For all the tech boffs

[keith]

Hey guys my chain sits fine on 2nd gear at the front and the smallest ring at the back, but on 1st gear at the front and the largest ring at the back the chain touches the front derailleur. I have tried adjusting the front derailleur but the chain still touches. Please help! Being a new guy a wanna learn all this stuff myself instead of having to run to my LBS everytime. Shot

[Guest poser]

Well, I am most assuredly not a tech boff - but you should not be riding those gears. You are doing what is called cross chaining (I think). This screws up your chain and cluster big time. You are forcing the chain into an acute angle which is too severe  so it has to rub against the deraileur - and it will do so no matter how you adjust it.

 

 

poser2008-04-13 11:37:31

[TheRev]

Some might not agree, but most chains and derailleurs are not made to be used at such an angle, ie largest front and largest back.

 If you read your manual that comes with a new bike most that I have read warn you against this, and encourage shifting so as to keep your chain straight. Angles cause serious stress on the chain and damage the rear derailleur and the chainrings.

For those that remember the old gripshift technology, it allowed one to get around this problem of chains touching the derailleur because you were able to do micro adjustments left or right. But this did not make the behaviour any less damaging to your equipment.

[keith]

Hey guys sorry I wasn't clear but I'm not riding with the chain at an angle. The chain rubs when I'm on the smaller ring in the front, and the larger at the back- the chain is straight. But when I am on the bigger one at the front and smallest at the back- the chain doesn't rub, and the it is straight. Make sense?

[TheRev]

Keith, not sure how much you know about the limit screws on the top of the front derailleur. They set the outer and inner limits that a derailleur can go. Your problem might just be that the derraileur cannot travel close enough to the left (ie toward the frame) to allow you the clearance you need. It could also be that your cable is too tight, thus not letting the derailleur go far enough left.

There could be two other problems.

1. Your crank and BB are too far away from the frame and thus not aligning with the rear cassette and/or not allowing your front derailleur to go far enough left, even if you release the set screw all the way.

(solution - check BB is all the way in and that you have the right length BB.)

2. The other possibility is that you have mixed components. eg, a 7/8speed narrow front derailleur with a 9/10 rear  cassette.

(solution - get a wider cage front derailleur.)

 

If you don't come right, post a pic of your set up from eye level and not too far away.

[Veloce]

I agree with TheRev. make sure that the limit screw (should be labelled "L" for that chainring) is not limiting the movement of the derailleur. This is the most likely cause seeing as you say you hev already tried adjusting the cable tension. 

[keith]

Thanks a lot guys I have a look tonight! Much appreciated

[stratus5]

I had a similar problem a while ago. Even with the limiter screw at its minimum adjustment and the cable slack the derailer would still touch.

 

The frame was slightly narrower than standard, so the derailer used a plastic shim (2mm?) where it clamped to the frame. This shim width was just enough to push the derailer into the chain.

 

The fix was to take out the one half of the shim on the chain side. I put some tape on the frame at this point to stop it getting scratched. It still tightened up properly with only one shim, but as the derailer now sat 2mm closer to the frame it no longer touched.

 

Craig

 

[Climate]

Another alternative may be the angle of your front derailleur, relative to the chainrings.  The outside of the cage needs to be parallel to the chainrings ... sounds like your cage may be sitting slightly proud nearest the back of the bike.