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A friend has an older road bike with index gearing.Back cassette is fine, but at the front the chain keeps slipping onto the granny ring if your on the lower gears at the back.
What's the best fix for this please?
Chainline may be out, or the middle ring may be worn - or all tranny components.
If you can screw the barrel adjuster out on the left hand shifter
I was assuming gears set correctly and front mech not slipping down to granny position.
Order for setting up a band-on front mech.
1) Position on seat tube with cage parallel to chain rings, so outer cage clears big ring by say 3mm when shifting up. Take care that all teeth on the outer ring are the same height when doing this.
2) With the chain under slight tension on the largest rear sprocket and the smallest front chainring, adjust the lower limit screw so the inside cage plate just misses fouling the chain.
3) With the LH shifter selecting the small chainring, pull taut and attach the cable. Ensure any barrel adjusters are set perhaps screwed a third out at this point.
4) With the bike on a workstand, turn the drivetrain and select big ring. While doing this repeatedly, adjust the outer limit screw so that the front derailleur cage travels far enough to shift the chain up, but doesn't throw the chain outboard. Ensure that the outer limit screw isn't screwed in so far that the mech cable is under high tension when big ring is selected.
5) Use the barrel adjuster to ensure the rest position of the front mech when big ring is selected positions the cage in a good position for most rear sprockets to be used without the chain fouling the front mech cage.
This is how I do a road double. The order of parts 4 and 5 can be interchanged. Note that sometimes the chain will always foul the front mech in certain cross gears. Comments above regarding a bad chainline or worn drivetrain may be correct.
ampthill - Member
If you can screw the barrel adjuster out on the left hand shifter
on a triple the only thing that you can adjust* (*other than below and i await correction!) that holds the mech in correct position is cable tension - and as above implies cable needs to be tighter - that is pull mech out so chain doesn't contact it when on smaller cogs at back and then drop down at front - a quick check would be a finger pull on the front cable while changing down at the back - mind you as suggested also 9Xout of 10 better starting from scratch than fannying
* front mech may not be correctly positioned or have been knocked out of square or bent
plus all the other comments about worn stuff
edit just trying to clarify reason for suggestion not correct!