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As winter settles in and the snow falls, I’ve noticed a number of amusing things regarding drivers and their misconceptions about the capabilities of an SUV. While driving home in my Jeep Wrangler on I-95 the other night, I watched as drivers [New York, New Jersey and Connecticut] slipped and slid through the ice in their expensive cars. I listened to my police scanner to the numerous accidents around me, so many accidents in fact that one particular transmission caught my ear.
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Highway Patrol: ‘I need the location for the next incident.’
bq. Dispatch: ‘You don’t need a location, there are so many that you can just drive up and down the shoulders and you’ll find ‘em quickly.’
Anyway, I digress. My point is that many people assume that because their vehicle has AWD (all wheel drive) and some form of traction control, that the car will always have traction thereby meaning they can continue to drive normally. Wrong Wrong WRONG! To begin with, it is necessary to understand what options exist.
- Part Time 4WD
- Full Time 4WD
- AWD
- 2WD Rear
- 2WD Front
Starting with the worst, two wheel drive obviously limits the number of tires actually contributing to traction. It’s generally understood that front wheel drive handles better in slippery conditions than a rear wheel drive vehicle. AWD differs from 2WD only in that two additional tires are turning. The misconception is that all AWD means that four tires always turn. That is not how your car works.
The drive shaft from the engine connects to a differential which distributes power to the axles and in turn rotates the wheels. But what is a differential? Put simply, it is a collection of gears that allow the two axles to rotate at different speeds. This is necessary in order for a car to turn (as the outside tire must traverse a greater distance than the inside tire) without having the wheels skip. Obviously, wheel skip introduces a loss of traction and thereby control. While the differential serves this crucial purpose, it is also the differential that creates the "one wheel turns, the other doesn’t" problem. The differential gearing transfers the power into the path of least resistance, which is often the wheel that has no frictional contact with the ground.
Back to all wheel drive. So long as all four wheels have equal footing, it is true, all four tires will turn with an equal amount of power and traction. However, just as two wheels on an axle are connected by a differential, the front and rear drive shafts are connected by a differential as well. This means, if one wheel of an AWD vehicle loses traction, the problem is amplified because ALL of the power goes to that one wheel while the other three sit stationary. This is why, without a form of traction control, an AWD vehicle is useless in any condition other than dry pavement.
A full time 4WD vehicle is essentially just an AWD vehicle that has the ability to "lock" the center differential on command. What this means is that when the lock is engaged, the front and rear drive shafts can be thought of as a single, solid shaft. Hypothetically, if one front wheel were to lose traction, it would spin while the opposite front wheel remained stuck. However, because the rear drive shaft is locked, both rear wheels will turn which will free the vehicle and – ideally – move the car forward such that the spinning wheel regains traction. The center drive shaft should only be locked in conditions that warrant such behavior because it will have negative traction effects on dry pavement when the vehicle attempts to turn (for the reasons noted above about differentials).
A part time 4WD vehicle is essentially a 2WD vehicle for all normal conditions. When 4WD is engaged, the front and rear drive shafts are locked immediately. Part time 4WD vehicles do not have the option of having the transfer case operate as an open differential. The drive shafts are either engaged and locked together or only one shaft is engaged. Typically, part time 4WD vehicles also have additional low-gearing options giving them tremendous amounts of torque to free themselves or tackle difficult obstacles.
Up to this point, it is obvious how part time 4WD, full time 4WD and AWD each have their strengths and weaknesses given particular road conditions. Additional technologies contribute to increase the likelihood of having traction in the event of wheel spin. The most fundamental device is called the locker. The locker principle was alluded to in the discussion on full time 4WD for locking the center drive shaft. Lockers also have their place in the differential, preventing the axles from spinning separately by locking them together "like a spool." A variety of lockers exist that can engage automatically or manually on demand in order to guarantee that two wheels will always turn. This sort of option typically only exists for off-road 4×4s and trucks.
