How does 4wd help a car on the road

Factual Questions
#1

I get most of my information on car performance from Top Gear, so bear with me here.

The other night, there was a repeat of a test drive of the Jaguar X-Type 4wd estate. In mildly snowy (looked like a inch or so of snow on the ground) conditions, it was described as better than the front wheel drive vehicles it passed and a rear wheel drive car was described as “terrifying” in that weather.

Going up hill I could understand, weight transfers to the rear of the car, the rear wheels get more traction. Other than that, going downhill and around corners, the car didn’t seem to have any advantage other than the weight of the 4wd system pushing the wheels further down and improving traction perhaps, but in an inch of snow that seems really slight. And wouldn’t the rear spin away anyway?

Off road, I can understand the advantages of wheels that can grip and pull away whenever possible. On a road, it’s less clear. Top Gear itself tested a 4wd Audi, front wheel drive Alfa and rear wheel drive BMW to demonstrate the difference between them on a road. The Alfa understeered, the BMW oversteered, the Audi was inbetween the two and was described as having the best and worst of the other two transmissions. Other magazines have stated a preference for traction control, it keeps a car going round a corner without the extra weight of 4wd

The Jag didn’t seem to have any tricks either, not even snow chains.

#2

4WD (really, AWD in these cases) can help on dry pavement by better using the available traction.

Consider a car accelerating around a curve. In a FWD car like the Alfa you mentioned, the traction available to the front two wheels is split between holding the car on its path (a lateral force) and accelerating the car (a longitudinal force). The rear wheels’ traction isn’t doing much other than providing a small amount of lateral grip.

In the RWD BMW, the situation is a bit better since you’re not asking as much from the front wheels, but it’s still possible to cause oversteer by overcoming the traction available to the two rear wheels.

In an AWD car, the AWD system typically splits power between front and rear in some way that makes sense, and in a way that neither requires too much traction from the front wheels (thus limiting the traction available for cornering), nor overcomes the traction of the rear wheels.

So, for a given acceleration scenario, you’re requiring a lower maximum amount of traction from the driving wheels in an AWD than in either a FWD or RWD. Power is more evenly applied to the road, which typically results in better dry handling and more predictable wet handling.