CIM motors on Kitbot Chassis

[Jarren Harkema]

Hi everyone, this is our rookie year, and we have decided to go with the wide Kitbot chassis with a few modifications. Because we also get the green tote, we are given 2 additional CIM motors. I was just curious to hear from experienced teams, what would be the up and downsides to having 4 CIMs on the robot as opposed to just 2 that the Kitbot instructions call for? Thanks for the help, this forum is a huge resource!

[mrnoble]

I will preface by saying, welcome! And, that I haven't really looked at the kitbot frame yet, so what I say might not be valid; maybe the mods necessary to make it work are to big for a rookie to take on.

That being said, having four CIMs is a big advantage in pushing matches, and (to some extent) in getting your robot up to top speed quickly. These days, there are very few robots running 2 CIM drives, and you will be at a distinct disadvantage when going head-to-head. Some robots will even use six CIMs!

Hope that helps. Best of luck!

[cgmv123]

The short answer is that a 4-CIM drivetrain gives you more acceleration, torque and pushing power. The battery will be used more, but unless you have a significant number of other motors that run for a significant amount of time, battery isn't likely to be an issue.

[Walter Deitzler]

Originally Posted by cgmv123

The short answer is that a 4-CIM drivetrain gives you more acceleration, torque and pushing power. The battery will be used more, but unless you have a significant number of other motors that run for a significant amount of time, battery isn't likely to be an issue.

The main advantage to having the extra CIM is acceleration and I believe (check me on this) that is will decrease the load on the individual motor, making your breaker less likely to trip during a tough pushing match.

I advise you go with 4.

[Jarren Harkema]

Thanks, guys, for all the input! I will bring it up to the team tomorrow!

[AlecS]

Originally Posted by LedLover96

the torque stays the same, much the same way the output speed stays the same.
I advise you go with 4.

The overall torque available at the wheels of the drivetrain is sum of the motor torques times the gear ratio. Adding 2 motors would give you twice the torque at the wheels.

[Joe Ross]

Originally Posted by LedLover96

The main advantage to having the extra CIM is acceleration and I believe (check me on this) that is will decrease the load on the individual motor, making your breaker less likely to trip during a tough pushing match.

Not only will it reduce the current draw of each individual motor, it may also reduce current draw in general. Since the torque for each individual motor is cut in half, most likely the motors will be operating at a higher efficiency, reducing current draw in general.

[Anthony4004]

Originally Posted by LedLover96

Actually, while the extra CIM does give you more acceleration, the torque stays the same, much the same way the output speed stays the same.

The main advantage to having the extra CIM is acceleration and I believe (check me on this) that is will decrease the load on the individual motor, making your breaker less likely to trip during a tough pushing match.

I advise you go with 4.

Yes! Last year our team had a big problem with our robot browning out from too much load on the two CIMs we had for a 6WD, go for the 4 CIMs!

[Lil' Lavery]

The advantages are numerous and pretty well documented in this thread and eleswhere. I highly encourage you to go with 4 CIMs in your drivetrain, especially as a rookie team.

One downside that hasn't been mentioned so far is weight. Adding two CIMs adds around 5 lb.s to your drivebase. Never forget the weight limit. That being said, it's some of the most productive and well placed weight on your robot. Keeping as much weight as possible down low is always a good option.

[cgmv123]

Originally Posted by Lil' Lavery

One downside that hasn't been mentioned so far is weight. Adding two CIMs adds around 5 lb.s to your drivebase. Never forget the weight limit. That being said, it's some of the most productive and well placed weight on your robot. Keeping as much weight as possible down low is always a good option.

5 lbs for the CIMs plus extra weight associated with the required speed controllers and wiring. Copper is heavy!

I too recommend 4 CIMs for all the reasons stated above.