The power needed to propel a vehicle can be
determined by combining the force that needs to be applied to
the vehicle to move it, with the vehicle speed at which this
propelling force must be sustained. What the calculator does is
help calculate the size of this drive force, you then tell it
the speed at which you want the vehicle to move under the drive
conditions being considered and it tells you the drive power.
Don't worry here if I'm not mentioning torque (isn't
that what you need in drive wheels though?) - drive torque,
through the traction generated by the vehicle's wheels, produces
drive force at the tyre/road contact - it is this drive
force that moves the vehicle. At the design stage it's easier to
frame the calculation around this drive force rather than the
drive torque.
A disclaimer might be in order first. The results produced
are what can best be described as design estimates. The physics
of this problem is generally well understood, but it is simplified here;
perhaps more importantly though the data you provide about your
vehicle (especially if it is at the design stage) is likely to
be estimated and the accuracy of the calculation results will
reflect both the accuracy of your input data and the
simplifications of the method. Use the results as a guide only!
Inputs
The total drive force that has to act on the vehicle to make it
move (or keep it moving) can be estimated by adding together
individual force components that arise from different physical
effects. These are: force to push up a gradient, force to
overcome the rolling resistance of the wheels on the drive
surface, force to overcome aerodynamic drag and force to
accelerate the vehicle's mass. If you also want the vehicle to
push or pull something then the force required to do this may
also have to be added. There may be other effects but these are
usually the main ones.
You can see straightaway that different driving circumstances will
give rise to different sizes and combinations of these forces -
eg for the purposes of the calculation is it on a hill or on the
flat, is it running at steady speed or is it accelerating
etc etc? You need to decide what driving condition your
calculation relates to, are you trying to calculate
maximum required power (to set maximum motor size for example),
or power under average running conditions (to determine average
energy usage and hence battery life for example), or power under
specific known conditions (eg to answer a question like "what
happens if I drive on grass instead of on tarmac?") or indeed
for any other circumstance you might think of.
So, input no.1 is your brain power - the calculator does some of
the math, it doesn't assess intelligently either your inputs or
the results outputs - that's your job.
The other inputs are:
(Note for a version of the calculator with US (imperial) units
click here.)