On 1/7/11 8:43 AM, HarryK wrote:
In , payer3389
@mypacks.net says...

On 1/6/11 10:25 PM, I am Tosk wrote:


I still hate front wheel drive.. Just doesn't make sense to have the
steering and drive on the same axle,

Please explain.

Well, I can as the real Harry, because if you remember, I took many
mechanical engineering courses. First of all, when the driven wheels are
also the steering wheels, there are parts that are under a LOT of stress
at times, such as the CV joints. Then if you lose traction like in snow,
no steering. Simple as that.

There are many advantages of rear wheel drive over front wheel drive,
and they have been discussed for many years by auto engineers. A quick
Google came up with this:

1) "Balance": The car rides on four patches of rubber, each about as big
as your hand. An ideal car would distribute its weight evenly, so each
tire had to bear the same load, and none would give way earlier than all
the others. The ideal weight distribution, then, would be split about
50/50 between front and rear (actually, 48/52 to help with forward pitch
during braking). "A rear-drive car can typically approach that," says
Zellner. Engineers can move the front wheels forward, so that the engine
– which doesn't have to be connected to those wheels -- sits behind the
front axle. Meanwhile, the driveshaft and rear differential (necessary
to send power to the rear tires) add weight in the rear. Front-drive
cars, which must connect the engine and transmission to the front axle,
typically have their engines mounted way forward and can't do much
better than a 60/40 front/rear weight distribution.

2) Center of Gravity: This is the point the car wants to "rotate around"
in a turn. On a rear-drive car, it's "about where the driver sits," says
Zellner. In a turn, in other words, the car seems to be rotating around
you – you're at the center. It's a natural pleasant effect, suggesting
you're in control, the way you're in control when you're walking or
running around a corner and your weight is centered inside you. (Analogy
No. 2: It's like wearing stereo headphones and having the sound centered
between your ears!) A front-drive car, in contrast, with its massive
front weight bias, wants to rotate around a point in front of the
driver. So in a corner, the driver isn't just rotating around his spine.
He's moving sideways, as if he were a tether ball on the end of a rope,
or Linus being dragged when Snoopy gets hold of his blanket. Not such a
pleasant feeling, or a feeling that gives you a sense of natural control.

3) "Torque Steer": One of the most annoying habits of many powerful
front-drive cars is that they don't go straight when you step on the
accelerator! Instead, they pull to one side, requiring you to steer in
the other direction to compensate, like on a damn boat. This "torque
steer" usually happens because the drive shafts that connect the engine
to the front wheels aren't the same length. Under power, the shafts wind
up like springs. The longer shaft -- typically on the right -- winds up
a bit more, while the shorter left shaft winds up less and transmits its
power to the ground more quickly, which has the effect of pulling the
car to the left. (This winding-up phenomenon occurs the moment you step
on the pedal. After that, the wind-up relaxes, but "torque steer" can
still be produced by the angles of the joints in the drive axles as the
whole drivetrain twists on its rubber mounts.)

Veer madness?Veer madness?Engineers try various strategies to control
this veering tendency, but even designing shafts of equal length (as in
all Cadillacs) doesn't completely solve the problem because the engine
still twists a bit in its mounts and alters the angles of the drive
shafts. True, some manufacturers -- Audi, for example -- are said to do
a particularly good job of repressing torque steer . But even a top-rank
company such as Nissan has problems -- its otherwise appealing new
front-drive Maxima is said to be plagued by big-time, uninhibited torque
steer. Rear-drive cars, meanwhile, don't really have a torque-steer
problem that needs repressing. Their power goes to the rear through one
driveshaft to a center differential that can a) have equal-length shafts
coming out from it and b) be more firmly mounted.

4) Weight Shift: Suppose you just want to go in a straight line. What's
the best way to get traction? Answer: Have as much weight over the
driving wheels as possible. Front-drive cars start with an advantage --
but when any car accelerates, the front end tips up, and the rear end
squats down. This transfers weight to the rear wheels -- away from the
driving wheels in a FWD car but toward the driving wheels in a
rear-drive car, where it adds to available traction. In effect, the laws
of physics conspire to give RWD cars a bit more grip where they need it
when they need it. (This salutary effect is more than canceled out in
slippery, wet conditions, where you aren't going to stomp on the
accelerator. Then, FWD cars have the edge, in part, because they start
out with so much more of their weight over both the driving and the
turning wheels. Also, it's simply more stable to pull a heavy wheeled
object than to push it -- as any hotel bellhop steering a loaded luggage
cart knows. In snow, FWD cars have a third advantage in that they pull
the car through the path the front tires create, instead of turning the
front tires into mini-snowplows.)

5) "Oversteer" and the Semi-Orgasmic Lock-In Effect: In a rear-drive
car, there's a division of labor -- the front tires basically steer the
car, and the rear tires push the car down the road. In a FWD car, the
front tires do all the work – both steering and applying the power to
the road – while the rears are largely along for the ride. That, it
turns out, is asking a lot of the front tires. Since the driving wheels
tend to lose traction first, the front tires of front-drive cars
invariably start slipping in a corner before the lightly loaded rear
tires do -- a phenomenon known as "understeer." If you go too fast into
a curve -- I mean really too fast -- the car will plow off the road
front end first. In rear-drive cars, the rear wheels tend to lose
traction first, and the rear of the car threatens to swing around and
pass the front end -- "oversteer." If you go too fast into a corner in
an oversteering car, the car will tend to spin and fly off the road rear
end first.