Shimano Derailleur Adjustment

The outer limit is too close in or the cage is misaligned or the BB is too short or....

http://www.parktool.com/blog/repair-help/front-derailleur-adjustments

Set the rear derailleur to the lowest gear (big cog) and the front to the lowest gear (small chain ring). Disconnect the derailleur cable. Screw in the adjuster for the cable tension all the way (this adjuster is on the shifter of a mountain bike or the down tube of a road bike). Pedal to make sure the chain is not rubbing on the derailleur cage. If it is, adjust the "L" setting on the derailleur until the chain moves by without touching the cage. Now pull taut and reattch the cable and shift the front to the big ring while pedaling. If it does not shift up well or completely, hold the shifter past its normal shift and adjust the "H" screw so that the derailleur allows the shift. Shift into the highest cassette gear and keep pedaling. The chain should not be touching the front derailleur's cage when you pedal. Adjust with the barrel adjuster until the chain moves freely without touching the derailleur cage.

i hope this helps
-matthew

Derailleur adjustment takes time to master. Try this out:
First, wash your drive train out really thoroughly and use a degreaser (Simple Green works really well) and a chain brush on the derailleur and chain. Rinse it off and let it dry, then relubricate the chain (and ONLY the chain - never lubricate your derailleur). Now flip your bike onto the bars and saddle so you can crank while you shift manually. Use the middle chainring up front to adjust the rear gears. Okay, shift into your highest gear in the back and then release the derailleur cable. Use this opportunity to make certain your "H" setting is good; the derailleur's jockey wheel should be perfectly aligned over the small cog. Now manually push the derailleur to the lowest gear (while pedaling) and set the "L" so the jockey wheel is immediately outside the last cog. crank back to the high gear. Turn the barrel adjuster for the cable tension to its half-way point (figured by unscrewing it all the way and then counting revolutions as you screw it back in). Pull the cable taut and reattach it. Shift into your middle gear (or if it is an even number of gears, pick one of the two middle gears). Use the barrel adjuster to get the jockey wheel exactly over the correct cog. Pedal and shift through the gears. In each gear, back-pedal to make sure the chain is not skipping. You should be able to free-wheel without problem in every gear on a properly adjusted drive train. If the gears are requiring much adjustment on the barrel adjuster, then you may have a bent derailleur hanger. This can be straightened manually without tools if you have a good eye and patience. If it goe that far, come back and repost "bent derailleur hanger".

i hope this helps
-matthew

Wash, degrease and scrub the chain and derailleurs with a good degreaser (like Simple Green). Dry the chain and relubricate it with an appropriate bike chain lubricant (not oil or WD-40). Shift the derailleurs to the smallest chain ring and cog on the cassette (low front gear, high rear gear) and then loosen the cable at the derailleurs, pull the cables taut and reattach them. If the derailleurs still need adjustment, here is a procedure for it:
Flip the bike over (so you can pedal the bike manually while you shift) and shift the front to the middle chain ring. Shift the rear to the highest gear (small cog). Release the cable from the pinch bolt on the derailleur. Adjust the screw marked "H" on the derailleur until the jockey wheel on the derailleur is aligned perfectly over the small cassette cog. Set the cable adjuster on the derailleur to the middle of its range (find this by screwing it all the way in and then count how many revolutions it makes until it screws out completely. Screw it back in one half of the total revolutions). Reattach the cable and shift to the lowest gear (the biggest cog). Over-shift and hold on the shifter, then adjust the "L" screw on the derailleur until the jockey wheel sits just past the last cog. You have to hold the shifter to do this. Now shift into the middle gear (or one of the two middle gears if the bike has an even number of gears) and adjust the derailleur using the barrel adjuster you previously set to the middle of its range. Adjust it so that the derailleur is sitting perfectly over the appropriate gear. Shift up and down the cassette while pedaling and in every gear, reverse pedal (freewheel) to make sure the chain does not hop. Adjust as necessary until all gears are smooth.
Now the front:
Set the rear derailleur to the lowest gear (big cog) and the front to the lowest gear (small chain ring). Disconnect the derailleur cable. Screw in the adjuster for the cable tension all the way (this adjuster is on the shifter of a mountain bike or the down tube of a road bike). Pedal to make sure the chain is not rubbing on the derailleur cage. If it is, adjust the "L" setting on the derailleur until the chain moves by without touching the cage. Now pull taut and reattch the cable and shift the front to the big ring while pedaling. If it does not shift up well or completely, hold the shifter past its normal shift and adjust the "H" screw so that the derailleur allows the shift. Shift into the highest cassette gear and keep pedaling. The chain should not be touching the front derailleur's cage when you pedal. Adjust with the barrel adjuster until the chain moves freely without touching the derailleur cage.
That should be it, unless there is another mechanical problem causing the missed shifts, like a bent derailleur hanger or damaged teeth on the cassette cogs or a bad derailleur. If it is still problematic, come back for further information.

