How tight should my chain be? I recently put new tires on my bike, and in so doing, removed and replaced the wheels.  It seems that now my chain has more slack in it than before, and if I pedal backwards while in the smaller cog in the main set of gears the chain rubs on the deraillieur.  Is this normal?  Why is my chain more slack now that it was before, if the wheels are the same?  What shold I do to fix the problem?
Dérailleurs exist not only to change gears, but to add tension to the chain as well. If you are noticing chain slack, and nothing else has changed or been replaced, the reason for chain slack is that the dérailleur has lost this ability.

This could be for a few reasons:


The dérailleur has lost some of it's 'springy-ness'. This can happen over long periods of time, but also can be indicative of damage, but is less likely.
The dérailleur has shifted positions. Some dérailleurs bolt directly to the frame, but some do not, and instead are positioned in the dropout. If you have this type of dérailleur, and recently removed your wheels, it's possible that the dérailleur has 'slipped' in the dropout.