cim motor power question

will the cim motor with a 1 to 3 ratio on the 6 inch tire (the little wheelchair tires with the gear given to us by first this year) be able to move something that weighs about 180 lbs? we calculated it and at around 2700 rmp it will move about 18 mph do you think that with the 1 to 3 ratio and the 6 inch wheel the motor will have enough power to get us off the line?

It will definetly move…but what you have to look more specific into is how fast do you want it to move…and if you are willing to sacrifice torque for speed, and speed for torque…any more questions let me know?

18mph is A LOT! The fastest I’ve ever seen a FIRST robot geared is 15 feet per second which is about 10.2 miles per hour. Most robots geared that fast also have a selectable low gear to push around. If you are planning on one gear ratio, I would recommend approximately doubling your reduction.

From my experience, you won’t want to gear nearly that fast, even for high gear with a shiftable transmission. A 15 ft./second robot is very fast. If you are using two CIM’s/side and are not shifting, you probably shouldn’t go higher that 7-8 ft./sec. (IMHO) unless pushing is clearly not going to be a part of your game.

Are you saying 3:1 geared straight off the output of the motor to a 6 inch wheel? :ahh: If this is the case you will be kicking a lot of circuit breakers. Our robot last year started off with a similar problem (7.2:1 geared to a 12 inch tire) and the robot would barely move itself. When the driver put the joysticks to full throttle forward or reverse, the robot would just jump because the current draw kicked the breakers almost instantly. When they reset themselves, it would jump and then kick out the breakers again. We finished the season with about a 30:1 gear reduction between the motor output and the 12 inch tire, and the robot was still very fast. As for your desire to go 18 miles per hour, I don’t think you will be very capable of doing so in the confined space that you must operate in. You could easily destroy most of the field IF you could get up to that speed.

I’m not talking about something for the bot. my engineering class (at school) will soon be building electric go karts. they have 1 hp (3 peak) and 3k rpm 24 volt motors. the go karts weight 165-175 without a person. last year my team won, and this year the teacher said no minibikes or scooters (we made a 3 wheeled car but the no 2 wheeler rule is for safety) but he did say that if we make our own electric minibike we could race against the go karts. i have 3 cim motors at my disposal, my question is that will they, bolted to a 25 lbs of frame and tires, be able to push 150 lbs me to at lest 18 mph?
if not help me design a powered cart, the van door motors are too slow and the windows are too weak, but the cim’s r too fast and my team wont let me take the drills off of last years bot!

Alright - now that it has been re-stated and we are aware of the real question that is being asked, leme see if i can add anything useful.

My quick calculations are as follows:

Linear speeds (w/ 3:1 & 6" tires):
CIM @ free speed ----------- 29 mph
CIM @ 75% (3750 rpm) ------- 22 mph
CIM @ 70% (3500 rpm) ------- 20 mph

based simply on that I’d say it isn’t a bad gear ratio to shoot for, its a little high but the real question comes to how much torque and power you will have as a result.

**–**So now lets take a look at how much power the CIM’s have to offer …

Based on the motor spreadsheet the CIM’s have a peak output power of 343 watts each - this means 3 of them theoretically will give you roughly 1.38 HP as your peak.

**–**Now lets continue to look at what kind of torque to expect out of this gearing …

Each CIM puts out roughly 1.8 foot-pounds of force, which means at the wheel you will theoretically have a maximum of 16.2 foot-pounds of force (1.8 x 3 CIM’s x 3:1 gear ratio). Now given you are running 6" wheels you will actually have a theoretical maximum of 32.4 pounds of force at the ground. Given this number and the nature of the traction on those 6" skyway wheels you may want to think about some other tire that will give you better traction, although i don’t know what kind of surface you will be running on.

I went ahead and also ran all these numbers out for the same setup only with a 3.5:1 gear ratio, and my results are as follows:
Free speed — 25.5 mph
@ 75% ------- 19.1 mph
@ 70% ------- 17.8 mph
4.83 HP
18.9 ft-lbs @ wheel

This would leave you with more torque and power with a little bit of sacrifice on speed. But all this depends on the efficency of your drivetrain, if your gears mesh well, chain and sprockets are in line, and you’ve got decent bearings for all your wheels. Remember - my torque and power calculations are theoretical maximums, not a number you will be continually putting out - as well as they don’t account for any drivetrain innefficencies, but that dosen’t mean you still won’t have an advantage. Also to consider - The torque is stall torque, at 117 amps! - you will hopefully have breakers in your system keeping you from ever reaching this.

Now another thing i noted in your information was that the go-karts you are racing weigh 165 - 175 lbs w/o a person - this would set you at a huge weight advantage over them assuming you meet your 25 lb chassis goal.

