RE: awd vs rwd braking

[Luis Marques <marques@rtis.ray.com>]

Eaton Dave <dave.eaton@minedu.govt.nz> wrote:
> > All else being equal (read equal weight, wheels and tires), the car
> > that approaches 50:50 weight distribution while braking at over 1G
> > will have the best performance.  In other words, something like a
> > Porsche 911.  RWD or AWD. 
> 
> ummm, the porker is nowhere near 50:50 weight distribution. 

Precisely.  It has more weight at the rear, so when it decelerates, it
approaches 50:50 better than any other car with static 50:50 or
front-biased weight dist. since the weight transfers to the front.

> also large contact patches tend to work against you in the wet.  

Yes.  That's why they have rain tires.  Everything in this life is a
compromise.

> to much rearwards weight distribution though and you'll get snap
> oversteer once you turn the steering wheel.  

Yes.  And that's why you brake in a straight line, then get on the gas
and turn.

> basically also any car that brakes at 1g brakes at 1g, regardless of
> the weight distribution. 

Yes.  But the same car could brake at 1.2g if it had more of the weight
in the rear.

> with 50:50 weight distribution, the rears aren't working anywhere near
> as much as the fronts.  the ideal for braking in a staight line would
> be a *rear*wards weight distribution.  just like a porker 911.  

Uhhh... Isn't this the same thing I'm saying?  50:50 _static_ weight
distribution is great for cornering, but a compromise under braking. 
What I said was that 50:50 _dynamic_ distribution (under heavy braking)
was the optimum for braking.

> this is where the torsen with it's shifting of engine torque forwards
> assists by taking the engine braking torque from the rears.  the newer
> "active" setups do this also...

It doesn't matter where the braking torque comes from - be it the engine
or the brakes.  The fact remains that a car with 50:50 weight dist.
under braking will stop the fastest.

> > Oh, about the gen. I quattro, a locked center diff doesn't split
> > torque 50/50, it varries between 0-100% forcing the front and rear
> > ends to runs at the same speed.  
> yes, so this has the effect of forcing braking torque through the rear
> wheels.  which is exactly not what you want....

Yes, it's precisely what you want if you want to keep a wheel from
locking.  All that locking the center diff will do is act as a
mechanical ABS.  It doesn't give more "stopping potential" to the car. 
It just allows it to reach its limits - like ABS.

> no the locked centre (no abs) will not come close to a torsen (w/abs)
> setup in braking terms.  in the wet? nope.

Are you sure?  Audi didn't run any center diffs or ABS in some of their
BTCC cars and the word was that they slowed down like if they dropped an
anchor.  In practical terms the locked diff and the torsen with ABS
should have similar braking performance (all else being equal of
course).  Without ABS, the locked center diff has the advantage.  And
they both have and advantage over open diffs.

> > We would need a ballast that moves back and fourth inside the car. 
> > Humm... a certain DTM Mercedes comes to mind.
> the m-b was an interesting concept, and one which seems to work pretty
> well in practice....

And it was RWD...  Can we drop it now?

Luis Marques
'87 4kcsq