Options for spoke replacement

[annirak]

The spokes on my rear wheel keep breaking. In the first 3 months I had the bike, I broke 4 spokes and got them repaired. When 2 more spokes broke, I had the wheel rebuilt; they used heavy duty stainless steel spokes. 8 months later, I had 6 more broken spokes. Every one of these spokes was broken at the hub side. This time, the LBS rebuilt the wheel with washers at the hub, saying that this would reduce the strain on the spoke heads. I suspect that part of the problem is how the holes on the hub were bored/smoothed.

I have a Nuvinci N330 hubgear, which may be the source of the problems, if the spoke holes have a sharp edge. However, there's a lot of weight applied to that hub: I weigh 75kg, I ride with panniers, and I pull a Weehoo igo Venture, which weighs 13kg and my kid weighs roughly 18kg. Based on the placement of the rear wheel and the child, less than half of (trailer + child = 31kg) is applied as downforce on the rear wheel.

If the washers do the trick, and I don't need another wheel rebuild for another year, then everything is fine. If I need another wheel rebuild within a year, I want to have a plan for how I'm going to fix this more permanently.

As mentioned above, the LBS has put in heavy duty stainless steel spokes and they've used washers at the spoke heads to try and reduce the breakage at the spoke heads. Are there other materials I should consider, rather than stainless steel? Should I be looking for more flexible spokes? What else can I try?

 

[MichaelW2]

What type of heavy duty spoke are you using?
The spoke holes can apparently accept 13g so a 13/14 butted spoke is probably the best size. You also need the rest of the spoking recepie to be good, ie cross pattern, length, tension.

 

[Ajax Bay]

Please share how many spokes and numbers of crosses. The load you've described is reasonably close to the median that the wheel will have been designed for. Did these "6 more broken spokes" give up one at a time?

 

[Yellow Saddle]

The spokes on my rear wheel keep breaking. In the first 3 months I had the bike, I broke 4 spokes and got them repaired. When 2 more spokes broke, I had the wheel rebuilt; they used heavy duty stainless steel spokes. 8 months later, I had 6 more broken spokes. Every one of these spokes was broken at the hub side. This time, the LBS rebuilt the wheel with washers at the hub, saying that this would reduce the strain on the spoke heads. I suspect that part of the problem is how the holes on the hub were bored/smoothed.

The concept of a heavy-duty spoke is a fallacy. Spokes don't break because they are too "light duty" or thin, but because of metal fatigue. Spokes made thinner in certain places and left at original thickness (2mm) at other places are far more durable than thick spokes. If your wheelbuilder suggested anything heavy duty, he/she is ignorant about wheel building. More broken spokes within a short time (presumably short distance, since time doesn't matter), shows that it was a botch job.

I have a Nuvinci N330 hubgear, which may be the source of the problems, if the spoke holes have a sharp edge.

Yes, that causes problems. Not because of the edge cutting the spoke but because it forces a sharp bend into the spoke that adds stress to the outside of the bend.

However, there's a lot of weight applied to that hub: I weigh 75kg, I ride with panniers, and I pull a Weehoo igo Venture, which weighs 13kg and my kid weighs roughly 18kg. Based on the placement of the rear wheel and the child, less than half of (trailer + child = 31kg) is applied as downforce on the rear wheel.

All of that can be overcome with double-butted spokes and stress-relieving the wheel as well as spoke line correction. Weight per se is not a problem.

If the washers do the trick, and I don't need another wheel rebuild for another year, then everything is fine. If I need another wheel rebuild within a year, I want to have a plan for how I'm going to fix this more permanently.

Washers occasionally serve a purpose in wheelbuilding, but without knowing what the hub flange is made of (alu, steel?) and how thick it is, no-one can say whether washers are good or no-good in this scenario. A photo will help.

As mentioned above, the LBS has put in heavy duty stainless steel spokes and they've used washers at the spoke heads to try and reduce the breakage at the spoke heads. Are there other materials I should consider, rather than stainless steel? Should I be looking for more flexible spokes? What else can I try?

As mentioned above, the wheelbuilder is naive. I bet the spokes didn't even break at the heads, but at the elbows. Heads never break off unless they are straight-pull spokes.

Show us some photos, see if you can measure the flange width, show a picture of a spoke hole and we'll come forward with suggestions. Any suggestions until we've established the base facts will be mere speculation.

 

[annirak]

What type of heavy duty spoke are you using?
The spoke holes can apparently accept 13g so a 13/14 butted spoke is probably the best size. You also need the rest of the spoking recepie to be good, ie cross pattern, length, tension.

I don't know what sort of spokes they were. The LBS didn't specify a brand name or model name. I'll have to find some calipers tonight and measure the endpoints and middle.

It's 32-spoke, 2-cross, but bear in mind that the flange diameter is 125mm, which would make 3-cross difficult. The manufacturer's documentation recommends 2-cross.

