My rear deraileur was switching the gears for me

Hey guys today i went bibking with a friend and i noticed that my rear gears were switching all by themselfes... ,i understand mechanics but what it the heaven is causing this to happen.The rear deraileur is a Deore LX,the bike is practically new.I checked the cables and they apear to be OK...but my guess is that the B tension screw maybe is out of set...or am i wrong ?,any advice .
Thank you.
Dan

 

Hi there,

I don't know anything about fixing bikes at all. But my mom's practically new Cannondale road bike started doing the same thing. She took it to the bike shop and they put on a new chain and that fixed the problem. Her chain came with the bike and so was pretty new but they said it got messed up because it had some sort of coating on it that was supposed to be cleaned off before putting it on the bike in the first place.

 

errr are you sure this bike shop knows what they're talking about? cannondale chains are covered in wax when they are shipped to the store. they are to be assembled with this wax on. it will wear off within thirty days of riding, and then it can be lubricated with typical lube.

sounds like the bike in this thread needs a simple cable tension adjustment. all good bike shops offer a free tune-up after 30 days of riding that involves adjusting the cable tension. cables on a new bike will stretch resulting in poor shifting, which may cause the bike to "shift by itself"

 

I'D SUGGEST PUT IT HALFWAY THROUGH THE GEARS ON THE BACK CASSETTE, PEDAL THE BIKE WHEEL IF IT JUMPS OFF THE COG! EITHER TIGHTEN OR LOOSEN THE DERAILEUR CABLE ACCORDINGLY. IF IT IS ONLY HAPPENING IN EITHEER THE TOP OR BOTTOM GEAR THEN MAYBE YOU NEED TO ADJUST THE SHIFT (H/L TENSION SCREW) IF IT WASN'T HAPPENING BEFORE TRY CLEANING YOUR CABLE.

 

I don't mean to hijack your thread. I've searched with no sucess (tricky wording of the problem I think)

I'm having a similar problems except it doesn't change gear, the deraileur moves as if it's going to change, it makes me nervous cause it always happens as i'm pushing down every other revolution and I feel like I'm gonna sack myself. It occurs on the 3 highest gears. I tried adjusting the barrel by both tightening and loosening with no sucess. The cable look good as well. Any thoughts.

 

HAVE YOU TRYED CLEANING THE SPRING AND THE DERAILEUR HINGES, I FIND THAT WHEN I HAVE BEEN ON MUDDY TRAILS ON MY MTB, THAT WHEN I LEAVE IT TO DRY AND DON'T CLEAN IT ADEQUATELY, THE DIRT DRYS AND BUILDS UP ON THE SPRING STICKING TO THE GREESE OR LUBE, THIS DIRT WILL ALTER THE LENGTH OF THE SPRING, SINCE THE DIRT ADDS TO THE DIAMETRE OF IT, TRY CLAENING THE SPRING, I USE A TOOTHBRUSH, BUT OTHER THAN THAT YOU COULD POP OUT THE PINS IN THE HINGES, DISASEMBLE IT, CLEAN IT THOROUGHLY "DESEIL WOKS GREAT" OR EVEN A TYPICAL DE GREASER,THEN REASSEMBLE THE DERAILEUR, HOWEVER THIS IS TIME CONSUMING.

THAT IS ON THE UNDERSTANDING THAT YOU HAVEN'T GOT THE CABLE TO TIGHT WHICH WOULD PREVENT IT FROM CHANGING, THAT IS THE USUAL PROBLEM THAT PREVENTS IT FROM CHANGING, WELL THAT AND A BUILD UP OF CORROSSION OR DIRT INSIDE THE CABLE LINER.

FROM MY EXPERIENCE THAT IS, THATS THE USUAL CULPRIT FOR ME, WHEN SHIFTING STOPS SHIFTING TO THE HIGHER GEARS 7 8 9

 

Sorry, I wasn't more clear the bike changes gears fine. The problem is when I'm in the any of the 3 highest gears the deraileur will just "act up" and attempt to change without me doing anything.


thanks for the response!

 

what speed groupo are you using, and what speed deraileur, your not trying to run a 9speed cassete with a 7speed deraileur are you?

Thats the only other conclusion i can come up with, other than your running a chain that have one or 2 stiff links that cause the problem when you get in smaller cogs.

buut as i stated when mine goes dicky, the spring requires cleaning, all the gaers 1-7 run fine, its when i'm dopping from 7 to 9 that it won't drop because of the muck on the spring.

what speed groupo are you running? other than that, if it is all clean and everything is as it should be, you've had your chain set on the 5cog of the cassette, you've got it running smooth on that, and you have it flicking 1 up 1 down smoothly, progress to 2 then all the other gears should be the same. as smoothly, well unless you've worn cogs on your cassette >last check if all else fails.

 

There are MANY things that can cause a Der. to act funny. The most common would be incorrect cable tension and friction in the inner wire do to dirt or corrosion. If the chain drops down the cogs switching into a higher gear: tension the cable by turning the adjuster to the left a quarter turn at a time. If the chain jumps up the cogs to a lower gear: adjust it to the right (less tension) a quarter turn at a time. Lube inner wire in both cases.
Other possible problems: Dirty Drive System, Bent Der. Hanger, Bad cable, Etc. A worn chain and cogset will not cause "ghost shifting" but will instead tend to skip or "pop" within the same gear.
--Hope that helps...

 

Being "practically new", I would suspect slack cables and would try tensioning them first.

Matt

 

see what sheldon suggests.

http://sheldonbrown.com/derailer-adjustment.html

or

http://sheldonbrown.com/autoshift.html

 

Must be magic!

 

if when your in all your largest gears, and your jocky wheels are hanginging at 90 degrees or one under the other, and the top cog ain't quite touching the top cog, then usually its set okey. i only ever use that to alter the alignment of the jocky wheels, just to ensure that there is correct tension on the chain.

check the message above.

i have 9sp deore on my MTB, works great for me

 

BINGO BANGO... upon closer inspection one of the links is siezed! The bike is brand new, how could this happen?

 

probably just a tight linkage when its been put together, a little dirt got down the side and tightened it up more.