It isn't that I simply "don't like" hub motors. It is that they seem
like a absolutley brilliant idea on the surface, but when you actually
try to implement them, the multiple subtle hidden disadvantages far
outweigh their initial advantages, and kill the concept very
effectively. When you see hub motors in a highway capable vehicle
proposal, run away from the investment, don't walk. The vehicle is
doomed because they haven't done their engineering homework.
Hub motors are terrific in low speed vehicles. They work great in
bicycles. Inboard separate motor (not hub motors) all wheel drive is
fantastic in race cars. For an ordinary consumer vehicle, hub motors
make no sense.
Every mainstream automotive manufacturer has tried building a prototype
car with hub motors. They each then discovered the major disadvantages.
They each then quietly abandoned the concept of hub motors, never to
revisit again. Porsche was the first to try hub motors in 1897. The most
recent attempt was Mitsubishi in 2007 with the MiEV (now renamed the
iMiEV.) The original prototype had four hub motors. The next generation
of prototype was reduced to two hub motors. The next prototype pulled
back to two inboard motors. Finally, they threw in the towel and they
have a single motor driving a differential like everyone else (and they
should have done in the first place, had management listened to the
engineers.)
Here is an overview of the history and a description of the technology:
https://en.wikipedia.org/wiki/Wheel_hub_motor
Unsprung weight is discussed as the major disadvantage, but they really
don't go into the other details. Increased cost, greatly reduced safety,
product liability, and a decrease in reliability are why you don't see
separate motors driving opposite wheels on a highway capable consumer
vehicle.
Think of a controller failure jerking the car into the oncoming highway
traffic and you can begin to visualize just a single aspect of the
multitude of engineering hurdles of hub motors.
Bill D.
On 5/15/2016 4:00 AM, Jay Summet via EV wrote:
I found this video about the Riversimple car very interesting, both
from a technology and business model sense.
https://www.youtube.com/watch?v=utmkddBFUg0
The car has 4 hub motors, specifically because they want all braking
to be electric. They have a large bank of capacitors to absorb 50 kW
of braking that are also used to power accelerations.
For people who don't like hub motors, the video shows that they can
work in an actual driving test mule, and their justification for using
them make sense (lots of excess electric braking power, ease of
packaging to reduce aerodynamic drag, etc).
More interesting than the technology was the business model they are
proposing. Instead of selling the car, they "lease" it via a service
contract (X$ a month and $N cents per mile). This way, the cost of
manufacturing the car can be higher, as long as the total cost of
operation is less than a gas car. It also makes longevity,
reliability, and low total cost of ownership a motivator for the
company, aligning their interests with those of the customer.
(Disclaimer: The whole system is powered by an 8kW fuel cell, but I
could also see a small sized lithium battery serving as the main
motive force, as they only need to draw 8kW at a time from it.
Acceleration is very good due to the capacitor bank, but top speed is
limited to 60 MPH average (with limited bursts above this using the
capacitor) due to the 11 HP equivalent (8KW) supply.)
Jay
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