Quote Originally Posted by liza View Post
Originally posted by Oakleaf:
I might change out the chainrings before replacing the cassette - five teeth in front will make MUCH more difference than two in back.

Does anyone know what's the maximum differential in a compact? Can she do 53/34, or would that result in poor shifting and/or a lot of cross-chaining?


Whether or not you can go down to a super-small inner ring depends on the Bolt Circle Diameter of your crankset.

I believe standard road doubles (53/39) have 130mm BCD's -- if this is the case with your cranks, you are limited to a 38 small ring.

Most compact doubles have a 110mm BCD; if somehow you ended up with a 110 mm BCD, you can go down to like a 33.

Sheldon Brown has a little information about this on his website, and you will get more if you google some combination of "BCD" and "chainrings".
Correct. She probably cannot change her rings without changing the entire crankset, which will cost mega $$. It would be much much cheaper to go with a cassette change if the rear derailleur can handle it (she needs to look at what's on there now). A good Ultegra cassette costs under $100. A new crankset can run well over $300.

Plus, even if she could change the rings, she's going to have to change both of them to get a compact inner ring, because the front derailleur won't be able to handle more than about a 16T difference.

Also, don't underestimate how big of a difference the rear cassette can make. Example: a 50/11 is a bigger gear than a 53/12. So say she's only got a 12-25 on there now. Moving to a 27 or 28 will be a big jump. She'll sacrifice some of the range towards the top end with such a large spread, but that's not necessarily a bad thing. It could get a little annoying on flat roads or in groups, but I think that's a better trade off than having gears that are too hard all the time.