[LINK] Standards, please! The third coming of electric vehicles
david.boxall at hunterlink.net.au
Sun Apr 22 10:09:38 EST 2012
On 22/04/2012 2:02 AM, Frank O'Connor wrote:
> - Why not install multiple smaller engines in an electric car?
> - Why not install them in the wheels?
A small matter of physics. That arrangement might work at very low
speeds and/or on extremely smooth roads. The problem comes when you hit
Hitting a bump accelerates the wheel upward. The degree of acceleration
is a function of the nature of the bump, speed of the vehicle and
diameter & width of the wheel, among other things, but the wheel ends up
going upward at a given rate.
The suspension is then required to stop the wheel and return it to the
road without adding to the problem by slamming it down too hard. That's
usually done with a spring and a shock absorber. The spring absorbs the
energy of the ascending wheel and the shock absorber slows its return,
so it doesn't rebound violently.
So far, so good, but the energy required to stop the wheel's upward
motion has to go somewhere. It goes into the body of the vehicle, where
it's ultimately felt by the passenger(s). The amount of energy absorbed
by the vehicle is a function of the speed and weight of the wheel.
Adding a motor increases the weight of the wheel. Vehicles employing
so-called wheel-motors have been made (might still be in production),
but none give a very comfortable ride, particularly at speeds above
One solution is to move the motors back into the body of the vehicle.
That reduces the weight of the wheels, while increasing the weight of
the body. The increase in body mass increases its capacity to absorb the
energy of a (now lighter) upward-moving wheel, further enhancing ride
characteristics. Unfortunately, the necessary axles and constant
velocity joints add to costs, though you can get away with using cheaper
motors (wheel-motors, needing to minimise weight, are notoriously
David Boxall | "Cheer up" they said.
| "Things could be worse."
http://david.boxall.id.au | So I cheered up and,
| Sure enough, things got worse.
| --Murphy's musing
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