Join Date: Jun 2006
Location: Boston Area
Bikes: Univega Gran Turismo, Guerciotti, Bridgestone MB2, Bike Friday New World Tourist, Serotta Ti
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Originally Posted by ncscott
So I was assuming that I would need a smaller gear, but it appears I may not. According to Sheldon Browns gear calculator, the Downtube combo of a 48t chainring and 32 max cassette is 2.2 vs my roadbikes 39x30 of 2.6. Does this make sence that the differences in wheels will cause the gearing to be the same??? So it apears that the 48 tooth chainring on a 20 incher acts much like a 39 tooth on a full size bike.
So with the gear ratios very similar will they climb the same up the hill on my commute? (I would just ride the bike up the hill but I haven't bought it yet, but I'm much closer as my car is now posted for sale!!)
Actually it's more like a 48 tooth chainring on a bike with 20 is more like a 35 tooth chainring on a bike with 700 C wheels.
It may be easiest to think of this in terms of "gear development". Gear development is the distance your bike will move for one turn of the crank. The magic formula is
So for the Downtube, the lowest gear development is 94.25 inches.
For your road bike the lowest gear development is 110.27 inches.
So the lowest Downtube gear should be a bit easier to push then the low gear on your road bike.