Spoke Strentgh (My spokes are breaking like mad...)

[liveforphysics]

Rims and Tires Reference

Spokes:
Material sciences already provide methodology for understanding stress and strain. We have to consider the following properties…

• Hardness; resist wear and penetration.
• Malleability; ability to deform before rupture, effects manufacturing.
• Toughness; ability to resist high-impact (shock) loads.
• Ductility; ability to deform under tensile load before rupture, ability to elongate.
• Brittleness; tendency to fail with very little deformation.
• Elasticity; ability to return to the original shape after elongation and without memory, springiness.
• Plasticity; ability to deform nonelastically without rupturing .
• Stiffness; ability to resist deformation. Stiffness varies directly as its’ modulus of elasticity.
• Alloy; the blend of two or more materials, one being metal, and exhibiting metallic qualities.

The other spoke aspects we also need to consider are:

• Thermal Expansion (example: copper, aside from being ductile, would make for a poor spoke because of high thermal sensitivity, though not too bad for bi-metallic temperature gauges).
• Thermal Stress; geometries that change shape to thermal conditions, and affected by uneven changes over the specimen length.

I would like to believe you're pulling our leg when you write this sort of thing, but it's like you put so much effort into it that it's hard to believe you're not serious.

As far as picking a spoke material, the properties you would want to know for making a reliable wheel are just modulus and yield point. Modulus doesn't matter though, because we're using stainless steel, and it's just a couple points of being a constant for all steels.

That leaves yield point in tension.

When you're going to yield in narrow things and drawn things and forged things comes down to grain orientation and structure/temper, this makes any bulk materials property sheets useless for your decision.

Having a high yield point is important. If you just had a high ultimate strength but low yield point, it would be less likely to have a catastrophic failure, but it would come out of true more easily. We want a reliable wheel that we can just ride and ignore for years. That means a high material yield point in tension.

Every decent spoke mfg uses roughly equally excellent stainless alloys well suited to being a spoke, so it's kinda irrelevant to selection anyways.



What matters? The size not being too thick to stretch a good distance in tension when laced to a bicycle rim. The profile of the spoke having strength where you need it, and being supple everywhere else (the butting pattern it uses), and every mfg claims they have a process of forging the butted section that best orients the grain structure to increase that yield point as high as possible, and it's true, it does. Here is an example of a typical spoke mfg claim for the yield strength increase in the butted area of the spoke:

"Sapim butted spokes have two major benefits: less weight and more strength! The SCFT-system (Sapim Cold Forging Technology) ’stretch‘ the spoke while retaining the linear molecular structure of the material, thereby increasing the spoke strength at the middle by at least 48%.

As an added bonus, the thin middle section of the light Laser and Race spokes also provides better shock absorption. The Strong, with its extra material in the bend, fits more tightly into the hub hole making it ideally suited for tandems and other heavy duty applications."

Which brings us to the profile. This matters.

A non-butted spoke shouldn't exist, it's simply a bad design.

If both rim and hub holes are small, I would run a 14awg-15awg-14awg double butted spoke profile. (EG: Sapim) If the hub hole is massive, I would run a triple butted 13awg-15awg-14awg profile. (EG: DT Alpine 3).

That would be your criteria to make the decision on spoke size. Triple butted if your hub has monster holes, otherwise a double butted design.


That's how a sane person does it.

 

[johnrobholmes]

Rim's with tipped nipple holes would also be handy, as it lets you run more angle in your spoke, which allows more spoke length to fit, and more spoke length means more material distance to stretch in a spoke, as well as closer to tangent angles leaving the hub to decrease the rise in spoke tension relative to torque load from the hub, but those forces are quite low relative to the hitting a pot-hole forces, the big advantage comes from the additional spoke length providing more distance of spoke length to stretch over.

So longer spokes give more yield. We should really be trying to get some rims that would allow for 2x lacing then.


Anybody else willing to get a rim profile made?

 

[Chalo]

We should really be trying to get some rims that would allow for 2x lacing then.

Anybody else willing to get a rim profile made?

That would go a long way towards making hub motor wheels just as reliable as equivalently built normal bike wheels. Maybe more so, since they wouldn't be trashed by dropped chains.

It's not the rim cross-section that needs to change; it's the spoke holes. Ideally, they should be deeply dimpled into little hemispherical bumps before being drilled with their spoke holes oriented eyeball fashion in the direction the spokes exit. That would constrain us to just one lacing pattern, but we already know which one we would want.

I have a couple of new old stock Araya BMX rims, single-walled and not very big, which have dimples and oriented spoke holes just as I describe. They are kind of cute, but not really of any use for heavy duty bike wheels.

An alternative would be to drill the dimples with really huge holes, but back them with matching spherical washers that have holes just big enough for the nipples. That would be a little fiddlier to build up, but it could be laced in almost any pattern.

