Front Mech Fettling

Mountain Biking Magazine

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Technical & Maintenance

Front Mech Fettling

On the face of it, the front derailleur is a paragon of crudity. It’s just a couple of bits of metal pushing the chain off one chainring and hopefully on to another. But the positive spin on crudity is simplicity. Unfortunately, setting the front mech up isn’t always all that simple, and with narrower chains and more gears it’s now fiddlier than ever. Here’s how to go about it…

1. This compact set of tools will see you right for the front mech. Cross-head screwdriver, 5mm Allen key, pliers.

 

 

 

2. As with all transmission-related fettling, and in fact most bike maintenance jobs generally, this one’s a whole load easier with your bike in a workstand. Then you can spin the pedals with one hand, click the shifters with the other and you’ve still got two feet left to do a little dance. Or something.

 

3. As with football, pitched battles and probably some other stuff, correct positioning is vital. There are two key axes to worry about. Here we’re looking at vertical alignment. You want to have about 2mm between the outer cage plate and the big chainring (with the big ring engaged). If anything, we’re running a bit close here. To adjust, loosen the mech clamp bolt and slide the whole thing up (or down, as appropriate) the seat tube.

4. The other vital axis is the alignment with the chainrings. You need to get the outer cage plate parallel with the chainrings. This isn’t as easy as it sounds, particularly since current mechs have intrigingly-shaped cage plates that often seem to lack handy straight bits to line up with. If you’re resetting a well-used mech it’s probably got bent as well, but persevere. You can always tweak it afterwards.

5. If you’ve fitted a new mech, don’t put the cable on just yet. If you’re working with an old one, let the cable off. Now set the low gear limit. This is the stop that prevents the chain from zipping off the little ring and landing on the bottom bracket shell. This is well worth avoiding. Chances are you won’t get this right first time, but this’ll get you in the ballpark. With the chain on the big sprocket at the back and the little ring at the front, turn the limit screw (labelled ‘L’) so the inner cage plate just clears the chain. On this top-swing mech the low limit is the outer screw. On bottom-swing mechs it’s the inner screw.

6. At this point you may notice that you’ve got some sort of new-fangled full suspension bike complete with a gert big swingarm that gets right in the way of the adjuster screws. If this is the case, you’ll have to use some sort of wholly inappropriate tool like a penknife or something. Not pretty, but it works. Kind of.

 

7. Cable tension time. Make sure the shifter’s clicked to the low gear position and the barrel adjuster on the shifter is most of the way in. Also check that all the cable housing is intact and the cable’s running freely. Then pull the cable through with pliers and clamp it with the bolt provided. Don’t heave the cable through, just tension it so it doesn’t sit slack.

8. Spin the pedals and try a shift. You’re looking to land in the middle ring with one shift. If it doesn’t get there, wind the barrel adjuster on the shifter out a little and try again. If it overshoots, wind the adjuster in. If it appears to be bang on, chance your arm with another click and see if it reaches the big ring. If it doesn’t, make sure the outer limit screw isn’t wound in too far. If it does go in to the big ring, shift in to the small sprocket at the back and set the outer limit screw so that the outer cage plate just clears the chain.

9. So far so good. Drop back down to the middle ring. If it won’t go, you’ve got too much cable tension and need to turn the barrel adjuster inwards a little. If you run out of adjustment on the barrel adjuster you’ll have to reset the cable tension at the mech clamp bolt. If you land in the middle ring OK, check for the chain rubbing on the cage with it in the small and big sprockets out back. You’ll probably find that it rubs in one position. In theory you should be able to make it not rub in either, but in practice it’s nigh-on impossible, particularly if you have a wide-range rear cassette and short chainstays. We like to prevent rubbing in middle/big ‘cos we use that gear a lot, but aren’t too bothered by middle/little rub. Use the barrel adjuster to tweak the position of the cage until you’re happy.

There’s a good chance that a test ride will show up all sorts of deficiencies. Consult our handy table for the solutions:

Troubleshooting
Problem Solution
Mech won’t shift from big to middle Turn barrel adjuster clockwise
Mech won’t shift from middle to small Check low limit screw. If that’s OK, turn barrel adjuster clockwise
Mech won’t shift from small to middle Turn barrel adjuster anticlockwise
Mech rubs on small chainring Wind low limit out slightly
Chain falls off when shifting to small ring Wind low limit in slightly
Chain falls off when shifting to big ring Wind high limit in slightly
Chain won’t go into big ring Wind barrel adjuster and/or high limit screw
Chain rubs in big ring Turn high limit out slightly. If no joy, check cage alignment
General erratic shifting performance Clean, lube or replace cable. Check mech for wear – if there’s a lot of slop in it, replace.

 

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