Belt/mid-drive BMX ... Current draw with wheel off the ground?

Hi everyone, im new here ...
Ive tried using the search function to answer my question but i just get lost in a lot of info which isnt helping me much.
I have a bmx ive built with a Turnigy sk3 (192kv) motor. Im running a vesc 4.12, currently on 8s (10s large lipo pack waiting for after initial test ride). I have a 2 stage belt reduction, both approximately 4:1 using htd5m-15 belts. Ive just finished building the tensioners and have added some tension to both belts. I have powered it up and spun the rear wheel off the ground and was wondering what current you would expect the motor to be pulling at max rpm just sat spinning? Im using an app connected to the vesc via bluetooth to get the readouts and at max rpm im seeing 4amps. Does this sound about right with the friction etc taken into account. Im not sure if im in the right ballpark, i was hoping to use this to judge the tension i should have though i guess that will be down to whether or not the belt skips when i ride it.
Thanks in advance for any help you can provide!

EBMX_monkey said:

Ive just finished building the tensioners and have added some tension to both belts. I have powered it up and spun the rear wheel off the ground and was wondering what current you would expect the motor to be pulling at max rpm just sat spinning?

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That depends on the motor itself, and the conditions it's run at. Normally it's pretty low for most motors, a couple of amps or less. Really big motors with low RPM and high torque (like big hubmotors) can have higher unloaded current draw, but smaller motors with high RPM and low torque (RC motors without a reduction system) can be pretty low.

If you want to know the difference between true unloaded motor draw and the drag from the unloaded drive system, you'd need to disconnect the motor from the drive system and test both.

dupe caused by forum glitch (can be removed by moderator)

Free running speed should always be very low except for the initial spin up. Friction from belt drive will add a bit more than a hub motor spinning but still nothing compared to proper load moving the bike and you. But without knowing how powerful the motor is it's all just guess work really. What is the motor rated at?

Just ballparking, and assuming a 20" wheel, I'm coming up with over 100W just to spin the wheel in the air at almost 20mph....seems a bit high.

The thing to start with is what's the no-load current with no belt on the motor? Then anything above that times your voltage is your friction losses and wind resistance on the wheel and tire.

FWIW, on less than 4 amps at 74V my big motors can spin a big heavy 20" OD scooter tire at over 80mph with the tire throwing off a lot of wind. Without the wheel bolted on the no-load current is 2.7amps at 1260rpm

Thanks for the replies so far!
John you are correct, i have 20" standard bmx wheels and the app was showing around 120w spinning the wheel at max rpm. I have since disconnected the 1st stage reduction belt and just spun up the motor by itself and its showing around 3amps unloaded. So that seems a bit high? 1amp doesnt sound too bad i guess for 2 belts
Battery pack was showing 30v whilst doing this test (started at the 8s' storage voltage of 3.75v per cell)
Oh and the motor is 'supposedly' 2750w but im pretty sure thats over-rated.
Maybe i will make a little video of my setup, im currently just figuring out how to attach my throttle cable to the potentiometer which is wired to the servo tester board ... then its pretty much complete.