The calculation is based on 3 seconds as Matt says.
It is also based on a lever arm of 275mm (the wheelbase of the car).
Torque is force x distance so by holding the motor can the lever arm becomes the can diameter so you have a force sbout 5 times higher. so 14.3g for acceleration to 50000rpm in 3 seconds
3 seconds is a figure I pulled out of the air based on a car accelerating down a straight, i.e. motor under load.
The torque generated by brushless motor is ball park 200nmm. this equates to a force of 432g in your hand.
This torque corresponds to 0.03 seconds to reach maximum rpm when no external load is applied.
But add the tyres in and that time is instantly increased by a factor of 10 due to the extra inertia. so it takes 0.3 seconds for the car to max out when in the air. - You're still only looking at a 1% of the cars weight at the wheels.
The main point is that how fast the motor spins the wheels is much more important than the direction of rotation of the motor.
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