Need some help on spokes

[Ox_Wagon]

Edman, I fully agree with you.  The other problems with all models are if you don?t set up the model as close as possible to reality all that you get, as you said previously, is a nice wallpapers.  <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Some comments and questions on your constraints:

 

1) What do you mean by ?The spokes are constrained to have zero displacements, but free rotation at the hub?. It sound like you are fixing the hub and that no torsion load can be transferred to the hub by the spokes. If so it sounds good.

2)?They are rigidly connected to the rim.? This unfortunately does not reflect reality. I know you did not agree with my constraint but we can discus it.  Check the campy pic and do the rim and spoke test.  You could get away with it if you apply pre-tension to the spokes and insure that the pre-tension is high enough to insure that even after applying the 100N force to that rim that all the spokes are still in tension.  According to Mister Bornman?s experiment and your reverence, a pre-tension that gives you a top spoke tension that stays the same before and after the force is applied should do the job.  

3) ?There is no pretension applied?. This plus the rigidly connection of the spokes to the rim is the thing that is causing most of the problems.  It means that the moment you have ANY deflection on the rim the spokes would be in compression.  This is shown in your pic  (2500x)

 

4) ?A 100N force is applied to the rim between the bottom three spokes?.  I assume that you fixing the hub displacement as mentioned in your first point.

 

5) ?it does not model buckling?.  The net guy also used this assumption.  IF your spokes are always in tension this is a valid assumption.  

Have a think about it. I looking forward to your comment on my comments.

 

PS: Enjoy the festive season.  I am almost out of here.

Ox_Wagon2007-12-21 03:58:41

[Minion]

 

1) What do you mean by ?The spokes are constrained to have zero displacements' date=' but free rotation at the hub?.[/quote']

I reread my wording on this and it is a bit ambiguous - I apologise. The hub end of each spoke is constrained to behave rather like a ball-and-socket joint. It cannot displace from its position, but it can rotate in place.

2)?They are rigidly connected to the rim.? This unfortunately does not reflect reality. I know you did not agree with my constraint but we can discus it.

If you can assume that the spoke is always in tension (i.e. any compressive load in the spoke does not exceed the pretension)' date=' then the rigid connection assumption becomes valid (also see next question).

If I was running this as a proper model with known loading (e.g. a failure analysis), I would run this simple model first. If I saw that some of the spokes were exceeding the pretension and going into compression, I would have to consider doing the more complex model with proper contact. Until then, the simple model will suffice.

3) ?There is no pretension applied?. This plus the rigidly connection of the spokes to the rim is the thing that is causing most of the problems.  It means that the moment you have ANY deflection on the rim the spokes would be in compression.

This is where superposition comes in. It says that, provided my models is:

a.) Linear i.e. the response (displacement) is directly proportional to the load and,

b.) The deflections do not significantly affect the model's shape,

I can get the response to multiple loads by adding the responses of each of the loads applied individually.

I can get actual spoke tension by taking the tensions in the model shown and adding them to the values of the pretensions.

 

This implies that any compression shown in the first model should be interpreted as a reduction in overall tension rather than a pure compression. (So long as it doesn't exceed the pretension, in which case the the spoke separates from the rim and assumption a.) is violated)

 

4) ?A 100N force is applied to the rim between the bottom three spokes?.  I assume that you fixing the hub displacement as mentioned in your first point.

True

 

5) ?it does not model buckling?. IF your spokes are always in tension this is a valid assumption.

As I said in response to 2 and 3' date=' provided the 'compression' shown does not exceed the pretension then it'll be OK. Once it does, buckling's not a problem since a spoke cannot support support pure compression, as you have stated.

Enjoy the festive season.

 

You too.

I'm now on leave for two weeks! http://www.freesmileys.org/smileys/ad/barmy.gif

 

[Minion]

 

And for my last engineering trick of the year , what the FBD for you case should look like:

 

L = spoke length

A = spoke diameter

E = spoke Young?s modulus (200GPa for steel)

k = spoke equivalent stiffness

T = tension

P = pretension

W = rider weight

x = hub deflection

 

So we get:

 

and bottom tension is:

 

Combining the two gives:

for equal spokes and pretensions

 

Similarly for the top tension:

 

The load is thus shared equally between the spokes of the magic two spoke wheel with completely rigid rim.

 

Edman2007-12-21 08:44:06

[Johan Bornman]

 

I'm learning here (to understand why the wheels that my LBS true/build don?t last), so I got to ask this:<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Why would/should my LBS shop fit spokes that have play in the first place? The washers might be only a fix for something that was done wrong in first place. Shouldn?t there be standard dimensions/tolerances that should be adhered to?

