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
