Need some help on spokes

[Minion]

 

Just to clear up:

I posted the Wikipedia link mainly as a piece of trivia. The information from the 2nd edition of the book is pretty much contradicted by the 3rd (and presumably more accurate) edition.

 

The relevant part of the article (ref 2 is 2nd ed., ref 4 3rd):

Reaction to load

A load applied at the hub causes the wheel to flatten slightly near

the ground contact area. The rest of the wheel remains approximately

circular. The tension in all of the spokes is increased except for the

few in the flat spot.^[2]

However according to^[4] and^[5]

when a radial load is applied to a wheel at the hub eg by a rider

sitting on the bicycle, the tension of all the spokes do not increase

significantly, with only the spokes directly under the hub decreasing

their tension.

Edman2007-12-20 03:50:18

[Ox_Wagon]

Sound right

[Minion]

 

I would be interested to see how the FEM looks like if it is properly constrained. If it is your model would you be so kind and humor me and run it with the spokes constained that they can only take a tension force. Pleaseeeeee

 

Just a note on the picture I posted: it really is just a pretty picture. I haven't properly checked my constraints or results yet, or done any hand calculations to make sure I'm in the right ballpark.

 

The problem with an accurate (i.e. spokes can move etc.) model of a wheel is that it becomes highly non-linear due the possibility of contact separation - it would probably take around a week to set up and solve something like that.

 

I can look at modifying my model to add a pretension to the spokes to make it more accurate in that respect.

 

I'm limited to 60min of internet time in 10min bites, so I don't know if I'll be abel to reply before this evening.

 

[Minion]

The problem with the FBD is that it is statically indeterminate. You can't solve it using basic forces. You need to account for the strains in the spokes (i.e. the hub will deflect downwards). This will lengthen the top spoke (increasing it's tension) and shorten the bottom spoke (decreasing its tension). The load is thus shared between them.

The supports are actually OK (although the top one should not really be rigid). I'll try post what I think the FBD should look like when I get home.

 

[Johan Bornman]

 

 

OX, I'm going to give you a dunce cap and make you stand in the corner. Still haven't done the experiment, have you? Do your homework.

Yes, the spokes in the picture you have there are in compression. However, that is an overloaded wheel where the load exceeds the collective tension in those three spokes. In other words, the spokes were tensioned to 1500 Newtons each and Fat H sits on the bike, with all his 451kgs (note the 1 kg please) weight on that one wheel. It is now an unstable wheel and will collapse.

 

In the model you propose, the top spokes elongate, the bottom ones push out past the nipple beds. This should cause a snapping noise as the wheel rolls. My bike doesn't do that.

 

Further, don't believe everything you read on mavic.com.

 

Mavic displayed that wheel at Interbike some months back and my colleague handled the spokes. They can handle compression - my arse!. He laughed and told me they are just thin carbon fibre tubes that buckle just like a 2mm spoke. They can't take any (meaningful compression). Do the calculation for yourself - you'll find it under Euler's buckling in that engineering book of yours.

 

And even further, here's two pictures of an ox wagon wheel. How appropriate is that? You'll notice that the spokes have no way of handling tension. They're merely inserted into a hole in the rim with nothing that stops them from pulling out, should the hub start to hang from the top spokes and spokes thus from the rim. I visited an ox wagon museum in Magalliesburg last year sometime and phaffed over those wheels for ages. You'll be interested to know that the payload for an ox wagon is 16 tons (imperial). That's four tons per wheel which you claim is all taken in tension by the spokes. Please explain that.

 

 

 

Oh, and please do the experiment.

 

 

 

Johan Bornman2007-12-20 04:40:37

[Ox_Wagon]

Edman, thanks for your inputs, please don't put too much effort into it is only for interest sake.  I would not even now where to start.  It would be a major problem to model the unloading of nipple as it lifts of the rim.  I think that is why all the internet FEM analysis takes the short cut to rigidly fix the spoke to the rim.<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Mister Borman, I agree that a lot of the info on the Mavic site may be marketing hype especially if you look at R-sys rear wheel and see that they use  a normal spoke system on the drive side precisely where you would like to increase lateral stiffness.

On the ox_wagon wheels it is rather appropriate.  I am heavy and I am slow.  That is precisely why I choose the login name.  I agree that they have no way of handling tension and you can see how bulky and heavy the spokes are (Generation One Wheels, Refer back to the Mavic different generations of wheels, sorry I could not help my self).  The same way the ox_wagon  spoke can only take compression  so a ?Modern? generation two wheel can only take tension.  If we can agree on that we have made major progress.  

