>>1426268everything
>>1426274says is right, however, the problem with swapping rings is they are dependent on the bolt circle diameter, so your small ring has to be large enough to bolt on. I happen to know that compact gearing's 34t needs a 110BCD or lower, and if you have traditional gearing, you probably don't. you could always lower your big ring, but the top end isn't what you're concerned with.
this is what I did:
was all set to buy a smaller ring when I ran into the above problem. my BCD was ancient race standard 144mm. so, I bought a Shimano Megarange cluster for the rear, which goes to 34t (and is available in 7 speed if you wish to go that route.) did not realize about what the other poster said, learned the hard way that you can't cram a 34t thru a short-cage derailleur.
so, I copped a long-cage salvaged off a crash for cheap at my LBS. it worked but on steep hills, or under heavy load, and especially both, I still needed more bottom-end gearing.
so, eventually, I copped a compact crankset, 110BCD as I said, that allowed me to get a 34t small ring. I'm super happy with this combination but the upshot is I gradually replaced my entire drivetrain except front derailleur.
so, unfortunatly, there are physical limitations on changing your gearing both front and rear. how you should proceed depends on how much lower gearing you need and what you have to start with.
