The Importance of Gear Inches

If you never think about gear inches, you probably don’t ride a fixed gear bike. With a geared road bike you have a range of gears, and you select whatever feels good for the moment. You might discuss with your mechanic changing the gearing if you are going to be racing flat crits, or climbing mountains in Spain, but otherwise you stick with the gearing options that came with the bike (maybe a 53/39x12/25 or a 50/32x11/23), which typically give you enough range to cover most of your riding.

But with a fixie there are no options. Whatever gear you have on the bike is what you are using on the flats, on the descents, on the climbs, sprinting for the yellow sign, etc. And if that gearing isn’t right, you will quickly be wishing you could change gears. Sometimes you forget and you start to push sideways on the brake lever, and then you remember …

The gearing that comes with a fixed gear varies. A true track bike would come with tall (high) gearing appropriate for the track but which would not get you up anything that resembled a hill. A single speed/fixie from your LBS might come with 48x17 gearing, and probably has a flip flop rear wheel with a freewheel on it as delivered. You buy the fixed cog and mount it on the other side of the wheel and remount the rear wheel to use the fixed cog. The 48x17 or 48x16 gear will work for normal riding on the road, but won’t be of much use if you try climbing hills like in Ellicott City.

And Ellicott City was the ride last Sunday. Coach Birner invited the wheelsucker to join, and the wheelsucker promptly asked "what gearing should I use, coach?" Coach Birner replied "39x16". This presented the wheelsucker with a problem. While his collection of fixed gear parts has been growing rapidly it does not yet include either a 39 tooth chain ring or a 16 tooth cog. A spare 39 tooth chain ring from a road bike would work – the wheelsucker’s road bikes and his fixed gear all use 130mm chainring bolt spacing – but both the 17 cog and the 15 cog in the wheelsucker’s collection would be too far off the 16. The nearest the wheelsucker could get to a 39x16 was a 42x17. By now you are probably wondering how to compare gears. The answer is: Gear Inches.

The formula for gear inches is number of teeth on the chain ring divided by number of teeth on the cog multiplied by wheel-and-tire diameter. The math-aware amongst you will immediately note that whoever dreamed this up forgot about Pi! Not only that, some people use actual wheel and tire diameter (say a wheel and 700x23c tire which converts to something over 27 inches), while others use the old 27 inch wheel diameter.

Here are the actual (not nominal) gear inches for some combinations (assuming a 700x23c tire):

50x14 93.9 gear inches
48x14 90.2 gear inches
47x14 88.3 gear inches
46x14 86.4 gear inches
48x16 77.3 gear inches
48x17 72.7 gear inches
38x15 66.6 gear inches
42x17 65.0 gear inches
39x16 64.1 gear inches

The 50x14 gear is appropriate for roller racing, and for pro level track monsters who push a monster gear. 47x14 is a mere mortal track gear. But it turns out that a difference of one gear inch is significant.

For example, coach Birner had suggested the 39x16 to the wheelsucker for the Ellicott City ride, but both coach Birner and Jay Murphy actually showed up with 38x15. A 39x16 is 64.1 gear inches, but fortunately for the wheelsucker he actually showed up with a 42x17, which is 65.0 gear inches. However he was still 1.6 gear inches short of what Mike and Jay were using.

Now it has been quite some time since the wheelsucker sat in a classroom studying mathematics, but by his calculations, one crank of the pedals with a 42x17 results in the bike rolling 5.178353 meters, while one crank of the pedals with a 38x15 results in the bike rolling 5.309867 meters. And this works out to the wheelsucker having to pedal at a faster cadence than Jay, just to stay with Jay. So if Jay is pedaling 160rpm down a hill, the wheelsucker has to be pedaling at about 164. And the sad truth is that the wheelsucker cannot pedal faster than Jay Murphy.

So the wheelsucker’s collection of fixie parts is being increased by the addition of a 38 tooth chain ring and a 16 tooth cog. And with any luck the wheelsucker will be riding the 38x15 combination at the Ellicott City ride next Sunday. Going with a slightly higher gear inches is tempting, but then the wheelsucker would have to push on the pedals harder than Jay, while chasing Jay up a long climb, and that does not sound like a winner, either.

If the math confuses you, simply download one of the iPhone gear inches apps and use that. But consider than any time you alter the number of total teeth (add the number of teeth on the chain ring to the number of teeth on the cog), you need to change chain length by adding or removing links.

Maybe you should stick to the geared bike after all.

The Wheelsucker at the Gym

After years of being somewhat of an endurance athlete, the wheelsucker’s inability to sprint, big ring climb, or jump have led to a serious effort to put some muscle on. This means weight lifting twice a week, doing Joe Friel’s Maximum Strength (MS) workouts. This is three sets, each of six exercises. The first exercise in each set is legs (leg press, squat or step up) done with high weight and low reps, as is the second (seated row) and sixth (standing row). The remaining exercises are lower weight and higher reps.

Joe Friel has load goals for Freebar squat, leg press, step-up, seated row and standing row that are a percentage of total body weight. The wheelsucker has been doing the MS workouts for awhile and the weights have increased somewhat, but he is nowhere near goal weights yet. Perhaps losing a lot of weight and maintaining current strength could work?

The wheelsucker warms up for ten minutes either on a treadmill or on a spin bicycle, and cools down at a high cadence on a spin bike. The idea is to not lose cadence while building muscle.

The MS workouts are supposed to be done in conjunction with plyometrics, but the wheelsucker’s calf is quite sore and he cannot do the plyometric workouts yet.