The specs on most allowed motors can be found here:
http://www.usfirst.org/sites/default...torInfo4.1.pdf
Basically, the choice of which motor to use comes down to a few important items:
- power
- torque
- speed
- heat dissipation
I'll address these items in reverse order. (a word of warning... the descriptions here are all for typical scenarios. I'm sure someone will post an example that proves something here isn't a "rule")
The CIM motors, due to their size and completely enclosed design, are typically the best to use for the drive train. They can take the most abuse and dissipate heat better than the other motors. Many of the other motors have internal fans (you can often see them through holes or slits in the casing) that push air over the armatures to help cool them. These motors are very bad to stall, as the air stops moving, your current draw increases, and your heat increases.
Speed and torque go together, really. When designing something, you often either need to meet a certain speed (a shooter that is set up like a pitching machine, for example, will require a minimum RPM to shoot the ball effectively), or a certain amount of torque (tipping the bridge down, for example, will require a certain amount of force applied on the bridge, which can be provided as torque from your motor). These requirements can often help to define which motor to select. For a shooter, you'll go with a fast motor that can be geared to meet your speed requirements. For the bridge tipper, you'll go with a slow motor that can provide enough torque on your lever arm to push it down.
Gearing can allow you to "trade" speed and torque, much like the gears on a bicycle do. When you're on a bike and reach a hill, you'll shift gears. That way, you can peddle faster, but move slower - you've transferred your speed into torque to help you get up the hill. When going down the hill, you shift gears in the other direction - you peddle slower but move faster, because you don't need as much torque anymore. Keep in mind that all gearing causes a loss of power (although not so much that it's worrisome in most applications), so you want to avoid any more gearing than is really needed for your application. For example, you could gear a window motor to power a shooter... but you would need so many gears to do that at a decent speed (plus the gears that are integral to the motor itself), that it's not really practical.
Finally, we get to power. Power sort of sums everything up. A more powerful motor can deliver more torque at the same speed as a less powerful motor. Since it also draws more current, it also ends up heating up more (assuming heat dissipation is identical for the two motors, which we know it isn't when comparing, for example, a CIM and a BB 550).