I’ve been pretty happy with my gearing on the Steamroller, and was wondering what it would take to get a matching build on a 29’er instead of a fixie.  It was actually a little harder than expected just in the gearing alone.

Conceptually, gearing is used to increase or decrease the distance covered by one rotation of the cranks.  Increasing gearing increases speed, but also the torque required to accelerate to that speed.  A physically larger tire possesses more rubber and kevlar, thus is of higher weight, which would require more torque to overcome.  Lowering the gearing allows the desirable acceleration, which is less important in sprinting off the line, but as climbs are a series of microaccelerations, more important on the climbs.

The first thing is I would probably want it to roll at around the same speed of my current bike for the same pedaling, so the rollout should be about the same.  There’s a few things worth noting here, namely the differences between a 29’er wheel and a regular 700C wheel.  First of all, a 29er rim has the same circumference as a 700C rim, thus the same diameter.  It does however take fatter tires, which means the effective circumference of the wheel increases, thus increasing rollout.  To compensate for this, a lower gear would be required to achieve the same rollout.  The second thing is that the Surly Ogre comes with a BCD 94mm 4 arm crank, which takes a 36T ring as the biggest Surly chainring for that BCD.

Currently, I’m running:
700x28, 47x18 Current set up
Nominal gear inches: 70.5
Actual gear inches: 69.87
Rollout: 219.5 inches.

My old set up is:
700x28, 47x19
Nominal gear inches: 66.79
Actual gear inches: 66.19
Rollout: 207.9 inches.

I intend to use a Schwalbe Big Apple tire 29x2.15, which is a 710g tire.  It has more inertia than regular tires, so I should target something closer to 208”, which allow me to accelerate at the same rate, and err on the side of lower for running tires at a lower pressure with increased resistance.  As a side note, decreasing the tire pressure should put it somewhere in the 196-206” range, or about 40x16-40x17, which is a nice gear to run over most conditions, and what I used during the recent snow.

So with all of this in mind, plugging in a few numbers I get:
ISO 622, 700x56 (29” x 2.2”), 36x16
Nominal gear inches: 60.75
Actual gear inches: 65.54
Rollout: 205.9 inches.

This works out quite well, as 16T cogs are easy to get hold of too.

Ref:
Gear and tire calculator: http://pastehtml.com/view/1cv4z52.html
Sheldon Brown on 29” wheels: http://sheldonbrown.com/gloss_tp-z.html#29