It's the tire not being seated in the rim. Most tires have a ridge above the bead that sits at the top edge of the rim. If that ridge is higher (further away) from the rim at one point than at others, whether on one or both sides, that's where it's not seated. That's where you get the 'hop'.

It can also be unseated BELOW the bead, and you'll see this as that ridge dropping BELOW the top edge of the rim, and that'll give you a 'dropping' feeling rather than a 'hop', like the front end actually falls a bit- which is just as bad. That comes from the tire not being properly inflated when it was mounted. Some shops actually OVERinflate the tires when they're first mounted, to force the tire to seat fully all the way out, but that scares the crap out of me. I'd rather inflate it properly, then tap the sides with a rubber mallet to coerce it to pop out. Sometimes lubing up the rim or tire with dish soap helps slip things into place.


Nick

The best tool for blowing your brains out takes high octane!