Just bought a 21x3.5 Ultima Fat Spoke rim and paired it with a 120/70-21 Michelin Commander II tire. Issue is, I have taken the wheel and tire to a Harley dealership twice to have the tire mounted on the wheel and balanced. Once I ride about 10 miles down the road, the tire goes flat to the point of the bead unsetting. The dealership doesn’t seem to be able to figure out the issue. They cant find any leaks or major bends in the rim.

Anyone have any thoughts or ideas as to what the problem could be?

Your help is greatly appreciated.

 

well, the tire is recommended for 3.00 to 3.75 inch rim so that is good.
if this is a new style rim, it should have a safety bead to prevent the tire for going to center, a wild ride indeed.
if the taper of the tire bead does not pass over and behind this safety bead, the tire will probably do as mentioned. sometimes, a lube and higher install pressure will force the bead over.
i have seen this before but in a diff light, darkside tire install.

 

If your using a used rim, you may want to look closely for even the slightest bit of oxidation around the bead seat location. Light sanding or DA sander should clean it up for god seal.
Good luck.