Taco To The Limit
One of the staple dishes here in Southern California is the fish taco. I find myself at Wahoo's Fish Tacos at least twice a week. Although they're my favorite, a variety of other places do them well. I've paid various prices, had them prepared different ways and still have trouble saying what the best method is. Grilled, deep-fried, sauted-they all have merit: it's a matter of taste.
The same goes for driveline layout. Recently, I've driven some of the finest examples of front-wheel-drive, rear-wheel-drive and all-wheel-drive cars to grace asphalt. The frontdriver was a Hotchkis-equipped MINI Cooper S, capable of some of the most amazing feats of direction changes I've ever seen. The car doesn't understeer (as everyone expects from a front-driver), while sliding the back end through a corner and initiating four-wheel drifts are completely possible.
Representing the rear-wheel-drive camp is Lotus' amazing Exige S 240. Being half the weight of most American SUV drivers and a quarter the weight of their vehicles helps this thing corner like Jesus on roller skates. Being rear-wheel-drive allows for easy mid-corner corrections and will respond obediently to any driving style thrown at it.
For the all-wheel-drive team, a number of suitable four-trackers have graced our testing regime lately. Everything from Audi R8s and RS4s to 997 Turbos have turned perfectly good rubber to dust in our hands. The Audis make excellent use of all four wheels, but cars like modified twin turbos absolutely need to use all four patches of rubber for forward propulsion.
The first concern when talking about drivetrains is: why does it even matter? The easy answer is because cars have a limited amount of grip generated by weight pushing tires on to the road. This finite grip must be divided between braking, cornering and accelerating forces. When trying to achieve maximum cornering gs, you want to use as little grip as possible for accelerating and vice versa. This is the downside of front-wheel-drive cars. As the power is fed in to accelerate, the tires will break traction if they're already at the edge of their abilities with cornering force.
Many enthusiasts prefer rear-wheel drive because the front tires are tasked with forcing the car around a turn while the rear puts down power. The rear has an advantage over the front in acceleration because drive torque and g-forces cause a car to squat during acceleration, creating even more grip in back. Hopefully, we've all felt the results of trying to put down too much power in a rear-wheel-drive vehicle. The back end loses grip and a slide ensues. If the car doesn't break the rear tires loose, the loss of grip on the front end causes the vehicle to understeer toward the outside of the turn.
All-wheel drive seems like the obvious answer. And it may well be. With the use of torque-biasing differentials that can transfer power to wheels with most grip, it's possible to get maximum acceleration and cornering gs at any time. Anyone who has driven a Quattro car in snow or rain can attest to all-wheel drive's superiority in low-grip situations. The downside is the parasitic loss of power in the drivetrain. While some all-wheel-drive set-ups are seamless in operation, some make for interesting vehicle dynamics while transferring power from wheel to wheel if the driver isn't accustomed to it. We won't even consider the added expense and complexity of all-wheel drive.
All this talk of dynamics and grip is based on driving at the very limit, which is a small percentage of a car's life. Front-drive is more effi cient in terms of power consumption and packaging. It's also cheaper to produce. Set up correctly, frontdrive handling can nearly match that of other variants and be just as much fun to toss around. Ultimately, it will limit the amount of power put to the ground, but many platforms can handle power in the 250 wheel-hp range with no issues whatsoever.
Rear-drive can be hugely entertaining, but requires a more experienced driver and preferably electronic intervention to be fast in inclement weather and/or with extreme power. All wheel drive may be considered the best of both worlds, being excellent for the average driver, having the ability to put down the most power and offering the greatest cornering stability.
Each layout has pros and cons. It really comes down to suspension tuning and the intended style of driving. All will do the job, it's just a matter of the method used to get it done.