I am a cycling newbie. My bike has 18 speeds with two chainrings in front and nine in back. I just did my first rear tire replacement. Most of that went well, however when I took it out for a short test ride and went through all the gears to make sure they were working correctly, I discovered that they weren't. The front derailleur is working fine, but the rear is stuck in first gear. I've watched the park tool video about adjusting the derailleurs, but I'd still like to know what might have caused this situation so I can avoid it happening again. I should also mention that during the process of changing the rear tire, I opened the rear brake because one crappy video on YouTube advised to "make sure" to do so. But my bike has hydraulic brakes and then they were stuck shut. I gently pried them open with a clean 2-in metal putty knife, and the brakes worked beautifully in my test ride. Could that have had any effect on the gear shifting? Any advice you can give would be greatly appreciated.
The brakes are probably not related to your problem - if the rotor was getting hung up then the wheel wouldn't be able to bolt in.
So what may have happened is that either your chain is somehow out of position as it passes through the derailleur, or you've undone a pinch bolt that holds the gear wire in place. Both could push the chain onto the highest/hardest gear cog with the fewest teeth.
If your chain is stuck on the easiest gear, then something has pulled a whole lot of cable from the rear derailleur. It may be your right-hand shifter is somehow damaged and not unspooling cable, or the cable is pinched in the tight position somehow.
Thank you Criggie and DoNuT for your rapid responses. I have taken my bike into a shop to get it back in riding order. I’ll get more info from the shop technician about what was going on and why the gears got so wonky. I’m also maintaining a file of all of your great responses to assist in case of problems when I do future rear tire changes.
Watching YouTube videos has also been helpful (and at times unhelpful) but what I really want is an IN PERSON bike maintenance class. It seem so incredibly basic; I mean, a metropolitan area of 1.4 million people should have some bike maintenance classes available, don’t you think? But NOOOO! I’ve searched and searched online, and nothing. Before Covid, there were a couple of classes available, but not anymore. Even the local Bicycle Society had no suggestions but calling each bike shop directly and inquiring if they offer classes! I guess I’ll be spending some time on the phone the next few days.
Again, thank you so much for your help. - Stevie