As director and team coach for the Aventon Factory Team, part of my job is to advise riders on gearing and equipment. Riders often agonize over their choice of gears. They spend hours thinking about: “ What gear should I use for my race.” Gear choice is especially important on single speed and fixed gear bikes. Track racers, fixed crit racers, and urban fixed riders have an important choice to make, because once they’ve made that choice, they are stuck with it for the rest of the ride. Geared riders have more options and probably think about it less, but there are still some important considerations. What many rider often forget, is that gear choice and crank length have an important relationship, and changing one ultimately affects the other. That is our topic today: How crank length affects gear choice, and more specifically: “How do gear choice and and crank length interact for fixed gear riders?” and “What is the best crank length and gear choice for fixed gear riders?”
I’ll start with this: RPM doesn’t matter. I know that sounds ridiculous at first. But 120RPM is not always the same. That’s because what does matter is muscle contractile speed. Muscle contractile speed is determined by a combination of both RPM, and crank length. Think of it like this: At 120 RPM, you are turning your feet at 2 revolutions per second. That is the same with 165mm cranks, or 180MM cranks. But the rider with the 180MM cranks is turning a bigger circle, her leg has to flex and extend over a greater distance on each revolution, therefore having a faster muscle contractile speed. In the early 2000s, researcher Jim Martin tested riders on cranks as short as 145mm, and as long as 210MM. Martin firmly established that there was no effect of the crank length on performance, as long as contractile speed was the same. To get the same contractile speed, a rider on 210MM cranks is pedaling at a lower RPM than a rider on shorter cranks. He can pedal a lower RPM on the longer cranks, but the speed of the foot moving around the circle and the muscle contractile speed can be the same. Another way to look at it is to watch your wheel as you pedal along at 20MPH ( don’t crash into anything). The outside of your tire is going in a circle at 20MPH, but the outside of the hub is only going a fraction of that speed. I haven’t done the calculations, but I guarantee we are talking about single digits here.
Most fixed gear riders use a bit of trial and error to decide what gear they are going to use. It comes down to a combination of terrain ( or track surface) as well as personal preference, fitness levels, and the balance between top end speed and acceleration out of corners. So what crank length should rider run for track races? Or what crank length should be used for fixed gear crit races like Red Hook Crit? Crank length is a big topic. But for riders that are doing fixed gear races like the Red Hook Crit, I think the answer is simple: Choose the shortest cranks you can comfortably ride, the obvious reason for this is ground clearance. Track riders don’t have to worry about ground clearance, and geared riders can stop pedaling as they rail the corners. But a fixed gear rider is committed to pedaling through the corner, so pedal clearance is paramount, and we’ve determined that crank length doesn’t matter so much as contractile speed. The shortest crank length that is widely available is 165MM, so that probably where you’ll wind up. If you are coming from a longer crank arm, you’ll also have to adjust the gears you ride. Since a 165Mm crank means you are turning a smaller circle, you’ll actually want to go down in gear size. You’ll be pedaling a higher RPM, but your foot and muscle contractile speed will be similar to a rider on a bigger gear and longer cranks. So how much should you change your gear when going to shorter cranks? As a general rule of thumb, you should go up by 1 chainring tooth for every extra 2.5MM of chainring length, and down 1 tooth for every 2.5mm reduction in crank length. So if you were riding a 50X15 on 170 cranks, and you go down to 165 cranks, you should go down to a 48 in the front, and keep the 15 in the back. If for some reason you decided to go up to a 172.5 mm crank, you would go up to a 51 tooth chainringin the front. All of these would give you similar contractile muscle speeds at any given forward speed. In reality gear choice is a bit of a personal preference, but most riders tend to settle around the same spot. So what gear do most fixed gear crit riders at the highest end of competition use? At races like the Red Hook Crit, most of the men are on approximately a 48 or 49 X 14 gear ( a 52X15 is right in between those two gears) with 165 cranks. While most of the women are on a ~ a 49 or 50X 15 with 165 cranks. Some riders ride bigger gears, but I think that is ill advised, as acceleration out of hairpin corners will be sacrificed, and riders will be better suited by developing more speed skill rather than moving to a larger gear. With track racing, the gearing often depends on the event, the track surface, and the elevation. But most of the riders are within a few percentage points range of each other. They’ve found the optimal pedaling speed not necessarily from RPM, but from muscle contractile speed.