I have td one derailleurs front and back on my son's bike. The front derailleur cage drags on the big chainwheel teeth (the small bar that connects the sides of the cage is what is dragging). Should I adjust the height of the derailleur? I also cannot get his rear derailleur to index reliably. I have the high and low stops set right and I can index reliably from 5 down to 1 but not from 1 up to five. Any suggestions?

Riding small-small is called cross-chaining, and is not recommended. On a double chainwheel bike at least the smallest and sometimes the next smallest should not be used. On a triple it's more like the 3 smallest. Nevertheless, the derailleur pulleys should take up the slack at least, if you have not made any changes to chain or derailleur. The spring on the pulley cage may be broken or weak. You need to get in-person help at a bike co-op or bike shop.

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

There are no pulleys on the front derailleur. Only the Rear derailleur has pulleys.

The Front derailleur consists of two plates that work the front gears (chainrings)

The Rear derailleur consists of a long arm with 2 pulleys to wrap the chain while shifting the rear gears (cassette).

Pulleys range from $10- a pair to $75-. Your local bike shop should have a suitable replacement set of pulleys, depending on the model of your derailleur it is sometimes less expensive to just replace the whole unit.

It will entail taking the chain apart again (known as "breaking" the chain).
The chain feeds UNDER the lower cog on the derailleur, BETWEEN the 2 cogs, and OVER the top one... then BEHIND the gear cluster and back OVER it. Run the other end AROUND the front chainwheels, through the front derailleur and re-connect. You may find it helpful to lay the bike on its side (sprockets up) and have someone hold rear derailleur to keep tension off the chain.
Good luck to you.

You have a 21 spd and you have two derailleurs, one front and one rear. To shift you must always be pedalling forward, not backwards or you can chip teeth on your chainrings and cog. To shift, remember left hand controls front, brake and derailleur, the front derailleur is what gives you the ranges, gears closest to the bike are the easiest, so, small ring it the front are your easiest range, middle, mid range, large hardest but fastest. It is best to remember that it is the number of teeth that are hitting the chain to give you the speed you want. Rear Derailleur is your fine tuner the largest is the easiest, same as above. NOW DO NOT KITTY CORNER YOUR CHAIN, SUCH AS LARGE COG IN REAR TO LARGE IN FRONT CHAIN WHEEL, AND DO NOT USE SMALL IN REAR TO SMALL IN FRONT CHAINWHEEL. The reason you do not do that is: you can stretch your chain, yes you can stretch your chain and it serves no purpose, if you are pedalling to easy change your range(front). I hope this helps you understand how to shift now. You may contact me for any further questions, I have been in business for 38+ years

If you learn to shift correctly this won't happen..

Google bicycle shifting technique, for example:

http://coachlevi.com/cycling/complete-beginner-guide-to-bicycle-gears-shifting/

You should NEVER run cross-chained (big-big or small-small). The drive train performs best when the chainline is as close to straight as possible. Excess sideways stress on the chain will wear it out prematurely and increase its normal efficiency.

To demonstrate this for yourself, put the bike in the same combination of gears you're having a problem with. Get behind the bike and look staright along the top of the chain. As you can see the line of the chain is a mild zig-zag where it is parallel to the bike at each gear but needs to angle across the drivetrain in between. On some bikes the chain might even come in contact with the next larger chainring when used this way.


If you can shift to the big-big combination without snapping the rear derailleur off, you're lucky. The big-big combo uses up all available chain length and the rear derailleur will be pulled toward the front of the bike, sometimes so far that it is in danger of being snapped off.

THERE IS NO UPSIDE TO CROSS-CHAINING.

As you can see, the rear derailleur is not long enough to **** up all the slack this gear combination creates. There are other combinations that would yield the equivalent gear ratio without either stressing the drivetrain or dropping the chain.

The basics of shifting are these:
1) The big ring and right side of the cassette in back are for speed.
2) The small ring and the left side of the cassette are for high torque, lower speed

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

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

Front or rear?
Rear: Flip the bike onto the bars and saddle, then shift into third while turning the crank. Use the barrel adjuster on the back of the derailleur to align the derailleur over the proper cog. If this doesn't work, you may have a bent cog or a bent derailleur hanger. Does it only happen in third? Inspect your third cog for any deviation.
Front: Shift into the small chainring and release the derailleur cable. Pull tight and re-attach. Shift into the big chainring now. If it still won't go, use the barrel adjuster on the shifter co tighten the cable until it does shift. If necessary (because the derailleur will not go to the big chainring on account of a mechanical impedance) adjust the "H" screw to allow the derailleur to move far enough out to shift on to the big ring.
If all else fails, check with the bike shop.

Hope this helps,
matt

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.

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.