The reason why your motors start and stop is because your geared the drive system for a too high of a speed. As a result, you are putting too much load on the motor, causing it to draw more current then the circuit breaker allowed.
See, when you use gears between the motor and wheels, you have a ratio between the sprocket’s teeth. That determines how fast your wheels are turning, and how strong it’s pushing. Think of it this way. If the motor spins at 100 RPM, you have a 8 teeth gear on the motor, and a 24 teeth gear on the wheel, for every rotation on the motor gear, you turn the wheel 1/3 rotation. That’s a 3:1 ratio, where you decrease the wheel speed to 1/3 of the motor, and increase the pushing force on the wheel 3 times.
if you have a 2:1 ratio, you decrease the wheel speed down to 1/2 of the motor speed, and increase the wheel’s pushing force to double. That’s pretty much the easiest way to explain about gear ratio.
Back to the reason why your motor turn on and off. As the load on the motor increase, the amount the motor draw increase. Now, load is the force the motor feel when it’s doing work. In your case, when you push the goal, you motor feel the force reflected from the wheels through the chain.
Now, remember, with different gear ratio, you can increase the wheels’ pushing force and reduce their speed. Turns out, the stronger you gear the system, the less load the motor will feel. That is why, when you put on a bigger gear on the wheel, your motor can run without turning on and off.
If you read Paul’s post about ratios, then you would understand how ratio isn’t important if you don’t know the wheels size. So, without really knowing what system you have, it is hard to tell what ratio would be the best for you.
In general, we can tell if the ratio is good by looking at the robot’s speed. Just an easy approximation.
1~2 fps (feet per second) is really really strong, but very slow. 3~5 fps is what most strong pusher robots goes at, not too slow, and a decent amount of pushing force. Great for pushing matches with robots.
6~8 fps is about the medium between the speeds… not as strong as the pushers, and not as fast as the speeders.
9~12 fps are fast robots that’s design for speed, and play the game quick. Even though there are not much pushing force, you compensate by your speed in getting to the objects fast.
13~16 fps are for super fast robot. At this point, it is pretty easy for you to trip your circuit breakers because of the amount of load on your motor. You can get really fast, but you probably don’t want to go into a puching match with other robots.
I put up a white paper in the white paper section @ http://www.chiefdelphi.com/forums/papers.php?s= on the third page called “WRRF motor selection workshop lecture notes…”. It’s pretty much the summary of what I learnd on the motors presentation by the chief delphi engineers. Take a look at it for more detailed explaination of motors and gearing.