Do you know how a 4X4 works?

[Pete Lamberty]

Hi Gang! Over the last few days Chicago has received about a foot of snow. Some of us got into a discusion about four wheel drive vehicles. Naturally all the good and bad points were made, but there was one point that, to me, just doesn't make sense. I just don't get it! From the way that I understand it, all four wheels DO NOT receive the same amount of power all of the time. If a 4X4 is stuck in the snow (vehicle is in all wheel drive mode) the wheel that has the LEAST amount of friction gets the most power. Even to the point that the other wheels may, depending on which brand of vehicle it is, recieve no power at all. So to repeat myself here. The wheel that is slipping and spinning the most and not getting any BITE on the pavement or snow is the one that gets the most power. I would think that if I was stuck I would want the wheel that has the BEST contact with the pavement to get the most power. The one with the most friction. Thats the one that would get me going. Can anyone explain this to me? What am I missing? Also, let me tell you that I do not have a 4x4 so I have never experienced driving one. I figured that someone here will be able to set me straight. Thanks!

[Jason Beam]

Not being anything resembling an expert, I don't think what you describe is unique to 4x4 vehicles. I'm reasonably sure this is a phenomenon caused by the differential or pumpkin in the axle. I've heard it called many other things, like Posi-trac or Positraction. There are other systems out there that I think are related to this like "Limited slip differential" which I am guessing addresses the very issue you mention.

You aren't missing anything, though. The situation you describe CAN and DOES happen on many vehicles, not just 4x4. It's kind of a path-of-least-resistence thing, I guess.

I assume newer cars are starting to address such a thing with computer controls directing the power to the proper locations instead of wasting it on spinning. Not knowing much about it, that's my uneducated guess :)

[Joe Pelonio]

We currntly have 2 4x4's and I used them both during our last week of ice and snow. My Ranger has limited slip rear axle, so while in 4-wheel mode the fronts both go, but the rear will apply the power to the wheel that's grabbing. Basically it keeps them both going at the same speed unless turning or in a situation with slippage at the rear.

On my wife's Liberty there is part time and full time. Full time works like you said, the power goes to the wheels that are grabbing only. That mode can be used on dry or just wet pavement with no damage and no excess tire wear or loss of gas mileage. Then it also has a "part-time" mode, which is just like the Ranger since it also has a limited slip rear axle.

What's gets me up the hills in 4wd is the fact that hopefully one of the 4 wheels (better yet more) are able to grab. If the rears are on ice the fronts may not be. In a 2wd vehicle with only 2 drivers and no limited slip you could be down to none of them grabbing and you get stuck.

The real problem is going downhill, 4 wheel drive doesn't help you stop when you apply the brakes on ice. Still, I can take a steep one in 4 wheel Low range because the engine runs at so many rpms that you have to actually hit the gas to go downhill, so you can barely creep.

[Dan Lyke]

From the way that I understand it, all four wheels DO NOT receive the same amount of power all of the time. If a 4X4 is stuck in the snow (vehicle is in all wheel drive mode) the wheel that has the LEAST amount of friction gets the most power.

What others have said: In older 4WD you had to not only choose 4WD on the transmission, but also get out and lock the hubs. This meant that, at least in the front, there was no differential effect, and putting the transmission in 4WD meant that there was no differential between the front and the back. So, at worst, one of the back wheels would spin faster than the other, but the front wheels would remain locked together.

This was bad in situations where the front wheels couldn't slide a bit, like clean dry pavement, because the different number of rotations between the wheel on the inside of a curve versus the outside meant that there was a lot of extra stress on the axles.

I believe that modern 4WD (especially "on demand" or "full time") has electronic brakes on the planetary gears of the differentials that lock the differentials if the computers sense wheels turning out of spec with what could be expected given whatever other telemetry of the car they're working with.

There is, of course, no reason you couldn't apply this same information and technique to a 2WD differential, and I've no doubt that some of the higher end sedans do this as well.

But, much in the same way that insurance companies have discovered that anti-lock braking systems don't actually reduce accidents, no amount of technology is a substitute for good judgement, and if people think "I have a four wheel drive, therefore I'm safe driving in icy conditions", I want to be somewhere else.

And, much like ABS, the computer control is only as good as the data it can acquire. You can get ABS to lock up the brakes if you can convince it that the car's not moving (sometimes with slow speeds and extremely slippery conditions), and I'm sure there are various ways to get yourself into trouble with traction control. Good driving habits are good driving habits, and you may need to do things like pump the brakes anyway to help the computer figure out what's going on.

[Steve Clardy]

Unless it has locker third members [posi] in both front and rear, only two wheels pull when in 4x4.
1 front, 1 rear.
Usually right rear, and left front pull, going forward.
In reverse gear, the right front and left rear pull.

