Tuning the Front Derailleur
||If the old bike isn't shifting smoothly on the front end, the problem can
be in the cable (stretch or hangup), the chainrings (worn teeth or ring warp), the chain
(stretch or warp), or the derailleur itself (position, cage shape, or limiting adjustment).
let's check everything BUT the derailleur. Check the cable. Make sure it's still tight
when the chain is on the smallest chainring. Pull it to be sure it slides and rebounds
freely. A bottom-pull derailleur with cable-housing (where the cable goes downward
from the derailleur into a sleeve) is especially prone to sticky cables.
See the section on cable cleanout and cable
Checking the cable tension and testing for cable
|If you have a grip shift, take it apart and clean it (a
cotton swab and mild soapy solution works). Grip shifts tend to get gritty
inside. See our section on cable replacement
for full instructions.
The grip shifting knob pulls away from
the body, exposing the cable and ratcheting mechanism.
||Check the teeth on the chain rings. Worn-down, bent, or broken teeth won't
transfer the chain properly. Check the rings for warp. You may be able to straighten out a
slightly bent chainring by clamping it in an adjustable-end wrench. Replace any chainrings
with significant damage.
Inspecting the chain rings for damage.
|Check the chain. (A warped chain is more likely to create
trouble on the rear derailleur.) Make sure it sits correctly on
the rings with no stretch or twisting. See the section on chain
maintenance. Replace a damaged or stretched chain.
Quickly check that the derailleur clamp is still in the original position,
aligned properly so the outside edge of the cage is parallel to the large
Check the height. The bottom edge of the outside edge of the cage should
lie about 1 to 2 mm above the large chainwheel when you push the shift
lever so bring the cage over top. The derailleur should sit just high enough to allow
shifting into the big chainring. See the section of derailleur
replacement for adjustment of the derailleur clamp.
Do you need a new derailleur? You can waste a lot of time trying to tune a
derailleur with worn-out hinge mechanism. You should buy a new derailleur if:
(1) The cage won't hold its shape. (2) The cage can be moved and forth more than
a whisker when you wiggle it with your fingers. (3) The mechanism sticks or
catches despite cleaning and lube.
Now let's start the tuneup. Loosen the cable.
Shift down to the small
(low gear) chainring. Shift the cassette (rear derailleur) so the chain is
on the biggest cog. Adjust the low-gear limiting screw so the chain is
about 2 mm (just less than 1/8 inch) from the inner wall of the cage. The low-gear limit screw is usually the outside screw.
If the chain rubs on the outer side of the plate while in the middle cogs
(rear) and small ring, move the limit screw so the front derailleur cage is
closer to the chain while in the small-ring,
big-cog combination (further away from the bike frame).
Setting the low gear (small chainring) limiting screw.
Also, if the chain tends to fall off the small ring during downshifts, adjust the
low-gear limit screw to
move the end-position of the cage towards the bigger rings.
||Pull the cable end with pliers to take up the slack, and
tighten the screw. Shift up to the large chainring. Shift the rear derailleur
so the chain is on the smallest cog. Set the high-gear limit screw so the
chain is about 2 mm from the outer wall of the derailleur cage. Adjust the
limit screw so the
chain is taken up by the ring easily, and doesn't rub on the inner side of
the cage while in the middle cogs.
Shift up and down several times. If the down-shifting (going from
big to middle ring) tends to skip the middle ring, increase the high-gear limit (allow the
cage to move further over onto the big chain-ring).
Setting the high-gear (large chainring) limiting screw.
Toeing in the cage: Hot Tip.
If you have trouble with
sluggish shifting, you can toe-in the front of the derailleur cage. (Typical
symptoms? The front derailleur barely transfers
the chain to a larger ring, yet hesitates or fails to drop the chain back to a
smaller ring on downshifts. You tune the shifting on the workstand, and it seems
fine. But when you actually ride the bike uphill and downshift to the little
chainring, the chain drags and rattles against the cage without ever dropping
off the middle ring.) Here's your fix: Pull the crank so the derailleur is
free from the chainrings. Grasp the outer side of the front of the derailleur
cage with pliers. Bend the outer front of the cage inward. Bend from a point
about 1 inch down from the front tip of the cage. Bend just a tiny bit -- a
millimeter or two. That configures the cage to "kick the chain off"
the bigger ring more quickly. Now readjust the limit screws and cable tension.
If it's still a bit sluggish, bend the inner plate slightly inward at a point
about 1/2 of the way between front and back. That makes upshifts (transfers to a
bigger ring) easier.
||Adjusting the cable tension determines the position of the derailleur cage
relative to the chain. Move the cage slightly left (towards the small chainring) if you
need faster, more secure downshifts. Move it slightly right if you're having trouble
getting into a higher gear (moving to a larger chainring).
the cable tension.
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