Hi Mike!
In the link you sent, I believe they cracked a (DT swiss?) EX rim and replaced with Andra 30s, i.e. the Andra's did not crack.
You're absolutely right; I should have read more carefully before posting the link in haste.
You are higher in weight and yet have no cracking, but those higher weights are only with derailleurs?
Yes, the tandem at present has a derailleur drivetrain with a Rohloff waiting in replacement. The current hubs are Suzue high flange, about 73mm as I recall, but still much smaller in diameter than a Rohloff so spoke angles are indeed less severe. The freewheel hub is double-threaded and a heavily used Arai drum/drag brake is on the offside, so the spokes and rims are subject to braking as well as drive forces. No problems to date.
And a significant portion of that weight is on a trailer?
My Danneauxbuilt 2-wheel trailer weighs 17lbs/7.7kg empty with hitch assembly. Full, the trailer has weighed up to 125kg/56.7kg...usually split between water and food for two. Tongue weight carried by the tandem's rear axle is a fraction of that, usually about 10-15% depending on how the load is distributed in the trailer.
The 40's hole surfaces appears to be planar, relative to the 35's, and so nipple washers on the 35's may not work well.
<nods> I see your reasoning in thinking the nipple washers might distribute the stress over a larger area, but I am concerned a) they will be incompatible with the 35's internal profile as you are, and b) they seem likely to offset the advantage of the rim's directional drilling.
As mentioned, I have Andra 30s on two of my singles. I decided to bypass the 35 in favor of the wider 40 for my tandem, the goal beong to maximize internal spacing between the sidewalls, so I did not spend a lot of time researching the 35s. When looking to source the 40s, I did find some listed "Rohloff-specific" directional drillings and others simply mentioned only "directional drilling" and a "smart bead" for the nipple (keeping in mind some of the ad copy was machine-translated from German to English). I'm not at all sure they are the same.
Mike, did you use Sapim's Polyax nipples? They have a rounded profile that nicely matched the dimpling on my Andra 30 and 40 rims.
Yes, the photos helped a lot, thanks. It brings to mind two memories from my past wheel builds, both having to do with the notch-sensitivity of aluminum:
1) Back in the day, Mavic produced a couple of classic box-section road rims, the MA-2 (polished) and MA-40 (hard anodized); both had clinched ferrules. While the MA-2s lasted forever (I am currently running a pair with more than 32,000mi/51,500kms on them), the MA-40s had a tendency to crack where the ferrules were clinched into the hard anodizing, creating a notch where cracks originated and then propagated from.
2) A lot of early 1980s touring bikes used Araya 16A(3) or 16A(5) rims. These used a channel section similar to the old Wolber Super Champion Mod. 58 -- but without ferrules. When these were supplied OEM in factory-built wheels, the spoke tension was usually on the loose side and the rim had no problems with cracking. However, a few of my bike shop friends increased tension and began to find longitudinal cracks starting at the rim holes. When I undid the nipples, we found they had gouged crescents into the rim's inner surface and that where the cracks began. Of course, these were machine-built wheels and no lubrication was used in the build, so the gouging may have been a direct result of the process as much as the design. I had no problems with the one I built.
This is not a universe of cases, but it makes me wonder of perhaps your rim might have had some burred spoke seats or perhaps the nipples you used might not have been fully compatible with the seat profile. Just a thought. It'll be interesting to hear what Ryde says in reply to your inquiry.