Please share how many spokes and numbers of crosses. The load you've described is reasonably close to the median that the wheel will have been designed for. Did these "6 more broken spokes" give up one at a time?

I don't know when the 6 spokes went. The hub is very wide: the right and left flange offsets are both 26mm. Because of this, the wheel was still close to true even with 6 broken spokes. Because it's a disc brake hub, the loss of true didn't cause any noticeable rub. The spokes stayed very close to their original positions because they broke at the "J" which meant that there wasn't anything obviously wrong with the wheel at a glance. What tipped me off was a loss of stability in the rear of the bike. I'm now checking for broken spokes regularly.

The concept of a heavy-duty spoke is a fallacy. Spokes don't break because they are too "light duty" or thin, but because of metal fatigue.

Yup. I may have read too much into what they said. I assumed that what they meant was that they would use spokes with a higher fatigue limit and/or higher tensile strength. I assumed that they knew that this is what was required, but were translating into "layman's terms" in order to communicate the concept to me without the nuance of expertise in the field.

Spokes made thinner in certain places and left at original thickness (2mm) at other places are far more durable than thick spokes. If your wheelbuilder suggested anything heavy duty, he/she is ignorant about wheel building. More broken spokes within a short time (presumably short distance, since time doesn't matter), shows that it was a botch job.

The original replacement of several spokes was on a factory-built wheel, not built by them. No spokes were replaced between the first rebuild and the second.

Yes, that causes problems. Not because of the edge cutting the spoke but because it forces a sharp bend into the spoke that adds stress to the outside of the bend.

Indeed, and I assume that this is why I'm getting failures. What I really need is recommendations on how to avoid this if the washer-based-fix fails.

All of that can be overcome with double-butted spokes and stress-relieving the wheel as well as spoke line correction. Weight per se is not a problem.

Do I understand correctly that double-butted spokes can be manufactured to overcome the introduction of the stress point in the J-bend when there is a sharp edge on the flange?

Washers occasionally serve a purpose in wheelbuilding, but without knowing what the hub flange is made of (alu, steel?) and how thick it is, no-one can say whether washers are good or no-good in this scenario. A photo will help.

I'll have to get the photo added later today. I couldn't find specifications as to whether the flanges are steel or alu, nor of the thickness of the flange. I need to find those calipers.

As mentioned above, the wheelbuilder is naive. I bet the spokes didn't even break at the heads, but at the elbows. Heads never break off unless they are straight-pull spokes.

The wheelbuilder may be naïve. They may also just be trying to communicate. They didn't say that the heads broke off. Those are my words. The point I was trying to make is that all the spokes broke at the hub-end, and all in the same way.

Show us some photos, see if you can measure the flange width, show a picture of a spoke hole and we'll come forward with suggestions. Any suggestions until we've established the base facts will be mere speculation.

I'm not going to unlace a spoke for this purpose. The wheel is currently fine. It's been fine for a few months. I'd meant to do some research on what to do when the spokes next broke when I had to get the wheel rebuilt the first time. The cost of not doing that research in advance was that I still didn't know what to ask for when the wheel had to be rebuilt a second time. Next time, I'll be ready. Provided that the washers aren't the answer.

Photos will follow in a little while.

 

[Yellow Saddle]

Yup. I may have read too much into what they said. I assumed that what they meant was that they would use spokes with a higher fatigue limit and/or higher tensile strength. I assumed that they knew that this is what was required, but were translating into "layman's terms" in order to communicate the concept to me without the nuance of expertise in the field.

Spokes don't break in tension, they break from fatigue. Therefore, tensile strength is just about irrelevant.

The original replacement of several spokes was on a factory-built wheel, not built by them. No spokes were replaced between the first rebuild and the second.

Indeed, and I assume that this is why I'm getting failures. What I really need is recommendations on how to avoid this if the washer-based-fix fails.

Plenty of factories don't have a clue about wheels and don't employ engineers. That's such a pity really, since the science is well documented.

Do I understand correctly that double-butted spokes can be manufactured to overcome the introduction of the stress point in the J-bend when there is a sharp edge on the flange?

No. They can't be manufactured to overcome poor hub design. A good hub has flanges from aluminium (has to be softer than the spoke material), spokes holes which are chamfered on both sides and a flange thickness of approximately 3mm. If the flanges are too thin, the J-bend forms a cantilever where it exits the flange and causes excessive flex which leads to fatigue cracks. To overcome a thin flange you can add washers to reduce the effect but the right way to do it is to stress-relieve (tension beyond its elastic limit) the spoke so that the J in the bend becomes more like a Z. That eliminates the cantilever.

I'll have to get the photo added later today. I couldn't find specifications as to whether the flanges are steel or alu, nor of the thickness of the flange. I need to find those calipers.

Magnet.

The wheelbuilder may be naïve. They may also just be trying to communicate. They didn't say that the heads broke off. Those are my words. The point I was trying to make is that all the spokes broke at the hub-end, and all in the same way.

