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Ever since I got my bike (Surly Long Haul Trucker), it has been prone to shifting on its own. This is almost guaranteed to happen when I stand up on the pedals, and happens regularly when sitting as well. I have the feeling that this is a dangerous condition.

It seems from reading about the problem that it is due to the cable catching somewhere and creating a clutch effect. (read about it here) There does not seem to be a way to adjust the friction of the shift lever. I've tried:

  • greasing the cable guide under the bottom bracket (no effect)
  • backing off the tension of the cable with the tension knob on the cable casing (no effect)

Can anyone relate how they overcame this? My next step is to further loosen the cable by adjusting the connection to the derailleur?

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There are bikes that have automatic transmissions, so I've changed the title and first sentence. Please feel free to roll back the edit if I've changed the meaning. –  Neil Fein Apr 20 '11 at 16:31
    
I have a bike with an automatic transmission, that's also how I read it first. It's not fun when you don't actually want it to change gears just then. –  Torben Gundtofte-Bruun May 1 '11 at 21:05

4 Answers 4

up vote 7 down vote accepted

Neil is right, most all "auto shifting" or "ghost shifting" is the result of cable-tension problems. If the cable is a bit loose, the derailleur will try to shift "up" to a smaller cog. If too tight, it will try to catch the next larger cog. Cables stretch, especially after a short period where the new cables stretch to the point they're stable. after that, they should be good for a long time. Try this.
Shift to the highest (smallest) cog and try to pull the DR cable away from the frame. If there's a lot of slack, it's too loose. If it feels very tight.... With either condition, make sure the shifter is in the highest position, and loosen the cable at the DR and then pull it snug. Re-tighten the securing bolt and run it through the gears; it should be very close. Ideally, the chain should jump smartly to each cog both up and down, and seat solidly on the cog. If it's just a bit off, you can play with the barrel adjuster either at the shifter or at the DR. Another cause of such problems is a bent rear DR "hanger", the part that the DR attaches to. Some of these are very soft aluminum and can be easily bent.
Make sure the DR cage is exactly parallel to the cogs and the hanger is not visibly bent.

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1  
If you can't find anything else, try crossing the gear cables under your down tube. That way the slight frame bending as you pedal is somewhat neutralised. The best way to do this is swapping the cables at the top of the downtube, but that may be new cables and outers if you don't have enough spare cable. –  Мסž Apr 20 '11 at 22:19
    
In this specific case, given that it's a Long Haul Trucker I'd doubt that frame flex is the issue. Those things are ridiculously overbuilt. –  lantius Apr 20 '11 at 23:58
    
After shifting to the highest cog and adjusting the cable tension, I haven't had an auto-shift in 3 days! Thanks –  mcgyver5 Apr 25 '11 at 16:27
    
What's a "DR cable" and where is it going to be on a bike? I have the same problem. –  Nicholas Jul 18 '13 at 8:09

Grab hold of the derailleur and try and wobble it. If there is any play it will move to towards the wheel under pressure, the chain will try and stay straight and will try to jump up to the next largest cog or slip between cogs. Hopefully it’s just loose and can be tightened with a hex key, else the housing is worn and its time for a new one.

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Check your frame for hairline cracks. One of my bikes did shift to a larger sprocket when going uphill and back down immediately after the incline. The situation actually lasted for an astoundingly long time, several Mm. I only found out after the right hand chain stay finally gave way completely. Apart from that particular bike, I never experienced ghost shifting.

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This is definitely worth checking for. Unfortunately you will need two people - a rider to sit on the bike and provide force to open up the cracks, and a second person to find them. –  Мסž Apr 20 '11 at 22:17

I've heard this referred to as ghost shifting, and a proper setup on your drivetrain will almost always make it go away. Someone else may be able to speak to the exact circumstances that cause this, but adjusting cable tension (see M. Werner's answer about this) and front and rear derailers, along with keeping your drivetrain clean and properly lubed, will go a long way towards eliminating this problem.

(While this has worked well on my full-sized touring bike, I have another bike where I've been unable to completely eliminate ghost shifting.)

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