Differences between FWD and RWD platforms

[Yakozan]

Stumbled across this brilliant piece of text on the net.

It's really worth a read.

1) "Balance": The car rides on four patches of rubber, each about as big as your hand. An ideal car would distribute its weight evenly, so each tire had to bear the same load, and none would give way earlier than all the others. The ideal weight distribution, then, would be split about 50/50 between front and rear (actually, 48/52 to help with forward pitch during braking). "A rear-drive car can typically approach that," says Zellner. Engineers can move the front wheels forward, so that the engine ? which doesn't have to be connected to those wheels -- sits behind the front axle. Meanwhile, the driveshaft and rear differential (necessary to send power to the rear tires) add weight in the rear. Front-drive cars, which must connect the engine and transmission to the front axle, typically have their engines mounted way forward and can't do much better than a 60/40 front/rear weight distribution.

2) Center of Gravity: This is the point the car wants to "rotate around" in a turn. On a rear-drive car, it's "about where the driver sits," says Zellner. In a turn, in other words, the car seems to be rotating around you ? you're at the center. It's a natural pleasant effect, suggesting you're in control, the way you're in control when you're walking or running around a corner and your weight is centered inside you. (Analogy No. 2: It's like wearing stereo headphones and having the sound centered between your ears!) A front-drive car, in contrast, with its massive front weight bias, wants to rotate around a point in front of the driver. So in a corner, the driver isn't just rotating around his spine. He's moving sideways, as if he were a tether ball on the end of a rope, or Linus being dragged when Snoopy gets hold of his blanket. Not such a pleasant feeling, or a feeling that gives you a sense of natural control.

3) "Torque Steer": One of the most annoying habits of many powerful front-drive cars is that they don't go straight when you step on the accelerator! Instead, they pull to one side, requiring you to steer in the other direction to compensate, like on a damn boat. This "torque steer" usually happens because the drive shafts that connect the engine to the front wheels aren't the same length. Under power, the shafts wind up like springs. The longer shaft -- typically on the right -- winds up a bit more, while the shorter left shaft winds up less and transmits its power to the ground more quickly, which has the effect of pulling the car to the left. (This winding-up phenomenon occurs the moment you step on the pedal. After that, the wind-up relaxes, but "torque steer" can still be produced by the angles of the joints in the drive axles as the whole drivetrain twists on its rubber mounts.)

Veer madness?
Engineers try various strategies to control this veering tendency, but even designing shafts of equal length (as in all Cadillacs) doesn't completely solve the problem because the engine still twists a bit in its mounts and alters the angles of the drive shafts. True, some manufacturers -- Audi, for example -- are said to do a particularly good job of repressing torque steer . But even a top-rank company such as Nissan has problems -- its otherwise appealing new front-drive Maxima is said to be plagued by big-time, uninhibited torque steer. Rear-drive cars, meanwhile, don't really have a torque-steer problem that needs repressing. Their power goes to the rear through one driveshaft to a center differential that can a) have equal-length shafts coming out from it and b) be more firmly mounted.

4) Weight Shift: Suppose you just want to go in a straight line. What's the best way to get traction? Answer: Have as much weight over the driving wheels as possible. Front-drive cars start with an advantage -- but when any car accelerates, the front end tips up, and the rear end squats down. This transfers weight to the rear wheels -- away from the driving wheels in a FWD car but toward the driving wheels in a rear-drive car, where it adds to available traction. In effect, the laws of physics conspire to give RWD cars a bit more grip where they need it when they need it. (This salutary effect is more than canceled out in slippery, wet conditions, where you aren't going to stomp on the accelerator. Then, FWD cars have the edge, in part, because they start out with so much more of their weight over both the driving and the turning wheels. Also, it's simply more stable to pull a heavy wheeled object than to push it -- as any hotel bellhop steering a loaded luggage cart knows. In snow, FWD cars have a third advantage in that they pull the car through the path the front tires create, instead of turning the front tires into mini-snowplows.)

5) "Oversteer" and the Semi-Orgasmic Lock-In Effect: In a rear-drive car, there's a division of labor -- the front tires basically steer the car, and the rear tires push the car down the road. In a FWD car, the front tires do all the work ? both steering and applying the power to the road ? while the rears are largely along for the ride. That, it turns out, is asking a lot of the front tires. Since the driving wheels tend to lose traction first, the front tires of front-drive cars invariably start slipping in a corner before the lightly loaded rear tires do -- a phenomenon known as "understeer." If you go too fast into a curve -- I mean really too fast -- the car will plow off the road front end first. In rear-drive cars, the rear wheels tend to lose traction first, and the rear of the car threatens to swing around and pass the front end -- "oversteer." If you go too fast into a corner in an oversteering car, the car will tend to spin and fly off the road rear end first.

