Answered by Cars.com senior editor Joe Wiesenfelder:
This question helps clarify what all-wheel drive and other four-wheel-drive systems do, and that is to help you accelerate and keep moving on slick surfaces. Technically, that’s all they’re intended to do. If you spin your drive wheels, especially when turning, you tend to slide sideways, so there’s an added benefit. All-wheel drive’s job is to transfer power (actually torque) from the wheel or wheels with the least traction to a wheel that has more traction, and that keeps wheelspin to a minimum. (Traction control accomplishes a similar effect with two-wheel drive, but it’s always helpful when more wheels can help push.)
Four-wheel drive adds weight to a car, so theoretically it will extend stopping distances. More mass means more inertia, and that takes more friction to stop. All other factors being equal – same model, same tires, same everything except the drive system – the one with four-wheel drive probably won’t stop as short or corner as well on dry, wet or icy pavement. But you can’t ignore that the added weight also presses down more on the tires, which can help offset the shortcoming. It’s on loose snow where additional weight might actually help, by causing the tires to dig down deeper and gain traction. This is why we used to put sandbags in the trunks of rear-wheel-drive cars before the days of traction control. In the big picture, though, all-wheel drive doesn’t improve braking, and it can degrade it.
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The veyron also has what are probably the most massive brakes ever put in a production car. That helps a bit with the stopping. But yes, tires are everything in inclement weather (and very significant otherwise).
Overloading is never preferable. That definitely makes a difference in the maintenance of the vehicle.
-Martin
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'all wheel drive' is a marketing term. It has no engineering significance.
Off-road drive, where you mechanically (or by friction) lock one driveshaft to another can be helpful for using the drivetrain to slow your vehicle, if you are moving in a straight line. [off-road drive is found in about 95%+ of '4x4' pickups, very few actually have a center differential-NV244, etc.]
Four wheel drive (where you have a center differential) can be helpful if you are using the engine to slow the drivetrain, regardless if you are going straight or turning. ABS works normally here, where in off-road drive the ABS duty cycle is reduced-just a while back, 10 years or so, it used to be disabled when in '4x4'
Mass cancels out when calculating braking. This is how a Bugatti Veyron (over 4400lbs) can brake just as well as Lotus Elise-less than half that.
The tires are everything. (see Blizzaks)
and at high speed aerodynamic induced drag & downforce rules (Veyron pulls just over 2g peak deceleration at over 240mph)
Basically, don't rely on the drivetrain, just treat its effect as gravy or sugar coating in extreme inclement weather.
In tires we trust.