Shimano rear derailleur help?

Question

I am having trouble with my Shimano LX rear derailleur. It keeps shifting by itself and clunks up and down between gears. This is the second bike that it has happend on, first it was a Gary Fisher Cake 3 DLX now it is on a Kona Stinky. What is the problem? Are the shimano shifters compatible with the rear derailleur of SRAM? And if i up graded to an xt or switched over to a SRAM x-7 or x-9 would that work? The bike has ben tuned up and still has the same problem. Could it be the chain is the worng length? Should i switch to a long cage derailleur instead of a mid cage? Please help if you have any usefull help!

Answer 1

Upgrading probably isn't necessary.....the LX is decent stuff. I've run a couple of LX rears without issues. As long as it isn't bent or just worn smooth out it'll work just fine. Sounds like you need some simple derailleur tuneup and equipment checks. If the derailleur was set up correctly and the cable is tensioned as it should be, then definitely have a shop check the hanger for alignment -- lots of shops do NOT do this when they "tune up" a bike, but it's vital for shifting performance and they're easy to bend. Also, some f/s bikes have issues when the cable housing is cut too short or misrouted....when the rear compresses, it can ghost shift if those things aren't correct.

Something a lot of people overlook is a clean drivetrain. If you have super-muck built up on the jockey pulleys, inbetween the cogs, or if you have grass and twigs jammed between the cogs, it'll really cause the chain to ride high and misshift or skip. Dirty chains don't always glide over the teeth the way they should either.

Unless it's been broken and had links taken out during the repair, your chain length was probably set up right from the beginning by a shop. If it's too short, yes, you can get ghost shifting when the shock rebounds, but it's probably not that. Eyeball your chain under tension to see if it's twisted or has bent links, and measure it for wear to see if it's time to replace it. Check out the teeth on the front and rear cogs for damage, too.

Shimano shifters will only work with Shimano rear derailleurs (well, any brand but Sram's). If you want to run a Sram rear, you need to get Sram shifters because the cable that's taken up with each click of the shifter is different between the two brands -- Sram rears are manufactured with this in mind and so the Shimano shifters won't work with them. You can, however, can use Sram shifters with a Shimano rear derailleur as long as you get one of Sram's models that are made for it (Rocket, Attack).

Check Park Tool's website for great how-to pics on tuning up your drivetrain and cutting your chain to the proper length. Much easier to see it there than try to describe both repairs in text!

If you're running the standard xc gearing with a 44T up front and a 34T in the rear, you should get a long cage derailleur. A mid- or short-cage isn't always a benefit, but if you want to run them just make sure you aren't trying to run them with more "capacity" than they're designed for. If the Stinky is set up with a double up front instead of the triple, you can use a shorter cage. Tooth capacity and cog max's are explained well at Sheldon Brown's site and on Shimano's European pages (but not the US pages for some reason?). Hope this helps some!

Answer 2

SRAM use a 2:1 pull ratio on their gears, whereas Shimano use a 1:1. This means that if you change the derailier, you must change the shifter.

Your problem is quite common. Cable stretch. You need to get your gears re-indexed. This will cost aboht £6 at your local bike shop. Failing that, try the help sheets at:

www.parktools.com

Lucl

Answer 3

Strange. Goes up and down gears by itself? I haven't a clue how it would go onto a larger cog by itself, that needs your input. I've been a bike mechanic and co owned a shop too. Rode for decades as well, never heard of a bike down shift by itself.

If the bike was tuned in a shop (you don't say) bring it right back they're fix it for free if it was recent.

Answer 4

check your cable tension and the limits on the highs and lows, also it could be your chain - if it is too wide it could catch the cog thats next to it - and make sure the derailleur is not bent and that the derailleur hanger isnt bent.

also if you have old shift houseing you may want to grease the cable.