[/quote']

 

They do this because they still have stock of the huge batch of DT-Swiss spokes in the system. DT never admitted fault and thus never recalled them. They simply reverted back to (almost) the original length. Alternatively they are just naive and believe what they read in bike magazines.

 

As for standards, there are none that I know of. Any hub manufacturer can make his flanges as thick or thin as he likes. However, Shimano and Campag lead the pack here and their flange width is basically the industry standard. The problem came in when DT-Swiss took over Hugi, which had the thicker hubs. DT then thought it could dicate to the industry what the flange thickness will be.

 

Shimano and Campag (and the very important Joytech) didn't budge. In the US, Wheelsmith capitalised on this mistake and got a big foot in the door. Many of the big professional wheelbuilders where so miffed, they stopped buying DT and switched to Wheelsmith and SAPIM. The Americans being as patriotic as they are, seemed to favour the American Wheelsmith company.

 

Should I accept the practice as "standard" or can I go back to my LBS and tell them to fix when I spot that they used washers on my wheel. (Currently my LBS don?t use washers but their wheels still don?t last)

[/quote']

 

I don't even think many of the bike shops are aware of the problem. I have no doubt they are happily mixing long and short-elbow spokes and don't understand why they're getting poor results. Some of them are so desperate with their poor success at wheelbuilding that they try not to take on any wheelbuilding and just sell botique wheels.

 

To answer your question, I just don't know what you should do. On the one had, the poor builders are lost in the wilderness because the DT agent never communicated the problem and they'll try anything to attempt to get a good wheel to the customer. Remember, they just as much as you don't want you to return with broken spokes and other problems.

 

I'll say, if your wheel is built with washers and it doesn't break spokes, leave it. You can build a good wheel with washers, it's just not necessary or pretty, for that matter.

 

If your bike shop builds wheels that don't last, it is not the washers' fault. They should simply learn how to do it. Being biased, I would suggest they do a formal course. The alternative to a formal course is a mentor. I haven't come across one that understands the theory, which in my opinion is essential for understanding wheels.

 

[Johan Bornman]

Funny story....we had tandem wheel problems' date=' breaking spokes everytime we ride. Had the wheels rebuild by the same guy that build the first time, using the original spokes. But this time we also used the infamous DT washers. Two years down the line we did not break one spoke or trued the wheels once.[/quote']

 

You can build good or bad wheels with washers. You can build good or bad wheels without washers.

 

My guess is that it isn't the washers that made the difference, but the build and stress relieve.

 

If one or two spokes in a wheel break from fatigue, the others are also fatigued and will break sooner rather than later. It is not a good idea to only replace broken spokes - unless the builder doesn't understand stress relieving and you'll be back there every few weeks in anyway, whether you have new or old spokes.

 

Spokes break from fatigue, not age. You don't say how much you rode in those two years. But if the wheels work, they work. Don't fiddle with them.

 

[Johan Bornman]

Ok' date=' with the Spoke Washer Anonymous (SWA) group session in full swing I have to add that I haven?t had any problems with the wheels again.  They previously had sapim dubble butted spokes which is as far as I know also very good spokes.

 [/quote']

 

DT-Swiss, SAPIM, Wheelsmith and WTB all make good spokes. It is the build quality that makes the difference between them breaking or lasting.

 

The impression that I got from the LBS was that the washer' date=' being very soft, would deform and give you a better fit between underside of the spoke head and side of the hub flange that isn?t properly aligned.  Mister Bornman?s explanation about the washer being used to correct too long J-bends also makes a lot of sense.

  

[/quote']

 

Well, I hope the techie was just trying to spare you a technical conversation. Brass, ironically is much harder than aluminium and doesn't deform nearly as much as aluminium.

 

Although spoke heads do break off, this is not the normal mode of spoke failure. They normally fail in the j-bend. starting with a crack on the outside which then propagates inwards. The last little bit just snaps off in a tensile break and leaves a sharp little lip. It is the continuous flex at the J-bend that eventually fatigues the spoke.

 

Here's a photo I took of a straight-gauge spoke that broke in the normal fashion. Have a look at the direction of the crack indicated by the countours in the crack zone. Note the last little lip that stretched and snapped - the only "snap" in the break, the rest was just a crack.

 

Here is a photo I had a metallurgist friend take of a spoke that snapped in a tensile break when a jockey went into the rear wheel. Spokes very seldom break like his as only blunt trauma causes it. Interestingly, it didn't pull out of the rim, which was sill perfectly usable.

 

 

 

The other end of the spoke would have been more cone-shaped to fit inside this cupped side.

 

 

[Guest Big H]

Johan, wanneer gaan jy jou Kersinkopies dioen???????

[campyonly]

Just a question - edman, oxwagon, johan - how many wheels have each of you actually built, and what was the spoke failure rate?

 

[Minion]

 

Just a question - edman' date=' oxwagon, johan - how many wheels have each of you actually built, and what was the spoke failure rate?

