After doing a little research while helping out a friend, Logan shares some quick tips on how to calculate gear ratios and gear inches for a Pinion P/C1.12 Gearbox. Find details here…

Figuring out the low and high gearing of your drivetrain is fairly straightforward. For example, we have a simple gear inches calculator here on the site that’s easy to use. However, deciphering the gearing of a Pinion transmission is a little more complex. I realized this the other day as I was helping out a friend who’s in the process of ironing out the specs on a new Priority 600HTX he has on order. More specifically, he is trying to nail down which size rear cog/pulley to specify.

Ron is an older gentleman and is looking for gearing that’s pretty easy on the steep grades in Oaxaca, Mexico. He mentioned wanting something a step or two friendlier than the 17.1 gear inches he has on his current mountain bike, which is equipped with a 30 x 10-51T derailleur-based drivetrain. Pinion has a calculator on their site to help folks figure this out, but it’s only displayed in meters of development. And, to make things trickier, the drop-down list for rear cogs only goes up to 32-tooth (note that this is hackable by simply entering in the numerical value in the box; see below).

Calculate Gear Inches for Pinion Gearbox

A more useful and versatile tool is Gear-Calculator.com. For those unfamiliar, there are a lot of options within this app. You start the process by choosing your drivetrain and then making customizations to suit your needs. In this case, I selected the Pinion P/C1.12 from the list on the left and then changed KMH to MPH to generate gear inches instead of meters of development. Still, things got a little dodgy when trying to customize the tire size. I needed to understand the gear inches of each shift based on a 29 x 2.6″ tire, and the drop-down list on their app only goes up to 29 x 2.35″ (60-622). To accomplish this, I had to figure out the circumference to adjust the calculation. The approximate diameter of a 29 x 2.6″ tire is 753mm (you can find that listed here), which has a circumference of 2365mm. Plugging that number into the field resulted in the chart shown below (and linked here). Ron could then use that to tweak the cog and chainring sizes to find a sweet spot.

Calculate Gear Inches for Pinion Gearbox

Another thing to note is that Gates Carbon Drive only offers a few larger rear cogs (32, 34, 36, and 39), so after considering these options, it appears Ron is going with the 34-tooth cog paired with a 32-tooth front sprocket. That will net two useful low gears, one around 18 gear inches and one around 21, plus an ultra-low gear of around 15.3 gear inches for those extra-steep climbs into the Sierra Norte.

If anyone has any additional thoughts or tools that might come in handy, please leave a note in the conversation below…

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