It's not so much the exact number of gears, as having a transmission that usually finds itself in the right gear. A transmission is effective if it can arrange to be in the right gear for whatever the game demands at a certain point.
It might be argued that there are two factors here--being in the
right gear, and being in
any gear. A 1-speed is always in gear, but it is arguable as to whether it is often the right gear. A 2-speed spends some time shifting between gears, but offers more opportunities to be in the right gear. And so on. The value of a transmission depends greatly on how much time you spend shifting, as opposed to putting some power to the ground. If you shift slowly, perhaps a 2-speed is best. If you shift quickly, or can cope with power transmission under partial engagement of your shifter, you can potentially deal with more gears, without spending too much time in neutral. One exception, of course: a CVT is a little strange, in that it is always in gear, and (if well-designed) can always be in the right gear. In
that respect, it is the optimal solution.
Of course, most teams can't build a CVT nor use one effectively. And a CVT will often exhibit complexity and sacrifice significant efficiency for its flexibility, meaning that a straight gear drive might be a better solution for some teams.
Given the robot speeds and torques at work in FIRST, the current state of the art in pneumatic shifting (one piston per pair of gears, most of the time), and my bad experience with electric shifting in 2003, I'd tend to think that 2 or 4 would be preferred (in terms of actually driving the robot). Actually, I really mean 2 or 3, but the way that many of these transmissions work makes having 4 gears the same as 3, only with a duplicated ratio in the 3-speed (222 is an exception--I think that they've got something good going with their 2004 3-speed transmission). The downside--and it's a major one--with more than 2 speeds is generally complexity. If complexity is undesired, then perhaps 2 speeds will suffice.
Aside: the 2003 gearbox from 188 was supposed to be pneumatically shifted, but we decided to ditch the compressor for weight early on, before the gearboxes were finalized. We tried to shift them with the Globes, but the mechanism wasn't especially well-designed (I feel a
mea culpa coming on...
), and the Globe motors couldn't be controlled with servo-like accuracy--they tended to over-rotate, and didn't return to predictable positions. We ended up modifying it to run in high. Good thing too--that year, our low gear would probably have caused even more #25 chain failures, due to the 6 motors and small sprockets (the chains weren't completely my fault!).
Now, if the leadscrew shifters that have been proposed recently (and perhaps previously implemented) can shift more quickly and smoothly than a dog shifter (a 2004 Woburn gearbox shifting instantly at top speed is really a beautiful sight to behold
), then a 3-, 4-, 5-, or more-speed transmission is
potentially useful. It just needs to be designed so that it isn't wasting too much time shifting. Is that sort of thing impossible with our resources? Certainly not! But it won't be easy.
Overall, until someone comes up with a capable enough design, I think that the 2-speed will continue to be the most practical number. It is relatively cost efficient and buildable. By doubling the number of gears, you gain a 2x benefit that only diminishes with increasing the number of ratios (1.50x, 1.33x, 1.25x, etc.). Similarly, it spends little time shifting between gears, because each gear has a wider useful range--it can therefore make do with an inefficient shifter (though obviously, a better shifter doesn't hurt a thing!).
Some teams will be able to commit the resources to take advantage of the benefits of more than 2 ratios; there's nothing fundamentally wrong with that--it's just that their designs will have to be that much better (irrespective of the number of ratios), in order to make proper use of those extra gears. And if they figure out how to shift faster, and to build and service it more efficiently, in a couple of years, a significant minority of FIRST teams may be following in their footsteps with designs based on their groundbreaking work.