There is no comparison - AWD is best in the snow. However, what follows is a LONG writeup that I did last year at the request of a co-worker the morning after a long night at the bar after worK…
FWD versus RWD
I admittedly have a strong bias for rwd cars. In fact, unless some significant financial event occurs in the future, I will have ceased ever owning fwd cars starting in January 2000 until the time I can’t physically drive anymore.
Why? I love cars and have never been able to rationalize driving a compromised platform, even for a daily driver. I will compromise on food, clothes and just about anything else but not my cars. I have always felt that some “fun facts” support my bias such as:
Question:
What do the following auto manufacturers have in common?
Aston-Martin
Bentley
BMW
Ferrari
Lamborghini
Mercedes-Benz
Porsche
Corvette (not a manufacturer but been around for over 50 years)
Answer:
They offer some of the best-driving cars on the market and they do not sell fwd cars bearing their nameplate.
Question:
What do the majority of public safety vehicles such as ambulances, fire trucks and police cars have in common?
Answer:
Invariably, they are rear-wheel drive and they, more than anyone else, must be able to handle all kinds of weather.
This last fun-fact always raises the counter “fun fact” from fwd-ers and people that don’t know much about cars that “everyone knows that fwd is better than rwd drive in the snow”. If you press the person stating this “fact”, you typically don’t get a real answer, just some anecdotal personal experience which is interesting but not exactly a fact-laced response. I’ve heard this tired argument so many times with no facts to back it up that I decided to check on this myself to see if it were/were not true. More importantly, I wanted to know why it was true or not true based on true facts. The reality is that your “typical” fwd car will be better in the snow than your “typical” rwd car but not for the reason that you might think. It really comes down to a typical driver’s ability to drive cars, not the drivetrain layout itself. Weak driving skills sustain this conventional view.
Comparing Snow Performance of FWD versus RWD
Test Car #1:
1998 Chevrolet Camaro Z28 (I picked this car because I used to own one and drove it daily, even in the snow)
Curb Weight: 3439lbs (source: edmunds.com)
Front/Rear Weight %: 55/45 (approx.)
Weight on Each Drive Wheel: 774 lbs (weight according to f/r distribution, divided by 2 for each drive wheel)
Peak Torque: 335 ft-lb at 4,000rpm (source: edmunds.com)
Limited Slip Differential from the Factory?: Yes
Fitted snow size tire for traction comparison: 205/50-16 (valid per tirerack.com)
Test Car #2:
1998 Hyundai Tiburon 2 Dr Hatchback
Curb Weight: 2566 lbs (source: edmunds.com)
Front/Rear Weight %: 60/40 (approx)
Weight on Each Drive Wheel: 770 lbs (weight according to f/r distribution, divided by 2 for each drive wheel)
Peak Torque: 133 ft-lbs at 4,800rpm (source: edmunds.com)
Limited Slip Differential from the Factory?: No
Fitted snow tire size for traction comparison: 205/40-17 (valid per tirerack.com)
Surely, since the Tiburon is fwd, there isn’t even a comparison for snow driving, right?
Which One is Better?
Since we are talking about one drivetrain layout’s superiority over another, it would be best to make as many other things constant as possible. Therefore, let’s assume that:
• Both test cars are fitted with tires with an identical UTQG rating (uniform tire quality grading system – Treadwear, Traction and Temperature) and are constructed from an identical rubber compound.
• The tread design is identical and tire footprint is identical
• The weight on the drive wheels are identical (see above – the weight difference is 4lbs).
• The alignment settings of the drive wheels are identical (camber, toe)
• These cars will be tested on an identical surface
• Tire deflection is identical.
What is Traction?
When we talk about the ability of a car to drive on a slippery surface, we are talking about traction. Traction (frictional force on a system) can be expressed by Coulomb’s Law:
Fu = u*W
Where u is the coefficient of friction between the road surface and tire and W is the vertical load. The vertical load W is a separate, relatively complex formula with the primary inputs being weight and surface area. I had originally planned to work this through to come up with a reasonably good estimate of how traction increases in proportion to how the weight affects vertical load but a quick read of the SAE (and others’) documentation showed that the additional benefit of this information wasn’t worth the effort. If someone reading this would like to go through this exercise, I’d be interested in the results. (You’ll have to get into details like the arc of contact, true surface area, deflection of the tire, etc.)
Anyway. in our above example, we agreed that we would have identical tires, contact patches, and surfaces for our “test”. Therefore, coefficient of friction and surface area are constants in this discussion and essentially “fall out”.
This leaves weight as the only remaining variable. However, in our test cars the weight is essentially identical on the drive wheels so the available traction is IDENTICAL between the two cars. Since both u and W are both constants, one car cannot generate anymore traction than the other, e.g. available traction has NOTHING TO DO WITH DRIVETRAIN LAYOUT.
