The different type of 4WD sytems

saw_dust · Jan 3, 2003

I thought this post would be appropriate here rather than the Yamnar thread

There are 4 different types of 4 wheel drive.

1.Part time 4WD
part-time four- wheel drive is the most basic 4x4 system.it gives the driver the choice of when to drive around in two-wheel drive mode and when to engage four-wheel drive.
That sounds great until you realize that you really can't engage four wheel drive on pavement unless it is ver,very slippery.Thats because part-time 4WD has no center differental between the front and rear axles.When you engage part-time 4WD,you lock the front and rear wheels together-great for straight ahead traction,but making it very difficult to steer around a corner on pavement.In fact,if you use a part-time 4WD sytem on dry surfaces,you may experience odd handling characteristics and you can harm the transmission and other drivetrain components.
So why choose part-time 4WD?The benefits of the system are:
#It is less costly to build and therefore to buy.
#It is often very durable under heavy stress.
#When you don't need 4WD,you can disengage it.
Older part-time systems required you to stop the vehicle to engage or disengage four wheel drive(actually,to engage/disengage the front -axle hubs).Most modern SUV's,through the use of automatic locking hubs have"shift on the fly"capability so you can activiate four -wheel drive while the vehicle is moving.But with some,you may still have to stop and reverse the vehicle to disengage four-wheel drive.

2. FULL-TIME 4WD

Full-time 4WD is the most common sytem,found on many popular new sport- utility vehicles.Full-time 4WD offers a two-wheel drive mode for summertime or dry road driving,an "automatic 4WD" mode for changing conditions,plus 4WD High and 4WD Low modes for really tough going.The automatic 4WD modes makes this system conveninent for many drivers.
A center differental(or some other type of coupling,such as a viscous coupling or multi plane clutch)between the front and rear axles lets the 4WD work automatically,depending on driving conditions.These couplings react to wheelspin and progressively lock the front and rear axles together to optimize traction.
In other words,many full- time systems in the "automatic 4WD"mode will operate in two-wheel drive on a straight,dry road,but progressively engage both axles if one axle starts to spin on snow,ice,etc.And in situations where you know you're going to need serious four-wheel traction(i.e.,off-road)you can engage 4WD High or 4WD Low.(All full-time 4WD systems have a two-speed tranfer gearbox with High and Low Ranges.
But full-time 4WD does not have some diadvantages,many of these related to what mode the system is in.Remeber,full-time 4WD still requires the driver to determine when conditions are appropriate for two-wheel drive or fourwheel drive-unless it is in the "automatic 4WD"mode,the system will not react to changing road conditions.

3.Permanent 4WD
permanent 4WD is similar to full-time 4WD,but it has no two-wheel drive mode.The vehicle is always in 4WD,so you never have to determine whether conditions are right for 2WD or 4WD-the sytem is always sending engine torque to the front and rear axles.Vehicles with permanent 4WD are generally purposed-engineered for that sytem(rather than having it "tacked on")in fact,with one exception (Jeep Grand Cherokee),the models that have permanent four-wheel drive do not offer any other drive sytem.
Current permanent 4WD sytems also have High and Low Range gearing,and have some mechanism for equalizing torque front to rear(locking or limiting the slip at the center differental).With some permanent 4WD sytems(some Jeep,Lexus and Toyota models),shifting to Low Range automatically locks the center differental.With others(Mercedes,Land Rovers and others)the center differental does not lock in Low Range,but slip is controlled either electtronically or through a device such as vicious coupling.The latter arrangement can be useful if you want high traction and sharp maneuvering ability(e.g.,towing a trailer out of a muddy field or a boat up a slippery ramp).
On pavement,you can ,permanent 4WD automatically delivers driving torque where it's needed with no driver action.Off-pavement,you can shiftinto Low range for all the traction and control of the best part-time sytems.

4. All-Wheel Drive

The last four wheel-drive sytem to consider is all wheel drive,used by some manufacturers(Subaru,Audi and Volvo,for instance)on passenger cars and by some companies on light -duty sport- utility vehicles(which do not recommend serious off road driving).Think of all wheel drive as permanent 4WD without a two-speed transfer gearbox.Manufacturers of this system for the convienance and on-road traction it supplies-remeber,its totally automatic and always in four-wheel-drive mode.
If your idea of off-roading is a smooth dirt road,then all-wheel drive may be all you need.But for serious rock climbing,towing in mud and snow,ascending a steep unplowed driveway or even slushing through heavy snow,a two-speed transfer gearbox can be a necessity.

JackIL · Jan 3, 2003

SD,
Nice job of summarizing the various types of 4WD used on cars and trucks. For tractors, there are some slight differences.

For tractors that have front wheels smaller in diameter than the rear wheels (usually called "row-crop" style since that is the lineage) if the front wheels are driven there is a clutch (but no interaxle differential) through which power can be sent to the front axle or shut off. Normally this is a manual operation controlled by the operator. It may or may not require that the tractor be stationary to engage. On many larger agricultural tractors this clutch can be operated by either a manual or automatic mode. In the automatic mode, the front axle is disengaged at higher speeds and then re-engaged at slower speeds when both brake pedals are applied. It also normally disengages with slow speeds when the front wheels are turned sharply to facilitate smaller radius turns when turning at row-crop headlands. This feature is not found on typical compact tractors.

For full-sized agricultural tractors with equal diameter front and rear wheels and which steer by center articulation, there is typically no clutch between the front and the rear axles. Both axles are always "on." These articulated tractors are usually just called "4 wheel drives" and the other "row-crop" style are called by various names including (MFWD - mechanical front wheel drive by John Deere, AWD - all wheel drive by many other manufacturers.)