The next tool for preventing wheel spin is called the limited slip differential. These differ from the standard open differential in that a coupling device is incorporated to allow axles to spin at different speeds, as necessary for a differential to function, but not at vastly different speeds, which is indicative of traction loss. A limited slip differential can be coupled by viscous fluids or a series of clutches. Ultimately, however, the limited slip differential keeps the wheels with traction turning allowing a vehicle to continue moving. As an example, take a standard AWD vehicle. If the right rear wheel loses traction, it receives all the power while the remaining three wheels do not turn. In a Jeep Wrangler Unlimited, if the right rear wheel loses traction, the limited slip differential will transfer power to the left rear wheel and the locked transfer case will transfer power to both front wheels allowing the 4×4 to continue moving.
Lastly, AWD vehicles regain some of their usefulness in slippery situations with the addition of traction control. The most common implementation is called electronic traction control which, counter-intuitively, uses the brakes. This system will be employed on vehicles with anti-lock brakes because a computer already has sensors monitoring the wheel rotation. In the event that a wheel begins to spin beyond a normal threshold, the electronic traction control system will actually begin to activate the brakes on that specific wheel – independently pumping up to 100% braking power. This has the effect of stopping the errant wheel’s spin completely – which, utilizing the path of least resistance, will transfer power through the open differentials to the remaining wheels in contact with the pavement. The system works fine for mildly slippery conditions, however on ice it can be dangerous because the car itself is repeatedly applying full brake force on the sliding wheels which prevents them for regaining rolling traction during a skid. A mechanical alternative to electronic traction control exists in the form of the Torque Biasing Differential [pdf]. Through a series of worm gears, torque biasing creates a mechanical advantage to the high-traction side of the differential which helps to prevent wheel spin. These differentials are often found on the front axle of performance tuned road cars. Torque bias differentials are not suited to 4×4 applications because they do not handle having one wheel lifted from the ground completely. (Notably, the HMMWV uses a torque bias differential and compensates for the raised wheel by applying both throttle and brakes simultaneously like a manual EDL.)
In summary, contrary to what owners believe, an SUV with AWD and traction control really is nothing more than a mini-van with big tires when it comes to slippery surfaces like snow, ice and off-road conditions. As such, it is necessary to continue driving them in winter conditions with the same due diligence as any driver of a vanilla 2WD vehicle. While true 4×4s and off-road vehicles have a definite advantage in the snow and ice, they too must take due care as an out of control SUV will cause them just as much harm whether they’re firmly planted or not.
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Physics Lessons on the Median Strip by gnifyus :: NR8 :: Show
Just a few random comments. First, this was a great summary and explanation of the options available in these vehicles. I imagine though, in the case of the AWD vehicles, most of them must have the traction control by now. We had a 2001 Dodge Caravan with AWD and quite frankly it was awesome driving in the snow. Once in awhile I would have to drive down roads with 4 to 5 inches on them during or after a storm, and other than the resistance felt from the actual snow, it was no problem at all. It tracked straight and did not slip enough to notice even when starting on steep hills. (Unless on pure ice, of course.) This from the description of how the AWD in it works:
Under normal driving conditions, the device sends up to 90 percent of the power to front wheels, so the minivan drives like a front-wheel-drive model. Yet when on-board sensors detect wheel slippage in front, the coupling immediately transfers some of the power — up to 100 percent, depending on the amount of slippage — to the rear wheels. By channeling power to all wheels, Grand Caravan AWD remains stable on wet or dry pavement.
But, it was still a minivan. You couldn’t go "four wheeling" with it; there just wasn’t the clearance or the power.
The thing is, as you mentioned, people seem to not only think they can drive "normally" during snow and ice conditions, (which was probably too fast to begin with), but they suddenly feel they are immune from the laws of physics just because there is a ‘SUV’ or ‘AWD’ designation on their vehicle. It doesn’t matter what you’re driving, if you hit a slippery spot going around a curve, the centrifugal force is going to do its thing and off the road you go. Stopping is the other thing. Just because you can go a little faster doesn’t mean you can necessarily stop any faster than that rear wheel drive car you left in the slush at the last set of lights, especially if your vehicle is heavier.
