Originally Posted by skier219
Man, this is a tough crowd! Forget everything I said. Learn it for yourself. Go out in your car and while you're rolling down the road, apply the emergency brake gently (or start from a stop with the e-brake applied). Your car should pitch nose-up because the added drag (equivalent to less power) at the rear creates a moment. Now visualize how adding power (or removing drag) at the rear would provide a restoring moment. It's all about power allocation and how it balances the car, and braking works on top of the vehicle's balance. If this doesn't make sense, then forget the whole thing, it's not worth discussing anymore.
Applying the ebrake to a moving vehicle causes the nose to dive slightly, not pitch up as you unbelievably suggest. Perhaps you
should try it.
There are no AWD vehicles out there that apply
motive force during braking, as this would be an awful waste of energy.
Regarding fore/aft weight transfer, this is governed by the laws of physics. Outside of extreme
cases (stuff you see at dragstrips), the weight transfer that is actually generated by applying torque to a wheel (i.e. twisting the vehicle around the axis that forms a line between the centerlines of the wheels on an axle) are miniscule... in the same realm as lifting up or pushing down by hand on the appropriate end of the car. What is not miniscule
is the weight transfer that occurs when any braking or acceleration occurs.....largely from the unavoidable fact that the center of mass is above the road. Regardless of whether you brake the car with the front or rear wheels, there is still a moment from the point of interaction at the road leading to the CoM. Since there is no force being applied above
the CoM to counteract this, the vehicle rotates forward under braking, backwards under acceleration.
Visualize this: If what you claimed were
true, a FWD vehicle would dive
when accelerating. While this would do an awful lot for my 60ft times at the track, its unfortunately not the way things work.
|The only proper braking comparison of FWD/AWD/RWD is when all other factors are removed and you can directly compare the braking with the various drivelines.
This cannot be done. It cannot occur. An ideal configuration for a FWD vehicle is so different from that of a AWD vehicle or RWD vehicle that the "other factors" will be inextricably linked to any results. General trends across broad categories are all we will be able to note.
|I have not seen that sort of comparison done. I can tell you that an AWD WRX STi will handily outbrake a lot of FWD/RWD cars in it's weight class, which is worth noting.
Ever see the brake system on an STI? It isn't exactly typical of cars in that price range. Its damn good. Serious stopping power. You'd have to buy a Porsche with PCCB or similar to get a notably better braking system in a road car.
If you look at the braking peformance of the more sedate Imprezas, with manual tranny and real AWD....you'll find that they are not
that good when compared to other vehicles in the same price/weight range.
In this kind of price range though, generally the tires are the limiters. Better tires will lead to much better results. My 800 dollar Mazda will do 110foot stops (once, anyways, not subsequently as there isn't enough rotor mass) if I throw on sticky enough tires.
FWIW, a friend and I have brake tests every summer for fun. For the unitiated, a sub 130ft 60-0 stop is surprising and a bit violent.
|Regardless, the only point I was making is that AWD can be used to improve the braking and stability of a given vehicle. I am certain that if I took my Outback wagon and measured braking with and without the active AWD distributing power fore/aft, the braking would be worse without active AWD.
I'd love to see it. I think you are wholly wrong.
However, if you have the car I think you have, you can't (easily) disable AWD without disabling ABS, so that brings driver skill into the picture as well.
|Will the AWD vehicle be worse than a FWD vehicle when braking? Perhaps.
Well, whats the issue then? Thats my whole point. Thats why I wrote off the XC90 example. These modern AWD systems were designed in large part because
of the difficulty of interfacing classical mechanical AWD with modern ABS. Difficulty is probably a poor term, cost is a better one.
|But if active AWD can be used to improve the braking of that AWD vehicle so that it's better than dumb-AWD, then it's a benefit in my book.
That, it most certainly does. Just not through the mechanisms you assume it does.
It is better than dumb AWD because as you brake, the viscous coupling/clutchpack/whatever center diff isn't
trying to limit the rotational speed difference between front and rear wheels. This is fairly important in order for an ABS system to function well. A torsen center diff is a clever way around this, but its expensive, and has its own set of limitations. The modern electronic systems could be very well executed in theory, but many aren't. Thus people like watersurgeon get the idea that they all suck.