The drivetrain is the most important part of the robot. Should a chain slip, a gear break, or anything else like that, you are dead in the water. Effectively useless for your entire team. There are four main things that should be analyzed for an effective drivetrain: Motors, Wheel, Motor-Wheel connections, and Mounts.
For an effective drivetrain, the motor configuration on one side should reflect the one on the other. If you have a high powered motor on one side and a weak one on the other, you won't be able to go straight. Most motors can be used in twos on the robot: Two drill motors, two chulupa motors, 2 van door motors. Some of the weaker ones can be used 4 times, but they aren't strong enough to power a 130 lb robot.
This means that you can use two motors to power the robot, or 4 for some more strength. The best motors to use for a robot are the drill motors and the chalupa motors. Look around and you'll see other motors being used sometimes, but they are the most powerful ones out there.
The final thing is that motors are designed to run one way most of the time. They go faster and more powerful forewards than backwards, which can be a problem if you run the motors facing away. Either do some tweaking to the motor (not reccomended), or find a way to make the motors run the same way on both sides of the bot.
Remember to put some sorts of wheels on the robot. Most robots use the standard 4 wheel rectangular configuration, but don't let that hold you back. Some robots have made quite effective use of a tank style robot. Nothing has more pushing power than a tank, although it isn't too fast. Some use 6 wheels, but that is very rare as well.
Remember to look at the wheels themselvess. You want your robot to have good traction if it is a pusher, or good speed if it is a ball grabber. The wheelchair wheels are usually effective for ball grabbers initially, but those pushing robots should put some treads on the wheels to get that extra oomph of power.
There are two main types of connections: Chains and direct-drive. Chains have the nice advantage of being able to put the motor anywhere parralell to the wheel, and being able to power multiple wheels at the same time with one motor. However, those chains can, and will if you aren't careful, slip off the gears, stopping your robot in its tracks. If you choose to use chains to run the robot, be sure they are always tight, and check them before each match to ensure that they are. Some power is lost in the chains, especially if you double them up, so keep that in consideration.
Direct-drive style is very reliable. There are no chains to lose. Also there is the nice factor of no power being lost on the way to the wheel. The main reason why it isn't used too often is the fact that only one wheel can be powered per motor. Also there is a bit of a restriction on where to put the motor. If you are planning to go with a 2 wheel drive system, use direct drive to do it. Otherwise, I would suggust going with chains.
This is important, but fairly obvious. If your motor is not securely fastened to the robot, the robot will not be able to run effectively, if at all. Be sure that your motors don't move around a lot, and you'll be fine.
This game is designed to figure out if you understood anything I wrote above. Please play it to test your self.
Explanations for Answers
They are a bit far down. Just keep scrolling.
Here they are:
The wheels on the left are treaded ones and the ones on the right are normal. That problem could make sterring a bit tricky.
The wheels should not be set up like that. Normally it is just a problem of astheticness, but it might cause some problems later on.
The left motor has a power of 1, while the right motor has a power of 3. You need to increase the left motor's power, otherwise you're running in circles.
One wheel is not treaded.
Treads are good for pushers, for the extra traction, but racers really don't need them.