Re: 4000 turbo conversion advice

[Scott Fisher <sefisher@cisco.com>]

You ask:

> and it has less inertia out back so should be more
> predictable as weight is right above axle (what is this called,
> anyone?   It is similar to CG center of Gravity but not quite 
> as it is a Horizontal Balance?  

Polar moment of inertia.  If you've got a lot of weight waaay up front
and waaay out back -- imagine you've tied a bunch of sandbags to your
front and rear bumpers, just to visualize -- that's called a high polar
moment of inertia.  It means the car is reluctant to make changes of
direction, but once it makes a change it's reluctant to stop.  Think of
Newton's firsst law -- an object at rest tends to remain at rest, while
an object in motion tends to remain in motion.  A high polar moment of
inertia has the effect of making cars that are very stable in a straight
line, but which, once spinning, are harder to catch.  Many street-driven
sports and GT cars (Alfa-Romeo 116 chassis cars, Porsche 924/944/928,
the latest generation Ferrari and Corvette) have used front engine/rear
transaxle layouts to achieve a higher polar moment; like understeer,
it's something generally deemed safe for the average driver.

If on the other hand you've got all the weight in the middle -- imagine
pretty much any purpose-built race car since about 1963, where the
driver sits right in front of the engine (gee, and who had THAT idea
back in the 1930s???) -- the car has a *low* polar moment of inertia. 
That means it's easy to get it *started* turning but also easier to
*stop*.  This is one reason the Auto Union GP car had such a reputation
for being dangerous to drive: the only people who were accustomed to
vehicles with such a low polar moment of inertia were motorcyclists,
which is why the two greatest drivers in the Auto Union were Caracciola
(who started racing on two wheels) and Nuvolari (who by all accounts was
just so damned good it didn't matter WHAT he drove).

Moving any mass around in the car changes its polar moment.  So to go
back to your example, the Ur-Q's under-seat-mounted battery will give it
a slightly lower polar moment, compared to the 4KQ with its battery in
the trunk.  Polar moment is calculated using both weight *and* distance,
because it's actually applying leverage (essentially, a torque couple
around the car's center of rotation).  

Moving the battery behind the rear-axle centerline should also change
the effective weight on the rear axle, for the same reason -- the mass
acts through a torque couple about the car's center of pitch.  Of
course, there's also got to be a corresponding lightening of downforce
on the front wheels: you can't create or destroy weight by moving mass,
you can only move weight from one place to another -- at least till you
go to aerodynamic aids.  The leverage trick, though, is why modern
racing cars have the rear wings stuck out behind the car as far as the
rules permit: this gives them a cantilevered effect allowing the
designer to reduce the size of the wing (thereby reducing drag and
increasing the available power) while using the leverage to increase
rear-wheel downforce.  Stick better *and* go faster through the magic of
science!

--Scott "Knowledge is power -- or at least lower lap times" Fisher