|Taking The Mystery Out of the Front Derailleur|
|Written by Pete Dzirkalis, Just Riding Along, Bradford PA|
|Monday, 04 April 2011 21:25|
If you read my article Smooth Shifting on rear derailleurs, you probably learned some tricks that will be useful in adjusting the front derailleur as well.
The front derailleur can be tricky because the mechanic must first make several adjustments to align the front derailleur with the chainrings before making any useful adjustments. These alignments are made by eye. New front derailleurs come with a guide sticker on the outer plate to help line the outer plate with the teeth on the largest chainring.
Some front derailleurs are mounted in a fixed position on the frame and cannot be adjusted. If you have a fixed-position front derailleur (like and e-type or direct mount), you can skip the first step assuming the derailleur, frame, and chainrings on the crank were designed to work together.
One clarification before starting: Outer means away from the center of the frame. If you are looking down at the bike while you are sitting on it, the outer parts of the drivetrain and derailleurs will be the ones furthest to the riders right side.
Adjust your height and side-to-side angle: Loosen the cable fixing bolt if a cable has already been attached. Loosen the band clamp on the derailleur where it clamps around the seat tube of the frame of your bike to where it is tight enough to stay on, but loose enough to wiggle up and down.
Look at the derailleur from the drive side of the bike (right side). Get your face at the same level as the outer plate of the derailleur. Eye this up to where it looks like the outer plate will swing out and clear the big ring. Push the derailleur cage out with your fingers and set the bottom edge of the curve in the outer derailleur cage so it a few millimeters above the big chainring. Now adjust your angle. You want the cage to run parallel with the gears.
Look at the derailleur from above--looking straight down through the cage at the cogs. Swivel the derailleur until it is parallel with the cogs. Now here's the trick--now that you think you have the right position--watch the cage as you tighten the clamp bolt to set your position. Chances are that the whole thing swivels a bit when you tighten the bolt. You may have to compensate for this movement by starting with the cage just out of parallel, and watching it move into alignment as you tighten the band clamp bolt. You should be able to compensate for this movement and get it just right when the bolt is tight.
Double check your height of the outer plate again to make sure that didn't change. Also make sure the bottom of the INSIDE derailleur plate doesn't scrape the MIDDLE ring when you push the cage toward the outer position. You may have to break the rule of the outer plate being just above the outer ring to compensate for this. This situation is not common, but it's better to find out in the shop than on the trail.
Set your starting point: Put chain in the small chainring in the front and the small cog in the back (of your gears on the rear wheel) This is also known as the small-to-small combo--a gear you should be able to shift into, but a gear that is not a practical to use on the trail because the chain will be really loose and have a lot of sideways twist in this combo.
When your chain in this small-to-small combination, the outer plate of the front derailleur should be very close to rubbing on the outer edge of the chain--most derailleurs have a small bumped-out area here to prevent this rub. This is your starting point. if your outer front derailleur plate is more than a few millimeters away from the chain, loosen the L screw until it is very close.
If the plate is already rubbing here--tighten the L screw until it just doesn't touch the chain anymore. If your derailleur doesn't have the above mentioned "bump out", the outer may need to rub in this gear combo to get reliable shifting down to the small chainring. Make sure your cable is taut, and attach it with the cable fixing bolt. Grab the inner cable along an exposed area of its routing along the bike frame and give it a yank.
If this puts some slack in the cable, tighten the cable at the fixing bolt again. You may be able to take out a small amount of slack by turning the barrel adjuster outward at the shifter in a counter clockwise fashion. Pedal the bike and shift up to the mid and outer ring. If it's slow to go up, or the outer plate rubs the chain in the big chainring, your cable is too loose.
If the chain wants to skip over the middle or outer ring when shifting--or hesitates when shifting back down to the smaller rings--the cable is too tight. If it shifts up fine, but you can still force the shifter past the outer ring, your H screw may need to be turned in a bit (clockwise) to prevent the chain from falling off the outside. If it shifts to the middle fine, but won't go up to the big ring, and the shifter feels stiff trying to go into the big ring, the H screw needs to be turned out a bit (counter-clockwise) to make it up smoothly to the outer ring.
One last check:
put the bike in the little ring on the crank up front and the BIGGEST (inner) cog on the rear wheel. If you get a little rub on the inner plate of the front derailleur, loosen the L screw a bit to see if it goes away. If doesn't do anything, loosen your barrel adjuster at the front shifter a bit.
You may be able to see the derailleur move in toward the frame and get the chain to stop rubbing, but don't loosen the cable tension or the low screw too much, or it will throw out the adjustments we already did. Sometimes the derailleur bottoms out on the frame and it won't go in any further no matter what you adjust on the front derailleur--this may be a sign of other issues with bottom bracket length, or crank selection which we won't get into here.Hopefully this helps de-mystify those tricky front derailleurs.
Happy Wrenching, Pete
Just Riding Along, Bradford PA