotating reference frame?

So if a spoke already has a tensile force exerted upon it, to physically COMPRESS the spoke you would need to exceed the tensile force already being applied in the opposite direction. Reduction of tension is not compression until this happens.

OK.. or to put it another way, to relieve the strain on the pre-stressed component you would have to put it under compression. That reduction in strain would relieve the strain and shorten it by the amount allowed by it's youngs modulus. This would happen until the stress became negative and the spoke bent.

i just find it amazing that people will still argue until there blue in the face, when its quite clear to any rational thinking human being that they were wrong. but not going to admit it. strange

i just find it amazing that people will still argue until there blue in the face, when its quite clear to any rational thinking human being that they were wrong. but not going to admit it. strange

Not a problem, as I'm neither right nor wrong. You can roll my equivalence up to a tight cone and do with it what you will.

Didnt you just re-write what i previously wrote? I fear we are arguing semantics here.
Anyway its the weekend shortly. I'll leave you argue to black is in fact black, when in actual fact it is actually black.

The Dunning–Kruger effect is a cognitive bias in which "people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it." The unskilled therefore suffer from illusory superiority, rating their own ability as above average, much higher than in actuality; by contrast, the highly skilled underrate their abilities, suffering from illusory inferiority. This leads to a perverse result where less competent people will rate their own ability higher than more competent people.

I'll disagree about one thing. Sometimes wheels do need bedding in.

Usually it's when the owner has specified cheap spokes and wants a cheap build. Properly pretensioning a wheel takes time, and cheap jobs are done as quickly as possible, and near enough is often the result.

You'd need some pretty strong rim tape for a spoke to be in compression LOLWTFETC…..

and if the spokes ever were in compression they'd just buckle anyway. Surprised none of the other engineers have mention this mode of failure yet.

I suggest looking at the London Eye if you still don't get that it's the spoke tension that hangs the hub from the rim above. The London Eye uses flexible steel cables, like giant gear cables, as the "spokes". Try claiming they are in compression 😆

Interestingly because the "rim" is off the ground and it's the hub that is supported, by the ground (unlike a wheel), in this case it is the lower "spokes" that support the "rim" buy stopping it falling away from the hub, i.e. becasue of the tension in the cables.

Hope that doesn't confuse. I find it helps me to get my head around a principle when I look at another example of it applied in a slightly different way.

There we go..

Looking up here led me to here.
If you can make it past the picture of his unicycle without laughing then there's a very good analysis. It's not semantics.. It's finite element analysis.

Here's his conclusions:

* There are 31 tensile spokes. On average they contribute 1.436 N (0.14 kg, just under a third of a pound) each to holding up the hub.
* There are 5 compressive spokes. On average they contribute 191.097N (19 kg, just over 42 lbs) each to holding up the hub.

And furthermore:

* The bottom spoke contributes 345.216 N of lift. The top spoke contributes only 12.487.
* The compressive spoke that contributes most (spoke 19: 345.216) contributes more than 15 times as much lift as the tensile spoke which contributes most (spoke 3: 21.955).
* The tensile spoke with the greatest impact on the lift figures is actually one pulling the hub down (spoke 15: -28.712).
* The least contributing compressive spoke (spoke 21: 60.955) contributes nearly three times as much as the most contributing tensile spoke (spoke 3: 21.955)

So basically, the bottom spokes have a far greater supporting role on the wheel than the top ones.