The following information and images are courtesy of BMW of North America, Inc. All BMWs are RWD as it's part of their philosophy.
Accelerate with near full power in a front-wheel drive car and you'll quickly notice the resulting effect of rearward weight shift. The front tires will likely lose traction and spin even on clean, dry pavement. By contrast, hard acceleration in rear-wheel drive cars increases the rear wheels' grip on good road surfaces because of the rearward weight shift.
Stopping ability is enhanced by the superior weight distribution of RWD. With the rear wheels carrying a greater percentage of the car's weight load than on a front-wheel drive car, they can apply more braking force to the road and help shorten stopping distances. Since RWD contributes to even tire wear, it is more likely that tires on a RWD car will have greater tread depth. Unless tires on a FWD car are rotated religiously, the front tires may become worn and less effective in braking.
Near equal weight distribution helps give front and rear wheels more balanced traction. This balance gives neutral handling characteristics that make cornering maneuvers easier. Rear-wheel drive's more equal weight distribution also aids handling agility through a lower moment of inertia. FWD cars usually have higher moments of inertia, contributing to understeer and sluggishness in cornering. As a result, RWD cars feel more responsive, lighter, and more nimble.
Balanced Force Distribution
With FWD, both steering and propulsion forces tax the front tires' slip-resistance during cornering. That's part of the reason why FWD cars tend to understeer or plow forward, changing directions less quickly than the turning angle of the front wheels. Since RWD separates the tasks of cornering (front wheels) and propulsion (rear wheels), it more equally distributes the traction-threatening forces to all four wheels.
Torque steering is a negative side-effect of FWD caused by the delivery of power to the wheels that steer the car. During acceleration in a curve or from a standstill, the force of torque steering can pose a hazard by changing the direction of the front wheels unless the driver is alert and can exert counteractive force on the steering wheel. RWD does not exhibit torque effect because the engine is isolated from the steering gear.
RWD allows a longer wheelbase and a more forward positioning of the front wheels. The longer wheelbase provides better handling while the forward position of the wheels reduces the possibility of the front spoiler scraping on dips.
No CV Joints
FWD cars have four CV (constant velocity) joints connecting the engine to the front wheels. In comparison, RWD cars use universal joints which wear out much slower than CV joints.
Fore-aft weight distribution more balanced. Braking performance enhanced. Tire wear more even. Cornering easier, more responsive. Lighter than AWD configuration for better acceleration and cornering performance and better fuel-efficiency. Better hard acceleration performance on good surfaces than with FWD. Better cornering ability because steering and propulsion are applied at separate axles. Greater agility because of lower resistance to changes in direction (lower moment of inertia). Longer wheelbase for smoother ride. Absence of torque steering effect common with FWD. No CV joints to replace.
Good traction during mild acceleration on slippery surfaces. Lighter weight helps fuel-efficiency. Interior room enhanced by lack of longitudinal driveshaft. Less expensive to manufacture.
Traction enhanced on all road surfaces under all weather conditions. Faster acceleration "off the line" due to all wheels driving. Better road grip during cornering in adverse weather or slippery road conditions.
Now here's what some experienced people have to say on the matter:
FWD are shopping carts (statement #1). They are always understeered because the steering wheels always have both inertias, that of the motor and that of rotation.
With RWD, you should be neutral coasting (you don't have to force it into curves), oversteering on throttle (very convenient for getting out of corners fast but on course), and if need be understeering on compression or on lifting throttle (like, say, oh shit, entering a corner too fast).
The reason why most people prefer FWD is because they "hang on" to the road, the rear end being dead it is less likely to fishtail in winter, etc. etc. Sure they have more sedate and very front-heavy behavior, but the average driver's ambition is not to enjoy spirited driving but rather to retain control over the car... One thing: if you enter a curve too fast with a FWD, there is nothing you can do except for the FWD artists who can modulate a hand brake in an emergency...
It is a matter of balancing the forces involved in getting a car around a course. When you are trying to accelerate out of a corner, almost any car starts to understeer. With rear drive, you can put more power to the rear tires, and it does two things at once it transfers weight onto the rear tires so they have more grip and increase the load on the rear tires so that the handling stays pretty well balanced. Too much power will cause the rear tires to break loose, but when you get it right, no front driver can catch you.
I drove an old Porsche 911SC, and it was kinda scary (at first) the way it would actually go straight with the steering turned pretty far as long as you stayed in the power...ease off of the power and the rear end would drift out making the car turn as tight as you want...when it is aimed down the straight, just mash the gas, and it goes straight until the next turn. Just make sure you have the steering aimed the right way for the next time you lift off of the gas as it will turn immediately. My first try in the Porsche, I spun it after the finish line as I lifted to slow down, the wheel was still turned from the last corner.
