Rear derailleur shifts on big bump
 

This is with bar end shifters. If I drop off a curb, the shock can cause the RD to shift to a smaller cog and pulls the shifter accordingly. This isn't unexpected since the bar end shifters don't lock the cable in place solidly.

Is there a solution to this (other than tightening the friction on the shifter)? The RD is a 9 speed Deore - would an (old, 9 speed) XT RD solve this? If there was some damping at the derailleur, that would probably reduce this occurrence.

I don't see a solution to this but I thought I'd ask here.

You answered your own question.
Jon

Efficiency.

Is the derailleur (9-speed Deore) a newer Shadow type? Those typically have stronger springs and maybe more apt to pull a friction shifter back than a traditional derailleur, at least given the same friction adjustment on the shifter. You could try a derailleur with a lighter spring...or increase the friction adjustment on the derailleur.

Have you identified what the cause of the issue is, as throwing parts at it with out knowing the cause is a waste of time/money.

Not getting the idea that the cable isn't held in place by the bar end shifters (I am doing a lot of riding with this 6480 shifters, and they are rock solid), in friction or index mode, both of these are held tight (unless there is play in the shifter, route cause?)

Is there a cable routing issue? is it fully in all the stops?

No mention of the frame/dropout, is this straight, if it's a replaceable type, is it screwed in properly?

I know what the cause is: the jarring from the bump, together with the spring in the RD, shifts the RD into a smaller cog. The shock+spring overpowers the friction and click-stops in the shifter.

I could just increase friction on the shifter to address this, but I'd like to avoid that.

I think it is a Shadow type, judging by looks. So maybe this design with the stronger spring is simply problematic with bar end shifters, in which case going to lower-end RD might be the solution. :eek:

If you have a co-op nearby, or similar source of used bike parts, you could probably pick up a used 6/7/8/9 speed derailleur for $5-10 and try it on your bike. If your current one is a Shadow derailleur, you will likely have shift cable length issues, unfortunately. Traditional derailleurs have that extra 6" or so required to make that 180-degree loop that the Shadow design effectively avoids. But -- for relatively small money, you could try a used derailleur and longer cable just to test the hypothesis that the strong spring is overcoming the desired friction setting on the shifter.

If it does work, then you'll already be set up with the correct shift cable and housing length, and you could either stick with your used derailleur or buy a new higher end traditional derailleur like an RD-M591, considered by many to be about the best of the type, at least as far as mountain style derailleurs go. If it doesn't work, and you still get bump shifts, then you'd have to decide whether it's worth it to keep the parts you just installed or return back to your factory equipment...and then your only other move is probably adjusting the shifter.

You could also consider an indexing shifter. I presume from your post that you have friction barcons. Do they make indexing 9-sp barcons?

They currently are indexed. The shock of the bump + the spring overpowers the click stops.

I’d check to see if the cable housing is long enough, especially at the rear derailer. If it is too short, it could flex and pull the derailer. I’ve had this problem with full suspension bikes where the cable runs under the bottom bracket. The housing was too short and pulled on the rear derailer causing ghost shifts after the frame flexed.

Some earlier bar end shifters that had indexing also had a friction option. Controlled by usually rotating a bezel or ring on the outer end of the shift pod. It is possible to have this ring partially positioned so that the index detents don't fully engage. Generally there were aligning marks to insure complete positioning.

Perhaps more info like the brand and version of the shifters might help us help you. It's easy for us to assume and be wrong. Shimano had a known failure issue with some of their earlier 9 speed bar end levers where the outer housing would fail and not contain the internals. The symptom was a vague lever movement into the next detent and a lot of "overshift/free play" between detents. Andy

I have a chain that's too long on one of my bikes, which causes auto-shifts into a smaller cog when I hit sharp bumps while pedalling. Gotta get around to shortening it a couple of links.

You could work on your curb dropping technique to try to reduce the impact on the rear wheel...

Interesting. My chain is on the long side, could lose a couple links.

edit: as I think about it, this might be the problem. Too-long chain puts the RD arm in a horizontal position, a sharp bump swings the arm down, then it thwacks back up and knocks the RD into a smaller cog.

Shifting up when dropping off a curb isn't that big of a deal - it's kind of my test case. It's when I'm off road and going up hill and getting that ghost shift into a harder gear that's really irritating.

It's an indexed Microshift. Doesn't have friction mode (you can adjust friction but can't disable click-stops). Feels fine, nice and tight and clicky.

Check to make sure the RD parallelogram hinges/ hinge pins aren't worn out. Remove the cable and check for play. Loose (worn at the axle) jockey wheels might be a problem, too.

This is the bike. You can see from the angle of the RD that there's too much chain. It ought to ideally be at very close to that angle when I'm in my smallest cog, but I'm a fan of big-big combinations on the local hilly roads, so it works well in combos like 50x28, which I use a lot.

https://cimg7.ibsrv.net/gimg/bikefor...3e4d3d1916.jpg

A mechanic at a bike shop I knew, said that's how he adjusted chain length, was put it in the smallest cog and the biggest sprocket, and the RD should be right at the middle of its range of travel, IIRC.