Greetings everyone...hopefully I can find some help here.
I have a Nishiki mountain bike (I'm not a hardcore mountain biker by any means). Since my last tuneup, my bike has developed this nagging problem with the gears. The bike has Shimano grip-shifters, and the right-hand shifter broke on me last summer, so it was replaced during the tuneup.
The problem is that the chain will sometime hop around on the gears on the rear tire (it wont stay 'in gear'). There are 7 gears on the rear gear assembly (I dont know the technical term). At first, the chain would only hop around when the the shifter was in the 6 position, but now it will hop around anywhere from the 4 position on up to 7. Yesterday I was in the middle of a 25-mile ride, and the gears started hopping around pretty badly; thankfully I wasn't on any hills because the problem is really bad when I'm on hills. Sometimes the chain will hop right back into the gear it was in, but most times it will hop onto another gear entirely.
The other problem the bike is having is that sometimes when I'm really hauling ass, the right-side grip shifter will start rattling really bad. This is the same shifter that controls the rear gears (where all of my problems are) so I'm wondering if the two problems are related?
I had this same problem going on a few years ago, but it was nowhere near this severe. I took the bike into a shop to get looked at back then, but they told me they could find nothing wrong with it, but obviously the problem is still there and it's getting worse.
If anyone has any idea what's going on, please feel free to toss me your ideas. If you need some more info on the problem, I'd be glad to go into more detail.