Bicycle Mechanics - Broken Spoke Nipple Head

Have some new-ish wheels with about 1,000 mi on them. Carbon tubular, 50mm. On Friday, I popped a spoke nipple, NDS. LBS fixed it up that night. I rode about 50-miles each day, Sat, Sun & Mon (total of 150-miles) and popped another nipple head about 5-miles from home.

Near as I can figure, it must be


Pressure from below the nipple pushing up on the head, or
Twisting pressure that "unscrews" the head, or
Both.


I can't figure out what would be doing this. Anybody ever seen this or have any ideas?

251816

I don't know what causes it, but I've seen it from time to time. In fact, I saw it earlier this month. It happened to a friend while we were riding.

Have some new-ish wheels with about 1,000 mi on them. Carbon tubular, 50mm. On Friday, I popped a spoke nipple, NDS. LBS fixed it up that night. I rode about 50-miles each day, Sat, Sun & Mon (total of 150-miles) and popped another nipple head about 5-miles from home.

Near as I can figure, it must be


Pressure from below the nipple pushing up on the head, or
Twisting pressure that "unscrews" the head, or
Both.


I can't figure out what would be doing this. Anybody ever seen this or have any ideas?

251816

None of the above.

At least some of your spokes are too short to go all the way through the rim so you're loading your nipple heads in tension instead of compression and alloy nipples aren't strong enough for that so they break. This could be because the wrong spokes were used or the wheel was laced wrong.

Pull the rim tape and see where the spokes end. Even with the bottom of the nipple slot or above is definitely fine; below the slot you have less than 1mm before the spoke ceases to have threads above the rim. If all the spokes in a side are that far below the slot they're too short.

If just a few are too short there could be a bike-shop mixup.

The right fix is longer spokes although switching to brass nipples on the short spokes may work well enough.

If you send the wheels back under warranty insist on them being rebuilt with the right length spokes.

If they alternate between too short and too long in one half of the wheel it's a lacing problem.

If the spokes turn out not to be too short you may have a dud batch of nipples. The heads may have been made too thin or there may be some other manufacturing defect which weakened them. I'd have them rebuilt with the correct length spokes (assuming they aren't long enough already) and new brass nipples. To my mind the weight difference between brass and aluminum nipples isn't worth the downsides of aluminum; YMMV.

None of the above.

At least some of your spokes are too short to go all the way through the rim so you're loading your nipple heads in tension instead of compression and alloy nipples aren't strong enough for that so they break. This could be because the wrong spokes were used or the wheel was laced wrong.

Pull the rim tape and see where the spokes end. Even with the bottom of the nipple slot or above is definitely fine; below the slot you have less than 1mm before the spoke ceases to have threads above the rim. If all the spokes in a side are below the slot they're too short.

If just a few are too short there could be a bike-shop mixup.

The right fix is longer spokes although switching to brass nipples on the short spokes may work well enough.

If you send the wheels back under warranty insist on them being rebuilt with the right length spokes.

If they alternate between too short and too long in one half of the wheel it's a lacing problem.

Cool post. Could longer nipples resolve this problem as well?

All those spokes should at the very least come up to a 1mm below the flat of the screwdriver slot. If the spokes seem lower than that - too short is the problem.

Ideally you want the ends of the spokes to reach at least the flat of the screwdriver flat.

Judging from your picture, you have 16mm standard profile nipples. I bet the spoke threads are standard factory rolled 9.0mm to 9.5mm threads. Most but not all 16mm nipples stop the factory rolled 9.0-9.5mm thread right at the flat. A wheel builder working within that limitation will purposely use an ERD that aims a millimeter below to ensure they reach optimal tension BEFORE running out of threads.

Problem is though, too often they overdo it - especially when they are using boxed spokes in 1mm increments instead of a spoke machine that can operate easily at 1/2mm increments.

Go to Youtube, search for "wheelsbyfleming". Watch my video on nipple and thread behavior - chart at the very end includes a 16mm nipple on short thread scenario that I'm not too fond of.

=8-)

Thanks for the responses. & mrrabbit, thanks for the videos. I'm going to have my LBS (wheel originally built by another shop) re-lace/build the wheel, using brass nipples and double-check for correct length spokes.

Cool post. Could longer nipples resolve this problem as well?

No - that's exactly the problem here.

The wheel builder used nipple length to finish the distance between hub and rim and nipples are not as strong as spokes, especially when the nipples are made out of aluminum.

Longer nipples would provide more threads below the rim; but those don't make the nipple strong enough to hold up where it goes through the rim without a spoke's reinforcement.

Longer nipples should only be used when when the rim configuration results in the spoke wrench flats ending up inside the rim instead of outside where they're useful for truing. To provide the most tolerance for spoke length and rim diameter variations this in turn dictates using a longer nipple with the same amount of thread you have with a 12mm nipple (Sapim) or spokes that have longer thread (via a spoke threading machine).

DT spokes have 9mm of thread (it might actually be 9.5; but lets use 9mm for discussion purposes)

DT nipple slots are 1mm deep.

Measured to the top of the nipple DT 12mm nipples have 8mm of thread, 14mm 9mm, and 16mm 10mm.

With the 12mm nipple you have 1mm remaining before the spoke runs out of threads and nipple stop turning when your reach the top, or 2mm past the slot. If we assume that you're allowed to go 1mm below the slot (I wouldn't since the nipple is narrower there) that provides +/- 1.5mm of tolerance. With spokes coming in 2mm increments some rim + spoke combinations will let you choose a size that ends at the slot for +2/-1mm of spoke length tolerance or +2mm / -4mm of ERD variation or a size that ends at the top of the nipple for +1/-2mm of spoke length tolerance or +4/-2mm of ERD.

With the 14mm nipple you have 0mm remaining at the top, or 1mm past the slot for +/- 1.0mm of tolerance. You might be able to aim for the slot itself; or you might be stuck aiming for 1mm below the slot for +2/-0mm of tolerance.

With the 16mm nipple you have 0mm remaining at the bottom of the slot and +/- 0.5mm. Some rim + hub + spoke brand combinations won't work at 2mm increments because you need to aim for the bottom of the slot with 0 tolerance for long spokes / small rims or 1mm short of that with 0 tolerance for short spokes or big rims.

That's not viable with alloy nipples where spokes that are too short result in broken nipples.

I don't know how often I've posted this, but nipples are not structurally equal to spokes.

Think of a nipple as a simple nut (the head) with an extension which has 2 purposes; allow the nut to be turned without removing the tire, and providing a skirt to hide the threads.

For all practical purposes, only the threads in the head count and any spoke that does not engage at least three threads (2mm) into the head is too short, and the wheel inherently defective. It's very simple, nipples fail because the spokes are too short, and (with very rare exceptions) no other reason.

In the OPs shoes, I'd return the wheel to the maker and demand a replacement.

From someones post in the past

From someones post in the past

As I pointed, most likely in that same post, the drawing is not a correct drawing of a typical standard profile nipple...the curved profile extends a 1/2 mm below the flat and then turns in toward the barrel. Picture shows a rough approximation...

Getting the spoke to a mm below the flat typically means the thickness of the rim has been dealt with...though more is certainly better.

=8-)

I just knew that would get a rabbit-reaction.:D

I just knew that would get a rabbit-reaction.:D

Us control freaks are easy targets!

=8-P