Monday, December 15, 2003

The Physics of 4 Wheel Drive and Snow

Repeat after me: 4 wheel "go" does not equal 4 wheel stop.

A quick rant to explain something to you S.U.V. Drivers, most of whom are clueless menaces on the road. I say most, rather than all, on the statistical possibility that one of you has some intelligence and driving skills. So far, this assumption has proven excessively optimistic, but hey -- I'm a positive kinda guy.

In New York, we got hit with a surprise snowstorm Sunday (it moved up from the South unexpectedly fast). We drove back from the East End of Long Island Sunday morning as the snow was coming down pretty fast. I was driving the front wheel-drive, manual transmission car (as opposed to our real wheel cars), so traction was not a problem.

On the LIE, snow accumulation was about 4 inches. By staying in the center of the middle or right lanes where traffic was heaviest, you could make sure your tires were, for the most part, in contact with actual pavement, as opposed to the white stuff. The left lane and the HOV lanes were less travelled and had a few inches of snow covering them.

Here's where things got interesting: Many S.U.V.s and quite few Audi Quattros were hauling ass along the left lanes. I may have a heavy right foot, but I do not go 70 in the snow -- these idiots did. We got to witness a dozen or so fender benders along the way 60 mile trip home: The most memorable was seeing the results of a Nissan SUV spin out, and in an unrelated accident, watching in more or less slow motion, a Hummer (!) careening into the guard rail. (Seriously, how bad a driver must you be to slam the "vehicle that can go anywhere and do anything" into a wall?)

Here's the physics of the situation: 4 wheel drive is your method of locomotion. It's how you go. But here's the -- apparently -- unknown secret: It has absolutely nothing to do with how you stop; That's an entirely different system within your automobile.

But 4 wheel go does not equal 4 wheel stop. It has no impact on halting your momentum. Can you comprehend that? Oh, sure, if you drive a stick -- and downshift properly -- in some situations, you can use the engine to control wheel slip and improve control. But seriously, how many SUV owners drive sticks? And of those who do, how many of them know how to drive? Too few. Perhaps that's why I didn't see any of the Audis in accidents -- they sell a lot of 5 speeds manuals, and Audi owners have a rep for knowing how to drive. But I digress.

If anything, 4 wheel drive merely allows you to get yourself into more dangerous situations. We saw a Porsche 911 fishtailing all over the road. He was aware of his limitations in the snow and was going 30 mph. His torquey, rear wheel drive system had little in the way of traction in the snow -- and he knew it. He crept along in the right lane and still ran into some trouble. (On the other hand, I drove our rear wheel drive SLK home from White Plains Christmas Day 2002 in 6 inches of snow -- it took 3 and half hours versus the usual 60 minutes. I had no problem, but that's mostly because I am not an idiot.)

Now back to our 70 mph snow fools: In the event of loss of traction -- not too hard to imagine in 4 inches of snow -- you are merely a two ton hunk of steel sliding across a frictionless surface until you either regain directional control or run into some mass which stops you. Your Anti-Lock Brake System (ABS) is useless when your tires cannot make contact with the pavement. Modulating brake lock up when snowplaning on a 2 inch cushion of white stuff does you no good whatsoever. The technical term for this phenomena is called "Wheeeeeeeeeeeeeee!"

A message to you SUV (and all wheel drive) owners: Understand the limits of your vehicle. Learn to drive in inclement weather. You'll be doing your families, and the people you share the roads with, a huge favor.

Posted at 07:03 AM in Current Affairs | Permalink

TrackBack

TrackBack URL for this entry:
http://www.typepad.com/services/trackback/6a00d8341c52a953ef00d8342361a553ef

Listed below are links to weblogs that reference The Physics of 4 Wheel Drive and Snow:

» The Physics of 4 wheel drive and snow. from ad Hominem Links
4 wheel drive is your method of locomotion. It's how you go. But here's the -- apparently -- unknown secret: It has absolutely nothing to do with how you stop. [Read More]

Tracked on May 9, 2005 5:13:39 PM

» The Physics of 4 wheel drive and snow. from ad Hominem Links
4 wheel drive is your method of locomotion. It's how you go. But here's the -- apparently -- unknown secret: It has absolutely nothing to do with how you stop. [Read More]

Tracked on May 13, 2005 1:15:40 PM

Comments

THANKS for the article -- my neighbor and I had some wonderful laughs reading through it last night -- and now we are circulating it!