I’ll leave any further calculations or decesions regarding gear ratio up to you, hopefully some of this information will prove useful to you in the design and build of your go kart killer. :slight_smile:


The CIM motors have a peak power of 337 watts which is available at roughly half the no-load rpm, 2655 according to the spec sheet. That would be roughly 1.37 hp for three of them. Gearing does not create horsepower. It only trades force for speed and vice versa. The specified peak power of 337w/motor is, in fact, the actual peak power if the supply voltage is the specified 12 volts. Loaded to peak power rpm, each motor pulls 68 amps.

To maximize top speed of your vehicle, you need to have gearing that places top speed at the peak power rpm of the motor(s). This can be determined with lots of fancy calculations involving air drag, power loss in gears and bearings, etc. For your purposes, assuming you have a battery that can provide 68x3 or 204 amps for a while (and a way to control it–are you going to use Victors?) you can just gear for 18mph at 2655 rpm and see if you go at least 18mph. You won’t want to “full throttle” the motors from a standing start because you would be pulling LOTS OF CURRENT for an extended period of time which the motors and speed controls won’t like, and which the battery probably can’t provide anyway. My gut feeling is that, with reasonably efficient gearing and proper electrical control of the motors, you should be able to go 18mph with your three CIM’s as long as climbing hills isn’t involved. Unless you can get that speed with “lower” gearing that allows you to run the motors faster than the peak power rpm, you won’t be able to do it for a long time, because the motors will get hot and the battery will die quickly.

If you are using chains as part of your power train and you can easily adjust gearing with a single sprocket change, I’d start out with gearing that would provide 18mph at the “normal load” rpm of 4320. At that rpm, the motor draws 27 amps and can, presumably, be run continuously.

Good luck, and let us know how this project goes.

Thanks for the correction on the HP - my bad. I don’t know why i was thinking gearing increased it, but it was late and i guess my thoughts were a little mixed up.

i was just suggesting using the 6 inch tire and the 3:1 ratio because a smaller tires is much easier to move. we do have 10 inch tires that i wud much rather use, and if i found a 50 tooth sprocket i thin i can make it work.
10 inch tire and 5:1 ratio will yeild 25 mph at 4320 rpm, thats seems like a decnt top speed to me. and with the light weight frame the motors should be able to move off the line, this is looking good so far guys. now all i need is a 50 sprocket gear…does anybody think a 40 tooth would work with the 10 inch tire?
10 inch tire 4:1 ratio will yeild 31 mph at 4320- rpm…that seems a little fast can the motor get us off the line?
(i have a 40 tooth already so if this will work i would muchn rather use it)

The 31 mph @ 4320 rpm is probably “higher” than is likely to work very well. If you have a way to attach them, you can get bicycle sprockets from 9 or 10 to more than 50 tooth. Bicycle chain isn’t as strong as what you might be planning on using, but it should be strong enough.

How do you plan to control the motors? Are you using some kind of electronic speed controls? If you just “throw a switch” to start your machine, you are going to be pulling very high current longer than you will want to, but if you can work your way up to speed at part “throttle” with a speed control, things will be much better.

i didnt plan on throwing a switch, because full throttle or none isnt a good way to ride somehting with faulty or no brakes (im gonna have to engineer some way to stop). i dont think ill use any of the speed controls from the bot because our programmer and electrical teams probbaly wont allow it…but i may be able to make it so i can easily detatch it from our powered cart and strap it to the minibike, that way no complaints.

if anybody has suggestions for brakes please tell!!!

if anyone has any suggestion please tell me!!!
(spell check isnt working so i apologize because i refuse to proofread :slight_smile: )

For brakes you could use the same way the victors create brakes. short the leads together.


Just stick your foot out and hope for no objects in the way. You know, since you are building an electric bike, have you thought about using a derailing shifter from a bicycle? Sounds like a fun project, good luck! — I wish there was an engineering class in my high school.

first of all have any of u tried putting down your feet to stop a bicycle moving at 20 mph? all i can tell you is that you dont stop at all and all your weight is now centered over your groin (for a man this is easily as painful as childbirth if done to an extreme). i was thinking about making a rubber pad that would rub against the gear, it would work but i doubt that i would have the torque to stop at a treasonable distance ( we must stop from 15-0 mph in 15 feet, which is quite extreme but we managed to pass last year with our cart). if the motors have enough power i can set them so that when i release the throttle the motors apply toque to keep stationary, but would this hurt the motors? i am a little afraid of using a bolted frame, the rules for the go karts is no welded parts but if i have to bolt together a minibike frame i dont think that i would trust it moving at 18 mph, after all i am very lose to the ground.

Funny that you mention brakes. I was wondering what you had in mind for stopping, because this thing might easily meet and exceed your speed goal. Something that is crude and won’t stop you quickly, but will get the job done eventually is to drag a piece of wood on the ground, actuated by a pedal . I think soap box racers is where I’ve seen that technique.

we tried that with our go kart and it didn’t work at all, i will most likely be using a piece of aluminum ( 1x1 square tubing) wrapped with rubber and duct tape, i will position it so that with the pressure applied by my foot it will push on the rear or front wheel. this will work fine but ill have to find out how to mount the piece properly.