Talking of experiments, try this one. Insert a single spoke in to a rim with its nipple in place. Now pull on it, cool it can take tension.  Now push on it, oops it can?t take any compression. That is if you haven?t installed the rim strip yet   

On the twange test.  Funny enough after a lot of thinking, you may have noticed I am a bit slow, looking at what other people say, Edman quote out of the bike book I agree with the result of your experiment.  That the tension in the top spoke may be close to the tension of the top spoke with the weight of the rider on it. So doing giving the same amount of ?Twang?

Simply put, the deformation of the rim on the bottom of the rim reduces the pre-tension of the bottom spoke. If the pre-tension is large in comparison to the weight place on the wheel, which seams to be the case with most wheels, you can get the situation, with a bit of tweaking, where the reduction in tension due to the deformation of the wheel equals the weight now placed on the wheel.  The end result is that the tension in the top spoke stays approximately the same.   

Here is where most argument derails.  The popular argument goes, that because ?nothing? happens in the top spoke all the magic is in the bottom spoke which means that the reduction in tension is the thing keeping the hub in the air.  BUT if we have a look at <?:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Newton?s law we see that all the forces should equal zero. Which gives us:

Tension in the top spoke = Now reduced tension in the bottom spoke + The weight placed on the wheel.  

The fact that this new tension in the top suspending the hub is almost the same as the pre-tension in the top spoke is the thing that causes the problems.

Ox_Wagon2007-12-20 05:52:14

[greatwhite]

Just to clear up:
I posted the Wikipedia link mainly as a piece of trivia. The information from the 2nd edition of the book is pretty much contradicted by the 3rd (and presumably more accurate) edition.

The relevant part of the article (ref 2 is 2nd ed.' date=' ref 4 3rd):

Reaction to load

A load applied at the hub causes the wheel to flatten slightly near the ground contact area. The rest of the wheel remains approximately circular. The tension in all of the spokes is increased except for the few in the flat spot.^[2]  (A)

However according to^[4] and^[5] when a radial load is applied to a wheel at the hub eg by a rider sitting on the bicycle, the tension of all the spokes do not increase significantly, with only the spokes directly under the hub decreasing their tension.  (B)

 

I picked up on it because you made the comment in response something about misquotes and misunderstanding.

 

Symantics, I know, BUT: In isolation these 2 statements certainly project a different picture, yet they are both true and don't actually contradict each other:

 

from A: tension in all of the spokes is increased except for the few in the flat spot - it doesn't say how much the increase is

from B: the tension of all the spokes do not increase significantly

 

So from both these statements, we know the tension increases, but not a lot.

 

 

from A: tension in all of the spokes is increased except for the few in the flat spot - so these are either unchanged or tension has decreased

from B: with only the spokes directly under the hub decreasing their tension. 

 

So from both these statements, we know the tension decreases in the spokes below the hub.

 

What a surprise: this matches with the result of the FEA results - a small increase in tension all around the wheel except under the hub in the loaded zone, where there is a decrease in tension

 

This can also be demonstrated in practice by Johans experiment - pluck the spokes - they all be fairly similar except the few in the loaded zone which will have a lower pitch due to reduced tension. The more weight you load onto the hub, the lower the pitch goes in those few spokes (the others actually increase very slightly in pitch, but most folks won't pick that up).

 

 

[Ox_Wagon]

More semantics: "with only the spokes directly under the hub decreasing their tension" <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

It does not state: "with only the spokes directly under the hub being in compression" As show in the FEM pic.

 

The area below the hub is the unloaded zone and not the loaded zone.  

[greatwhite]

More semantics: "with only the spokes directly under the hub decreasing their tension" <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

It does not state: "with only the spokes directly under the hub being in compression" As show in the FEM pic.

 

OX - Right you are' date=' if you ignore the preamble

 

The area below the hub is the unloaded zone and not the loaded zone.  <?:NAMESPACE PREFIX = O />

 

Right again - my fault for not qualifying my statement - I meant the load from road to rim. Although 'unloaded' refering to the spokes would certainly be more accurate given the context of this debate.

 

Now the main question is:

How much have we actually helped mbaymike who made the original post?