Hopefully "The Physics of 4 Wheel Drive and Snow" will teach a few people a few things.

NOW, can you write something about "IT'S CALLED 'SPINNING YOUR WHEELS' FOR A REASON" --

so I don't have to threaten another idiot wheel spinner with my broom to make him stop --

(I only threatened to beat him and his car because IF he'd hit pavement, he would have smeared me up against and into my own vehicle! Y-Ouch! Then I really would have had to hurt him!)

Posted by: Katherine Taylor | Jan 6, 2005 2:09:52 PM

Bwahahahahaha! Well said and 200% true.

Posted by: John | Jan 15, 2005 2:52:10 PM

As to the physics of AWD, if I am driving a full-time AWD like a Subaru and hit someone from behind, is the impact double (or greater by some factor) of what it would be if my car had only two-wheel drive?

Posted by: Curious | Feb 5, 2005 12:19:05 AM

No,

The impact is a simple equation. The force is mass (the weight of the vehicle) times speed (how fast they were going).

If you want to get fancy, you can use relative speed -- how much faster they were going than you. Thats why a fender–bender rear-ender when both cars are travelling in the same direction causes only modest damage -- Speed differential of lets say a 20mph car hitting a 15mph is only 5;

That same car rear-ending a standing still vehicle 20 - 0 has a much greater differential;

Lastly, a head on collision -- the 20mph car hits the 15 mph travelling in the opposite direction looks like this:

20 - (-15) = 35

Get it?

Posted by: Barry Ritholtz | Feb 5, 2005 5:56:26 AM

Good Article. What most 4WD SUV drivers don't realize is that the tires on their vehicles are a compromise. The tires are designed to keep road noise to a minimum and to favor dry road, warm weather handling. Actually most cars are shod with this type of time. The shoes(tires) a car wears is more important than weather it's front, rear or all-wheel drive. In eight inches of snow, I'll take a rear wheel drive car shod with dedicated winter tires over a 4WD car shod with high performance summer tires anyday.

Posted by: Richard | Feb 10, 2005 8:35:28 PM

LOL, Physics it's not just for geeks.

SUVs are terrible in the snow I used to see 3x or more in the ditch then cars and usually deeper in or up-side-down.

When I used to commute long distance a lot, I started to notice something, lots of SUVs in the ditch in the wintertime, so I started keeping a count.

Something very interesting emerged. At the beginning of snow season there would be a 3:1 ration of SUVs:Cars in the ditch, usually upside-down. By the spring the ratio would get down to 1:1 but never better then that.

Mini-vans seemed to have the fewest snow problems. I rarely saw them on the side of the road. I would guess they have the most sensible drivers, or at least they are looking out for the kids onboard.

Posted by: Stephan F | Feb 23, 2005 2:21:38 PM

I need help with this.

I bought a used Nissan Pathfinder last summer and was driving in a snowstorm last week with the vehicle engaged in 4WD-High. I downshifted one gear to slow the vehicle (procedure I've used in snow for 30 yrs). When I released the clutch pedal the rear end skidded out from me but quickly came back. About 2 miles down the road I approached a light and the same thing happened when I released the clutch pedal after downshifting one gear. This time it caused me to skid into the opposite lanes and spin 180 degrees. Fortunately no cars were approaching or else there would have been a serious collision.

I am at a loss to understand the "physics" of why downshifting this vehicle in 4WD caused this problem. I also wonder if it will happen again and under 2WD conditions as well. Does anyone have insight into this?

Posted by: Richard | Feb 28, 2005 10:49:03 AM

re downshifting/rear wheels locking - it may be down to "intelligent" 4-wheel drive, although am not sure. We have a CRV, which runs as rear wheel drive until the rear wheels lose traction, whereupon it slips into 4-whl drive; if yr Pathfinder had this type of traction device, if the rear wheels locked - albeit only briefly - the rear end would be very hard to control.

