Derailleur wont derail!

First step is to find out where the problem is, you have 3 choices: Shifters, cable & housing, derallieur (and of course the connections)

Watch the cable when you shift:
- Does it move out of the cable housing correctly? If no, then your problem is either in the shifters or the cables/cable housing.

My first thought was the cables & housing, but you mention that they are fine. Next, hold and pull tight with your hand the cable. As you switch the lever in either direction, do you feel the cable pull and release correctly? If no then it is the shifters.

Are your shifters, derallieur, and cassette all compatible? I'm guessing you've thought about that.

My next thought is that either: There is too much slack in the cable, or, that the derallieur has excess play in the mechanism somewhere. The deralliuer should move responsively to any movement in the cable, until the H/L limits are reached. If the cable can move without any response in the derallieur, then depending on the derallieur: something needs tightening or, you need a new derallieur.

Hope that helps!

I'm guessing that it may be your last suggestion. I'll have a good look and let you know the outcome.

Thanks for your help.

Presumably you're talking about a rear derailleur? If so, the problems you're having could be caused by an incorrectly adjusted b-screw. This is a small screw located at the point where the derailleur mounts to the hanger on the frame, and it determines how close the top jockey wheel is to the cassette. Shift onto the biggest cog and adjust the b-screw until the top jockey wheel is as close as comfortably possible to the cog. Hope this hepls.

I'm guessing the closer the better?

John Picton said:

I'm guessing the closer the better?

Yep, but not so close it starts making a grinding noise. Adjust the b-screw with the derailleur shifted onto the biggest cog at the rear and the smallest at the front; if you adjust it in some different gear combo, then when you shift to a different gear you might find the RD rubs the cogs, which of course isn't good.

Will give it a shot. Cheers.

mjw_byrne said:

Presumably you're talking about a rear derailleur? If so, the problems you're having could be caused by an incorrectly adjusted b-screw. This is a small screw located at the point where the derailleur mounts to the hanger on the frame, and it determines how close the top jockey wheel is to the cassette. Shift onto the biggest cog and adjust the b-screw until the top jockey wheel is as close as comfortably possible to the cog. Hope this hepls.

Sorry, but that is just BS.You have sent him off on a wild goose chase. Congrats!

Care to explain why?

mjw_byrne said:

Care to explain why?

Well, I get a bit weary or writing the book on obvious stuff, but if the upper pulley isn't hitting the largest cog and causing an obvious noise, then it isn't an issue. Even if the B screw isn't hitting the big cogs, it could be adjusted for a more optimum position, but if it isn't quite optimum for the ARs, it still won't hose shifting as he describes.

John Picton said:

Somethings not quite right here! I know the basics, I place the chain on the smallest cog, I release the gear cable from the mech, I set the high and low correctly, I taughten the cable then refasten it to the mech then use the barrel adjuster to adjust the tension until the gears change according to the indexing when I move the levers.

Simple? Hmmm. Apparently not. It appears that the gears will either move up cleanly but not drop, or drop cleanly but not move up. On occaison, when moving to a higher gear when clicking the lever the derailleur will not move at all until the next click, then moves fine but is infact a gear behind!

I try to keep the whole lot as clean and well lubed as possible. It's irritating because the gearing works perfectly in one direction but not the other. Surely if they are indexed gears then this shouldn't be the case.

Any idea where I should start looking? I thought maybe dirt in the cables but they seem OK. Or perhaps it's a spring somewhere? Any ideas would be appreciated.

Are the cables new? If so did you lube them?

boudreaux said:

Well, I get a bit weary or writing the book on obvious stuff, but if the upper pulley isn't hitting the largest cog and causing an obvious noise, then it isn't an issue. Even if the B screw isn't hitting the big cogs, it could be adjusted for a more optimum position, but if it isn't quite optimum for the ARs, it still won't hose shifting as he describes.

Well, it seems to me that if b-screw is holding the upper pulley too far from the cogs, it will make the drivetrain reluctant to shift for obvious reasons (chain just bends from side to side without derailing, you get the idea).

And I would second the advice about making sure the cables are lubed, housings are cleanly cut and so on. Cable friction could certainly cause the problems in question.