Every little bit helps, but spoke thickness can have a lot more effect on stretch than length does. The 15ga center of a butted 14-15ga spoke has less than half the cross-sectional area of a 12ga spoke. So for our purposes, that's the same as making it more than twice as long.

Chalo

 

[amberwolf]

What would be involved in making a "handheld" tool you could use to dimple an existing rim, around the existing spoke holes? I imagine it would involve a die on each side and either a handheld hydraulic or ratcheting crimper like some connector crimpers, or a die block on each side, a surface to put one block against that the rim can't touch so there's no rim deformation except where we want it, and a nice big hammer.

If I had or could make a die to do it I'd be tempted to experiment with this.

If I took some steel blocks and shaped them with a dremel or grinder, so they fit the rim profile of a particular rim, but with a pit on the inside one and a dome on the outside one, should this theoretically work?

To offset the existing holes in the direction needed, one would just offset the die blocks by that much in the opposite direction, right?

 

[liveforphysics]

Found a nice article that covers what I've been trying to explain in a much more elegant fashion. This is written by someone who understands the dynamics of a bicycle wheel, and can write in a clear elegant fashion, and a non-nerdy-word-choice fashion that I think a wider audience will be able to read and get a good understanding.


"Spokes are mostly made from steel. On very expensive wheels, you can get titanium, carbon fiber, or aluminum spokes, but steel is the standard choice for several reasons. First, steel is cheap. Second, it is strong and has good fatigue resistance. Third, it's easy to cut smooth, strong threads in it for the nipples.

So the main things you need to ask yourself is: what shape (profile), how many spokes, and what thickness? First, shape. Almost all spokes are round in cross section. If you want the wheels for time trialling or triathlon, where you'll be going very fast and aerodynamics are important, you can get ones that are a bit flattened in profile. However, if you get them too flattened, you won't fit them through the hole in the hub. You can get special hubs for this if you're desperate for very areo wheels.

Second, number. Low spoke count wheels are trendy nowadays. But let's face it: spokes really don't weigh all that much, and they are really what gives the wheel its strength. So you really gain very little by using few spokes. Consider, for example, Rolf wheels. They have very few spokes, but the rims have to be heavier in order to provide structure for the wheel in the large gap between spokes.

Still, if you're very light and ride only on the road you'll be putting less stress on your wheels than if you're heavy or ride off-road, so you can get away with fewer spokes. As a general guide: for road riding, 32 spokes make a good durable wheel, while if you're large, ride off-road, or go touring, 36 spokes is better. If you're light and want a light wheel, 28 spokes will do. 28 spokes is also fine for a racing wheel, where you're willing to trade off some strength for speed (reduced spoke count makes for reduced areo drag) and lightness. For off-road riding, the greater strength of MTB rims (their smaller diameter and wider profile) means that most people can get away with 32 spokes. Heavy people or people who want to carry a significant load off-road, dirt jumpers, or downhillers would be better off 36 spokes. Tandems generally need more than 36 spokes: 40 or 48 is the norm.

Third, thickness. Here, you not only have a choice of how thick you want the ends of the spoke to be, but also a choice of having spokes that are narrower in the middle. The spokes that are thinner in the middle are called double butted. Double butted spokes are definitely the way to go. They are slightly lighter than straight gauge spokes, but their main advantage is that they make a stronger wheel. The thinner middle section allows them to stretch a bit when the wheel is hit, spreading the load to neighboring spokes to help distribute the impact. This helps to prevents the elbow near the spokes head from flexing (which leads to fatigue and spoke breakage), and also takes the stress away from the rim wall near the nipples.

One of the most common myths about wheels is that double-butted spokes will make your wheels weaker. Where people get this idea from, I have no idea. Spokes almost never break in the middle, only at the ends (usually near the spoke head), so common sense should tell anyone that a wheel built from double-butted spokes will be at least as strong as one built from straight gauge spokes. In fact because of their greater elasticity, double-butted spokes result in a stronger wheel.

In general, the thicker the spoke is near the head, the stronger it will be. However since spokes mainly fail from fatigue (see above, under What makes a strong wheel?) if the wheel is built well, this does not make a big difference. So don't go thinking that to have strong wheels, you need to have super-thick spokes. In fact you'd be better off by having more thinner spokes to distribute the load better.

I tend to use 14/15/14 gauge (2mm/1.8mm/2mm) DT stainless steel spokes in all my wheels, mainly because that's what the local bike shop stocks. I wouldn't hesitate to use 15/16/15 gauge (1.8mm/1.6mm/1.8mm) spokes if they were as easy to come by. You can also get spokes that are drastically thinner in the middle than they are at the edges, such as DT's Revolution spokes, which are 14/17/14 or 15/17/15 gauge. I wouldn't use these because it's hard to prevent spoke windup (twisting of spokes) with these spokes as they have so little torsional rigidity. However, if you're getting the wheels built by someone else, use them by all means as long as you don't mind the extra cost."

