Bicycle Mechanics - free wheel options

Currently I have Dura Ace 53/39 chain rings and 12/27 freewheel on my road bike. That gives me more than enough options for riding in Michigan, but I'll be riding some actual mountains later this summer and would like a lower gear or two.
I see Shram has a Shimano compatible 11-32. Could I use that (or something similar) or am I at the capacity of a Dura Ace road derailler with my 12/27?

32 is likely on the big side for the Dura-Ace rear derailer but it might work. If you're running 9- or 10-speed, you can put on any Shimano alpine (MTB) rear derailer to handle the 32 (6/7/8-speed Dura-Ace is not compatible with any other Shimano but there are ways of kludging it to work).

If your Dura-Ace is short cage (which it likely is since you have a double crank) another issue with 11-32 is chain wrap. You'd need a rear derailer with capacity of 35T but the short cage Shimanos wrap only 29T. I've exceeded that by 3 or 4 but 6 might be a problem.

Shimano says their road rear derailleurs have maximum cog compatibility of 27T but experience shows most will accept a 30T with no problem and a 32T is a sometimes yes/sometimes no proposition. Basically you will have to give it a try and see if it shifts ok and runs quietly without rubbing on the largest cog.

Any Shimano MTB rear derailleur will accept a 32 or 34T cog and may be the lowest cost way around the problem if your DA doesn't like the 32T cog.

I think you have a cassette, not a freewheel.
To use anything larger than a 30t cog you will probably need a mountain style derailleur, and the new cassette needs to have the same number of cogs as the old one, 9-speed to 9-speed or 10-speed to 10-speed, you haven't said what you have. Switching cassettes to match the riding terrain is a good idea, just make sure everything is compatible. A 9-speed mountain derailleur should be compatible with a 10-speed cassette, the shifters, cassette, and chain all need to be of the same "speed" (the chain may be backwards compatible). The disadvantage of a mountain type cassette is the big ratio jumps between cogs, but chainging back to a road cassette when needed is easy.

Al

There are no Shimano or SRAM 10-speed MTB cassettes.

Check this link and page down to just under the 10 speed cassettes (RE: 11T cogs)
http://sheldonbrown.com/harris/k7.html#10

What I wrote was there are no Shimano or SRAM 10-speed MTB cassettes. IRD is neither. Also, since neither Shimano or SRAM make 10-speed MTB cassettes, I'm willing to assume neither company thinks 10-speed is rugged enough for MTB use.

What I wrote was there are no Shimano or SRAM 10-speed MTB cassettes. IRD is neither. Also, since neither Shimano or SRAM make 10-speed MTB cassettes, I'm willing to assume neither company thinks 10-speed is rugged enough for MTB use.

Not sure if you are responding to my post?

I was responding to th OP about using an 11 tooth cog on his current FH.

Not sure if you are responding to my post?

I was responding to the OP about using an 11 tooth cog on his current FH.
I was but perhaps not to what you really intended to say.

I assume the OP has a 9-speed drivetrain or he wouldn't be considering a Shimano or SRAM Xx32 or 34T cassette since neither make them in 10-speed format. Therefore the 10-speed IRD cassettes aren't an option.

That's right. Dura Ace 2x9 drive train.