Bicycle Mechanics - Rear derailer can't decide on gears

I seem to be having a problem with my rear derailer. I have an 8 spead shimano rapid fire connected to a shamano deore derailer. When riding in any of the middle 4 cogs on the back the chain has a tendancy to jump either up or down a gear. Sometimes it reverts to its former gear, other times it stays in the new one. I looked at the Park Tool Co. site to research the problem and followed their advice on aligning the derailer. The problem is that when on the bike stand, there is no problem when I shift gears. It is not a worn chain or cog (I don't think) as it happens under all types of riding, uphill, flats, accelerating, etc. Can anyone offer any advice on what would be causing the derailer to shift on its own. Oh yeah, both parts were replaced by the lbs last week.

I know you had a hassle with this shop, but you might want to take it back and tell them it is not working right (I would expect my customers to do this, I just worked on it, it should work right)

If that is not an option, try this. While riding the bike, try to figure if the issue is it is jumping up or down. If it is trying to drop onto a smaller cog, you need to give the adjuster on the der a quarter to a half turn counterclockwise(looking from the back) This is the most likely issue, the cable has stretched a bit and turning the adjuster counter clockwise will tighten it. See if it is better, it may need a little more to get it right.
If it goes the other way, then you need to go clockwise.

I don't know why they would stick a Deore der on your bike with eight speed, an alivio would work fine and be cheaper.

"I don't know why they would stick a Deore der on your bike with eight speed, an alivio would work fine and be cheaper."

I don't know why they did alot of things, but whats done is done so time to move on. The der is not fussy, it goes both was without warning. I also noticed that there seems to be a phantom gear when shifting. When going up or down a gear it sometimes takes a second push of the shifter to get the der to work. Oh well I guess I shall venture back to the lbs on Sat. to see if they can fix the problem.

This is also a good problem to start learning on. All you need to adjust the cable is 5mm allen wrench.
Try this. Shift all the way to the smallest cog and make sure the shifter is also clicked all the way to eight. If there is a lot of slack in the cable, loosen the binder at the der and pull the cable snug(not tight) Eyeball the der from behind and see if the pulley is lined up with the cog. If it is not pretty close, you can line it up using the screw marked H. Now click the shifter up one gear (While turning the cranks) and see what happens. If it hesitates or does not shift at all the cable needs to be tighter. If it shifted to the second cog(seventh gear) shift it to the third cog(sixth) and shift it back down to the second cog. If it hesitates on this shift the cable is to tight. If you still have a shift issue, the calble may be kinked and binding in the housing or the housing may be damaged.

Did you change the cable / cover at the same time ?

This is also a good problem to start learning on. All you need to adjust the cable is 5mm allen wrench.
Try this. Shift all the way to the smallest cog and make sure the shifter is also clicked all the way to eight. If there is a lot of slack in the cable, loosen the binder at the der and pull the cable snug(not tight) Eyeball the der from behind and see if the pulley is lined up with the cog. If it is not pretty close, you can line it up using the screw marked H. Now click the shifter up one gear (While turning the cranks) and see what happens. If it hesitates or does not shift at all the cable needs to be tighter. If it shifted to the second cog(seventh gear) shift it to the third cog(sixth) and shift it back down to the second cog. If it hesitates on this shift the cable is to tight. If you still have a shift issue, the callable may be kinked and binding in the housing or the housing may be damaged.

Thanks Rev C., For taking the time to share your bike repair experience.
I find your instructions very useful and am going to go through your posts to see what other gems I can find.
From reading your posts and trying a few repairs\ adjustments I have a much better working knowledge of dérailleurs. :)

This is also a good problem to start learning on. All you need to adjust the cable is 5mm allen wrench.
Try this. Shift all the way to the smallest cog and make sure the shifter is also clicked all the way to eight. If there is a lot of slack in the cable, loosen the binder at the der and pull the cable snug(not tight) Eyeball the der from behind and see if the pulley is lined up with the cog. If it is not pretty close, you can line it up using the screw marked H. Now click the shifter up one gear (While turning the cranks) and see what happens. If it hesitates or does not shift at all the cable needs to be tighter. If it shifted to the second cog(seventh gear) shift it to the third cog(sixth) and shift it back down to the second cog. If it hesitates on this shift the cable is to tight. If you still have a shift issue, the calble may be kinked and binding in the housing or the housing may be damaged.


Followed your advice and there were still problems so i took it to another lbs for a check. The der hanger was bent when it was installed. The mech figures that that it was a not quite new one that was installed. Anyway the problem is fixed and I learned how to fix my der in the future. Thanks for the help and guidance. Found a new lbs as well. Nicer people and better prices.

Good deal.