i hope this helps
-matthew

The "H" screw is the one that is letting your chain jump of f of the ring. If the screws are not labeled, the way to distinguish the two is to shift the front as high as it can go and hold it there manually. Screw either adjusting screw in. If you are screwing the correct one, it either will not move at all or move only very slightly.
First thing is first: flip your bike over onto the bars and saddle. The best method to adjust these settings is to shift into your small ring and let loose the shifter cable. Now shift your rear derailleur into the lowest gear and manually pedal the bike while adjusting the "L" screw on the front until the derailleur cage allows relatively free rotation of the chain. Now set the barrel adjuster on your down tube fully turned in (clockwise). Pull the cable taut and reattach at the derailleur. Shift into your middle ring (or high ring if you have a double-ring crank) and then shift the rear to the highest gear. Use the barrel adjuster to get the derailleur cage to just clear the chain while cranking. If you have already succesfully set your "H" screw to a point where the chain no longer jumps off, then you are all set here. If not, hold the front shifter in while cranking and set the stop so the chain cannot get off of the big ring, but far enough out so you can purposely over-shift by just a bit. Sometimes if you shift early, that little bit of over-shift is very helpful.

i hope this helped, but if you would like to see pictures of this happening, go to parktool.com/repair and check out their tips on derailleur adjustment.
matthew

Oh, it is too bad you are in the Bronx. i'd have that fixed right quick. There are probably a pair of culprits at work here. One, your derailleurs no doubt need adjustment. Two, your rear derailleur hanger may be bent. These two can both cause the problem. Start with the rear hanger, because that will be easier to see.
Flip the bike onto the bars and saddle and take a look at your rear derailleur. Is it going straight up and down and is it aiming straight with your frame? If it isn't, you have found your problem. The hanger needs to be straightened. This can be done without any tools if you have a good eye. Hold the derailleur by the body and bend it to make the alignment good with the cassette. If this is NOT the problem (but i really think it is) then here is a walk-through on adjusting your derailleurs and hopefully this will solve it.
Here is how to adjust the rear (first):
-Set your front gear to its middle position.
-Shift the rear into the highest gear.
-Disconnect the derailleur cable.
-Screw the barrel adjuster on the shifter (or down tube if it is a road bike) to fully turned in clockwise.
Screw the barrel adjuster on the rear derailleur to its halfway point.
-On the back of the rear derailleur you will see two screws. One will be marked "H" and the other "L". These mean high and low. Adjust high to get your jockey wheel on your derailleur perfectly centered over the smallest cog on the cassette. Manually shift by pushing the derailleur (while cranking the pedals) to the largest cog (1st gear) and set this stop on the derailleur "L" where the jockey wheel is over the cog, but past it where it would be touching the outside of the cog if they were side-by-side.
-Pull the derailleur cable taut and re-tighten it.
Shift the rear to the middle gear and adjust the derailleur using the barrel adjuster to center the jockey wheel directly over the cog it should be.
-Now shift up and down the cassette, cranking as you go and in every gear, back-pedal to make sure there is no problem with the gears hopping. If they do, adjust as necessary with the barrel adjuster.
Here are some tips for the front:
-Shift into low gear on the front and rear derailleurs. Make sure the chain is not rubbing on the front derailleur. If it is, adjust the screw marked "L" until the chain just clears the derailleur but does not touch it.
-Pull the derailleur cable taut and re-tighten it.
-Now shift the front into the middle gear and while it is still in the low gear in the back, crank to make sure the chain is not rubbing on the front derailleur. If it does, adjust the tension of the cable with its barrel adjuster.
-Shift the front into high gear and shift the rear to the highest gear, cranking as you shift. If the chain is not rubbing, you are all set.

