Advice re: nipple choice carbon tubeless mtb wheel build

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trailgumby
Posts: 121
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by trailgumby

I'm building up a set of tubeless race wheels for my DS 29er race bike, and am having trouble deciding on appropriate spoke nipples. I'm looking for some experienced comment and advice.

Components bought so far are:
Hope Pro II EVO 142x12 rear hub 28H
Lefty front hub 28H
Light Bicycle 27mm wide bead hookless XC rims

To buy
Sapim CX-Ray spokes, which I'll probably be lacing 2-cross.

That leaves spoke nipples. Top of the short list are:
Sapim Polyax 14mm 7075-T6 hex-drive
Sapim Polyax 14mm brass

Sapim reckon their 7075 alloy nipples are stronger than brass, which I'm giving some credibility from my use of that alloy when racing R/C model cars, and the lighter weight is very attractive. However, I am concerned about the corrosion risk in a carbon rim with tubeless sealant, although they claim their anodising process again makes them more corrosion resistant than nickel-plated brass. Not so sure I'm convinced by that one.

The other concern I have is with the bevelled shoulder on the Polyax nipple, whether that increases the risk of cracking the rim at the spoke seat.

Am I being unduly concerned, or should I be using a vanilla nickel-brass nipple? :noidea:

Thanks for your help.

Orfitinho
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by Orfitinho

Polyax nipples are very often used with carbon rims. Your worries about rim cracking because of these nipples therefor seems to be quite overprotective. Are their any reports of rim cracks because of Polyax nipples?

The risk for the nipples to come in contact with the sealant is quite limited. If it should happen, you will not be able to inflate the tire, as the air leaks out of the rim bead.

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trailgumby
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by trailgumby

Thanks for setting my mind at ease on the nipple shape.

I have read comments elsewhere about alloy nipples corroding when used with tubeless sealant. The speculation was it was the ammonia fumes that triggered the reaction.

Does anyone have experience to confirm if this is the case or not?

mattr
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by mattr

Aren't the latest formulations all ammonia free?

And FWIW I'd only use aluminium nipples (of any grade) on raceday only road wheels.
I've had enough hassle with aluminium nipples to last me a lifetime. (Yes they are *almost* as reliable as brass, but almost isn't good enough when it's that much hassle to swap them!)

trailgumby
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by trailgumby

Not sure about the formulation. I'm using Stan's.

These will probably be race day only wheels.

bm0p700f
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by bm0p700f

Lace the wheels 3x, disc brake forces are large and you want to maximise torsional stiffness. I use alloy nipples with carbon rims and no issues. It is spoke tension that crack rims. The LB rims can handle 1200N. Why do you need to swap out alloy nipples. Never had one break in service. (sapim nipples that is).

Valbrona
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by Valbrona

Alloy nipples can corrode and are more easily damaged by a poor-fitting nipple key or an accidental slip. The only downside to brass is extra weight.

The use of hex head nipples can make spoke length determination difficult on account of differences in design. If you put a 14mm hex head nipple next to a 14mm regular nipple they will be the same length but the point at which the nipple rests against the rim will typically be different. This is certainly the case with DT hex head vs. DT regular nipple.

pushstart
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by pushstart

I would use the alloy nipples. I have never had corrosion on numerous sets of tubeless rims (using Stans sealant). Also, after buying a proper spoke wrench, have never had installation problems. I use the round PolyAx washers too, which I think helps with any risk of snapping alloy nipples during tensioning (eliminates any surface irregularity, may not matter with carbon but I do it anyway). And motor oil, per Musson's guidance. (Used to use boiled linseed oil.)

