Checking your derailleurs
If you ride a bike with derailleurs and you’re having trouble shifting or losing your chain there’s a few checks and adjustments you can do before taking it to your mechanic. Generally if the derailleurs have been set correctly they won’t move out of adjustment and problems changing will be due to a worn chain and/or cassette or stretched cables. Cables can be tensioned via your adjusters. If it’s a new or newish chain/cassette and you’re still having problems there are a few checks to carry out. You may have knocked the rear derailleur and bent the hanger or maybe they just weren’t set up correctly in the first place.
So firstly just take a look from out back and make sure the derailleur hanger is straight and inline with the cogs. A special tool is used to check and adjust this but it’s expensive and unless your knocking the derailleur all the time it isn’t justifiable.
If your chain is slipping off either end you’ll need to adjust the H or L (or both) stop screws.
Different models will have the screws in different places but it’s the same adjustment. It’s easier to do it with the chain removed, so hopefully you’ve got a quick link or similar. In 1st gear adjust the L screw so that the derailleur pulley and big cog line up. In top gear adjust the H screw so that the pulley is in line with the small cog. This will ensure the chain does not over travel and come off the cogs and damage the spokes etc.
Now that everything is straight and lined up you can adjust cable tension. This is done at either the shifter end or the derailleur end(or on some models both ends). When in top gear the cable should be just a little loose so as not to be under tension but still tight enough to shift into the next gear. Change up and down the gears a few times to check operation. Well that’s about it for the rear.
The front is very similar. Adjust the stops so that the guide is over the chain ring but not so far to the outer edge that the chain will come off when changing gear. Again the cable should not have tension on it when on the small chainring but enough tension to shift to the next chainring. Tension is usually adjusted at the shifter end.
Lastly check the height of the front derailleur. When the guide is over the middle ring the edge of the guide should be 2-3mm above the large chainring at the closest point.
Now, all this assumes that your cables, cable housings,cassette and chain are all in good order. If after you’ve done your checks and adjustments, things still aren’t right it’s probably time for new parts. Chains can easily be checked for wear with a chain checker mentioned in the previous post.