Why the Difference in Apparent Snow Performance Between the 2 Drivetrain Layouts?
The real, physical, relevant difference between these two cars are available torque. More torque means it is much easier to overcome available traction. More torque means you have to be easier on the accelerator. This makes it harder for the Tiburon to overcome available traction (at 133 ft-lbs) when compared to the Camaro (at 335 ft-lbs), even given the fact that the Camaro has a limited-slip differential and the Tiburon does not. Even if the Tiburon were rear-wheel drive, it would still be harder to overcome available traction in the Tiburon than in the Camaro.
Here are three funny pieces of information:
• Most fwd cars DON’T come with a limited-slip differential, which will greatly assist in available traction since you will be directing torque to both wheels rather than one wheel. Most rwd cars have a limited-slip differential.
• When going up a hill, weight transfers to the rear of a car regardless of drivetrain layout. That’s just high school physics. Since available traction increases with weight, going up a hill will REDUCE available traction in a fwd car and going up a hill will INCREASE available traction in a rwd car. For our Camaro/Tiburon comparison, fwd is WORSE than rwd going up hills in slippery conditions in terms of available traction. For some reason, people’s perception is exactly the opposite on this.
• Climbing steep and very slippery hills can be done in a rwd car via wheelspin and steering through it. If you invoke wheelspin in fwd car, steering ability is greatly reduced or not even available (if you don’t believe me, try it sometime). I used to live at the top of a big hill and had some fun with my neighbors and their “superior” fwd cars a few winters back. I drove up the hill that they couldn’t drive up in a 1983 Monte Carlo SS with Eagle GT tires on it. …and that car didn’t even have a limited slip differential. Because of the steepness and length of the hill, the only way up the hill was a high starting speed and later on invoking wheelspin to keep the car moving. Since there was a bend part-way up the hill, the fwd cars lost steering and had to back out of the throttle. This caused them to lose forward motion at that point and they were stuck. In my rwd car, I kept the wheels spinning and kept going all the way to the top.
Where did the average person get the idea that drivetrain layout determines snow capability rather than available torque?
My theory is that the brainwashing started in the mid-1970’s with the auto manufacturers. The federal government started mandating CAFÉ standards for the automotive manufacturers. CAFÉ (Corporate Average Fuel Economy) is an average fuel economy that an automaker must respect or pay a penalty if the fuel economy of the cars that they produce exceeding this amount.
How does an auto manufacturer increase fuel economy? Well, the obvious ways are to reduce power and reduce weight, which is exactly what they did. Front wheel drive was a great way to do this. At that time, the auto manufacturers could only fit smaller (more fuel efficient) engines in front wheel drive cars so that the engine and transaxle can fit up front (you can get fwd V8 cars nowadays but every time I see one advertised, I have to pick myself up off the floor after laughing so hard – I can’t imagine a dumber idea). The bonus here is that the transaxle, half-shaft arrangement is much lighter than the transmission, driveshaft, differential, axle-shaft configuration. So we get a lower-powered car that is also lighter – double bonus for fuel economy. The final feature that made this attractive to the manufacturers is that factory assembly of a fwd car takes 2 less processing steps than a rwd car, so it’s less UAW workers to pay and provide benefits for PLUS the assembly plant can be smaller (source of this paragraph: a discussion with a GM engineer I met at an symposia in Anahiem, CA in 1988 – this guy had a pretty low opinion of the intelligence of your average citizen). This allows for two “invisible” cost savings that the buyer will never see.
This still left the auto manufactures with a problem – how do we sell a car that doesn’t make any torque (torque accelerates a car, not horsepower) and make the customer happy about it? Well, we give him improved fuel economy and then we tell him that it’s a superior platform for driving in snow. …and thus the ruse was started – to fulfill CAFÉ requirements. The funny thing is that we were duped into accepting half (or less) the power that we were accustom to (in the early 70’s) and somehow, competent marketing made the public happy about this.
FWD CARS DON’T HAVE ANY MORE AVAILABLE TRACTION THAN RWD CARS – THEY JUST DON’T HAVE ANY TORQUE THUS MAKING IT EASIER FOR ANY MORON TO HAMMER THE ACCELERATOR AND NOT GENERATE WHEELSPIN AND OVERCOME AVAILABLE TRACTION.
That is why fwd cars are “better” in the snow. Most people are unable to drive cars that make high power because they don’t know how to use the accelerator.
If you could actually BUY a rwd car with the identical torque curve as a fwd car, it would actually GO BETTER up hills in the snow because weight transfer would INCREASE available traction rather than reduce it.