[Travis Hirst]

You have to understand that 4x4 and all wheel drive are not the same. Even in the 4x4 catagory there are different types being part-time, full-time, on-demand and the old lock in the hubs type. These all react in different manners. The all-wheel drive car reacts much like you are thinking in that it will give more power to the slipping wheels or wheel. The old type 4x4 system does not do that it give power to the front and rear equally when the hubs are locked in. I have to agree with Dan in that there is no substitute for good old heads up driving. 4x4 is not and will not stop you any faster in the snow and ice than will a 2x4. The only thing 4x4 is good for if you think it's better in the snow and don't respect the snow is getting you to the scene of the accident faster!

Travis

[Joe Pelonio]

See if this helps

http://4x4abc.com/4WD101/4WD-AWD-autoAWD.html

[Steve Clardy]

You have to understand that 4x4 and all wheel drive are not the same. Even in the 4x4 catagory there are different types being part-time, full-time, on-demand and the old lock in the hubs type. These all react in different manners. The all-wheel drive car reacts much like you are thinking in that it will give more power to the slipping wheels or wheel. The old type 4x4 system does not do that it give power to the front and rear equally when the hubs are locked in. I have to agree with Dan in that there is no substitute for good old heads up driving. 4x4 is not and will not stop you any faster in the snow and ice than will a 2x4. The only thing 4x4 is good for if you think it's better in the snow and don't respect the snow is getting you to the scene of the accident faster!

Travis

Sorry, got to disagree with ya on that one Travis;)
I've drove 4x4's too many years on bad roads.:)

[Travis Hirst]

I should make that a little clearer. Some people think that just because they have 4x4 that they are invincable in the snow and ice. If people think that then it will not. Goes back to the next sentence in that post that if you do not respect the snow and the driving conditions then you will probably run into trouble.

Travis

[Steve Clardy]

I should make that a little clearer. Some people think that just because they have 4x4 that they are invincable in the snow and ice. If people think that then it will not. Goes back to the next sentence in that post that if you do not respect the snow and the driving conditions then you will probably run into trouble.

Travis

True. I'll go with ya on that one;) ;) ;)

[Frank Chaffee]

Sorry, got to disagree with ya on that one Travis;)
I've drove 4x4's too many years on bad roads.:)
I second that.

And Travis, I agree with you in that people had best be aware of the capabilities of their machine as well.

[Jason Roehl]

My brother has an AWD Subaru WRX. That's power to all 4 wheels all the time, but there is a center viscous couple that allows differential power to the front and back axles, normal is something like a 60/40 split, I believe.

Trac-Loc and Positraction are trademark names from Ford and Chevy respectively (I forget Dodge's) for a limited slip differential. The limited slip has a clutch pack in the differential to allow for slippage of the clutches--thus different numbers of rotations on the tires--on hard-surface turns, but the clutches engage when one wheel spins much faster than the other, bringing both tires to the same speed & torque. These eventually wear out and start acting like an "open" differential--what I have on my truck. I call it "1WD".

For serious off-roaders, they will often have limited-slip in their front axles, and put what is called a "locker" in the rear. This is either an air- or electric-activated (from the cab) device that hard-locks the two half-axles together so that both tires get the same amount of power/torque. Rock-crawlers may even put a locker in the front. There are no clutch-packs to wear out, it either works or is broken.

Locking and unlocking hubs in the front of a 4x4 is simply a way to reduce wear and tear on the axles when not in 4x4. Unlocking the hubs means that only the wheels spin freely, instead of both half-axles also. Since the front and back axles of a vehicle travel different distances in a turn, if the tires have a good grip, you can do damage to a 4x4 system (the transfer case) if it is on and the hubs are locked. Some guys do a cheap version of this by welding their spider gears in the differential, but that means that their rear-end hops around turns on dry pavement because it is a permanent modification that tends to eat tires.

This is kind of an overview of the truck-type systems. Newer SUVs and such have sophisticated, computer-controlled systems that can control the power and braking to individual wheels to recover from skids and slides and such.

[Don Bergren]

My previous full size van had four wheel drive in it. I bought it new and had a 4 x 4 conversion done that was an aftermarket setup by Quigley Motor Company. It had a floor shift with 2 WD, 4 WD - Hi, N, and 4 WD - Lo. I had automatic front hubs because I don't do so well dealing with locking hubs. The rear end was a limited slip differential. It could go where most full size vans could never go.

It was great in the snow, but driving in the snow wasn't something I did carefree just because I had a four wheel drive. But if it did get stuck it always got itself out.

These days I still drive a full size van, but it doesn't have 4 WD. I don't miss it at all. The gas milage is sooo much better without the 4 WD conversion.