What's the best way to fly off the road? Safety types prefer frontwards -- understeer. Why? To control an oversteering skid, where the rear wheels are heading for the weeds, you have to both slow down and counterintuitively turn the wheel in the opposite of the direction you're turning. In a front-drive car, with the front wheels slipping, you slow down and keep turning the way you'd been turning to get around the corner in the first place -- a more natural maneuver, since you're pointing the car in the direction you want to go. This is why, for safety reasons, even rear-drive cars sold to average consumers tend to have their springs and other suspension bits set up to make them understeer -- to make the front tires slip first, despite the car's innate oversteering tendency. Only by applying lots of power in a corner can you actually break the rear end of a bread-and-butter rear-drive car like the Mustang loose -- a maneuver favored by sports car freaks, but one you try at your own peril.

Big American manufacturers (all heavily invested in front drive) like to say that for 99 percent of drivers, driving at normal speeds, FWD's inherent understeer and better traction in the wet makes it preferable -- both safer and easier to drive quickly. It's only the 1 percent of speed freaks who enjoy breaking the rear end loose and then catching it with a bit of "reverse lock." Here's where I emphatically dissent.

It's pretty clear to me, after driving hundreds of different vehicles over several decades, that rear drive offers a big aesthetic advantage to ordinary drivers at ordinary speeds in ordinary conditions. Why? The lock-in effect I mentioned earlier. Suppose you go into a corner in a rear-drive car at a reasonable, safe, legal speed. Nothing's about to skid. But you can still feel the front end starting to plow wide a bit. What to do? Step on the gas! Don't stomp on it -- but add a bit of power, and a miraculous thing happens. The front end swings back in, the car tightens its line. Cornering traction seems to increase. And the car feels locked into a groove, balanced between the motive power from the rear and the turning power in the front.

Hit the brake?
You don't have to be a race driver to feel this. You can be a defensive driver and feel it. You can be driving a 1973 Ford Maverick with leaking shocks and you'll feel it. Accountants feel it on the way to the office and housewives feel it on the way to the Safeway. Even Ralph Nader probably feels it. It's a good part of what makes driving a car a sensual act. (What's happening, technically? None of the tires is at its limit of adhesion. But the added speed is making the front tires --which [since they are undriven] have plenty of surplus traction -- apply more force to the road surface to change direction. Meanwhile, the rear of the car is shifting outward, ever so slightly -- not a Bullitt-style power slide, but a subtle attitude adjustment that cancels the plowing effect. The power "helps you through the corner," as Zellner puts it.)

This doesn't happen in a front-drive car. The best an ordinary driver can hope for in a FWD car is that it "corners as if on rails" -- no slippage at all. No plowing -- but also no semi-orgasmic "lock in." More typically, if you hit the accelerator in a fast corner, things get mushy up front (as they did that evening near Jayne Mansfield's house). The lesson the FWD car seems to be teaching is: Try to go faster, and you're punished. Front-drive cars are Puritans! In a rear-drive car, you hit the accelerator and things get better! Rear-drive cars are hedonists. (This is assuming you don't hit the accelerator too hard.)

No front-drive here
I'm not saying there aren't sophisticated techniques that allow FWD cars to do better. A recent issue of Grassroots Motorsports tested a humble FWD Acura RSX against a classy rear-drive BMW. The Acura actually turned laps a bit more quickly. How'd that happen? The Grassroots people realized that by stepping on the brake hard enough on entering a turn, the rear of the Acura could be made to swing wide, canceling out its inherent understeer. (This is the same effect you get by stepping on the gas in a rear-drive car.) But normal drivers aren't going to mash the brakes and go sliding through turns like a rally champion. Nor does braking to achieve "lock-in" seem as satisfying as accelerating to achieve lock in. I suppose I shouldn't knock it until I've tried it -- but I'm not going to try it! That's the point. Housewives heading to the Safeway aren't going to try it either. The joys of rear-drive are accessible to them -- it's the joys of FWD that are reserved for the skilled Grassroots Motorsport elite