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Just looking at Moonglu ( http://www.moonglu.com/shop/category/wheels/mountain-bike-wheels/) and they list the Stan's Arch rim as "filling the gap between Crest & Flow".
For some reason, I had it in my head that the Arch was an older model of Stan's rim, and needed drilling & additional faff/rimstrips to set up tubeless with non-UST tyres - am I wrong on this?
They are an older model, but they build into a wheel that's apparently stiffer than crests and lighter than flows. I can't say I've noticed my Crests being flexy at all, though.
Theres a new Arch Ex with the new bead socket doofer.
I've just got some Arch's on Hope hubs from Merlin for my 456 Ti and they seem to be just great. Have gone tubeless with the usual yellow Stans tape and some sealant with no problems. The Crests seem to be limited to 80/85kg and the Flows are a bit heavier so I think they are a good compromise for something that can take a bit of abuse but still be pretty light. They seem to be much more popular for 29ers for some reason.
I'm bang on the weight limit for crests and ride like a baby rhino, they (hope hoops) have seen me through the Welsh Ride Thing and an aborted Bear Bones 200 bikepacking ride, plus the Dyfi Enduro and have not gone out of true or given me any problems. Super easy tubeless too.
I've got arches and the wheels are stiffer than the ones I had before with crests, but that could be the build. The weight gain isn't huge so could be worth a punt for the extra strength. tyres came out a little wider on the crests, I think because they are slightly wider and I did have some problems with the drilled holes in the arches when i went tubeless, stan's recommend using teh wider yellow tape for arches.