[Closed] Wobbly wheel after one weekend- acceptable?

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.

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 ****ting 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 [u][b]physically[/b][/u] 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.

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 [b]net[/b] 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),

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 [u]is[/u] 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 [url= http://en.wikipedia.org/wiki/Reaction_to_a_radial_load_%28tensioned_wire_spoked_wheel%29 ]here[/url] led me to [url= http://www.astounding.org.uk/ian/wheel/ ]here[/url].
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.

[img] [/img]

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.

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. [url= http://www.astounding.org.uk/ian/wheel/ ]here[/url]

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

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.

Compression is the cause and contraction is the result

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

Ok any[i] wire spoked wheel[/i] - 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.