 

[amberwolf]

Not to distract from the discussion, but the Fiberfix was mentioned before, and I thought anyone interested in them might like a read of this page describing their use:
http://www.peterwhitecycles.com/fiberfix.htm

 

[Ykick]

I started a thread about spinergy fiber spokes a while back:

http://www.spinergy.com/catalog/tech_special_feature_v2.php

They look flexible enough to tie in a knot but also heard they don't really stretch much and can break stuff.

JRH is right, the industry need a purpose built rim to start with. Something that can properly accommodate large hub flanges. Currently, we just use the best current thing usually intended for another purpose.

 

[GCinDC]

An alternative would be to drill the dimples with really huge holes, but back them with matching spherical washers that have holes just big enough for the nipples. That would be a little fiddlier to build up, but it could be laced in almost any pattern.

And/Or: How about using washers that can be compressed..

 

[liveforphysics]

I like the larger hole and spherical or belleville washer idea. Spreads the force of nipple pulling out of the rim over a much larger area, kinda like the function of an eyelet, only a bit larger. The downside is the larger hole in the rim would weaken it a bit, but in a wide rim it should hardly have an impact at all.

Or a belleville washer, it features a cone profile that should be friendly towards tipping some offset in the rim spoke hole, and as a bonus it's made of very tough spring steel. It would be getting loaded backwards of how it's normally loaded if you used one alone.

Just got an idea. You could stick a small belleville washer (or two, they can be stacked nested to double spring constant, or facing each other to double displacement distance) on the back of a spherical washer (to provide the angle), and the velleville stack could offer some additional linear movement on each spoke, perhaps 2mm per spoke from the belleville washer stack in addition to whatever linear movement you can gain from the spoke stretch (which isn't much with the very short spokes). This could perhaps make a short-spoked hubmotor wheel capable of distributing an impact load over a very wide group of spokes, just like a normal long-spoked bicycle wheel.


This could be the long-term spoke reliability solution for big diameter hubmotors laced up with short spokes.

 

[nicobie]

I like the larger hole and spherical or belleville washer idea.

This could be the long-term spoke reliability solution for big diameter hubmotors laced up with short spokes.

I think you are on to something here. If I can find the right size, one or maybe two of these in each rim would solve my problem getting spokes in these 18" alloy moped rims. My excellent wheel guy can only do 14g spokes

They have a 11mm recess with a 5.7mm hole.

Thanks Luke.

EDIT: Bingo!

http://www.centuryspring.com/Store/item_detail.php?StockNumber=CDM-124206

 

[liveforphysics]

Looks like this site offers them with 10mm OD, and either 3.2mm ID or 4.2mm ID or 5.2mm ID.

http://www.springmasters.com/disc-springs/din-2093-1.html

Looks like you could get one with a 4.2mm ID, that would be around 1000N average between 50% deflection and totally flattened out. That's only 224lbf though... And a quality 14awg spoke can handle >700lbf, so I would nest 3 of them in a little stack under each nipple. This way the belleville stack would be bottoming out right about as the spoke would be near breaking, so it seems like this would give you a pretty matched setup, and should do an excellent job at letting a short spoked hubmotor wheel have enough compliance to distribute stresses to a wider group of spokes to share handling impact loads.

I think we might be onto something here.

 

[liveforphysics]

I just measured my 14awg brass spoke nipples on deathbike, the fattest point on the shank you would be passing through the ID of the belleville washer is 4.05mm, so it looks like the belleville's with the 4.2mm ID would be perfect.

 

[liveforphysics]

EDIT: Bingo!

http://www.centuryspring.com/Store/item_detail.php?StockNumber=CDM-124206

Those could be perfect, looks like you will want 3 of them per nipple to get the force right.

Rather than paying the high onesy twosy price, I bet you could find a bag of 1000 of them for cheap, and then sell them to the rest of us.

 

[nicobie]

Good idea, I'll have to look around for a bulk supplier as we would need at least 108/wheel.

Think that 11mm recess will be big enough for the 10mm washer when 75+% deformed? The source you found had 8mm ones listed.

 

[Chalo]

EDIT: Bingo!

http://www.centuryspring.com/Store/item_detail.php?StockNumber=CDM-124206

Those could be perfect, looks like you will want 3 of them per nipple to get the force right.

Rather than paying the high onesy twosy price, I bet you could find a bag of 1000 of them for cheap, and then sell them to the rest of us.

I gotta say-- it's such a good idea, I wish I'd thought of it myself.

I do wonder whether they'll behave predictably when sitting on a concave seat. If they get loaded above 100% deflection without support underneath, it seems like there would be some chance they'd pop inside out.

