Front, Rear, All, or Four Wheel Drive

My background as a driver is diverse. I don’t think I’ve driven less than 30 thousand miles in a calendar year since the mid-seventies, and today I drive somewhere between 40 and 80 thousand miles each year. I love driving.

I was raised on a farm in Wisconsin.

My Dad never bought a new tractor. He did, however, believe in diversity. He has had two John Deere A’s, a B, (the first tractor I ever drove, I believe I was 8 years old) and two G’s. an International 300 utility as well as an International 706, a Farmall M and a Farmall H, an Allis Chalmers WD-45, a British Leyland, an Oliver 1850, a Fordson Major and a Ford NAA tractor. (My favorite was the Fordson Major) The most challenging operation on the farm was backing a four wheel wagon around a corner. I could do it but only if I was “In the zone” so to speak.

He never had a four wheel drive truck before the late 1990’s.

I have driven not only farm tractors, but construction equipment, over the road trucks, fire trucks, ambulances, sports cars, limousines, and automobiles built from every decade since the 1930’s.

I have test driven both front and rear wheel drive law enforcement vehicles through cones in both dry conditions and in pouring down rain.

Being from Wisconsin where we can have snow on the ground from September through May, I have driven many miles in snow with all manner of vehicles.

I have plowed snow with a 1969 Bronco, a 1972 F100, (both manual trans) a 1980 F 250 (auto trans) and various newer trucks.

I have chained up to drive a 2 wheel drive pickup through Lolo Pass between Idaho and Montana in the nineties, and not chained up to sneak out of California during a bad snow storm just a couple of years ago.

I have driven a fully loaded farm truck, a 1967 International 1300, with no brakes. I believe it was a 20 mile round trip and it was unloaded for the first half of the trip. We (my Dad and I) decided we could fix the brakes after dark but could not load the truck with firewood after dark. So, knowing ahead of time that there were not brakes, we made the trip. That was, however, on dry pavement. (The “good old days” weren’t all that good).

I once towed a two wheel drive truck with my two wheel drive truck with a chain, at night, in a snow storm, for 50 miles. My Dad was steering the towed vehicle. He was a little stressed by the eventual 55 mph top speed.

I once towed a two wheel drive pickup behind another two wheel drive pickup, this time with a tow bar, for 160 miles. All was fine until the ice storm started about 60 miles into the trip. I arrived couple hours late.

I have driven up a two track trail in the foot hills of Colorado that had a couple sections of 10% grade with 6 inches of snow on the ground and more falling rapidly without using four wheel drive just because I was irritated by the dude who questioned whether I could make it with four wheel drive. The fun part was when he met me at the door and said, “I see you made it.” My response was, “Yep, but if there had been more snow, I might have had to use the four wheel drive”.

And there was the time I drove through 4 inches of untracked snow at 4 AM in a 1983 Caprice Estate wagon and just for the fun of it wanted to see how fast I could go. I ran out of engine (305 cu in) before I ran out of stability; it would go no faster than 80 mph. Damn, I wish I had a video of that trip. Today I use a dash cam.

I like driving in snow. No, I love driving in snow. And the rest of this article is devoted to snow primarily. Not ice, not rain, not hail. Snow.

Let’s give a few definitions:

Front wheel drive. The front wheels share the drive and steering function of the vehicle.

Rear wheel drive. The front wheels steer, the rear wheels drive.

All wheel drive. The front and rear wheels provide drive to varying degrees as determined by a control module in response to wheel slip.

Four wheel drive. The front and rear wheels provide drive and are fully mechanically connected to the power train. (With a few exceptions)

Open differential. This gizmo is what allows the outer wheels of a drive axle to turn faster than the inner wheels when negotiating a turn. The down side of this is that when one drive wheel loses traction and spins, no drive is supplied to the opposite wheel. Practically the means that a two wheel drive is only a one wheel drive in slippery conditions and a four wheel drive is only a two wheel drive in slippery conditions. There are “limited slip” options, the principles of which I will not address here.

