I was watching the BMX racing on the 2012 Olympics and the gearing seems incredibly low. About half way down the start ramp they are already going too fast to keep pedalling and they barely seem to be able to pedal effectively any other time through the race.
Why don't they adjust the gearing to be higher so they can get more power down through the course?
The tracks do not have many long stretches and so cornering and getting ahead of the pack is a lot more important than going fast in the flats. The gearing on a race BMX is higher than on one you will buy out of the store. Most BMX will come with 44:16 gear ratio where racers will use 46-48:16, and I have seen higher. (There are different rear sizes but the standard is 16 when talking about Gear Ratios, even if you ride 9t or 14t rear you almost always know the conversion to 16t.)
Most racers will start with 44/45:16 then move up when they build their leg muscles to handle the higher ratios, but even then you really need that hole-shot a lot more than you need the higher end speed.
If the tracks had longer straights like a mountain bike course you might see more bikes with multiple gears and even higher gearing. As it stands, the berms are where you make your moves and getting in and out of the berms require more power than raw speed. I raced 44:16 against guys with 46:16 and never placed lower than 3rd. You want more power than over all speed on a race track.
On street or in park you want even lower gearing, especially when you are trying to get that last pop before jumping up onto a rail.
This is all my guessing based on BMX racing when I was younger, but getting ahead of the pack out of the starting gate is a rather big deal. Once you're behind or right next to someone else, you sometimes have to tap the brakes to avoid them, so there's a huge advantage to this sudden rush of acceleration.
This would be a clue as to at least an upper limit on gearing. You might have some advantage to higher gearing after the starting gate, but you wouldn't have the quick burst and you'd probably spend much of the race making up for it.
Much of the gear selection from at least stock BMX bikes has to do with age; I remember having a lot of difficulty with how high mine was geared because of my age. This would be why some youth-oriented BMX bikes have low gearing, although this doesn't apply at all to professional circuits.
There are two things no one has mentioned:
This is a CONTACT SPORT, track have one PERFECT line, and is the one that first guy will use, and there are pretty low chances to have few other guys in the same race, with a better line or even the chance to select a better line. So elbows, cuts, hits and everything is allowed to get that PERFECT LINE.
A few years ago, there were SINGLE SEEDS LAP to qualify TOP bikers (single biker lap) where some of them used to change the gearing.
Basically, there are only short track sections where you can pedal and apply full power, and the whole stuff is around that issue. There were several attempts to use shifters, but nothing too effective (shifting and BMX aren't good friends)
https://vimeo.com/31119376 (you can see BARRY NOBLES with 3 crashes because of XTR fails)
With that in mind, is all about RPM and legs speed. Having in mind that most powerful muscles acts for a very short time in a pedal stroke, then you need to see how to have more strokes instead a long one.
It is because BMX racing is sprint racing. The riders stand and spin very quickly. There are a variety of crank lengths available which can help achieve higher cadences (RPM) based on your skill and leg length, and which in turn affect gear choice.
