Originally Posted by
syburgh
Be careful: IME the Synergy extrusion is not really up to the rigors of a wide tire at the high pressures needed for tandem. I converted our 700C tandem to Synergy + Grand Bois Hetre, which we run at 80psi (450lb team). We've broken three Synergy rims in less than 12 months riding on occasional gravel roads (mostly roads, probably about 3k miles). The wheels fail by going severely out of true after each ride (need lots of truing every dozen miles) and then eventually the rim cracks at some random time of the day or night (sounds like a gunshot). We have had the same experience with US and Australian-made 36h O/C rims. Our wheels were built by one of the more internet-famous builders of tandem wheels.
Stoker loves this setup and would not consider going back to narrow tires, so we are on Synergy #4 and #5 now (both are fine with <1k miles)
I've enjoyed Synergy rims on my single bikes for many years (and miles), but am looking forward to 36h 650B downhill rims for the tandem (our frame is ready for discs). Considering Pacenti DL31 and Velocity Blunt for next step, but neither is compatible with rim brakes.
Our 650B tandem had the same issues with 38mm tires at 75-80 psi. I got some of the rims replaced by Velocity and they continue to say the rims will work. We have replaced four rims all rebuild cost out of pocket. Mine cracked along the middle of the brake track within 1,000 miles.
Had both sets rebuilt with Velocity A23 rims. These rims are "tubeless ready" which means they have a much shorter rim wall and shallower well which hopefully will make them resist outward force of the wide tire. I did some calculations and slightly shorter unsupported rim wall dramatically increases the strength at the bead to resist for wide tires. I have only a few hundred miles on the wheels. They are much more difficult to mount tires on due to the shallower well between the rim walls. Hopefully they will work out.
The A23 is not listed as 650B on the Velocity web site but Longleaf bicyles ordered a special run and has 32 & 36 650B rims for sale. They also carry 40 hole 650B rims of some variety if you have some older hubs
Also bought some heavier Velo Orange rims to give a try should the A23s crack.