adelii

On my Jamis XLT 1.0 I have noticed that every now and then the bike starts changing gears by itself, Its usually while im goin uphill, but it does do it occasionally while going on the street on flat surface. It kind of feels like the chain is slipping. Very weird feeling and its getting very annoying! It only happens sometimes. But usually happens if i ride long enough. Does anyone know whats going on? I have to fix this problem i have been riding a lot lately and i dont want to fiddle with anything because frankly, i dont know whats happening.

LowCel 

Try adjusting the adjusters on your shifter. Try taking it counter clockwise one click then ride it a while and see what happens. If it doesn't fix it try another click. Just remember how far you have gone so you can take it back to the original position.

Also, have you crashed it on the drivetrain side lately or hit your derailleur on anything? It's very possible that your derailleur hanger is slightly bent. If that is the case your shop will be able to check it with a Park DAG-1 derailleur hanger tool. Shouldn't cost but a few bucks to have them check it out.

adelii

I dont think i have hit it on anything, but who know if a rock hit it or something. It does make noise though from the hanger, i noticed yesterday while i was changing the rear tire tube.

NoRacer

There should be a cable adjuster (a.k.a., trim adjustment) for both DRs. If the rear DR is downshifting automatically, then there is too much tension on the cable. You need to release just enough tension that the RD stays on the target cog.

To release tension you have to effectively increase the length of the cable by turning the cable adjuster in the proper direction enough to cause the chain to stay centered on the teeth of the target cog.

I usually do these types of adjustments while the bike is on my trainer, but it can be done 'on the fly' if necessary.

A word of caution: Do the adjustments while the chain is on the middle cogs and not the far ends UNLESS you open up the 'H' and 'L' limits using the adjustment screws on the rear derailleur--something you should probably leave alone unless you are truly intent on finding out how they work. Anyway, if you try to adjust the cable while it's being mechanically limited by the limit screw, you could see no progress and you could break the cable or the adjuster or both by trying to forcing the adjuster past the limit.

Haggard

Switch to SRAM.

Have you made sure your cassette and chain are perfectly clear of debree. Every time my bikes have ever shifted on their own, it's because there was a vine or a twig I couldn't see wedged between the gears. Just a thought.

NoRacer

Also, check that there is no muck where the spring is located.

NoRacer

Here's the Deore shifter. The cable adjuster is 5:

adelii

Ok thanks guys for the help, ill have a look at the dr, i did clean some twigs and stuff last night while changing the tire, maybe i have fixed the problem already, ill have to take it on another spin before i go crazy. The other thing is that it only shifts by itself under load, it never shifted by itself if i just put it on a stand and turned the pedals.

NoRacer

Here's something from Sheldon Brown, too:

https://sheldonbrown.com/autoshift.html

Tequila Joe

Ghost shifting can be hard to diagnose. I had this on my last mtb and tried everything. I finally fixed it by installing a longer cable housing to route the RD cable around the bottom bracket. It seems that when the rear suspension compressed, like when climbing, it pulled slightly on the cable causing it to shift. Before you try that, I would give the barrel adjuster on the deraileur a try. Pull it back and give it a 1/2 twist in the clockwise direction.

T.J.

koine2002

+1 on the lengthening of cables. My LBS found that adding 1/2 an inch or so to that all important short piece near the RD improved shifting in all aspects by about 25%.

mtb-chop

Check ParkTool's website here for derailleur stuff (adjustment, housing length, etc.). The page on shift housing length has some good pictures that illustrate the above observation.