Front mech issue

[LondonBikeCommuter]

I've got an Ultregra group set on my cyclocross bike which is used as my daily commuter.

Right from the start the front mech has been a nightmare to keep in alignment. The main problem being that when the chain is on the largest ring at the front the crank rubs against it with each revolution of the pedal. Its already been replaced once under warranty after the rubbing cracked the metal but in reality it gets set up and within a couple of weeks its rubbing again.

Is this a design flaw or am I destined to be permanently adjusting it? Bike warranty runs out in a couple of weeka .
Thanks

[CREPELLO]

Do you ever get it into alignment, then it goes out?

[531colin]

What happens for the thing to go out of alignment?
Possibilities include (but probably are not limited to).......
Cable goes slack
chainset moves/has play
front mech. fitment to frame loose/moving (causing or adding to cable slack)

OOOps! just read your post....you say the CRANK rubs the mech, not the CHAIN rubs the mech.....
that means the mech. creeps closer to the crank over time???....that can't be the cable going slack.

[LondonBikeCommuter]

Do you ever get it into alignment, then it goes out?

Yes, had it sorted a month ago and now its started to rub on the crank again. There's a noticeable scratch/groove where its rubbing.

Up to now it's been adjusted free but not sure after warranty expires.

[CREPELLO]

Do you ever get it into alignment, then it goes out?

Yes, had it sorted a month ago and now its started to rub on the crank again. There's a noticeable scratch/groove where its rubbing.

Up to now it's been adjusted free but not sure after warranty expires.

To hear that the mech has worn a groove in the crank should be a serious concern, because this could weaken the crank significantly in the long term. I'm also baffled as to how the mech would creep outwards, which is against cable tension. Any unintentional movement would normally be inwards, I would have thought. So I can't help any more, but to say this is an issue I would want to seek to rectify under warranty, if I could prove that it's not 'user error'.

[cycleruk]

Obviously something is moving.
Which part of the mech' touches the crank arm?
Is the mech' still in line (parallel) with the chainrings or has it turned slightly?

[Mick F]

Is this just under load, and doesn't happen if you back pedal?

[pete75]

It sounds like adjustment won't cure the problem. Maybe you need crank/mech replacement. Insist it's sorted out under warranty, though you are entitled to repair or replacement long after the warranty expires under the sale of goods act http://www.adviceguide.org.uk/nireland/ ... cement.htm

[fatboy]

It sounds to me that it's twisting around somehow such that a bit of it is getting to be further out than it should. You say you use it for coummuting. Bike racks at work are perfect places for stuff to get knocked.

But it also sounds like the combination of Mech and chainset isn't right. Maybe a photo or two might help.

[grani]

I had a problem with a Shimano 105 5700 double front mech where I got it second hand and only had one half of the shim to take down to 28.6mm seat tube. When it was tightened it just about seemed fine and I used it like that for a while.

On a morning commute the chain was in the big ring I hit a pothole and it made the mech turn slightly on the seattube and start rubbing the chainset. It seems although under normal riding conditions the mech is fine but when the cable is under tension the vibration and jolts on a rough surface can make the mech twist and move.