# is it wind resistance



## sergiu tofanel (Jan 13, 2014)

That looks like a very "draggy" vehicle. Based on your anecdotal measurements, one can calculate power expenditures for rolling resistance and aerodynamic drag. 

at 70km/hr, your vehicle expends the power as follows:

17% of power (or 13A) to overcome rolling resistance and 83% (or 57A) to overcome aerodynamic drag. 

1t 100km/hr, assuming a current draw of 185A, the vehicle expends

10% of power (or 18.5A) to overcome rolling resistance and 90% (or 167A) to overcome aerodynamic drag.


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## Duncan (Dec 8, 2008)

Hi Owen

Sounds about right
The Device - about as un-aerodynamic as possible
Used 50 amps at 50Kph
and about 200 amps at 100Kph

Since then I have done a bit of work on things like wheel alignment - but I suspect its still the same


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## MisterSid (Feb 26, 2014)

evnz said:


> I have been driving for a while now and I am trying to understand why I can use just over one amp per km/h (unless I am booting it ) up to 70 km/h and at 100 km/hour I use 170-200 amps


Drag can account for a big part of the equation, but not an increase of that much.

It takes about 3x more energy to push a car a long at 100 vs. 70 km/h

Some rough figures would be;

50km/h = 1.3kW
70 = 3.5
100 = 10.2
200 = 81.5 

This assumes a 0.36 drag coef, 2.2m2 frontal area and no rolling resistance (add about 7%), etc. The flat rear of the cab could also start increasing your drag as you get up speed.

I do not know what accounts for the rest. Made your motor is hitting the drop off in torque at this RPM ... maybe?


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## sergiu tofanel (Jan 13, 2014)

The increase is due to the drag. Any wheel misalignment or drivetrain friction will require a linear increase in power. If the speed doubles, the power requirements double. Not so for aerodynamics. The aerodynamic power expenditures are proportional with the cube of velocity. In other words, a velocity increase from 50kph to 100kph increases the aerodynamic power requirements 8 times! If you can take accurate current vs velocity measurements (on a flat course, calm day), it's rather easy to calculate rolling resistance and aerodynamic coefficients.


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## John (Sep 11, 2007)

MisterSid said:


> This assumes a 0.36 drag coef, 2.2m2 frontal area and no rolling resistance (add about 7%), etc. The flat rear of the cab could also start increasing your drag as you get up speed.
> 
> I do not know what accounts for the rest. Made your motor is hitting the drop off in torque at this RPM ... maybe?


I think your cd estimate is a touch low. A newish Ford ranger with an enclosed tray is still about 0.40. I would estimate Hilux cd at least 0.45. A locost is about 0.70. Aero drag (Da)=1/2*v^2*p*cd*A. p=air density (kg/m^3), v=velocity (m/s), A=frontal area. Da100=1/2*27.78^2*1.25*0.45*2.5=542N. Power due to drag = drag times velocity. Pa100=15.1kW

Drag due to rolling resistance = Crr*m*g. Estimate mass at 1400kg, Coefficient of rolling resistance (Crr) at 0.012. Drr=0.012*1400*9.81=165N.
Drr*v=Prr. Prr100=4.6kW. Power total (100km/h) is 19.7kW. This falls short of the estimated 26kW+ used. There is usually a gap between power due to rolling resistance and aero dynamic drag and power consumed. Other losses aren't acounted for such as energy to accelerate the vehicle, climb hills, wind drag, and electrical losses.

Calculated power total at 50km/h (Pt50) = 4.2kW,
Pt70 = 8.4kW
Pt100 = 19.7kW
Pt200 = 130kW

A modest head wind of 5km/h would boost the power requirement at 100 km/h to 22kW. Climbing a 5% grade would need about 19kW at 100km/h on top of the rest.


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