3x sounds good, though practically speaking 2x is probably just fine too. I laced 2x on my disc commuter (28h) wheels because I had spokes in that length and they have been fine for a few thousand miles (which I realize isn't much in the scheme of things).

bm0p700f
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by bm0p700f

there you go use a proper fitting nipple key. Park Green tool fits very well but the best is the 4 sided sapim key. Corrosion became a problem eventually but I have alloy nipples on winter wheelset and they never need truing so the fact the nipples maybe seized is rather academic, in fact it helps in a funny sort of way because the wheel can only go out true unless the rim is damaged. When the rim wear out the spokes get cut out and replaced anyway. On a disc brake wheel were the rim does not wear out, spoke breakages due to fatigue mean the other will follow anyway so just cut the out and rebuild, spokes are quite cheap spoke breakages ruin rides why take the risk when one has gone through fatigue after 10,000 miles.

TheKaiser
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by TheKaiser

All of these responses are good general advice for the build the OP is talking about, but no one has really directly addressed his concern about corrosion.

I understand that most of you, and 1000s of others have used carbon rims with alu nipples without any issue, but there is a heck of a long thread over on MTBR about Enve carbon rims causing nipple corrosion which led to failure, and then a number of people on the "Cheap Chinese Carbon" rim thread (which is mostly about these Light Bicycle rims that the OP is using) were complaining (with photos to prove it) about the same issue.

I had wondered myself if quality anodizing would protect the nip, but from my experience, the pressure and twisting from tensioning the wheel wears through the ano layer, leaving bare aluminum at the contact surface. Ano quality varies a ton, and I had hoped that a true hard ano would take care of it, but from the reports I have seen I am not so sure.

Other things I had wondered about were adding a bit of extra epoxy right at the hole to add extra insulation, but there isn't much room to do so around the cylindrical portion of the nip that penetrates the rim wall. I had also wondered about washers, but the same goes for them...they won't prevent that contact at the penetration.

Anti-sieze at the contact surfaces was another possibility, but I think there was a dude that tried that too, without much luck.

On the plus side, it doesn't seem to be a problem on all sets, for reasons unknown, so hopefully you will be one of the lucky majority. Humidity might have something to do with it, or maybe frequency of stream crossings or something too.

It does seem a shame to buy super light rims and then add brass nips, so I would be tempted to try all of the above combined and hope for the best.

P.S. Glad the OP raised the nipple pull through due to polyax shaping question. I had also pondered that. To everyone else here unfamiliar with the LB rims, the MTB models have had some issues with cracking at the nip holes, so the concern is not unfounded. Experience with other brands may not apply on this point. Washers do seem like they would help this issue, even if they don't on the corrosion one.

mattr
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by mattr

Good point actually, i was focusing more on the use of sealant, but of course, the sealant won't get anywhere near the nipples. Even though it has a history of causing rim bed corrosion.

Carbon/Aluminium corrosion has actually been quite a problem with frames, mainly where the bottom bracket shell (aluminium) is inserted into the carbon frame. I understand a lot of manufacturers use either an extra resin layer, or add a layer of fibre glass, to prevent the galvanic reaction.
I'd suspect it would be a more significant issue with drilled spoke holes (exposed carbon) than molded holes (resin and possibly clearcoat between Al and CF).
I'd still use brass nipples myself. For the reasons i stated earlier!

thisisatest
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by thisisatest

At some point, whether it is when you get a flat. Or when you change tires and pull slightly on the rim tape, sealant gets into the rim cavity. Then you get corroding nipples. I've seen it on Enve rims, seen it on Roval rims. Sometimes it's amazing how little of a spoke nipple is left while still holding tension.

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trailgumby
Posts: 121
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by trailgumby

In the end the choice was made easy for me. My wheelbuilder told me he wouldn't do the job if I wanted alloy nipples.

"Alloy nipples are good for about a year before some of them turn to powder and start pulling through." I think this experience was founded on Enve rims, which I've heard reported by another Enve owner on my local ride networking site (nobmob.com)

He was quite complimentary about LB rims. The only one he's seen crack was an AM build where the guy cased a jump badly and cracked the rim all the way though from ERD to to tyre.

Interestingly, he said the local Sapim distributor charges north of AUD6 apiece for CX-Ray spokes! And he was charging $7. He encouraged me to get them on-line.

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