Chalo

 

[John in CR]

EDIT: Bingo!

http://www.centuryspring.com/Store/item_detail.php?StockNumber=CDM-124206

Those could be perfect, looks like you will want 3 of them per nipple to get the force right.

Rather than paying the high onesy twosy price, I bet you could find a bag of 1000 of them for cheap, and then sell them to the rest of us.

I gotta say-- it's such a good idea, I wish I'd thought of it myself.

I do wonder whether they'll behave predictably when sitting on a concave seat. If they get loaded above 100% deflection without support underneath, it seems like there would be some chance they'd pop inside out.

Chalo

These are the kind of gems that make ES so great. I learned good stuff about spokes, and then blam a unique idea that will give use better stronger hubbie wheels.

John

 

[GCinDC]

i've got giant dimples in my M/C wheel and use two washers there (to hold 12g spokes in 8mm holes), so i was wondering if they helped w/ spoke tension, then i thought, hey reading chalo's i thought hey

what i like is the irony of it considering all the discussion of compression just earlier in the thread. :lol:

 

[liveforphysics]

what i like is the irony of it considering all the discussion of compression just earlier in the thread. :lol:

Spooked wheels are complex, and a bit counter intuitive.

Anything complex and counter intuitive is going to have someone who doesn't get it giving backwards advice.

Do to the lack of recent responses, I think he might have grasped it. Hoping at least.

 

[neptronix]

Spooked wheels are not only counter-intuitive, but they're also really scary.

 

[betarambo]

I think you guys are really onto something, particularly for the build I am doing. I really want to mount a 14" motorcycle rim to my MP3. The MP3 is 12" outside diameter and the rim is 13" inner diameter. To make it work The spokes will be about an inch long (longer than the gap because they get some angle from the motorcycle rim dimples) and pretty much threaded right to the bend. After reading all of this and thinking about trying to stretch the 1/4" of spoke that is left to some tension I was thinking this idea is dead in the water yet again. The other issue is that I was planning on bicycle guage spokes which means the nipples fall right through the holes in the rim.

Now, armed with your new idea, maybe I can kill two birds with one stone. If I can spec out some of these spring washers with the right ID and OD maybe I can get all my tension on the spoke from COMPRESSION on the spring and get a washer to keep the nipple in the rim at the same time. Maybe.

They are relatively cheap so maybe I can order some to play around with. In the meantime, I am still moving forward with my backup plan of running a creepy cralwer 20X2.5 on a 19" bike trials rim. Even with that, the spokes will be pretty short so this new idea might be pretty handy.

Thanks to all!

To add another thought to the general conversation, I have seen wheels on trials motorcycles that could see some real compression in the spokes. Montesa and Beta both have had some years where the back rim has a rib so that it can use two short spokes connected by a turnbuckle in the middle. Check out the picture at the link below to see what I mean. I liked these wheels because trials bikes run tubeless tires and no spoke holes means no air leaks.

http://www.google.com/imgres?hl=en&...&w=630&h=831&ei=CLbaT53uAYnE2gWDh4SMBg&zoom=1

 

[betarambo]

I strung up my MP3 to a 14" motorcycle wheel. I think this might be the shortest spokes anyone has done on a hub motor. They are under 1" long. I used Belleville washers as postulated here. I stacked 4 of them, 1 facing concave to fill the space for the larger motorcycle nipples and then three facing the other way in order to get the spring tension recomended above. The wheel is 32 spoke so I drilled the hub motor to fit. I took it to a great local bicycle shop (http://corralesbikeshop.com/ in New Mexico) to get it trued and it trued and tensioned up really well. The guy at the shop thought it wouldn't true but later he felt like the Belleville washers allowed us to get tension we would not otherwise have gotten. I rode it today for the first time once I solved the issue of getting a valve stem out through the super narrow opening. The wheel held fine and everything was great until I popped the internal MP3 controller by putting 48V into one of the hall inputs to the controller from the throttle. I will post later when I get it running again to let you know how the wheel holds up. Any tips on troubleshooting my controller are appreciated.
Wheel with tire:

Close up of short spokes:

 

[Chalo]

I strung up my MP3 to a 14" motorcycle wheel. I think this might be the shortest spokes anyone has done on a hub motor. They are under 1" long.

Well done.

How did you get 1" spokes? The shortest anyone has been able to cut for me has been about 65mm.

Chalo

 

[hjns]

Nice work.

However, it makes you wonder what the tensile strength is of such short spokes. But for the issue at hand, this was probably the easiest way forward.

 

[Farfle]

Lol, after going thru 10 spare spoke on the dual pie, this is rhe conclusion I came to:

 

[Chalo]

Nice work.

However, it makes you wonder what the tensile strength is of such short spokes. But for the issue at hand, this was probably the easiest way forward.

Same as any other spokes of equal gauge.

The short spokes lack only elasticity, which is why he used Bellville springs as nipple seats.

Chalo