Rear derailleur fitting problems - Now resolved

[urbanfireblade]

I think that everything you are using is compatible it just needs fettling some more......

I agree, i have once come across a rear derailleur i fitted that no matter what i tried, it just wouldn't shift correctly up and down, i could get it to shift up fine but shifting down it just wouldn't shift quite right, i swapped it for another old derailleur and problem solved. I guessed that the spring tension of the derailleur was to blame as everything looked spot on when inspecting it.

So try another derailleur and you may find the problem is solved. I'm placing my bet on that being it.

 

[marin man]

I reckon you're right as the spring may be a bit worn out so it will be a sod to set up.
Try the six/seven and if not get a nice xtr m900 and problem solved

 

[urbanfireblade]

Also crouch down behind the bike and check the derailleur is hanging vertically(ish) and not pi$$ed at an angle(bent hanger) and also that it isn't twisted left or right.

 

[marin man]

And if all else fails go to halfords and ask them or buy an apollo

 

[hamster]

The cog pitch is different.
5, 6 speed: 5.5mm
7 speed: 5.0mm
8 speed: 4.8mm

The cable pull per click is about half that of the cog pitch

The floaty top jockey wheel takes up a lot of inaccuracy (which makes shifting less tetchy). To get a mixed 7/8 system to work you need clean cables and a good floaty top jockey.

In time, it won't shift accurately, which is why people tend to use the thumbie hidden click approach: if all else fails, switch it to friction.

 

[gerryattrick]

Thanks for all the replies. They'll help when I get to use my new Lidl repair stand today for the first time.

I'm unsure about how it can be shifter/derailleur speed incompatibility as it shifts down OK and only jumps a ring when moving up the gears i.e. towards the smaller rings. I would have thought that incompatibility problems would be worse moving towards the bigger rings!

I have thought about fitting an 8 speed cassette, but won't I have to change the freehub and then have the wheel re-dished, making it an expensive solution?

I'll definitely fit new cable outers.

I hate to think it's the derailleur as that will be another unwanted expense, but I will try swapping it with a new Acera mech that I know works well to see if that's the problem before looking for another XT or DX.