Quote:
Originally Posted by warped
Out of interest I've just done a few rough calcs based on the following assumptions (I haven't got a scales to hand so the weights may not be accurate.)
2wd
Tyres weigh 50g each with 45mm effective radius
rotor weighs 100g with a 7mm radius and a gearing of 10:1
acceleration from 0 to a motor speed of 50000rpm in 3 seconds.
Without reproducing the calcs in full, the answers I get are as follows:
The torque due to inertia of the motor is approx 10% of the torque due to the inertia of 2 wheels, which is actually more significant than I was expecting.
For the acceleration above the torque generated by the inertia of the wheels is 35.3 Nmm.
So for a typical wheelbase of 275mm the instantaneous effect of the torque due to the tyres on a 2wd is the same as taking 13g from the front axle and moving it to the back axle of the car.
So take 10% of 13g and that will be the change in load due to the inertia of the motor. i.e. 1.3g
So changing the motor direction by adding a gear to the transmission results in a maximum difference in load of 2.6g for a 1500g car or 0.2% of the cars weight.
Which is a difference that I personally am not good enough to notice.
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Indeed, when I tried to calculate it, I got surprisingly very low numbers as well.
Acceleration in 3 sec:
Wheels:
I = 1/2 * 0.1 kg * (0.045 m)2
= 1.0125 *10exp-4 kgm2
a = 174,44 rad/sec2
T = I * a
= 0.0176 Nm
"Weight" from inertia wheels at 275mm:
0.0176 Nm / 0.275 m = 0.064 N or 6.5 g
Motor:
I = 1/2 * 0.1 kg * (0.007 m)2
= 2.45 *10exp-6 kgm2
a = 1744,43 rad/sec2
T = I * a
= 0.00427 Nm
"Weight" from inertia motor at 275mm:
0.00427 Nm / 0.275 m = 0.0155 N or 1.5g
But an acceleration in 3 seconds is a little bit slow in my opinion. Especially in the air, I think it's at rev in 0.5 sec at the most.
Also there must be an extra force playing on this matter besides inertia: think of a 4WD car that can make a backflip by holding the throttle open. At a certain point the wheels are at speed, but it seems they keep on helping flip the car. This cannot be pure inertia, because inertia only works when speed changes. It must be friction with the air of the wheels. So I think mechanical losses in the motor can have an effect too.