Posted by: Joss B | Mar 8, 2005 5:52:33 AM

lol

Posted by: lol | May 21, 2005 3:55:29 PM

Remember to slow front-drives and 4x4s before corner so as to avoid going from understeer to instant oversteer. Unplowed sides of road can pull you off the road. Anybody want to explain black ice ? Invisible instant loss of traction. As above, slow down and use gentle control input.

Posted by: opit | May 24, 2005 12:58:10 AM

As an Audi owner, I can attest that a larger percentage of Audi owners know how to drive when compared to typical SUV owners.

Also, Audi has a technology they call "ESP" which dynamically controls each wheel's braking system independently... so in the event of slippage, the system will compensate to correct any errors... whether accelerating or braking.

Also, lower center of gravity and less mass than an SUV make any AWD car (be it Audi or Subaru, etc) easier to stop.

I use two different sets of tires. Summer tires are for March -> September. Winter tires are used October -> February. All-Season tires are good for no seasons.

It's good practice when living near or north of the mason dixon line to own a set of each, as your ability to manage your vehicle on the road will be more greatly effected by your tires than almost anything else.

Posted by: Chad K | May 27, 2005 2:01:59 PM

Richard-

4 wheel drive can cause differentials to be locked thus increasing the chance of spinning out when one set of wheels wants/needs to spin faster than the other can.

This is a link from the Car Talk guys talking about the same thing.


http://cartalk.com/content/columns/Archive/2001/May/05.html

Posted by: zach.neal | Jun 24, 2005 5:08:40 PM

There could also be something wrong with your front differential. I bought a used Isuzu Rodeo last winter(suffice to say, my Jetta was WAY better in the snow). The problem with the rodeo was that the front cv joints or differential were shot. So it would actually REDUCE the amount the front wheels wanted to turn, if at all, and the rear wheels had a hard time pulling or pushing the front, and would end up with me doing a 360 cuz the front end wouldn't move.

Posted by: Mike Cohen | Sep 23, 2005 9:32:43 AM

Richard, your pathfinder just was demonstrating the laws of physics. When you slow down the forces are applied to the front wheels (it's why the front goes down). The back wheels therefore become light and you risk a slide. The same applies in reverse when you accelerate. It doesn't matter what drive system you have nor how you choose to slow down (brakes or gears). If you are turning and braking most of your grip comes from the outside front wheel. Distributing available grip to 2 or 1 wheels instead of 4 is generally a bad thing!

As for the article, absolutely correct bar the part on going through the gears in what you folks call 'stick' (I'm writing from Switzerland, we call it manual gearbox). There is little point going down the gears in a modern car as the brakes are good enough. You will tend to have more control of the brakes, they are distributed to all wheels with a front bias where you need it. If you do go down gears it's worth learning how to heel-and-toe (using the brake and accelerator at the same time with the same foot to match the engine speed to the new gear before engaging). As opit suggests the correct technique is to brake well in advance of the corner, set the correct gear and turn in. A slight brake at the point of turn-in can aid turn in as long as you're not on the limit as can a pre-turn (a tiny left turn just before a right turn to destabilise the car's natural inertia.) Then when in the corner don't use the brake instead balancing the line on use of the accelerator. 4WD cars understeer in, oversteer out.

Antilock and other electronic braking systems are actually worse (for an experienced driver) on new snow. If there is no antilock the locked wheel builds up a mound of snow at the front of the tyre which slows the car down. An antilock system releases this snow.

The only other advice is to use snow tyres in winter and to take snow driving lessons (or skid pan work). Snow is not difficult with a bit of practice and can actually be a whole lot of fun. A car will handle in a similar manner on snow at 20mph as in the dry at 100mph+ so some race track experience can be useful. Just get out of the way of those idiots.

Posted by: Andrew | Sep 30, 2005 6:28:42 AM

I'm looking at a Izusu Rodeo, what turned me off was the fact that it is rear wheel drive and a 5 speed. Beautiful vehicle, but how well in the snow???