Jack

Anonymous Poster · Jan 3, 2003

Wow, Saw dust!

Could not help but think you're the guy I need to talk to before I buy that 14" cabinet saw for my wood shop!

If you can go into that detail on grease and oil stuff, I can only imagine what a saw dust related topic might bring forth! /w3tcompact/icons/laugh.gif

Bill in Pgh, PA

turnkey4099 · Jan 5, 2003

Excellent. Type 1 is the most common (or at least was) and is the cause of all the 4wd things you see in the ditch during slick road conditions. Being in 4wd on slick roads is the worst possible thing you can do. Most drivers of these vehicles do not understand that you are dragging at least one wheel at all times and the more you turn the more drag/skid there is. Our local TV stations used to warn people about this a couple of times a year. Haven't heard them do it for a few years now tho.

Harry K

Trev · Jan 5, 2003

Someone once told me that first-time 4WD drivers get into trouble a lot because, although they can accelerate a lot better than the average vehicle, everybody has 4-wheel brakes! /w3tcompact/icons/laugh.gif We always seem to feel invincible until we remember this simple fact. /w3tcompact/icons/grin.gif

RanchMan · Jan 5, 2003

Being in 4wd on slick roads is the worst possible thing you can do.

Er, uhhh... I, have to disagree with that statement. /w3tcompact/icons/sad.gif

I’ve done lots of driving on icy/slick roads (both 2wd & 4wd) and 4wd is a LIFESAVER (literally). Seen far more 2wd vehicles in a ditch than 4wd ones under such road conditions, although 4wd vehicles certainly wind up there as well. The really big problem is that when folks who have 4wd on their vehicle engage it under such conditions, they then proceed to drive at higher speeds because they now have more control than they did in 2wd. (e.g. they are more comfortable now.) Greater speed and less attention/care to driving then often lead to the “Oh [expletive] - We’re heading for the ditch!!!”

Of course, you don’t have to take my word for it. Fortunately I had access today to 3 different vehicles today and decided to see what the manufacturers themselves said. (The quotes below were taken directly from the owners manuals or safety decals within the vehicles.)

Mfg. 1 - 4wd is for “...use on slippery, loose road surfaces.”

Mfg. 2 - “4-wheel traction is more suitable...such as driving at normal speed on slippery roads...”

Mfg. 3 - 4wd... “Use this for driving on wet, icy or snow covered roads.”

Again, this is from three different manufacturers, not just three different models from the same company.

The one time I would agree with the statement you made is if you are running lockers in your differentials. (Detroits, ARBs, TRD, whatever.) THAT makes things touchy for sure since the differentials in the axles are no longer able to let slippage occur to the point where traction remains on the opposite wheel. (e.g. you "over power" and wind up breaking traction on both tires. (or all four if using 4wd.))

Perhaps the reason your local TV stations no longer present such “helpful hints” is they finally realized that “Joe Blow” reporter knows less about the proper engagement & use of 4wd than the auto manufacturers. (To consider someone in the media would have such an epiphany is optimistic, but it could happen.)

Think about it this way - with our lawsuit happy society today, don’t you think that if 4wd was the worst thing you could use if you were driving on a slick road, the auto manufacturers would say “Don’t do it!” in their manuals???

No, there is no inherent problem with 4wd as you suggest. The problem is individuals who are careless/ignorant regarding its proper use. Those who own/use 4wd vehicles should take the time to read the owners manual and educate themselves about their vehicle & its 4wd system, when is the appropriate time to use it & when is not, and not to think that 4wd gives them a "license to speed" when unsafe road conditions are present.

hwp · Jan 5, 2003

Most drivers of these vehicles do not understand that you are dragging at least one wheel at all times and the more you turn the more drag/skid there is.

Count me in the group of morons who don't understand. What do you mean by this? My F150 4x4 has automatic hubs but they don't stop the differential from working. Or are you talking about situations where there isn't enough traction for the front differential to work? One of the things I don't like about the setup on my truck is that it has a positraction rear differential (one of Ford's "better" ideas). If I am going up a slippery hill with a fair load the locked rear differential will overpower the steer wheels and push the vehicle straight ahead with or without 4wd engaged.

Cacinok · Jan 5, 2003

"Most drivers of these vehicles do not understand that you are dragging at least one wheel at all times and the more you turn the more drag/skid there is."

I think what he means by this is that when you are in a locked 4wd and are turning the outer and inner wheels travel different distances. Because the differential is locked to the wheels binding on the inner wheel will occur and unless it is allowed to slip (e.g., when it is on snow) it will "drag/skid."

Four wheel drive, on most vehicles, is not true 4wd. If you've ever seen a (non locker or non positraction equipped) 4wd in the snow or mud you would only see one front wheel and one rear wheel spinning. Thus it's technically only a 2wd. There would not be the drag/skid problem on this type of system.

Also, people who drive 4wd's in the snow often have difficulty because when the wheels start to spin the vehicle will not just have the backend come around (as in a rear wheel drive vehicle), but the whole vehicle will slide sideways.

Clint

saw_dust · Jan 5, 2003

What do you want to know about the cabinet saw?

Anonymous Poster · Jan 5, 2003

Who me? /w3tcompact/icons/grin.gif

Well, when I buy a new one I will use the tractor to carry it down to the shop door...so I guess this is related/w3tcompact/icons/smile.gif

I am thinking about buying a Grizzly 14" cabinet saw...

Very price competitive...looks pretty good in person...just wondered if you have any experience with this saw...I have a couple grizzly tools that work satisfactory...

Man, if they were only painted orange rather than green! /w3tcompact/icons/laugh.gif

Bill in Pgh, PA