Multi Drive Fun...

[Jnadke]

Originally posted by Sachiel7
Now here's the thing:
We're only using an 8:1 gearing ratio.
Now, I know every team will say you need at least 20:1 w/ the CIMs, even I tell myself that, but we surprised ourselves this past year.

This is very much a meaningless number. It would be very easy to get by with an 8:1 gear ratio if you have the correct wheel size. The wheel acts like a lever, converting the rotational torque into a force. The smaller the wheel the less gearing you have to do.

speed = (free speed) / gear ratio * pi * wheel diameter / 60
Still, 9 FPS at that geardown would yield a wheel diameter of 0.25 feet or 3". That's a fairly unbelievable number. Are you including any sprockets/belts in your gear ratios? Something tells me you calculated them incorrectly. Either that or your top speed is wrong.

The goal of gearing down your motor is not necessarily to achieve enough pushing torque, but to maximize your acceleration. Virtually all your torque will go into accelerating a 130lb robot (gears weigh so little compared to the robot as a whole that very little torque is lost), so the acceleration up to the top speed can be modeled by F=ma. If you're only seeing 9FPS out of your robot then you most likely have not maximized your potential for your given wheel size. If you want to design your robot for speed then you should have enough torque to accelerate to top speed within 10 feet (usuallyt that's all the usable room there is on the field). The equations can be a little tricky, given that the torque of a motor depends on its free speed, but it can be done. Once you have set up your equations, you should then be able to solve for the gear ratio.
An easier way to calculate the minimum gear ratio is to say that the motor must go from drawing 120A to 80A within 1 second and from 80A to 40A within 5-8 seconds. This is from the breaker spec sheets of how long it takes for them to trip.

Having a large top speed is useless if your robot cannot accelerate to it rapidly. A robot geared for an 8FPS top speed will most likely traverse the field faster than a robot geared for 20FPS (assuming both are using single-motor drivetrains).