Originally Posted by
timg7
It may be hard to ensure that new chainrings are compatible with your existing cranks. The most economical thing to do might be to replace the entire crankset. If you have a square-taper bottom-bracket spindle, just beware that there are two incompatible standards:
http://sheldonbrown.com/bbsize.html#square
What gear-ratio changes are you thinking of? Changing the sizes of the chainrings will have consequences for the chain-length and the derailleurs.
Actually there are quite a few incompatable standards when it come to chainrings these days.
There are chainrings with different spider BCD (Bolt Circle Diameter) diameters - 110mm vs. 94mm - for the 2 outer chainrings.
There are chainrings with 4 bolt spiders and 5 bolt spiders.
There are some granny gears with 58mm BCD and some with 64mm and even 75mm BC.
And these are just current Shimano specifications. Add in Campagnolo and some of the other odd and obsolete stuff out there and you have real problems.
Best bet if you aren't sure what to get is to take your old chainrings to the LBS and ask for quality replacements. At the very least get them to tell you what you have so you can order them on the internet (although this isn't really a good idea if you don't know what is good and what is mediocre or even poor quality).