I'm stuck on middle ring, the mech wont change down and I can't change up because the lever is too tight it won't engange the outer ring...
What do the phillips screw head adjustments do and what does the twisty thing next to the handlebars do?
Right fool proof way to set up a front mech.
With the chain off push the mech cage by hand an make sure it clears the big ring by 2-3mm It its too low loosen the band around the seat post and move the mech. If the mech doesn't move far enough then adjust the high limit screw until it does.
With the chain on but the cable disconnected - put the chain on the granny ring and the biggest sprocket at the rear. Adjust the low limit screw until it just rubs on the chain.
Now attach the cable and make sure it is absolutely bastard tight (grip the cable with pliers and really pull. Have you barrel adjuster (the twisty thing on the bars) in the middle of its range so you have some adjustment up and down but if you have pulled the cable tight you shouldn't need it
This sorts out 99% of front mechs.
those cross head screws on the mech itself are the limit screws.
should/might be marked 'L' & 'H'
L = this adjusts how close to the seat post the mech can move (given enough cable slack) the more you tighten this screw, the further from the post it will sit....
H = this adjusts how far out the mech can move, i.e. two clicks of the shifter (on a 3 ring setup) this stop prevents the chain overshifting past the big ring
The barrel adjuster on the shifters is to fine tune cable tension so that the mech sits in just the right place to shift nicely and not rub the chain.
other things to consider....
is the mech straight, in line with the rings
is the mech bent/bashed
is the mech clamped high enough, to clear the rings in shifting but not too high
is the cable inner/outer rusted.
This video is a good explanation of everything said above.
First set the mech height so it clears all the rings and sits approx 2-3mm above the out ring's teeth.
Second set the mech so that the outer cage plate is parallel to the rings.
Thirdly set the limit screws so that the cage drop the chain onto the smallest ring and biggest range. Try doing this by moving the mech by hand at this stage.
Fourthly set the cable tension so that the chain moves between the rings happily. This will take the longest and can be quite faffy to get just so. Try initially setting it up so that when you're in the biggest sprocket at the back the chain is just scuffing on the inner mech plate when in the middle ring. Then screw the barrel adjuster on the shifter in until it just stops scuffing.
Once you've got the idexing sorted out properly, check the limit screws are still ok.
Good luck. It is a pain in the arse, but once you understand what you're doing its ok. Just do little adjustments and you should be ok.
If I convert from a triple down to a double chainring, should I lower the front mech as well or leave it?
That video is spot on. Pretty sure I can do it now just after watching vid just once. Thanks!