My Celica is not quite that extreme, but it is still a driver's car, and the throttle is an integral part of pointing it where you want to go. If I roll on the power smoothly, it will widen its line very slowly...if I jump into it, the rear will break loose. A mid-engined car is halfway between these, and I believe that is why it is the dominant race car design. I am still trying to move my balance point 1.5% to the rear so that I can equally load all four tires at max cornering load. Right now, I am depending on my huge front sway bar and fairly soft rear suspension to make it put down power. That is why it is lifting the inside front tire when I accelerate out of the turns, even with a front weight bias. If I stiffen the rear, it breaks loose too easy, but if it were more rear heavy, then I could run the rear stiffer and get less body lean and still be able to put down power. Watching the MR2 beat me last week was actually fun as he had the car dialed in and drove it to the edge. He had the inside front tire over 4 inches in the air, all the way around the sweepers. He could use more rear roll stiffness and possibly go even faster.
In a front drive car, the power still transfers weight to the rear but then the unloaded front tires get the power that overloads them and causes wheelspin and major understeer. The only fast way through a corner in a front driver is to dive in fast and hope it sticks. If you are too slow and try to accelerate, you will induce understeer. Most SP class front drivers run HUGE rear sway bars to try to keep both front tires on the ground under all situations so they can put down some power. Even the cars with limited slip diffs in the front just plow if they try to put down too much power though. The fastest front drive person around here is very smooth and never tries to accelerate in corners he just carries even speed at the limit of the chassis, then accelerates on the straights. He is remarkable to watch but is never close to the fast rear drive times.
Short answer, rear drive is FUN.
For pure performance work, you would only consider RWD. Why?
For acceleration: With the weight transfer going to the rear of the car, RWD is obviously the way to go as more acceleration increases rear grip. With FWD, the faster you try to accelerate, the less traction you end up with.
For cornering power (not handling): If the torque of the engine is transmitted through separate wheels from the steering wheels, then the tires will naturally have a better ultimate lateral loading limit before they start to slip badly. By this I mean that tires only like doing one thing at a time, i.e., they can't walk and chew gum at the same time! =) As a quick & nasty demonstration, get up to a high speed in your car then hit the brakes hard enough so the fronts are just on the point of locking up, then turn the wheel. The front tires will definitely start to slide badly and create bad understeer.
For handling (not cornering power): In a perfectly balanced chassis, the front and rear tires slip evenly when cornering. This is called neutral handling, of course. With RWD, the torque being transmitted through the rear tires tends to make the rear end step out if enough power is applied, but for the moment I'll only talk about a "stable" situation when cornering. With the car set up correctly, and if you have enough power, then you can actually steer the car with the throttle, and it is quite fast when driven that way. (Rally car style, as a lot of the old Formula 5000 grunters did. Bloody great to watch!) With FWD, the more power you try to feed through the front wheels, the more they start to slip and cause the car to widen out when exiting the corner. This isn't good if you want a high exit speed to hold down the following straight. The FWD mob do have some tricks they use to get by this though, but at the expense of tires. No matter what you do, a high performance FWD will eat tires, but a well-balanced RWD will use them far more evenly and slower too.
Part of the factor involved in making a car handle well (not to be confused with cornering power) is weight distribution. A well set up car of any sort will have a weight distribution of about 45% front/ 55% rear. You can even go as far as 40/60 with no adverse problems. In a lot of RWD cars, you can easily get 50/50 or sometimes better. In a FWD, all the parts that have any reasonable weight are stuck right up the front and so it's very difficult to get the weight distribution better than 55/45. But, as I've mentioned, they have their little tricks to get past that.
Another factor is that a typical FWD is generally a more compact car that an equivalent RWD and so are often a sizeable chunk lighter. Short of liposuction, it's hard to get the last little bit of dead weight out of the RWD car to even things up. =)
I guess the acid test is the real world. In all the purpose built racing cars (NOT modified road cars, etc. I mean real racing cars like the Formula's, etc.), not one successful one has ever been FWD. And physics says there never will be. I figure if it's good enough for the track, then it's good enough for me.
The other hassle is the radical change in driving habits needed to go fast in a FWD as they are very, very different from a RWD. If you've spent all your life in a RWD fast car, then you're going to have a lot of trouble making the changeover.
All this being said, the best FWDs do a damn good job! I must digress here I am thinking specifically of the 2-Liter Touring Car race at Bathurst, Australia last year. As expected, the RWD BMW's & 4WD Audi's dominated in the dry. But then it pissed down, non-stop. I fully expected the Audi's to run away and hide in the wet, but to my great surprise, one car made a phenomenal break from the rest of the pack. It was Jim Richards (a New Zealander "Webfoot" extraordinaire and a damn nice guy too...one of the best wet weather drivers in the world, bar none) in a FWD Volvo! I just sat there in disbelief! How on earth could a FWD completely blow away a 4WD in the wet? Well, I still don't know...all I can think of there were three things that made it possible:
1. Jim Richards.
2. The Volvo team had excellent foresight and set the car up to suit the conditions when the other teams didn't.
3. Jim Richards.
FWIW, I've never seen that since, so perhaps Jim's team did do a super job on his car that day, and the other teams have learned from it and made extra preparations.
So to sum up, yes, both can be made to do a very good job, but for an all round package, you can't go past a RWD. They have inherent advantages and are generally more reliable (from what I've seen) and cheaper in the long run.
[Tech Info] [Mods] [Lounge] [Gallery] [Links] [About] [Contact] [Sitemap] [New]
|COROLLA Performance Business Card|
|spookhalloween . judi poker . Franklin Haney's Executive Profile|