Mountain Biking - Ghost Shifting

I put this up on the bike mechanics forum, perhaps this forum might offer some more suggestions? thanks.
I have a new low-normal xt rear mech which is currently ghost shifting in certain gear combinations (14-44 seems to be the worst). anyway, its very annoying and dangerous because it could cause me to greviously injure my groin if the chain slips. has anyone any suggestions on how to fix this? i have played around with the adjuster on the back of the mech which works for a while but then it starts its old tricks again. it seems to be at its worst after a long, steep climb and then on the downhill its starts to shift in and out of about three different gears. any suggestions appreciated. i brought my bike to my lbs (both of them) but they are not exactly what you would call technically proficient when it comes to bikes, a bit of a drawback when you consider their occupations. anyway...

If adjusting does not help, maybe you have a bent rear derailleur or hanger? Also, I seem to remember Sheldon Brown suggests the reason for most if not all autoshifting is too much friction in the cable guide beneath the bottom bracket. See http://www.sheldonbrown.com/.

--J

Thanks, I'll try that so.

How old are your cable and housing?

Have you recently changed you chain without changing out your cassette?

Full suspension bike or hardtail?

Check the length of your housing from the shifters to the first frame cable stop. If the loop is too short it can cause ghost shifting.

Also if you have full suspension, the piece of housing from the front half of the bike to the bike half might be too short and when your bike cycles through it's suspension travel it could cause a lot of cable tension. (this could explain the 'after climbing' where you compress the rear suspension).

Another place to look is the loop from the cable stop at the frame to the derailleur. Again, too short and it could ghost shift.

If your shifting in smooth in part of your range, but not in the other. i.e., smooth shifts in the higher gears and crap shifts in the lower gears, that is usually and indication of a bent derailleur hanger.

riding a hardtail giant XtC 3. all the stuff is less than 6 months old. it got a bit of a hammering recently in spain so i will check the deraillieur hanger. perhpas a bit of oil at the cable guide beside the bb might sort it out. i'll have a look tonight but the kool aid acid test will be out on the trail.

Shift your bike into the big ring up front and if your chain is long enough the big cog in the back. Now, without pedaling, click your shifters into the small gears. This will provide slack in the shifter cables. Usually enough so you can slide them out of the cable stops on the frame. Slide your housings around. You can tell instantly if they are rough. Drip some lube on them and slide them back and forth lubing and wiping with a cloth to get the grit out.

Then, put them back on the frame stops. This way you can clean/lube your housing without having to re-tune your bike.

a2, thanks for the tip. I am (hopefully was) having a problem with ghost shifting also. I have an '04 Jekyll w/ XT front & rear deraileurs & deore shifters. Used your tip to lube the housings, also adjusted my rear deraileur tension..hopefully this does the trick. :)

Sounds like a bent derailleur hanger since its more prominent in your higher gears... any LBS should either have a replacement or a alignment tool to check it for you. The way I check without the tool is to take the hanger off (after removing the derailleur obviously), and lay the hanger on a flat surface, like a good countertop. If its warped you should be able to see it on a flat surface.