Broken Spoke Nipples
 

I have a Mavic 719 Disc rim laced to a DT Swiss hub with 32 spokes. This is the back wheel on my bike. What keeps happening lately is spoke nipples breaking off. I've had issues with broken spokes before on different wheels where spokes break near the flange of the hub but this is my first experience with the nipples themselves breaking. Does anyone know what this indicates? I think there may be some uneven spoke tension on the wheel that is causing this but am not clear on why the nipples are breaking instead of the spoke. Is one kind of nipple stronger than other kinds? I'm using regular brass nipples on this wheel. Are alloy nipples stronger? Thanks for any advice.

Brass or Aluminum?

Brass nipples are stronger than aluminum so this sounds odd. I woulda guessed you were using alloy nipples! who built the wheel, did they use proper length spokes? I'm wondering if the spokes were too short and didn't thread in far enough if this could happen?

When you say breaking, where on the nipple is this? where you see the spoke going in, or somewhere else, a picture would be useful for this

spokes are too short.

Odds are the spokes are too short.

Take a look and check whether the tops of the spokes reach close to the top of the nipple. If you're breaking nipples I'll bet it doesn't, and that's why your nipples are breaking.

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Think of the nipple as having two distinct parts. The head which is about 2-3mm thick and is basically like any other nut, and the shank of the nipple, which is a thin tubular extension allowing you to turn the nipple from the outside.

The head butts directly onto the rim or eyelet easily transferring the load the same way any other nut would if the spoke reaches into it. But if the spoke is short, ending below the nipples head, than the thin section shank becomes the last link in the structure transferring the spoke's tension to the head, then to the rim.

As they say a chain is as strong as it weakest link, and the nipple shank which was never intended to be a structural element - whether brass or alloy - will ultimately fail.

In fewer simple words - nipples used properly with spoke heads reaching the top of the head don't fail.

BTW- some time back someone posted 3 nice cross-section sketches of spokes in nipples at different heights, which nicely show the problem of too short spokes. If he sees this maybe he'll be so kind as to post them again.

I agree. Pull the rubber and rim strip off and see if the spoke goes all the way through the nipple, or close to it.

The OP doesn't even need to do that, except to see if all the spokes are short. When nipples break, it's always at or beyond the end of the spoke. If the broken section still attached to the spoke is the shank, that confirms the short spoke diagnosis.

I googled and found this Serotta forum thread.

http://4.bp.blogspot.com/_FkxX587Lsi...ple+thread.jpg

Agreed. I guess my method would show if all the spokes are too short, one side, or just some of them. If the OP is doing the nipple replacement, he could look at them then. If he is getting the nipples replaced by someone else, they may not look at the length of the rest of the spokes.

Thanks, that's what I was looking for. The picture make the issue obvious.

Actually that picture overdoes it a tad...

The absolute minimum to avoid chronic nipple separation of the barrel from the head is for the spoke end to penetrate the thickness of the rim AFTER proper tension has been achieved. You can be 1.5mm below the screwdriver flat of the nipple and you'll be okay - that is if you don't mind seeing a thread or two when using 10mm/12mm nipples.

In most cases though, the spoke end is barely reaching the edge of the rim - often the case when manufacturers are using 16mm nipples on standard 9.0mm threaded boxed spokes. Very common with the factory machine built DP-18s on Formula/Origin8 track hubs.

Use a flashlight - take a peek - if you think the spoke ends are down deeper than a 2mm - a rebuild is called for with the proper length spokes.

Nipples are nipples - they're not meant to be spokes.

=8-)

Low spoke tension can be the problem. I built up a rear for a 225lb.er with his Powertap hub. That hub has a little more dish than normal so that the nondrive tension was low. He was breaking nipples on the nondrive side. I added another 20kg of tension to the drive side so that I could add some to the NDS and he no longer had the problem.

Nice find! Only problem is the drawing is not to correct scale. The wall-thickness of the nipples are nowhere nearly that thick. Any exposed barrel section without a spoke in it will snap.

Did you see how deeply the spokes ended up in the nipples? Low spoke-tension may be the final-straw that did in the nipples used with too-short of spokes, but it's not the actual cause of the problem. Regardless of tension, any time you break a nipple, it's pretty much a sign of too-short spokes. Although I have seen some cheapo nipples with super-thin heads, about 1/2 of normal, and those snapped off the edges of the head in a clean ring.

Thank you for all the responses. This information is kind of disconcerting. The wheel in question is fairly high end, with Mavic 719 Disc rim and DT Swiss hub. It was built by Bicycle Wheel Warehouse. I assumed they would use the proper length spokes for building a wheel but now that nipples keep breaking, I'm beginning to wonder. This problem just started occurring about two years after I first put the wheel on the bike.The nipples break right where the flange of the nipple meets the rim. Could it be a case of uneven spoke tension? Although the wheel looks fairly true and round on the truing stand, I noticed some spokes are tighter than others. Thank you in advance for any advice.

Uneven spoke tension sounds like a reasonable idea.

If the wheel is 2 years old, how much have you used it? I take it, from the spec it's for MTB, and the rim is fairly heavy duty, indicating that it is designed for hard All Mountain use, has it just had a hard life of impacts; and the wheel is comming to the end of it's life?

