grandcanuck

I have an old (10+) mountain bike I use to commute. It was still working great but now I'm having problems with the rear derailleur and I'm not sure if its just an adjustment or it's just getting old.

When I push really hard, like going uphill, even more so if I stand-up, the gears often time change on their own. I'll be on a high (small) gear and it will skip down several gears, down to the first one sometimes. It's getting almost dangerous because here I am, pushing really hard and all of a sudden, the resistance goes away.

Any ideas?

I still get the bike serviced and tuned frequently and as I said, it's always worked okay. Started doing this just recently.

Thanks.

z415

Usually a RD needing adjustment would "auto shift", but I've heard of autoshifts going a whole spread of cogs, much less more than one cog. If you have mechanical experience concerning bikes, then you should go to parktool.com > repair help > and go through the RD adjustments. If not, then take it to get it looked it at your LBS (Local Bike Shop).

One possible way for a RD to auto shift a couple of cogs would be a slightly bent derailleur hanger - your derailleur attaches to the hanger which is attached to the frame. I had that problem a couple of times and usually a defunct hanger can be spotted with the naked eye. If your bike is steel, then chances are you won't have a hanger, so don't worry about that.

Look at your pulley wheels in you RD - they are the little wheels that the chain runs a reverse "S" pattern that increases the chain tension - if those show excessive signs of wear (they are pointy and grooved) then that would mean your RD is just getting old, and even then replacement of the whole RD might not be needed because you might be able to get the pulley wheels replaced.

With all that being said, it would be hard to diagnose without being hands on so my best advice it to take it in.

vpiuva

To shift that many cogs must be a shifter issue, but if this is the case it should be shifting from the largest cog to the smallest cog (releasing cable tension). Unless you have one of those oddball Shimano systems that shift backwards. What type of shifters and RD do you have?

grandcanuck

Thanks z415, thanks vpuiva...

vpuiva, to answer your question. My RD is a Shimano Deore LX and the shifters are Shimano SLR (the brake handle and shifter are together). Shifter model is one where, for the RD, the high button gets you a smaller gear and the lower button gets your on a bigger one. Up gear shift is one at a time, but you can downshift several gears at once by pressing the button deaper.

I inspected the RD and everythign looks in order. Nothing looks bent or broken. The small gears which guide the chain seem perfectly in line with the gears on the rear wheel.

I guess it's time to bring it it. I'm just trying to avoid expensive repairs on this bike since it's a cummuters and it may need replacement at some point.

Thanks again.