Okay, hopefully someone can help me with a question or two ... I shall start at the beginning.

Yesterday, I fitted a Mammoth onto an On-One rim. With a little coaxing plus Gorilla tape and laminate floor insulation, it went on tubeless and all seems well*.

Did the same with the front but it refused to seal. Had another look this afternoon, re-taped etc and all seemed well until it went down On furter investigation the rim's leaking at the join and no amount of anything will stop it. I thought sod it, I'll fit a tube.

Inflate tyre until it seats then start to let air out until about 12psi when the tyre slides off the bead . So, it's obvious from the leak that On-One rims aren't exactly a quality item but what's causing the tyre to drop off the bead into the well?

a - the rim
b - the fact I'm using a DH tube rather than a full size 4" fat bike tube
c - something else

Thank you.

*saying that I've not ridden the thing yet

That'd be an OnOne Fatty wheel? Had this problem fitting OO Floater tyres - they are really baggy on the Fatty rims, which don't have the best rim hooks and are not built to consistent tolerances. Maybe try fitting the tyre again, sticking 40PSI into it and leaving it overnight. You might find the tyre bead conforms to the rim hook.

Yes, Fatty wheel ... it's sat in the workshop pumped up rock solid