As for the rim / hub being high end, this does not have much if anything to do with the quality of the wheel build.

If the spoke were long enough, penetrating into the head of the nipple by 2mm or more, you wouldn't be having these problems.

The nipple wouldn't break below the end of the spoke because the spoke is supporting it there, any more than a seatpost would break 2" into the frame. The general guideline for the design of nuts calls for them to have a thickness equal to the diameter of the thread (actually a bit less is OK, but this leaves a margin). Likewise a 2mm spoke should engage the head of the nipple by 2mm.

Short spoke wheels are very common because folks are so paranoid about going beyond the top, and production builders don't want to bother with too many spoke sizes in inventory. In reality most spoke/nipple combinations allow the spoke to beyond the top of the nipple a bit to allow builders to err high which ensures against the short spoke problem.

There's a certain irony to all this. For almost a century wheelbuilders used spokes that were on the long side. Mind you this was using single wall rims, so the excess had to be trimmed off. Grind marks were common on wheels through the seventies, and there were even special narrow grinding wheels sold for the job.

Bike tool companies like Var and Eldi sold nippers made expressly for the job of cutting excess spoke flush at the head. Later on with the advent of hollow rims and excess spoke beyond the top of the nipple no longer mattering folks started going the other way and erring low. Part of the reason is that many spoke/nipples have less room to err high before the thread jams, but that's solvable by good selection of materials.

In any case I'm of the opinion that any wheel where nipples break where spokes end below the head of the nipple is inherently defective, though good luck getting manufacturers to accept responsibility.

People screw up. Maybe they calculated the spoke length wrong, maybe they didn't have the right spokes, maybe the box was labeled wrong, and clearly no one cared enough to look at the length. There are only two reasons nipple break. One is the spoke is too short. The other is the nipples are total junk, and can't handle the tension. Either way it's a defective wheel. Two years is probably longer than the warranty, so buy a new hoop, some spokes (of the correct length!) and rebuild it. Or get someone local to do it for you.

Yes, this might explain why they built it this way, but in no way excuses it. It becomes a question of integrity. Some people may send out poor product out of ignorance, not know there was a problem. Or, and worse in my opinion, because they simply don't care.

There's no way a builder doesn't notice the spokes are too short. So either he really doesn't know what he's doing, or totally lacks personal integrity. Either disqualifies him as a vendor as far as I'm concerned.

Bicycle Wheel Warehouse
 

Have other people had issues with this vendor? I bought the wheel online from BWW and it never gave me trouble for two years. Recently though, spoke nipples are breaking with sickening regularity (only on the back wheel). The wheel has been used on pavement only but sometimes I carry heavy loads on it. I haven't had a chance to look, but the short spoke theory does make sense. I'll just have to keep replacing nipples until I eventually rebuild the wheel. Thanks to everyone for the advice. http://www.bikeforums.net/images/smilies/smile.gif

If the spoke extends all the way to the bottom of the slot in the nipple, it's long enough.

+1, that's been the guideline for years. The spoke should end somewhere between the bottom of the slot and the top of the nipple. Or 2-3mm into the head which amounts to the same thing.

Short spoking is so common these days, that's it's almost par for the course. Truly careful selection of components is one thing that separates the artisan hand builders from the production shops including those who build by hand.

Your experience is typical because brass nipples while weaker than the spoke, they're strong enough to last a while. But wheels flex and the brass used isn't noted for high fatigue resistance. Also being a tube, the effects of flex are higher because the material is farther from the neutral axis. Couple that a spoke ending near the rim, and you have the perfect recipe for failure, ie concentrate the maximum flex into the smallest place.

Folks get away with it because the wheels do hold up for a while and by the time fatigue becomes an issue, they've either been crashed, or the owner figures he got good life and so doesn't complain.

1. After "adequate" or "proper" tensioning the spoke only needs to penetrate the thickness of the rim seat. It can be 1mm below the screwdriver flat of the nipple and the spoke, nipple and wheel as a whole will be fine.

2. "Between slot and top" spec is a carry over from the days of 10/12mm nipples and boxed spokes or 16mm nipples w/long threaded spokes. Today we have 10mm, 12mm, 14mm, 16mm and 20mm nipples with unfortunately mostly boxed 9mm-9.5mm threaded spokes. Hence the "short game" played often with 14mm, 16mm and 20mm spokes because 99% of the wheelbuilders out there don't have a spoke machine like some of us do.

3. Toss in crappy nipples intended for "el cheapo" bargain basement wheels into today's alloy wheels and things get worse. This is happening with a lot of basic and replacement machine built wheels distributed through shops.


I agree in the sense that just about any halfway decent wheelbuilder would recognize right away while building a wheel that the spokes are too short and that the wheel should not go out the door. They don't have to wait until the build is done - when pre-tensioning is done they'll know right away because they are fighting to clear the last couple threads into the nipple BEFORE the truing stage. Happened to me last week on a disc front - didn't bother to start truing - just replaced half the spokes and then got on with business. It sucks, but it happens.

If the OP verifies visually that the spoke ends are easily 2mm or more down under decent tension and true state - call the original vendor.

=8-)