[LINK] Standards, please! The third coming of electric vehicles

[David Boxall]

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 
a bump.

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 
walking pace.

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 
expensive).

-- 
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