Fair enough, but I won't let him off the hook. I prefer straight talk, even if it is to a customer. Even difficult topics can be explained in layman's terms if you know your subject well enough.

I'm not going to unlace a spoke for this purpose.

Chicken.

 

[PatrickPending]

In the past when I've had spokes go on a rear wheel - machine built - I've had a competent wheelbuilder rebuild the wheel with quality stainless steel double butted spokes. Always worked for me, you'll spend a lot more money replacing them as and when they break.....

 

[annirak]

Spokes don't break in tension, they break from fatigue. Therefore, tensile strength is just about irrelevant.

Provided it exceeds the minimum required to sustain max-load. For example, they should be strong enough to sustain dropping off a kerb with panniers & trailer, not that I do that, but it seems like a sane expected minimum tensile strength for the spokes.

No. They can't be manufactured to overcome poor hub design.

I didn't think so. However, my hub is non-negotiable. I'd rather sort out the spokes than replace my hub. It's a CVT and I really enjoy riding with it. I'd chamfer the spoke holes myself before I replace the hub.

Magnet.

Irrelevant. Many stainless steels are non-magnetic.

Chicken.

Probably. Also, I don't have a spoke wrench. Remember, I don't currently have broken spokes. I've just broken a lot of them, so I want to be ready for next time.

I tracked down my calipers. I'll measure up the spokes and flanges tonight.

 

[annirak]

Here are some photos of the hub.

 

[Yellow Saddle]

Provided it exceeds the minimum required to sustain max-load. For example, they should be strong enough to sustain dropping off a kerb with panniers & trailer, not that I do that, but it seems like a sane expected minimum tensile strength for the spokes.

No, you have the wrong idea of how spokes support a load. Jumping off a kerb reduces tension, not increase tension. I suggest you read the sticky at the top of this Mechanics and Repairs section for an understanding of how spokes work. Even the thinnest commercially-available spokes, which are 1.5mm, have more tensile strength than a nipple can tighten to or a rim can withstand without buckling. Tensile strength is NOT the issue, durability is.

Irrelevant. Many stainless steels are non-magnetic.

Try it, I doubt the hub is made from stainless steel. These things are forged or CNC'd and stainless doesn't cut the grade there.

Probably. Also, I don't have a spoke wrench. Remember, I don't currently have broken spokes. I've just broken a lot of them, so I want to be ready for next time.

Hmmm,

I tracked down my calipers. I'll measure up the spokes and flanges tonight.

 

[annirak]

The spokes are 2mm straight spokes. The hub flange is 4mm thick at the spoke hole, tapering to 3mm at the edge. The hub flange is non-magnetic.

 

[Yellow Saddle]

The spokes are 2mm straight spokes. The hub flange is 4mm thick at the spoke hole, tapering to 3mm at the edge. The hub flange is non-magnetic.

OK, your problem is apparent then. Straight-gauge spokes are never a good idea except for supermarket bikes which need to be as cheap as possible. Not only are straight-gauge spokes cheap, they are also quick to build with and can be used in automated building.
Secondly, the bend where the spoke comes out of the flange will be compromised by the over-thick flange. There is nothing you can do about either issue now, but next time, take the trouble to champher the flanges and fit double-butted spokes and stress relieve the wheel. There is plenty of good literature on this forum if you search for "Stress relieve" "spoke arrival angle" "spoke line correction" etc.

From the photos it seems as if your outbound spokes can do with spoke line correction. There's too much space there and washers won't do it.

Here's a photo of a very, very poor wheel. Look at the large cantilevers between spoke and hub. Those gaps have to be closed up.

This one is better, but not great.

For some reason now I can't find my pictures of good hubs. They're in a PPT presentation doc on another drive. I'll hunt it down and post. But, I'm sure you get the picture by now.

 

[Nigelnightmare]

I don't know when the 6 spokes went. The hub is very wide: the right and left flange offsets are both 26mm. Because of this, the wheel was still close to true even with 6 broken spokes.

JMTPW
I'd say the spoke tension was too low, you would have seen a greater deformation if even 1 spoke broke in a properly tensioned wheel.
It's also the most common cause of spokes breaking at the "J".
Also (looking at the pictures) the spokes ARE butted & the holes look to be chamfered (pic#3).

 

[Yellow Saddle]

JMTPW

Also (looking at the pictures) the spokes ARE butted & the holes look to be chamfered (pic#3).

You have a point there. Now that I look, I can see the short butt on at least one spoke. I wonder if they aren't those stupid DT-Swiss "superstrong" (whatever they call them) spokes which are 2mm at the thread, 2mm in the centre and 2.5 or 3mm at the bend? Opeeee! where's your vernier?

 

[Tom B]

Fascinating thread.

Taking it off at a tangent is it common to break a spoke at the Jbend? In 30 years I can't think I've broken a spoke at bend. Plenty at the thread end.