I have a fat bike (Giant Momentum Iride Rocker 3). It says this bike didn't design for offroading but I have one bike and I use it on single-tracks (not very technical) and doing long trips (about 75kms per day). I usually have heavy luggage. But nowadays, the rear wheel is wobbling side to side (when you look the bike from the behind it wobbles right and left, I mean the top of the wheel wobbles to the chain and to the kickstand. I hope you understand.) And it does this freely, sometimes to right; sometimes to left. So I don't think the wheel is out of trueing. Can you suggest me what to do? Because I don't trust the local bike shop he is trying to sell new parts and usually lies about broken parts. If you need more information I can share with you. And I'm sorry for my bad English. The bicycle has a freewheel by the way, not a cassette.