Problem with front mech rubbing on the crank arm.

homers_double

I have a slight issue which is confusing me at the moment and seems to have developed in the last few months.

When I'm in the big ring the front mech is catching the inner edge of the crank arm and has created a scratch.

Common sense tells me to wind in the top limit screw however if I do that the shift into the big ring still works but I have bad chain rub in the small sprocket at the back.

I measured the BB shell this morning after a ride to make sure I've not bodged anything, its 68mm. Checking the shimano manual online the chainset is good for a 68mm/70mm BB shell and makes no mention of a spacer - which is good because I haven't fitted one.

Bike is a Specializd allez and I have a 105 groupset which is 10 speed. The crank reference is 105 5750 which, looking at the manual references it being a compact.

Any thoughts guys?

dgunthor

sounds like the front mech has twisted? is it a band on or braze on front mech?

homers_double

Band on. When you say twisted do you mean bent or just sort of spun around the seat tube?

Ben6899

homers double wrote:

Band on. When you say twisted do you mean bent or just sort of spun around the seat tube?

Either the band has rotated on the seat tube or the mech is damaged and the cage has become twisted. I'll have a tenner on the former.

dgunthor

homers double wrote:

Band on. When you say twisted do you mean bent or just sort of spun around the seat tube?

spun around the seat tube - the outside of the front mech cage is normally pretty parallel with the outer chainring, is this still the case?

homers_double

I'll get it on the stand and check later, thanks for the tips.

dgunthor

just re-read your post, check the height of the front mech is set ok too - the outer plate should clear the large chainring by about 1.5 to 2mm IIRC

homers_double

From memory I set that up with the little plastic tab that comes on the rings when new. Will double double check though.

homers_double

Firstly, please excuse the quality of the phone snaps...

Secondly, yes I need to clean my chain...

View from the back wheel along the chain line. - Does the front mech look bowed out or should it be like that?



View from over the top of the mech/chainrings



Side view of the chain rings



Front view of the mech/chainrings



I had a bit of a fettle last night and have managed to wind in the limit screws to prevent the rub but (and I know this shold never happen and I dont do it) but when it's set to up in big ring/big ring there is rub on the front mech and at the other extreme, little ring/little ring.

I've always managed to set my MTB up so that this never happens, are tolorances tighter on road stuff?

dgunthor

front mech looks a little twisted inward to me (i normally set a little outward past parallel with the chainset), possibly a little low too

KINGGARY

That front mech is bent, it shouldn't bulge out.
I would suggest you have got the chain jammed at some point causing damage to the mech, I did it myself on an older 105 front mech as they are a little soft.
Get a large pair of pliers, possibly water pump plies and gently squeeze it back in line. the outer cage should be straight not bowed.

homers_double

I did think that, cheers KG.

keef66

+1 definitely bent cage. Surprised your front shifting isn't affected. Get bending!

homers_double

I shall get the pliers out m'lord!