Acceleration. There is a reason that physicists use this term to describe both increase and decrease of speed. That is because it is pretty much the same thing. Technically an acceleration is a change in velocity. Velocity is different than speed. Speed is simply a rate of travel in a straight line. Velocity incorporates direction. This means that a change in velocity may be an increase or decrease in speed or a change in direction. Why is this important to you? To increase your safety on the road, it is useful to remember that the force that allows you to increase speed is basically the same as the force that allows you to decrease speed. Therefore, your brakes can be rated in horsepower just as accurately as your engine. Also, your tire traction is just as important to decrease speed as it is to increase speed. About which more presently.

Principles:

First: The power train does not move your car and the brakes do not stop your car. These systems simply cause the rotating members (driveshafts, axles, hubs) of the vehicle to increase or decrease speed in relation to the chassis. The only thing that propels your car is the tires. Likewise, the only thing that stops your car is its tires. (Or a tree)

Having said that, if you really are concerned about moving and stopping in snow, you need four matching tires. That means four snow tires, not two. If you wish to disagree, I would like to hear of a vehicle (passenger vehicle or light pickup, not an exotic sports car) that comes from the factory without matching front and rear tires.

Second: There is no substitute for a competent driver. Driving is a skill that is learned and maintained through practice and discipline. Sometimes simply practicing can lead to the development of bad habits. My point here is that total hours behind the wheel does not guarantee competence. Most polls show that 80% of drivers report themselves as above average in ability. Think about that. Given the statistical likelihood that there are a few drivers that misreport themselves as below average when they are average or above, this means that more than 30% of drivers over-report their ability. It is one thing to be a bad driver. It is quite another to be a bad driver and not recognize the reality of your own incompetence. This is similar to what psychologists call the Dunning Kruger Effect.

Third: In spite of what you may have heard, speed does not kill. In the scenario described above it was not the fact that the car was traveling at a high rate of speed, it was primarily that the tree wasn’t. It is not speed itself but speed differential (what Physicists call Delta) that kills. Think about this. The rotational speed of the earth at the equator is roughly 1000 miles per hour. This sounds scary but bear with me. If you live at the equator your house is zipping along at 1000 miles per hour. Farther north or south it is somewhat less. As long as your neighbor’s house is moving the same speed it’s no big deal. But if your neighbor’s house were to stop, you would be totally screwed. (Assuming your neighbor’s house was east of you).

Fourth: Even though front and rear wheel drive have different capabilities, one cannot be stated to be better than the other under all circumstances. When someone tells me that front wheel drive is better, first I need to know what is meant by better. Better on the highway? Better in town? Easier to maneuver? Better in the rain? Better resale value? More crash resistant? There is rarely a consistent answer to any of these questions.

Modern technology has narrowed the gap between the capabilities of front and rear wheel drive. Today all automobiles sold in the US are required to have ABS, Traction Control, and Stability Control. These are great systems and provide a dramatic improvement in vehicle controllability. But briefly, all this technological improvement only really means one thing. You can now drive somewhat faster without crashing. Hopefully.

So, what do we know for certain? Not a whole lot…. but enough….if what we know affects what we do. For example:

It is easier to park a front wheel drive vehicle in snow. Because the drive wheels can be steered, the control of the vehicle at low speed is improved, often dramatically. This low speed maneuvering ability can also prevent some crashes.

It is generally safer to drive a rear wheel drive vehicle at highway speeds in the snow*. This is because of understeer. This may not prevent a crash but may lessen its severity.

*This assumes equivalence in a number of variables: Tire design and tread wear, vehicle loading, road conditions, and especially driver incompetence. (Safe drivers are safe whatever vehicle they drive)

All vehicles built over the last several decades are designed to understeer in the event of traction loss. This is considered a safety feature. Understeer means that the vehicle continues in the direction it is pointed in regardless of steering input. With understeer, if you turn the steering wheel to the left, your vehicle continues to travel in the direction it was before the steering wheel input. It “under-reacts” to the steering. This is generally considered safer than oversteer. If a vehicle oversteers, it “over-reacts” to steering wheel input. If you steer left, the vehicle turns more sharply than the steering input alone warrants. Think of drifting (fun) and spinning out (funeral).

