We have recently been exploring lower and wider gear range options for our Retrovelo Ponyvelo. We have two Ponyvelos a red single-speed and a blue 2-speed. The 2-speed is equipped with a Schlumpf Speed Drive internally geared crankset. The crankset is fully enclosed and is really nifty because it requires no cables. To change gears, you simply click the button on the bottom bracket with your h This changeseal and will engage the Overdrive option. The gear ratio on these Schlumpfs are 1:1 (direct drive) and 1:1.65 (overdrive).
Before I get into our blue Retrovelo newly equipped with an Alfine rear internally geared hub, I want to talk about gear inches.
Gear inches was originally derived from the diameter of the front wheel of the old Penny-farthing using its direct drive pedaling system. Gear inches are numerical measurements to gauge how low or high a gear is. On penny-farthings, one full rotation of the pedal is equivalent to one full rotation of the wheel. Nowadays, we do not use wheels as big as the front wheel of a Penny-farthing, but rather 700c, 26″ and in our case, 28″ are more prevalent. The Ponyvelo in question, takes a 24″ wheel. If your rear sprocket and your front chainring were the same size, it would be a 1:1 ratio, meaning one full rotation of the pedals would take you one rotation on the wheel. In our case, a 22T chainring and 22T cog would be 24 gear inches (due to the diameter of the wheel).
This gear inch (and ratio) will change if you change the size of the sprocket or chainring. For example, if the front chainring were 48T and the rear sprocket was 24T, it would mean that one full rotation of the pedals would turn the wheel two times. Thus, using a 24″ wheel, would be equivalent to 48 gear inches because of it’s double rotation.
The Shimano Alfine 8-speed rear hub uses its direct drive ratio (1.0) at Gear 5. At Gear 1, it is 0.527 of its Gear 5. For the Ponyvelo, we used a 22T rear cog and the front chainring is 27T. This means at Gear 5, the gearing is 27T x 22T. Below, I’ve compiled a chart to show the total gear ratios and inches for both direct drive and over-drive.
Thus, using direct drive on the lowest gear (Gear 1), you can see, one full rotation of the cranks will only turn the wheel 2/3rds, thus gearing it really low! This is perfect for the hills of San Francisco.
Below are some more build photos of this setup. Enjoy!
William Hsu | My Dutch Bike