 

I suspect the simple answer is - give it to Steven from Coolheat and let his folks deal with it if it is in the warranty period. I suspect he'll still have to speak to Steven even if it isn't, because he'll need special spokes anyway.

 

[Ox_Wagon]

Agreed Greatwhite, shame on us . 

 

Back to the original problem.This bit . At my weight I would typically stay away from anything exotic like that.  I must say when I lost a number of my spoke on my current wheel set I ask my LBS to build them like tandem wheels. Since then I haven?t had any problems.<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Ox_Wagon2007-12-20 07:57:45

[Johan Bornman]

 

Now the main question is:

How much have we actually helped mbaymike who made the original post?

 

I suspect the simple answer is - give it to Steven from Coolheat and let his folks deal with it if it is in the warranty period. I suspect he'll still have to speak to Steven even if it isn't' date=' because he'll need special spokes anyway.

 

[/quote']

 

I think nothing.

 

This whole debate was a great disappearance act.

The OP disappeared after being asked to articulate his problem.

Steven disappeared after dropping a statement bomb and then running away. Perhaps he got lost in the wilderness?

Then, the mysterious Doug Patterson never appeared.

Now I just wish that buzzing fly would disappear.

 

Well, it has been fun at least. Thanks guys, I am going to carboload with some hops juice.

 

[Kiwi]

Tell you what's interesting about this, if you go to Campagnolo's site and down load any of their manuals, all of them carry a disclaimer which say something along the lines of: "If you weigh 82kg/180lbs or more you must be especially vigilant and have your bicycle inspected more frequently (than someone weighing less than 82kg/180lbs) for any evidence of cracks, deformation or other signs of fatigue or stress." This is the same whether its for ergo leavers or wheels.

 

Shimano don't suggest any weight limit for their wheels if you go through their tech documents for the WH 550 wheels (SI-0058A-001   EV-WH-R550-F-2336A   EV-WH-R550-R-2337A   SI-4BE0D) I use the WH 500 wheels for training and have finally broken a front spoke (radial, standard J-Bend spokes rather than the straight-pull ones in WH550s). I weigh 85kgs to 87kgs.

 

If those WH550s have never been trued (so not incorrectly tensioned) I would most definitely get Shimano to look at them because for the spoke to pull through the rim that would suggest overly high spoke tension.

[Ox_Wagon]

I that case I will remove my statement about rider weight.  

I personally think if one is a bit heavier (>80kg) it is better stick to a custom build wheels with higher spoke count (32-36).  I know some shops tend to recommend the DRC ST17 rims.  So far I have not picked up any problems with mine.  

 

At this moment I would not even consider a flashy light weight wheel set because it is stupid to me to spend R3000 plus on a wheel set to save 500g while I am 20kg plus overweight which I most probably will damage because I am too heavy.  <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

[Guest Big H]

You are all a bunch of "toordokters" ..... great pretenders who flash big words!!!!!!!!!

 

Now seriously........ who would you trust, the doctor who takes your pulse and says you have high blood pressure or the specialist who does an ECG to determine your condition........ the doctor who takes your pulse sounds very much like the twanging kind of wheelsmith!!!!!!!!

 

Ek wonner wie is die vlieg wat Bornan van praat....... kan enige van julle dalk raai!!!!!!......mmmmmmm!!!!!!

[Man with no name]

You are all a bunch of "toordokters" ..... great pretenders who flash big words!!!!!!!!!

 

Now seriously........ who would you trust' date=' the doctor who takes your pulse and says you have high blood pressure or the specialist who does an ECG to determine your condition........ the doctor who takes your pulse sounds very much like the twanging kind of wheelsmith!!!!!!!!

 

Ek wonner wie is die vlieg wat Bornan van praat....... kan enige van julle dalk raai!!!!!!......mmmmmmm!!!!!!

[/quote']

 

Mmmmm..... I would trust a qualified mechanical engineer who does high tech stuff like designing carbon fibre inlays for nuclear reactors and perform carbon fatigue tests in a proper laboratory for an opinion on anything to do with carbon and even stuff like spokes and metal fatigue.

 

Oh.... wait.... this sounds like one of my team mates!

[BMCfan]

 

Ek wonner wie is die vlieg wat Bornan van praat....... kan enige van julle dalk raai!!!!!!......mmmmmmm!!!!!!

 

 

en ek wonder of mike reggekom het met sy post op bladsy 1