Originally Posted by
jefmcg
Nice bike.
To be clear, if you have the same chainring and cassette as you mountain bike, it's geared much lower. Wheel size needs to be factored in.
So your highest gear on the mountain bike would be 75.6", while on the brompton it's 46.7".
http://www.sheldonbrown.com/gears/
(gear inches is how big the front wheel of a penny-farthing would be to cover the same distance with one spin of the pedals)
I used the Brompton Gear Calculator:
http://xldev.co.uk/bgc.html
With a 39t front chain ring, the Gear Inch range goes from 25.7" to 77.8".
Using Sheldon Brown's calculator for my Reign w/ 26x2.125" tires, 170mm cranks and a 32t middle gear with 11-34t cassette, I get 24.4" to 75.7".
That's pretty close and more importantly, it feels just about right. Can't wait for the weekend to pull that weeride co-pilot trailer. should be fun.