Understeer usually happens when traction is lost on the steering tires in a curve and steering input has little or no effect. This usually occurs when speed is excessive. Traction may be maintained on the rear tires, thus keeping the rear of the vehicle from trying to pass the front.

Oversteer, on the other hand, usually happens when traction is lost on the rear tires. When a curve is encountered it causes the rear of the vehicle to swing around due to loss of traction while the front tires maintain traction. (This is the object of a law enforcement officer when he performs the “Pit Maneuver”. And yes, this is a simplification.

Yes, these are very general statements and yes, there are a lot of variables. Let’s mention a few. These are not necessarily in order of importance.

Variables:

Variable number one, vehicle speed. The faster the vehicle is moving, the greater the kinetic energy, and therefore the greater the inertia, the tendency for the vehicle to continue in a straight line. If people would slow down, they would often maintain control without the help of technology.

Variable number one A: The driver application of the accelerator. This is somewhat different than the vehicle speed. The driver can apply and release the accelerator pedal much more rapidly than the vehicle inertia can respond. Not only can a rapid acceleration cause tire spin, a rapid deceleration in slippery conditions can cause the engine drag to cause the drive tires to lose traction. This can be a problem if the drive tires are also the steer tires.

Variable number two, the driver input to the steering wheel. If the driver moves the steering wheel too rapidly, the inertia of the vehicle may not be overcome, front wheel traction may be lost, and the vehicle will continue in a straight line (the direction it was travelling when traction was lost).

Variable number three, the driver input to the brake pedal. Because of the physics of vehicle braking and suspension, all cars tend to pitch forward while braking. This is because of the interaction of three functions. The tire traction occurs at ground level. The center of gravity is above that. The suspension compresses in the front and relaxes in the back when the brakes are applied. This is often referred to as weight transfer. The front gains weight and the rear loses weight. This is why the front brakes on all light duty vehicles do a high percentage of the braking. (And this percentage changes based on how aggressively the driver applies the brake pedal). The upshot of braking in snow is that weight is transferred to the front tires, increasing braking ability and steering control on the front tires. But this only occurs up to a certain threshold, that threshold being where the forward inertia of the vehicle overcomes the traction of the tires. Then, if understeer occurs, and the vehicle continues in a straight line regardless of steering input.

Variable number three A: The speed at which the brake pedal is depressed. Because the driver can step on the brake more quickly than the vehicle can respond, it is fairly easy to cause a brake lock-up on very slippery road surfaces, which leads us to:

Variable number four: The condition of the road. Dry pavement provides the best tire traction. The friction between the tire and the road can be changed by:

• Concrete or asphalt.
• Oil and grease from other vehicles.
• Water, whether rain, overflow, or spills.
• Ice.
• And for our purposes. Snow.

But all snow is not created equal. Dry powder is different than heavy wet snow, which is different than sleet, which is different than rain mixed with snow. Freshly packed snow is different than snow that has been driven on by 2,000 cars over five hours. Driving through untracked snow is different than being the second car through. Windswept snow is different than fallen snow even if it is of even thickness which it usually is not. Windswept snow often occurs in patches, so the driver must be prepared to deal with patches of dry pavement, then snow, then dry pavement, etc. And if the sun is shining on patches of white windswept snow alternating with black pavement, especially with partly cloudy conditions that produce uneven melting and then re-freezing into black ice…. well, you get the idea.

Variable number four A: Slush. This condition has its own paragraph because of the extreme importance of the driver recognizing this danger. THE MOST DANGEROUS WINTER DRIVING CONDITION ENCOUNTERED BY MOST DRIVERS IS IN THE PERIOD OF TIME BETWEEN WHEN THE PLOW TRUCK SALTS THE ROAD AND THE PAVEMENT PECOMES SIMPLY WET. It is impossible to overstate the importance of understanding how to drive in slush. Most drivers are aware of the term “hydroplaning”. This is when the speed of the tire exceeds the ability of the tire tread to channel water around the contact points of the tire and the those contact points ride up on the water. If slush rather than water is encountered, the problem is increased. Slush can be much more difficult for the tire to channel through and around the contact points. The slush is thicker on the road surface than just water. Also, the slush can be plowed to the right and left of each tire that passes through it. This leads to ruts of wet pavement between significant stripes of slush. If the driver rides up on this slush, he can completely lose traction on either the front or rear wheels or both. If this does not scare you, I have failed in my intention; read on, I’ll keep trying.