Try this first:
Shift into the small ring and release the cable. Pull it tight (with pliers) and re-tighten the cable. It should shift now. If it doesn't, the solution is a little time consuming, so i apologize in advance.
First, flip your bike onto the bars and saddle. Now shift into the small chainring and your lowest cassette cog (the largest in number of teeth). Release the cable from the front derailleur and set the adjustment knob for the front derailleur to its center point. You should find it on your shifter. Now adjust the "L" setting on the derailleur while turning the crank so that its stop sets it in a position where it just skims the chain but does not touch it. Now pull the cable taut and re-clamp the cable. Shift into the middle ring and shift the rear derailleur to its highest and lowest gears, making sure the chain does not rub the front derailleur. If it does rub (it will be in either the lowest or highest cog on the cassette), adjust the front derailleur by way of the adjustment knob to clear the rub. Then shift into the big ring and you should have no problem. Set the "H" stop if necessary.

Front or rear? What is it doing?
Flip the bike over onto the handlebars and saddle and let us fix it.
Here is how to adjust the rear (first):
-Set your front gear to its middle position.
-Shift the rear into the highest gear.
-Disconnect the derailleur cable.
-Screw the barrel adjuster on the shifter (or down tube if it is a road bike) to fully turned in clockwise.
Screw the barrel adjuster on the rear derailleur to its halfway point.
-On the back of the rear derailleur you will see two screws. One will be marked "H" and the other "L". These mean high and low. Adjust high to get your jockey wheel on your derailleur perfectly centered over the smallest cog on the cassette. Manually shift by pushing the derailleur (while cranking the pedals) to the largest cog (1st gear) and set this stop on the derailleur "L" where the jockey wheel is over the cog, but past it where it would be touching the outside of the cog if they were side-by-side.
-Pull the derailleur cable taut and re-tighten it.
Shift the rear to the middle gear and adjust the derailleur using the barrel adjuster to center the jockey wheel directly over the cog it should be.
-Now shift up and down the cassette, cranking as you go and in every gear, back-pedal to make sure there is no problem with the gears hopping. If they do, adjust as necessary with the barrel adjuster.
Here are some tips for the front:
-Shift into low gear on the front and rear derailleurs. Make sure the chain is not rubbing on the front derailleur. If it is, adjust the screw marked "L" until the chain just clears the derailleur but does not touch it.
-Pull the derailleur cable taut and re-tighten it.
-Now shift the front into the middle gear and while it is still in the low gear in the back, crank to make sure the chain is not rubbing on the front derailleur. If it does, adjust the tension of the cable with its barrel adjuster.
-Shift the front into high gear and shift the rear to the highest gear, cranking as you shift. If the chain is not rubbing, you are all set.

There should be one or two limiting screws for the front derailleur that you can use to adjust how far out/in the derailleur moves to shift the chain over. You can ask a friend to lift the back end of the bike off the ground while you do the adjusting of these screws and then try shifting the gear while moving the peddles with your other hand.

Try this first:
Shift into the little ring and release the cable. Pull it tight (with pliers) and re-tighten the cable. It should shift now. If it doesn't, the solution is a little time consuming, so i apologize in advance.
First, flip your bike onto the bars and saddle. Now shift into the small chainring and your lowest cassette cog (the largest in number of teeth). Release the cable from the front derailleur and set the adjustment knob for the front derailleur to its center point. You should find it on your down tube. Now adjust the "L" setting on the derailleur while turning the crank so that its stop sets it in a position where it just skims the chain but does not touch it. Now pull the cable taut and re-clamp the cable. Shift into the middle ring and shift the rear derailleur to its highest and lowest gears, making sure the chain does not rub the front derailleur. If it does rub (it will be in either the lowest or highest cog on the cassette), adjust the front derailleur by way of the adjustment knob to clear the rub. Then shift into the big ring and you should have no problem. Set the "H" stop if necessary.