Winter driving is a learned skill I take for granted sometimes. The issue is fresh in my mind because I have a 16 year old son getting his license soon. The secret really boils down to: ‘’No sudden movements." Don’t take off fast. Don’t stop fast. Don’t go around a corner or curve fast, and leave yourself time to not have to be in a situation of stopping or cornering faster than the physics of the situation allows.
Last February we were driving back from Florida and hit a rare snowstorm in southern Virginia. I am always amazed at how many cars (most of them SUV"S of some sort) were backwards in the median strip or worse.
Jerry by Anonymous :: NR0 :: Show
My experience has been that traction control has some problems on steep hills when you’re going slow and you have to "spin" up the hill – the engine practically shuts off.
I must confess that I really like most Subaru’s in the snow and on ice. The only problem is that they don’t have enough clearance for the big snowfalls in mountain areas and they certainly aren’t a good mud bog 4×4 farm truck. I’ve even seen people in lake-effect areas jack their subarus up to counter this.
However, to me the biggest danger to safety is being on black ice without knowing or driving sideways a bit when you’re on mixed traction and then hitting road. That’s where Subaru shines because they don’t torque steer nearly as much as most other brands, they are a lot smoother in mixed traction, and they are low to the ground which makes it easier to pull back straight in-line when your tires start to grip again.
don't forget... by Anonymous :: NR0 :: Show
…there is a difference between mechanical locking fidderential and viscous differential. Most passenger vehicles, SUV’s and light trucks are viscous clutch plate type diff’s. Some Audi’s and Subaru’s have mechanical locking differentials. Subaru for instance, depending on model(STi, for example) has a electronic adjustable center diff and 1 each front and aft(3 total). Audi Quattro’s uses Torsen center differentials and some of the transverse mounted engine cars uses Haldex electronic locking diff’s(also found on some VW’s). When locked, it is a true 4WD.
Also maintenance of the diff’s makes a difference as the clutch plates do wear out. Since it is rather expensive to service the diff’s, most owner hardly ever have them service during the life span of ownership.
What about Auto 4WD by robertjw :: NR0 :: Show
My sister has an 07 Chevrolet Tahoe with an Auto 4WD setting. I’m not sure exactly how this system functions, but I do know that GM SUVs have had a similar system in place for the last 10 years. I also know that it works remarkably well. I had an opportunity to drive her vehicle during a recent snowstorm and the performance of the system was amazing. I saw little need for the 4wd High setting on the vehicle for the average driver (the 4wd Low is still advantageous when the vehicle is stuck and you need to crawl out of a situation).
A couple other comments.
Remember that there is very rarely a situation where driving on ice results in ZERO traction. Even in the iciest situations there is some traction and a skilled driver can maintain some control over the car.
Second, one of the big differences in FWD and RWD cars is the amount of control after the vehicle loses stability. In a FWD car ALL of your steering, power and most of your braking are controlled by the front wheels. If the front wheels slip there is little hope in regaining control of the car. In a RWD car the power function is in the rear wheels. In many situations, even if the front wheels slip and you lose steering and braking you will still have an option of applying power and using the application of that power to maneuver the vehicle.
4x4 in Desert. by Anonymous :: NR0 :: Show
I have a Mitsubishi Pajero 2005 4×4 with option of Manual 2WD (Rear), 4×4 Fulltime, 4×4 high & low including electronic traction control & ABS.
I drive this car in the Desert where the wheels are 6-10 inches deep in sand and it really require a lot of power to keep the car moving.. Infact I have felt that this car does the job extremly well in handling such situation. I put the Car in 4×4 High on low sand and 4×4 on deep sand conditions and it just work well..
Infact I have pulled out numrous cars stuck deep in sand spinning rear/front wheel of those people who beleives that their expensive sedan must have all the necessary equipment & Power to take them out of sand… !!!
I am not sure how it would work on snow/ice as we dont have it but it is quite stable during rain as well… I beleive the traction control & ABS keep it from spinning…
But nothing compared to Cautious driving in extreme condition, as mentioned "No Surprise/sudden moves..!!! is the key.
Good luck, drive safe and enjoy your rides.