 

[/quote']

I have built no wheels. I'm working solely from a theoretical standpoint, coupled with my day-to-day experience from designing and modelling various furnace structures, cooling systems and other ancillaries. So far none of those have failed in the course of normal operation

 

[Guest Big H]

kempekker-only you have now opened a festering wound just before Christmas. This is enuff to keep all the boffins tearing at each others throats for the next few days. This development will be interesting to watch and the assflies will also have a field day sniping at the so called self proclaimed "experts" !!!!!! teeee heeeee I like!!!!!!!

[Johan Bornman]

Just a question - edman' date=' oxwagon, johan - how many wheels have each of you actually built, and what was the spoke failure rate?
[/quote']

 

You should understand that wheelbuilding is not one of those things, like drawing a perfect still-life, that takes practice. As you build more wheels you don't get better, you get faster. If you follow the rules and understand the theory of how spokes support wheels, how metal fatigues, how to stress-relieve, how to identify a good or bad rim, how to measure components, which hubs to avoid etc, your very first wheel will be a perfect wheel.

 

My very first wheels are still in use fifteen years later. I had to replace the rim on the front wheel about three or four times because I wore out the braking surface. It's never broken any spokes, front or back. I can't estimate the kilometers ridden on those wheels as I never kept a record. It was built with Mavic MA-2s and I still have a small stock of 32-hole MA-2s in my garage. I may unfortunately have to retire them soon, since 7-speed screw-on clusters are extinct in SA (but alive and well in the US and Europe).

 

 

My mountainbike wheels were build ten years ago and I've replaced the eight outbound spokes on the right hand side in the rear wheel. I damaged them in a deraileur accident when the chain whent into the spokes and shaved half the metal off each one.

 

Also, you don't need any fancy equipment to build a wheel. I still have my first spoke spanner - a Spokey, that's now extinct in SA. My first set were also built without a wheelstand, bridge or tensiometer. I built them on an upside-down bike.

 

How many have I done? I don't know, but since I started to serialise them five years ago (to get rid of chancers wanting free repairs), I've done about 800.

 

How may spokes have broken? I have a pretty good idea of those, since my wheels (other than certain carbon rims that are not strong enough to stress relieve) are all gauranteed for life and I like to think people will come back for a free fix rather than go and pay for it somewhere else.

 

Two broke from metal fatigue and plenty from accidents. Accidents are accidents and those are not wheelbuilding issues. The two that matter broke off at the nipples from a wheel I build where I went 3X instead of 2X simply because the lenght for 2X wasn't available at the time. The 3X configuration on a 28-spoke wheel on a large flange hub causes the spokes to arrive at the rim at an angle, which puts stress on the threads. Getting the right spoke length and going 2X solved the problem.

 

Some of my wheels are now 15 years old but sadly, I suspect not many are still in use. Wheels have become a fashion business and people don't want to be seen with the fashion equivalent of the safari suit. They want boutique wheels.

 

I have taught about 120 people to build wheels and most of those are enthusiasts who don't want to do it for a profession, but find the concept intriguing and usually just end up building one or two or three sets of wheels. 99% of them build a perfect first set of wheels that never break spokes.

 

 

 

Johan Bornman2007-12-22 04:20:49

[Minion]

 

kempekker-only you have now opened a festering wound just before Christmas. This is enuff to keep all the boffins tearing at each others throats for the next few days. This development will be interesting to watch and the assflies will also have a field day sniping at the so called self proclaimed "experts" !!!!!! teeee heeeee I like!!!!!!!

 

*Edman gnashes his teeth and, with a rabid look in his eyes, launches himself at the throat of the nearest expert *

 

[cling]

Edman! Don't let him get to you!

 

[steven]

Sorry guys we wont be able to load all spoke tensions on the Hub as  this info is loaded on our server at the office. We will next year offer you a Hyperlink onto our website giving you all the technical info you will need to build a proper wheel with a tensionmetre or you will get stuck in the middle of a wilderness without it.

 

Merry Xmas and keep the wheels turning

 

sm www.coolheat.co.za

 

 

[Guest Big H]

Edman! Don't let him get to you!

 

 

cling.......... Do you really think Mr Bornman got to Edman, I think he gave him a real good run for his munny!!!!!! and kept him away from doing his Christmas shoppimh...............teee  heeeee !!!!!!

[Minion]

 

 

cling.......... Do you really think Mr Bornman got to Edman' date=' I think he gave him a real good run for his munny!!!!!! and kept him away from doing his Christmas shoppimh...............teee  heeeee !!!!!!

[/quote']

 

Anything that keeps me out of shopping malls at Christmas is the greatest favour anyone can do. 10min in one of those places, fighting through the people and I really DO start to get angry