Variable number five: The way the vehicle is loaded. This item is often only taken into consideration by drivers of rear wheel drive vehicles when they put sand bags or other things in the trunk. However, all who drive in snow should stop and think about how their vehicle is loaded. Wherever weight is added in a vehicle, the nearest tire gains traction, period. This may be the only absolute statement in this article. Wherever weight is lost in a vehicle, the nearest tire loses traction. About which more presently.

Variable number six: The condition of the tires. Tires vary in tread design, rubber compound, contact patch size, and amount of wear. Plus, there are four of them. If the vehicle is not well maintained, the four tires may have significant variation in the amount of wear and even pressure. If the driver is uninformed, the tires may vary in tread design, size, make, rubber compound, etc.

Now that you have heard what the variables are, what should you do to be safer in snow?

1 Slow down.

If you only read two words of this article it should be these. SLOW DOWN! Slowing down helps you maintain control. Slowing down lessens damage to your car, yourself, and your passengers in the event, in spite of your best efforts, you still lose control or in the event one of the “30%” does.

2 Use matching snow tires on all four wheels.

Yes, sometimes safety costs money. Snow tires have a rubber compound that is softer in cold weather than summer or “All Season” tires. Personally, I think “All Season” is an oxymoron. By the way, if you absolutely cannot afford 4 snow tires: put the two that you buy on the rear. It does not matter if you have front wheel drive. For SAFETY, put the snow tires on the rear. Your installer may balk at this because “Nobody does it that way”. (This is similar to the reasoning used by teenagers when they tell your daughter that “Everyone is doing it”.) There is a reason that rear wheel drive is safer than front wheel drive in snow. It is because if snow tires are installed, they are always installed on the rear. This maintains rear traction during braking and contributes to understeer.

3 Embrace “low force” driving.

This means slow deliberate applications of all driver inputs: Steering, Braking, and Accelerating. Developing these habits throughout the year means you may avoid being one of those drivers that has to learn how to drive in the snow every year. Oh, by the way, you also get better fuel economy, longer brake life, longer tire life, fewer breakdowns, and longer vehicle service life when you practice low force driving.

4 Consider how your vehicle is loaded before you head out.

Weight in the rear is just as important for a front wheel drive vehicle (perhaps more so) than a rear wheel drive vehicle because there is already a higher percentage of weight on the front wheels.

Consider the following scenario:

You have a front wheel drive vehicle, no one in the back seat, an empty trunk, a low level of fuel in the tank, and worst of all you have snow tires only on the front. You encounter slush. Here’s what happens. Your weight and best tires are on the front. You are driving in the narrow sections of pavement that are merely wet. You drift ever so slightly, and all four tires encounter the ribbons of slush. You apply the brakes. Because the front tires have the most weight, the best tread, and the softer winter rubber compound, they maintain traction as you slow down. But the rear tires have little weight, poor tread, and hard rubber; essentially no traction at all. No traction means no control for the driver. No traction in the rear means oversteer. Oversteer in slush means you do not drift into the relative safety of the ditch. It means you gradually or dramatically (depending on your road speed and other factors) turn sideways in relation to oncoming traffic. If you are lucky it happens fast enough that you do not have time to contemplate your life before the semi hits you broadside in the driver door. If you are really lucky you hit the ditch sideways and simply rollover. You might want to get in the habit of wearing your seat belt.

5 Maintain your tire pressures.

Yes, I know we have TPMS. Tire Pressure Monitor Systems have been around for many years. But there is no substitute for a proactive vehicle operator. Technical systems fail.

6 Understand the physics of driving, weight transfer, tire traction, and the all importance of paying attention.

I believe an article in USA Today once stated that the good drivers make the road safer for bad drivers. I really like that. I like the fact that by driving well you can protect yourself as well as others.

Think about it.