But is that likely to be caused after he installs shorter gearing? He didn't mention making any other changes to the bike IIRC.
He's saying it was put on and the axle is all the way back now, if you move the axle over an inch back or more, it's hella noticeable.
I interpreted it that they the chain kit was just installed. So who put it on, made it a link too long. That 1 link is easily an inch or more on the axle position/wheelbase. I can see it being that big a difference for him to make a post about.
Maybe I've misinterpretted the OP then. I read tires plural that are sliding. If he's just loosing the rear on the gas, and he's upright and sticking his leg out like a dirt bike rider, I reneg. But the axle all the way back and possibly not changing the front ride height makes that bike all fcuked up.
If he's a knee dragger and put his knee down to keep the bike up, then I would hope he shortens the wheelbase up so the bike turns as it used to and he doesn't get hurt.
If the OP is solely about him spinning a rear tire one time, then he's not used to the gearing change and how much easier the tire spins or his tire pressure could be shitty too.
I interpretted it as a new chronic issue of both tires pushing/sliding, and it ain't gonna happen without poor BP and a ton of lean and running out of tire. So maybe this change in geometry was enough for the bike to not do what it does every other time he has had it all the way over, etc etc..