Topic: Rolling resistance of electric cars

[piersdad]

                                     Rolling resistance of electric cars

Despite a great many attempts to calculate the rolling resistance of an electric car  using weight and formula it is never more than a guess as there can be no way to find out a worn bearing or just a bad bit of aerodynamics
   The only accurate method is to actually tow the vehicle along a flat road typical of the average surface the EV will be on.
A tow test both ways and the average of the two results will give a real result.
If the tow is at the actual road speed required then the aerodynamics will also be in the results.
This  method will take into account the wear on the tires ,bearings, brake grab, and anything else that is not just right.

Testing for rolling resistance.
In a test I did I only had a small 30 lb scales so it was attached to a lever system that allowed it to measure only ¼ the actual pull.
The tow rope was attached to a car but in the end near the car a lever was placed so that the actual strain was taken on a short 4 inch part of the lever while the scales took the strain on  a lever  12 inches from the fulcrum, the scales only read 1/4 the actual tow effort.
This lever will be able to be pulled by some one in the rear of the towing vehicle once the tow is in the test area.
During the start the tow rope will get severe strain and the lever system will mean the scales only need to be used once the tow is up to speed and the strain on the erope is settled down.
Once the results are noted this is the actual effort required to move the car.

In one test we did 4 runs and the e average pull for a Morris minor was 48 lbs at 30 mph
To calculate the power  to move this we use this formula

Pull in pounds X speed in feet per seconds  = horsepower
550
(550 foot pounds per second = 1 horsepower)

On this test 48 lbs drawbar pull
Speed 44 feet per second

48 X 44  = 3.84 HP
550

1 HP = 746 watts
Power to move this vehicle is there for

3.84 hp = 2864.64 watts
This does not take into account the losses involved in the motor and power supply to it.
The car weighed 1200 lbs so for every extra lb of weight of battery  etc we can expect an increase of energy of
2864  =  2.39 watts
1200

Another way to get a more accurate idea of actual conditions is to do 2 tests  one with the vehicle as light as possible and one with as much weight as possible the difference  then in effort gives a good indication of the power required to move the vehicle under every day conditions