Vehicles and Stuff

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(Updated 25-April '07)

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Power Calculator



Simple Vehicle Power Calculator


Calculator using Imperial Units


Basic Vehicle Data


Vehicle Weight     lbf

Vehicle Speed               mph


You decide if laden, partially laden or unladen weight is used - the more heavily loaded the vehicle is the bigger the power consumption. Speed is that at which vehicle power is to be calculated, note that the faster the vehicle goes the more power it needs. HINT - if you are towing another wheeled vehicle/trailer/implement adding its weight here will bring it into the calculation (underlying assumption is that the Rolling Resistance specified below applies to both vehicle and trailer).


Rolling Resistance


Coefficient of rolling resistance,     Cr   


Rolling resistance is dependent on ground conditions and tyre/wheel design. Click here for help and suggestions on what value to use. The calculator assumes a single Cr for the vehicle (ie all wheels/tracks have the same Cr). Normally the Cr figure relates only to the natural resistance of a wheel to roll - ie issues such as bearing friction are not included. If you are likely not to use low friction bearings you might increase Cr to compensate. Alternatively specify a drive transmission efficiency below.


Gradient Climbing


Angle of climb, 1 in             


Hill climbing can be a big contributor to required power - specify gradient in the form "1 in 10", "1 in 50", etc - anything flatter than 1:1000 will be taken by the calculator as on-the-level.


Aerodynamic Drag


Drag Coefficient, Cd     


Vehicle Frontal Projected Area,     in^2


Representative frontal width     ins.    Representative frontal height     ins.


Air Density    kg/m^3    


Drag is strongly speed dependent and is felt much more at higher speeds than low. If you have a slow moving vehicle this component of drive force might be well be ignored.

Drag coefficient is dependent on vehicle shape. Click here for help and suggestions.

Either input the frontal projected area directly or specify the width and height of the vehicle (the area will then be approximated at 0.9 x W x H).

A value is suggested for air density at normal atmospheric pressure.


Vehicle Acceleration


NOTE - This is an input to the power calculation, not an output. It allows the calculator to determine the power required to achieve the specified acceleration - you state the required acceleration.


Change in speed       mph

Time taken to achieve this speed change      seconds


The calculator can work out the acceleration figure it needs if you tell it by how much vehicle speed changes over what period of time. For example 0 to 60 mph in 10 seconds can be specified as a 60 mph speed change in 10 seconds, or a 6 mph change in 1 second. 40 mph to 60 mph in 2 seconds can be specified as a 20 mph change in 2 seconds. The calculator assumes that this magnitude of acceleration is present at the vehicle speed specified at the top of the page.


Any Additional External Force


Ext Force   lbf


This allows you to add in extra effects such as towing or pushing. If you have a tow bar pull you want to accommodate then you can add it in here.


Mechanical Transmission Efficiency


Effy  %


The calculated power is the ideal mechanical power required at the wheels to drive the vehicle under the conditions you describe. The motor output power will need to be more than this - because some is lost in the mechanical transmission. If you specify a transmission efficiency (in %) a figure for motor output power will also be calculated.





A new window will open with a summary of your input data and the calculation results. Use the browser's "Back" button return to this page to run a repeat calculation with different input data. This way you can build up a picture of how the results vary with design and/or operating condition changes.


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