Posted by: Donna | Nov 1, 2005 2:01:11 PM

In Mr. Ritzholtz's response to "Curious"s question about an impact from the year of a 4WD system:

"No,

The impact is a simple equation. The force is mass (the weight of the vehicle) times speed (how fast they were going). "


This response is only partially correct. This is true for cases where the vehicle is at a complete stop and either the brakes are engaged or the transmission is out of gear. Only then is the drive system irrelevant to the impact force felt.

When the transmission is connected to all 4 wheels, the vehicle is more resistant to motion, whether it be in the positive direction (the direction the vehicle is travelling before the impact) or the negative direction. The reason for this is that there is twice as much usable friction to turn the engine faster before the tires give way and slip. The end result is that the body of the car takes more of the energy of the impact instead of more energy being kept kinetic.

The difference between a 2WD and 4WD vehicle in a rear-end collision would likely be at negligible at any rate because the energy exerted in a collision more often than not far exceeds the friction coefficient supplied by the tires, but for the sake of accurately answering the question, there is your answer.

Posted by: Justin | Nov 6, 2005 6:32:15 AM

Before you people start using physics to bash SUV drivers take a course in it.

Force is not mass X speed, its mass X acceleration. You guys are also incorrently using the term speed in place of velocity, which is a vector that describes both direction and speed, but don't fret, its a common source of confusion for stupid people. And to answer Curious's question correctly: No, it makes no difference. Once your brakes are applied, the drive system does nothing but add weight to the car.

Stopping a car requires one to deaccdelerate (i.e. apply your breaks). The faster you are traveling in a given direction the more force it takes to bring your car to rest. An icy road lowers the magnitude of the frictional component that it working against the positive motion of your vehicle.

If you guys have this much time to search the web for false information, I might suggest finding a lady friend!

Posted by: Josh | Dec 7, 2005 2:11:55 PM

Yeah, we smart people know how to deaccdelerate our cars.

... oh wait.
1. you spelled deaccelerate wrong and...
2. deaccelerate is not a real word.

DOH!

Posted by: jodsh | Dec 13, 2005 5:38:50 PM

Ah, this reminds me of an occassion when I was returning from New Haven, CT on (-91 to my residence near Hartford. The weather that day had seen heavy snow fall during the morning and early afternoon, followed by a super heavy rain which had washed the snow from the pavement but left a thin wash of water on the road. A totally clear fron followed the storm and the temperatures fell from the low-40s to the low-20s in about two hours, freezing the water on the road.

While driving through North Haven and Wallingford and feeling the road become increasingly unstable with my FWD sedan, I slowed down to about 40 mph. I was immediately passed by two SUVs apparently heading for ski country and sailing along at 70 or 80 mph. They sailed along until the hit the black ice. The SUV in the extreme left-hand lane broke to the right in a spin, right in front of me, and wound up in the ditch tail-first. The SUV in the middle lane broke to the left in a spin and wound up in the center media facing backwards. I am convinced they administer IQ tests when they sell SUVs and refuse to sell them if the buyers pass.

BTW, a CT state trooper had been trailing along just behind these two jokers and headed over to the SUV in the right-hand ditch. I don't know if it was to offer assistance or write a ticket or both. I crawled on at 25 mph on the berm where the rougher tooth improved traction and just wondered how state DMVs gave licenses to the brain dead.

Posted by: PrahaPartizan | Jan 20, 2006 6:21:37 PM

Well having owned 4wd drive vehicles for the past 15 years. From Land Cruiser FJ40s to new Land Rovers, all I can say driving skills are not necessarily dependent on what type of vehicle a person owns. My Land Rover can corner as fast and as stable, if not more so than my wife’s Sable. So make of vehicle has a lot to do with what you get. Also, I put winter tires on my Rover, and it is very capable in the snow. On ice, not much is good except studded tires, but they are illegal in Minnesota. I think it is silly to think that all people can stereotyped because I have seen bad drivers in all types of vehicles.

Posted by: Jim | Jan 31, 2006 6:26:52 PM

Sigh. OK, folks, to straighten out the physics; I've taught it in college, and have done a lot of work in geophysics professionally. I've also been driving SUVs since before the acronym was invented (1972, to be exact, when I got an IH Scout in college), and I've spent a lot of time off-road since:

First off, it's true that 4WD is not "4 wheel stop." Stopping requires applying a force opposite the velocity of the vehicle, and that force is supplied by the frictional resistance of the tires against the road surface as their rotation is slowed by the brakes. That resistance is considerably lowered on snow and ice. Fundamentally, it's that simple.

