Is it possible to change the angle of a rear mech?

[swee'pea99]

I'm trying to fit a 32 tooth sproket on my road bike, and the rear mech won't *quite* cope with it. It's very nearly there. The thing will run. It's just that every five or six teeth, the sprocket tooth seems to correspond with a jockey wheel tooth, making the mech 'jump' slightly.

If I could find a way to make the whole rear mech fold back just a few mm, it stops the jumping. I can hold it with my finger, and it runs smoothly. But when I release it, it goes back to its 'natural' position, and starts jumping again. It's hard to describe, but I've tried making a brief vid to show what I mean. Starting off jumping, then smooth when I hold, then back to jumping again.

I've tried adjusting the one screw I can see, but it doesn't seem to make any difference.

Any suggestions much appreciated.

View My Video

 

[ColinJ]

Adjust the b-screw?

 

[Nebulous]

There's something called the 'b' screw. It needs to go as far in as possible to move the jockey wheel away from the cassette. If you screw it right in and you still don't have enough clearance there are several tricks you can try at your own risk - including putting the screw in the wrong way round to give you more clearance.

 

[Peteaud]

As above.

on the pic below its the b tension screw

 

[swee'pea99]

Fantastic! CC comes up trumps yet again. That's it. The B screw. There is one, and it is screwed all the way in. I shall try reversing it, as suggested. And if that doesn't work, get a longer one, as per Sheldon's page. Many thanks.

 

[swee'pea99]

Chapter 2

(This one could run & run...)

Ok, I tried reversing the screw, and fitting a longer screw, and nothing works. I think the root of the problem seems to be a mis-alignment of the B screw hole and the related 'ledge' in the dropout that the screw end is supposed to engage with/sit on. If you look straight through the threaded hole, you can see the 'ledge' only occupies the top half of the hole, rather than the whole thing:

As a result, when you try to reverse the screw, as per Sheldon's suggestion, all goes well till the tension increases, at which point the screw head suddenly pops past the ledge, and becomes totally inoperative:

Time for Plan B : the longer screw.

Again, all goes well at first...

...but when the tension gets high (which of course is the point at which I'm about to achieve the aim of the exercise - moving the whole mech back a few degrees), the screw again slips off 'the ledge', and stops serving its function.

Any ideas, suggestions, bodges?

Many thanks.

 

[lulubel]

Any ideas, suggestions, bodges?

Get a longer rear mech?

 

[RecordAceFromNew]

Seems to me the shorter screw slipped because it was too far out - it doesn't seem to be lined up straight, so it was able to slip - must avoid that happening or you will start damaging the mech's thread.

If you reverse the longer screw, since it has a squarer/bigger head, it might work. You want to set it only as far out as it would catch the ledge securely.

 

[Peteaud]

looking at the bottom pic it looks as if the thread is on the pi$$.

Is the screw / thread ok.

looking at pic 2 it also looks wrong.

 

[Red Light]

looking at the bottom pic it looks as if the thread is on the pi$$.

Is the screw / thread ok.

looking at pic 2 it also looks wrong.

I agree. Have you cross threaded it? If not and that is the way the thread was cut I would be inclined to twist the bit of metal with the screw through it round a bit to make the screw line up better when the mech is at the extreme. Either that or glue a bit of metal onto the "ledge" to move it closer to the screw.

 

[mickle]

What is the mech's recommended largest sprocket?

The thing is.. The angle of the rear mech's parallelogram is determined by the 'angle' of the inderneath of the cassette sprockets. Buggering about with it to enable it to clear the oversized sprocket will have a knock on effect, diminishing its ability to influence the chain over the smaller sprockets.

Having said that... You could try fitting a smaller top jockey wheel. Sometimes a swap of 11t for 9t is just enough to clear.

 

[Peteaud]

That just looks so wrong.

How tight (wobble wise) is that screw.

 

[sidevalve]

Try fitting a small nut to bottom of the longer screw when it's in position. If you can get one to fit [you may have to file away the sides a bit] and fix it in position with locktite [or even a dob of paint].
If you decide to bend the bracket be VERY carefull as small components like this are often unwilling to move much [if at all] before snapping.

 

[swee'pea99]

Thanks all. I agree - the angle of the screw thru' the hole does look 'all wrong'. I've never cross-threaded it, but it's a secobnd-hand bike, so I guess it's possible that someone did (having said which, you can usualy tell, and it's doesn't feel as if it's been cross-threaded). It's strange though - if it hasn't been damaged, the 'looseness' of the fit isn't really what you'd expect of a 105 mech. If it was some cheap 'n nasty, maybe. But at the 105 level, you expect a bit more precision, don't you?

I agree also that I'm hesitanet to start bending the bracket (tho' I can see the sense in it)...I suspect it would start to snap, then I would start to cry, and it would all get a bit superfluous. My favourite suggestion so far has to be swapping the jockey wheel. I didn't know they came in different sizes, but if they do and I could get one even a teeny bit smaller, I think I might be in business. My gut says it's a matter of fractions of a mm. (Maybe I could file the jockey wheel teeth down a bit...)

Thanks again. Much appreciated.

 

[Peteaud]

looking at the gouges (sp) i would just get another mech from fleabay.