Freewheel Compatibility Question
 

There is a Sachs freewheel for sale on eBay which has a tooth configuration of 13-30. The gearing is 13-15-17-20-23-26-30. And I'm thinking to myself...Setting that up with a double crank on a vintage bike (I'm not interested in modern compact crank arrangements) seems like a great alternative to a triple setup for biking in the mountains. The seller indicates this freewheel is compatible with indexed 7-speed systems. And I am wondering why this freewheel could not be used with the early Chorus Synchro RDs in a friction setup. Those RDs had an A-B setting specifically designed for freewheels with large tooth variances from smallest to largest cog which resulted in what Campy calls an open taper freewheel (as opposed to an acute taper freewheel where the tooth variance is small). Can I get some expert opinions on this? I have no desire to use the Campy Synchro shifting to make this work, so it's either friction capable or I'll just forget it. Thanks for any info!

If you're in friction mode you should be fine as long as the RD can handle a 30 tooth cog.

As a followup to my original post, how much difference would there be in lowest gear between a double crank with 39 in the front and 30 in the back and a triple crank with 30 in the front and 25 in the back? Any math geniuses out there?

http://www.sheldonbrown.com/gears/

http://www.wisil.recumbents.com/wisil/gearinches.asp

http://www.panix.com/~jbarrm/cycal/cycal.30f.html

The triple setup would be 8.33% lower.

Miamijim is almost correct. To pick a nit, (39/30)/(30/25) = 1.08333, making the double 8.33% higher than the triple. In turn, since (30/25)/(39/30) = 0.92307, the triple is 7.7% lower than the double. The difference is in which ratio one uses as the reference when computing the percentage change, i.e., 3 is 50% greater than 2, but 2 is 33% smaller than 3.

After visiting the Sheldon Brown link courtesy of Old Fat Guy, I did my own calculations and I came up with the same answer that you did. I appreciate the response from everyone. I learned something new today!