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Weird ghost shifting

942 views 12 replies 8 participants last post by  bryanus 
#1 ·
I have a problem with my new bike ghost shifting on me. I have read other posts on ghost shifting and like most people, my bike only does it when pedaling up hill or pedaling harder than usual. The derailleur does not shift gears though, it stays in the same gear. I just had knee surgery so I won't be riding for awhile, so I figured instead of taking this to a bike shop I would try to play with it myself in order to learn how to repair my bike. Does anyone have any suggestions on what to try first? I have a park tools blue book to go off of, and I am mechanically inclined just not with bikes. The bike shop where I purchased the bike went out of business shortly after I bought the bike so I can't have them fix it. The bike is an '09 Trek Fuel EX8 with a Shimano Deore XT Shadow rear derailleur, a SRAM PG970 9 speed cassette, and Shimano SLX SL shifters. Any help would be appreciated.
 
#4 ·
I would probably go through the adjustment process as a first step. Here's a link to a video that might help: http://bicycletutor.com/adjust-rear-derailleur/ .

As for the hanger, I like to always have a spare anyways, so you could certainly buy a second one and try it out to see whether it fixes the problem. If the new hanger fixed the problem, then great. If not, then you have a spare hanger. But I would still do the adjustment process first.

Park does make a derailer hanger alignment gauge that enables you to measure and know for certain whether you have an alignment problem. But buying your own gauge is probably not cost-effective.
 
#5 ·
In addition to the other suggestions...

  • Do you have a full suspension bike? If so, sometimes the cable housing length & routing can cause ghost shifting. This will usually occur when hitting a bump though. The suspension compression will cause the housing to be slightly pulled out of the stop causing a momentary shift.
  • Gummed up or dirty housing can cause all manner of shifting problems. In particular, you may find that even though you've adjusted the cable tension so that it's perfect, it will sometimes take two shifts to shift once. (I ride in dusty conditions and have to replace sections of the housing once or twice a year.)
  • A frayed cable can also cause shifting problems. The symptoms will often manifest as similar to those of gummed up housing until the cable finally breaks at which point you'll be stuck in either your high gear or low gear (depending upon whether you have a high normal or low normal derailleur)..
 
#6 ·
Agreed that cabling and their housings are the prime suspect for ghost shifting. If you don't feel comfortable re-cabling (and you shouldn't if you don't own a dremel), take it to a bike shop and ask them to do that for you. Also a good suggestion was made to go ahead and replace the hanger; it's cheap and you need a backup anyway.
 
#8 ·
Thank you for the derailleur adjustment video, that will be helpful. The bike is a full suspension but I never have a problem during bumpy sections. Also I haven't gotten the bike very dirty yet so I don't believe there are any problems with the cables being dirty or in need of replacement. I think I'll buy a derailleur hanger and see if that solves my problem. Thanks for the help!
 
#9 ·
Chad5080 said:
I think I'll buy a derailleur hanger and see if that solves my problem.
Before putting a new derailleur hanger on the bike, I would first determine if there's something wrong with the alignment of the current hanger. You should know too that even new hangers need to be aligned using the hanger alignment tool (Park DAG-1 or equivalent).

If the current hanger is not overly bent, but is off a little, I'd use the alignment tool to bend it back into alignment. If it's really out of whack, then replacing it would be a good idea.

That said, it's worthwhile having a spare hanger because they do occassionally break. Carry it in your pack with your spare tubes and whatnot.
 
#12 ·
dan0 said:
actually I would bet that the rear cable housing is too short, the shadow needs about 1/4 - 1/2" more housing than you think.
After thinking about it some more, I'm with dan0 on this one.

Pedaling hard or pedaling uphill is probably making the suspension squat. As I noted in my earlier post, if the housing is too short, it'll be pulled slightly out of the cable stop causing a momentary shift. But it's only momentary since tension on the cable will be released when the suspension extends again. Other types of ghost shifting aren't quite as predictable; sometimes the bike will auto-shift back; sometimes it won't.

Chad, you can check to see if this is the case by putting the bike in a stand with the rear wheel supported. (It's important to support the rear wheel because you'll be removing the shock and you don't want to let the swingarm drop when the shock is removed. On some bikes this won't matter. On others, you may well damage something.) Remove the rear shock and then move the swingarm upwards. Watch the housing to see if there is frame interference causing it to be pulled out of its stop.

If you lack a stand or are feeling lazy you may be able to still check this by deflating the rear shock most of the way and then compressing the suspension. You may need help from someone else to watch the housing though.

If there is interference between the housing and the frame, you'll need a slightly longer housing segment. You may need a new cable too.
 
#13 ·
I'm having this same issue with a 2009 Trek Fuel EX 9.8.

I just got this bike (used) an d took it on its first ride today. When I got it last week, I tinkered with it and did a lot of parts swapping with my other bike.

One of the things I noticed right away was that the routing for the rear derailleur housing was wrapped very strangely (around the seat tube). I thought this looked like it would hinder cable movemnt, so i routed it more cleanly, and the end result actually matches the way it routes on Trek's website.

Anyway, today's ride I had a lot of ghost-shifting. I've never had this problem before with my Epic, which has full-length housing for the rear.

The Fuel EX suspension design is pretty different than the Epic, and it routes the housing up top versus underneath. When riding, the frame pushes downwards and causes the segment of derailleur housing to bend upwards. Pedaling forces and bobbing flex the cable housing into bigger bends as the suspension moves through more of its travel.

As the housing bends/loops, there must be some additional tension on the cable, which causes the ghost shift (even though I made sure to make my cable long enough, IMO).

I'm still trying to figure out what i can do about this issue, but right now, i'm not sure if it can really be fixed. Maybe some thicker 5mm housing would help minimize the flex and subsequent tensioning of the cable? Or maybe go for a Nokon compressionless cable system?

I'll try a few things and report back if I have any success.
 
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