Road Shifter / MTB derailleur mix and match question

[Fat Boy]

OK, so I'm looking at buying a folder with road components on it. It has an 11-23 cassette. I would probably want to put a cassette on it with a wider range.

So, here's the question. If I go with an 11-32 or 34 cassette, I have to go with a mountain bike rear derailleur (Say, an XT) because a road derailleur will only accept a biggest rear cog of 27 teeth. Will the XT rear derailleur work with Ultegra shifters? The real question, I guess, is if they have the same gear spacing and if they do, do the shifters and derailleurs have the same actuation ratios?

I know this is kind of a weird question to pose, but I know from riding my DT that a 53-32 combination is marginal for really steep hills, but I like the taller gearing in the flats.

So does anyone know if I can have my cake and eat it too?

TIA

[invisiblehand]

Assuming that the road and mtb cassettes have the same number of sprockets--probably 9--then you can use the same shifter for both of them. Turns out that sometimes a road derailer can accomodate a 11-32 (and almost always a 30t max sprocket). Typically, you need to adjust the B-screw to fit the bigger cassette.

Check out Sheldon Brown's page for more details.

-G