This article is flawed by Anonymous :: NR0 :: Show
Without a detailed examination of different differentials and how different cars employ them, stupid remarks like "This is why, without a form of traction control, an AWD vehicle is useless in any condition other than dry pavement." shows up just how little the author knows.
And anyone who knows anything about cars can tell you the drive train is only about 20% in the handling/traction equation. Tires (tyres) contribute 50% of the cars ability and suspension 25%. The remaining 5% comes from everything else.
Scandinavians Appear To Be Immune To This by Anonymous :: NR0 :: Show
While the science appears sound, real-life suggests differently. While with the Marine Corps I spent time in Norway on numerous occasions. Lots of Volvos, Saabs, Audis, etc. with AWD. They routinely blew us off the road in our Humvees. While I wouldn’t want to take one of these vehicles off-road (nothing beats a true military Humvee off-road), on packed snow they made us look like kids in soapbox derby vehicles.
Ultimately it comes down to experience and driving ability. I’ve seen plenty of "real 4WD vehicles" in the ditch during light snow; usually because the driver felt invincible.
Learn to drive the vehicle you have properly. Learn your vehicle’s limitations. Learn your limitations. And remember: you can’t drive as well as you think you can. (Just ask your friends and family; they’ve been trying to tell you that for years.)
I love my Subaru by Anonymous :: NR0 :: Show
Living in Salt Lake City we get a good amount of snow. Not nearly as much as where I’m from, Buffalo, NY.
I purchased a 2003 Subaru Outback this year and as long as you know how to drive in the snow a Subaru will get you almost anywhere you need to go safely. It uses a viscous Limited Slip Differential. I’m not an expert on all the types of drive variations but I can safely say I am more than elated with the way my Subaru performs in the snow.
I’ve driven past quite a few SUVs stuck spinning their wheels because they were rear wheel drive, so if you live in an area that gets snow regularly and want an SUV I recommend you make sure to get some form of 4 wheel drive and not a rear wheel drive vehicle. I ended up digging a neighbor in his BMW 545 out just yesterday. I’ve also had a rear wheel drive vehicle for a short while and it was simply not safe in the snow (putting weight in the back or not).
Audi AWD by Anonymous :: NR0 :: Show
I have an Audi A4 Quattro AWD without traction control and I can tell you from lots of experience that this car is unbelievable in the snow. We just had a 3 week long series of storms in Vancouver, BC and I was literally plowing through foot deep snow. Many times I pulled over to help push people out of the snow and I would park the car in deeper snow than they were stuck in. I never got stuck once. I agree that if you had one tire on ice it would spin but in real life driving situations the car was amazing.
One piece of advice is to avoid wide tires on snow/ice and use winter tires, not all season, for max. traction.
AWD Mountaineer by Anonymous :: NR0 :: Show
I have a 99 AWD V8 Mountaineer with ltd slip rear differential. My testing ground is a 1/4 mile uphill washed out gravel driveway in Northern VT where the slope varies between 8 & 16%. Try walking up a snow and ice covered 16% slope and you may well be doing it on your hands and knees! In the winter 4WD and snows are a must. The only time I couldn’t make it up was attributed to Michelins with 80,000 on them. Cooper Discoverer Snows will get me up smoothly everytime, without the mad dash and thrash that most owners of 4WD’s insist is necessary to make it. The author’s theory of AWD may be true for some vehicles but certainly not mine. I have never been in a situation, ice or not where one wheel spins leaving the other three immobile and god knows I have tried! Another benefit to AWD that 4WD cannot boast, is even tire and tread wear without the necessary/constant need to rotate the tires. Any 4WD I owned was a guarantee to scalloped, uneven tire wear resulting in a loud rough ride and never getting much more than 40K out of a set of tires. I have 194K on my Mountaineer on the original snows and only the 2nd set of summer tires, I got 100K out of those original Michelins! I’ll take AWD over 4wd anyday!
very misleading arcticle. by Anonymous :: NR0 :: Show
Your statment “AWD vehicle is useless in any condition other than dry pavement.”
and is one the more ignorant statments I’ve read lately.