Dual CIM+CIMple box powered bike

i was contemplating mounting a cim motor onto by 21 speed Fugi mountain bike. i thot that it would be a good idea to mount it at about 8 inches from the peddles and make it chain driven. the chain would run from the small sprocket on the cimple box and around the 1st peddle gear out of three. this would reduce my gear range down to a 14 speed though. im not sure what im going to do about the battery, or powerswitch cus im not planning on having a jaguar or anything, just all or none.
is this a solid idea or design? would it provide good enough torque and speed without slowing down my bike or burning out the motor going up a hill?

Just speaking from my minimal experience here, but I would think overheating would be an issue. IIRC, CIMs are designed for light, intermittent duty. Because of that, they have no fans or anything for cooling. This works for FRC matches because they are 2 minutes, 15 seconds long, and the CIM usually doesn’t get hot enough to fail by overheating.

would it be possible to water cool them, by running a pump to circulate cold water over the motors and through a coolant reservoir?

The torque and speed will depend on your gear ratio. Without knowing a planned gear ratio from the gearbox to the sprocket (and the possible gear ratios from the small sprocket to the larger ones, and from the larger ones to the cluster on the wheel), it’ll be hard to answer that question.

An all/none switch may not be the best idea either. If you have a battery that can provide more current than the CIMs can handle, you may find yourself with two fried CIMs. You’ll want some form of protection for the motors–a fuse or breaker will work well. If nothing else, a switch with a fuse/breaker in it.

How many teeth are on the sprocket on the Cimple box output shaft?

How many teeth are on the sprocket on the “1st peddle gear out of three” ?


You should probably put more thought into this than “I guess putting the CIM onto a random gearbox should work”…

I think you were responding to post#1 but you linked to post#5.


i plan on using the sprocket that comes in the kit of parts ( 12 teeth) and the other gear has 28 teeth

Now we can do the math:

  • The CIM’s free speed is 5310 rpm

  • The Cimple box steps that down 4.67:1 to 1137 rpm

  • Your sprockets step that down 28:12 to 487 rpm

  • In order to run the CIM continuous-duty without burning it up, you would want to run it at about maximum efficiency, which is 87.5% of free speed (1/5 horsepower). So 87.5% of 487 is 426 rpm

  • 426 rpm is approximately 7 revs/second.

I don’t think you normally pedal that fast.


To go all-out with this, look into a NuVinci CVT for bicycles. $400, but pretty cool nonetheless.

Here is the one I built this summer. I think it is the same idea you are shooting for. I just added a clamp-on sprocket to the left of the rear wheel and a freewheel sprocket to the cimbox with 2 cims.

Note that the chain and sprockets in the kit are the #35 spec, with a 3/8" pitch, among other things. Standard bike chain is 1/2" pitch, so you will have to be sure to use a sprocket of that size to fit with everything else on board.

What is the total gear ratio, from CIM shaft to rear wheel ?


Off the top of my head I can’t remember the stock ratio of the 2005 AndyMark gearbox I used (to lazy to look it up), but from the gear box to the wheel is 2.65 : 1. I do remember that I calculated the wheel RPM to be about 140 at the cim motor’s peak torque band (geared way down to climb hills that cover Clemson).

That is the FIRST Kit gearbox from 2005, designed by Paul Copioli and sold by IFI. It has an reduction of 12.75:1. So, your overall reduction is 33.8:1. If we multiply this number by your desired max wheel rpm to be 140, then the max motor rpm will be around 4700.

Andy B.

Thanks Mr. Baker. Just checked the free speed of the cim motor which is 5310rpm and the Peak Eff = 4614 rpm.

so 4614(motor rpm) X (1/33.8) = 136.5rpm at wheels

Did you mean peak efficiency band?

140 rpm with a gear ratio of 2.65x12.75 = 33.79 gives a CIM speed of 4730 rpm, which is 89% of free speed

At 89% speed, the CIM torque is 11% of stall and the efficiency is 64.6%

The max efficiency is 64.7% at 4648 rpm

EDIT: I got interrupted while crunching the numbers and after I posted I saw your most recent post clarifying that you indeed meant peak efficiency.

It looks like a lot of fun to ride. Are those 24" tires ?


26" by 2.2". It is a lot of fun. Going to try to figure out how to mount a 3.5" cim (Haven’t used one in years)

I was thinking about doing something like this as well, would it be feasible to mount a variable resistor to the handle to control speed? I’m not good with motors or electricity at all, but I think it would go a long way for control or safety.

definition 1 yes

definition 2 no1

1 assuming by “variable resistor” you meant a variable load resistor in series with the motor. If you meant a potentiometer to create a control signal to be read by a microcontroller and used as an input signal to an appropriately-sized motor controller, then yes.