Bicycle Mechanics - Help with front shifter!

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tommyd49
08-01-08, 10:04 AM
On my MTB I am having the problem with the front shifter that it won't shift in to the top chainring without putting a ton of force in, so much that i can only shift it with my right arm, I don't have the strength in my right arm. Ages ago when I first started having this problem I sprayed WD-40 on it but that made it so the part you move with your hand slid over the actual shifting mechanism (this is a Shimano twist shifter BTW). I figured there are three possible causes:

1. This only started after I converted from 42 teeth to 48 teeth with a SRAM front derailleur. Is this because shifters designed to work with 42 tooth cranksets can't generate enough movement to get up to a 48 ring?

2. The shifter needs taking apart and servicing

3. The shifter is just buggered and needs replacing

If any one has got any ideas they would be much appreciated.

Here is a picture of the shifter if that helps

77801

robtown
08-01-08, 10:25 AM
Just a guess - but with the extra 6 teeth could your chain be too short? I've had a RD stretched all the way out because of a short chain and it exhibited the same behavior.
tommyd49
08-01-08, 10:45 AM
Don't think so: I installed a new chain and sized as it told me to on Sheldon Brown's website.

Al1943
08-01-08, 12:10 PM
Was the FD properly adjusted for the new 48 ring?
Was the shifter in the small ring position when the cable was re-attached to the FD?
With the shifter in the small ring position there must be no slack in the cable.
You may need to increase the size of the other two chainrings to reduce the differences in sizes.

Al
tommyd49
08-01-08, 01:03 PM
Yes to both of your questions - the more I think about it the more I think the problem is with the shifters.
Al1943
08-01-08, 04:33 PM
Yes to both of your questions - the more I think about it the more I think the problem is with the shifters.

Was the shifter working well before you changed the big chainring?
tommyd49
08-01-08, 04:51 PM
Yep there weren't any problems like this before
Bill Kapaun
08-01-08, 06:54 PM
Did you raise the FDER for the larger ring?
Have you backed out the H screw a bit (like a 1/4-1/2 turn). It's unlikely the new rings have EXACTLY the chain line as the old. If they sit out 1/2MM too much, it's still too much!
tommyd49
08-02-08, 01:26 AM
Well, I put a new front mech on when I upgraded, and set it up from scratch then