Now, how many remember the Dodge and Chevy 4 WD's (possibly Fords too) back in the 70's that had 4 WD - Hi Loc and 4 WD - Lo Loc? Lock those puppies in and you had true four wheel drive! Of course, the downside was that an awful lot of people tore up their drive trains with them. But it was fun while it lasted!

[Ken Garlock]

said the Indian when his dog died.

As I recall, some sedans use a computerized monitor of the wheel spin. When one wheel is rotating faster than this mate, the brakes are applied to the spinning wheel to force the torque to the non slipping wheel. That would be a lot cheaper than a box of gears trying to figure it out.

I am not a 4WD person, and even growing up in northern Ohio, didn't I need it. Good old studded snow tires did the trick for me. There is one comment that does leave me puzzled. The remark that 4WD will stop you faster than conventional 2WD. All I can say is that if that is true, you have found a way to violate the known physical laws regarding friction and should be in the patent office in the morning.

The only thing controlling stopping distance of a vehicle is the friction between the tire and the roadway. Second, you have 4 wheel brakes regardless of your power transmission mechanism. You can effect that distance by putting additional weight over an axle and hence creating more friction. That in turn can increase your stopping 'power' and decrease your stopping distance, maybe. There is however a penalty to be paid to kinetic energy. The more weight on board, hence more kinetic energy, the longer it takes to stop as that energy must be dissipated.

Kinetic Energy = 1/2*(M*V**2) One half the Mass time the Velocity squared.

[Steve Clardy]

said the Indian when his dog died.

As I recall, some sedans use a computerized monitor of the wheel spin. When one wheel is rotating faster than this mate, the brakes are applied to the spinning wheel to force the torque to the non slipping wheel. That would be a lot cheaper than a box of gears trying to figure it out.

I am not a 4WD person, and even growing up in northern Ohio, didn't I need it. Good old studded snow tires did the trick for me. There is one comment that does leave me puzzled. The remark that 4WD will stop you faster than conventional 2WD. All I can say is that if that is true, you have found a way to violate the known physical laws regarding friction and should be in the patent office in the morning.

The only thing controlling stopping distance of a vehicle is the friction between the tire and the roadway. Second, you have 4 wheel brakes regardless of your power transmission mechanism. You can effect that distance by putting additional weight over an axle and hence creating more friction. That in turn can increase your stopping 'power' and decrease your stopping distance, maybe. There is however a penalty to be paid to kinetic energy. The more weight on board, hence more kinetic energy, the longer it takes to stop as that energy must be dissipated.

Kinetic Energy = 1/2*(M*V**2) One half the Mass time the Velocity squared.

Ken. Not necessarily stopping faster, its the less chance of sliding quicker, easier, on ice. You have two axles engaged when in 4x4, thus, more drag when you get off the gas.

Try taking a 2w downhill on ice. Then a 4w down the same hill.
Common sense applies also, but 4w is safer in ice conditions.
You can steer better in these conditions also.
I've had at least 1 4x4 every year since 1974, sometimes 2 [have 2 now]
Our roads, weather, my work schedule demands 4x4's

[Mike Cutler]

"4WD will get you twice as far into the ditch":eek:, ;)

[Jerry Olexa]

See if this helps

http://4x4abc.com/4WD101/4WD-AWD-autoAWD.html


Good source, Joe. Thanks

[Ken Fitzgerald]

I bought my first 4WD in '78. I use them in the manner in which they were designed and go places that some folks wouldn't want to go. That being said, the reason for the large number of accidents is people being over condfident with 4WD ....thinking it's the answer in all conditions. We elk hunted in the mountains of Idaho....camp at an elevation of 7,000 feet. Our season is a month long. Our group always hunted the last 10 days of the season. Main reason....SNOW......it allows you to track the animals and it drives most of the hunters out. My group....we all had 4 WD and we carried 2 sets of chains for each vehicle. Often one of us would drive to the main road every hour during the night to keep it busted open of drifts ...and the next day we broke camp...loaded the pack mules and got out while we still could. I've layed on my back at 1:00a.m. in deep snow to chain up when we overestimated the capabilities of my newly purchased 4WD 1-ton Ford. A week earlier I brought all of the camping gear out of the same place in the back of my truck.....at least 1200 lbs. of gear.......I had traction.......A week later, my neighbor and I slipped back in....his p/u with a cabover camper...mine empty for elk. When the snow got deep, we parked his and drove mine...and the 2nd time I slid off the road into a ditch ....we chained up and stayed that way until we left.

4 WD has it's place....but overestimate it's capabilities....and you can get in a lot of trouble quicker than in a similar 2WD!

[Jason Roehl]

Here's a good primer on differentials of various types:

http://auto.howstuffworks.com/differential.htm