From Jean-Marc...
If I read it correctly, the suspected culprit with cracked nipples is too short a spoke length.
Yes.
Do you think that the longer nipples compensating for the shorter spoke should mitigate against this problem ?
No.
The problem -- and it can take awhile to manifest -- arises from the internally unsupported nipple head supporting the spoke tension at the rim. There are two stress risers, one internal (threads) and a larger one that is external, where the head meets the rim. The second is most problematic in nipple decapitation because it is a sharp, abrupt change in radius and cross-section at the point of greatest tension.
From George...
I can't speak for Dan, but I think longer nipples are not much help. It is my understanding that the extra length of the nipple is not threaded.
In my experience, some nipples do have extended threading, but I agree most of the longer nipples I have seen simply have longer unthreaded shafts. PLACEMENT of the threads in the nipple is also key. If they are in the same place, the extra overall length adds nothing in this case except longer wrench flats. OTOH, if it is at the end that will help to better grasp the too-short spoke threads but the specter of head decapitation remains Regardless, the problem comes when the spoke is too short and the internally unsupported column comprised of only the brass nipple itself is called upon to support tension all by itself. Eventually -- provided the spokes are properly tensioned -- something's gotta give in the weaker brass, and it eventually does -- >POP!<
As for extended nipple length and thread placement, it very much depends on the nipple brand, length, maker and application. I have attached a photo below of two nipples from my stock. The shorter one is by DT, the longer one is an early 1980s mass-production Hoshi, intended for machine-built wheels. The DT is shorter but actually has more threaded internal length. The Hoshi (_this_ Hoshi, for others vary due to different intended applications) simply has its extended length in the hub end of the nipple shaft making for longer flats so a machine can better grasp the nipple to spin it up on the mechanized wheelbuilding machine. Internally, the shorter DT actually has more threaded support, not due to brand but intended application (hand building rather than machine assembly) and again, thread placement in the shaft is important. I have a collection of fine dental picks with right-angle ends I use as depth gauges for my spoke nipples and I do check each batch I order to make sure threaded length AND thread placement in the nipple shaft are correct for my needs. Not every longer nipple is actually longer when viewed from the spoke's perspective!
All that said, what you can "get away with" (casting no aspersions on others and their preferences) depends very much on application and supply-chain availability/inventory. I built up a pair of nice 406mm (the smaller "20-inch") wheels for my custom Folder project where the spokes do not fully support the nipples. The difference in this case was I manually countersunk each.and.every spoke hole in the unferruled rim so the spoke heads had greater support...but it is still the brass column at the nipple head alone that supports the spoke tension. The spoke lengths/nipples were all I could get at the time. I do plan to rebuild the wheels later, but needed these to check on frame clearances when I brazed in the chainstay and brake bridges, so will do for now but not permanently even though spoke tension is correct. I just don't trust them long-term.
As for ferruled vs plain (unferruled) rims, it is not only a strength issue but also one of spoke well design, angularity, and placements/offset. Your Andra 30 rims have NO ferrules, yet are very "strong" thanks to the extra thickness of aluminum in the extrusion in the center, along the spoke line. If you got the Rohloff -specific version, the spoke drillings and seats are angled with the larger Rohloff flange diameter in mind. I've found ferruled rims build to tension more easily without excess "stiction" at the nipple because the nipples (typically brass...I have not found the disadvantages of aluminum nipples to be worth the slight weight savings over brass) have less friction against the ferrule, resulting in less spoke windup and less need to counter-twist the nipple to achieve true tension at the seat than with a plain rim. Spoke well design plays a role in wheelbuilding ease and nipple longevity also, as does spoke angle of approach/crossings and hub diameter. The absolute worst for nipple decapitation risk is too-short spokes coupled with a shallow approach and a plain-edged hole in, say a chromed steel rim with raggedly drilled unfinished holes as found on cheap wheels of yore. The hole edges tend to cut into one side of the nipple as it is tensioned, forming a groove just beneath the head, making the native stress riser there worse. The "solution" for builders of these wheels is low spoke tension, which is really no solution at all.
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These observations come from building only several hundred pairs of wheels, NOT the several thousand (or tens of thousands more) of, say, Thorn's captive wheelbuilder. I've had good luck so far and (knock on wood) my Danneauxbuilt wheels have never suffered a decapitated nipple or broken spoke to date, despite a number having accrued >45,000-55,000kms in sometimes hard service. I suppose having said that, they'll all start popping like popcorn, the curse of the unspoken fear stated aloud!
My "wheel guru" is Wheelsmith founder Ric Hjertberg, whose knowledge I can only hope to ever approach. If you get a chance, look for his writings online and at his "Wheel Fanatyk" blog:
https://wheelfanatyk.com/Also from Jean-Marc...
...the nipples and spokes are Sapim Race.
These are what Thorn used for my Nomad build in 2012 and they have worked well for me. Recently, they have come to endorse Rohloff spokes instead, due to full compatibility/Rohloff approval for warranty purposes. See...
https://www.sjscycles.co.uk/spokes/146-rohloff-double-butted-201820-spokes-with-nipples-silver-set-of-37/?geoc=US Note the answer to the question about nipples below the product description.
For more information on Rohloff spokes and warranty coverage from Thorn, see the latest edition of "Living With a Rohloff" available here:
http://www.sjscycles.com/thornpdf/ThornLivingWithARohloff.pdf Relevant pages are 34-35.
I wrote this on my phone as I often do, so hopefully the small screen, bright sunlight and autocorrect won't conspire to make a confusing result.
Best, Dan.