"Momentum" is not irrelevant, however, because force is the change in momentum with respect to time; dp/dt, in the language of calculus. (This reduces to mass times acceleration for constant mass; the proof is trivial, involving first-semester calculus, and is left for the reader. Oh, all right, here's an outline: momentum is mass times velocity: p = mv. Now differentiate with respect to time, remembering that velocity is the first derivative of position with respect to time.) Hence the larger the momentum, the more force you need.

For collisions, this also means that the force applied during the collision depends on the deceleration during the collision. The faster the momentum changes, the bigger the force. That's why it's useful to have bumpers or fenders that crumple, to spread out the deceleration, and it's also why collisions in which the target vehicle in turn slides are less damaging: that way the momentum is not canceled all at once.

There's another issue, though--the dissipation of kinetic energy during the collision. This is not the same as momentum. Kinetic energy is a scalar (i.e., a pure number, not a vector) and its magnitude is 1/2 mass * speed squared. (Remember that the speed is the magnitude of the velocity.) So a vehicle traveling twice as fast has 4 times the kinetic energy; 3 times as fast, 9 times, and so on.

Energy is conserved, so that that kinetic energy can't just vanish in the collision. It shows up in mechanical deformation, heat, noise, and so on. And since energy goes up as the square of the speed, high-speed collisions are considerably more damaging than low-speed collisions. This, btw, is why an 80-grain bullet traveling at ca. 1000 m/sec is so much more destructive than a fastball traveling at ca. 80 mph, even though they have roughly the same momentum.

All this was worked out in the 18th century, too, so it's not exactly cutting-edge.

Btw, Josh, the devices that slow down your car are not "breaks" but "brakes." And you "decelerate" when you slow down. You also might remember what Alexander Pope said some centuries back about the dangers of a little learning.

And Jim, I agree: driving skill shows little correlation with the type of vehicle one drives.

Posted by: slg | Feb 23, 2006 10:23:00 PM

Or, for a slight re-phrase, "you may have four wheel drive, but everyone has four wheel brakes."

Posted by: John | Mar 15, 2006 8:48:05 PM

I am shopping for a new 4wd vehicle, my previous one having slammed into an elk that was dashing across the road right after a turn in the pavement. I enjoyed my Ford Explorer, despite design flaws that made it turn over (mine never did). But now I would like something smaller that could still handle a place with snowy icy mountain roads 9 months out of the year. I prefer a stick and always put good winter tires on. Any suggestions would be appreciated.

Posted by: Cindy | Apr 13, 2006 11:22:43 PM

Excellent - So true, so true. The same thinking that went into buying the SUV gets flipped on it's head when the snow comes. Sweet.

Actually I like automatic better in snow. You can rock out of situations you could never shift fast enough for. Good clearance and studded tires are also essential.

Posted by: Keith | May 16, 2006 3:12:56 PM

Guys,

As a side note here, it is not a law of physics that FWD is better in the snow. The physics does not support this conclusion. Most modern cars have very nearly 50/50 front-to-rear weight balance. I have had plenty of problems with losing traction in a FWD car, equally as many in a RWD car. The car makers want you to believe FWD is better, because it is cheaper to manufacture a FWD car than the equivalent RWD car. As for AWD, which I currently have, it's nice, but I have to agree with one of the above posts, with lousy tires it does little good. The BMW X5, I currently drive, came stock with Michelin tires. They last a long time with a relatively hard tread compound. But, they were so bad in the snow, you needed AWD to at least be as good as my other cars (both RWD and FWD are in my stable). I have since replaced the Michelins and have very good results in winter. Thus allowing me to go too fast in situations where I can't stop fast. I don't know a good answer to this problem except to drive slowly, which I do in snow.

It always comes back to the driver. It's the driver, not the car. No matter how many fancy gizmos they give us to improve poor driver ability.

Posted by: ron | Jul 16, 2006 9:24:48 AM

The comments to this entry are closed.