This overstates the case. A solid axle car has about twice the unsprung weight
of an IRS car! I'll bet most folks couldn't tell the difference between a solid
axle Mustang and an IRS Mustang even if they drove them back-to-back. Likewise
most people can't tell the difference between independent front suspension and
solid axle for 4WD trucks, despite a huge difference in unsprung weight.
Mini Cooper racers can lose 35 lbs per tire by going to lightweight rims and
lighter tires! If you could do your hub motor package for 35 lbs you'd have the
same handling and ride as stock! The Mini Cooper is renown for its handling and
popular for racing, despite having fairly high unsprung weight (run flat tires
are heavy).
Yes, less unsprung weight handles and rides better -- so if you are going for
that extra fraction of a second in racing around corners it's a worthy
consideration. If you are paying major $$ for a car it's worthy. For most folks,
though (racers included) adding 30 to 50 lbs per wheel can be a worthy
compromise for handling and ride vs. cost, or off-road toughness, or drag racing
toughness.
Now there are other reasons that make hub motors an unpopular choice.
Off-the-shelf solutions are very expensive. They would be hard to protect in
deep puddles. The brakes would transfer ruinous amounts of heat into the hub
motor.
It would be better to use the tranny, or mount the electric motors similar to
the original motor mounting, and drive the axles though speed reducers.
________________________________
From: Joseph Ashwood <
[email protected]>
To: Electric Vehicle Discussion List <
[email protected]>
Sent: Mon, November 1, 2010 10:05:48 PM
Subject: Re: [EVDL] Anyone convert an Audi TT to EV?
-----Original Message-----
From: Al Swackhammer
Subject: Re: [EVDL] Anyone convert an Audi TT to EV?
> Try looking into a hub motor system. No drive line at all. More room for
> batteries and can make it either front or rear wheel drive.
I'm not sure that would work so well. A TT is a relatively small, so there
won't be much room for heavy batteries. A large mass of batteries would be
necessary to combat the unsprung mass of a hub motor system. This of course
wouldn't apply for very light hub motors, but those don't deliver much
torque.
For an ideal ride the sprung weight/unsprung mass ratio* should be infinite.
Each increase in the unsprung weight (weight of wheels, tires, brakes, half
the suspension, etc) needs to be accompanied by a large increase in the
sprung weight (body, occupants, engine, half the suspension, etc). This is
why your EV rides better than your ICE on the same springs. Normally in an
electric vehicle there is a large weight of sprung weight from the
batteries, but in a relatively small vehicle like the TT there won't be as
much battery mass. With hubmotors it is impossible not to increase unsprung
mass, the only way to combat the effects would be to increase the sprung
weight. Assuming you will be traveling at reasonable speeds aerodynamic
weight will be limited to relatively small amounts.
This is also the reason the oft-quoted, rarely observed Civic on 24s goes
flying. The unsprung weight is thrown in the air, F=MA, all that force hits
the lightweight civic and its the difference between being hit by a jelly
bean and a baseball bat. All the spring can do is extend the timeframe of
the impact, but negate it (softer springs extend the timeframe).
So if you want a slow vehicle, the light-weight hub motors won't add too
much weight, but for high power hubmotors you need a very large battery
weight to combat the losses. I don't think the TT will hold enough batteries
for this.
Joe
* This is one of those places where the difference between weight and mass
matters. The vehicle force is Vehicle Mass * Gravitational Acceleration,
this is weight. The unsprung force is Unsprung Mass * Impact Acceleration.
The Gravitational Acceleration is approximately constant on earth (small
variations depending on how far above sea level are ignorable). The Impact
Acceleration depends on the road conditions are varies constantly.
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