Generally speaking, All Wheel Drive has a differential in the transfer case so that while both axles CAN receive power at all times, the front and rear drive shafts can rotate at different RPM if/as needed (AMC Eagle, Subaru, Jeep Grand Cherokee, etc.). This differential sometimes can be locked, giving true Four Wheel Drive (my old '76 Dodge 3/4 ton, my Jeep Grand Cherokee, some Jeep Wagoneers, etc.)
In a Four Wheel Drive vehicle, the transfer case usually does not have a differential between the front and rear drive shaft. This means that it should NOT be driven on dry, hard surfaced roads in four wheel drive. To do so can result in wind-up in the drive system, which could adversely effect steering and handling, and could become severe enough to twist off a driveshaft or axle shaft. I have had this wind-up occur in a Four Wheel Drive to the point where I had to jack up a front wheel to release the strain before I could get it out of four wheel drive. As the wheel left the ground, it rotated almost half a turn! Something in there was REALLY stretched, I wonder how much more it would have taken to break something? I truly don't wish to find out!
This also means that all four tires should be the same size. I once bought an old Jeep Wagoneer that was stuck in 4-HI. I decided to drive it home anyway. The freeway was patchy snowpacked and icy. Every time I hit a slick spot I could feel the car jump. Finally, after a long stretch of bare pavement, I hit an icy patch and the release of the wind-up threw the car into the median, barely missing a bridge abutment. I discovered it had three different diameters of tires on it! That made for a very frightening ride, to say the least! It was better on the frontage road, where it was nearly all snow packed and somewhat bumpier.
"4WD often includes a low range with an additional ~2.5:1 gear reduction when needed.
No AWD has this feature that I know of in a common vehicle."
My '05 Jeep Grand Cherokee does. My old Dodge pickup did. The low range in my Jeep Rubicon is, IIRC, 4:1 gear reduction.
"Not an issue on 4WD cars with a transfer case."
Subaru is the only AWD vehicle I know of that actually does not have a true "transfer case"
Nearly all other AWD or 4WD vehicles have a transfer case between the front and rear drive shafts (there may be some others that don't have one, that I don't know about).
CM1, USN (RET)
2002 Fleetwood Southwind 32V, Ford V10
Toad: 2006 Jeep Rubicon LJ
Other toad: '06 PT Cruiser, Kar Kaddy dolly
Toy: 1977 Dodge W100 CC SWB, 3/4 ton axles & springs
"When seconds count, help is only minutes away!"
The AWD-4WD designation has truly been blured in modern pickups. My truck has it all, I have AWD, N, 4WD-HI and 4WD-LOW as well as 2WD. It sure works great in snow and mud with the Eaton Locking Rear Dif. God help me if I have a problem with this complicated drive system when the warrant is over.
I learned how to drive in the snows of Colorado in a one wheel drive 1951 Oldsmobile. Traction was sometimes provided by six buddys pushing.
The way I see it, 4wd is the ability to lock both front and rear axles when needed. AWD is where the front and rear axles are locked full-time.
No. mowermech gives an excellent description of 4wd vs. AWD.
4wd almost always does NOT have a center differential so when 4wd is engaged front and rear drive lines are locked together and must turn at the same rate. In 2wd the front (usually) axle is uncoupled and does not transmit any torque to the front axle. When you engage 4wd you don't lock the axles, you lock the two drivelines together. In many cases in 4wd vehicles, the rear axle has some kind of either limited slip differential, or in some cases a true locking differential such as the Eaton autolocker. This results in 3 wheels (both rear and 1 front) that MUST turn at the same rate in 4wd.
In a few cases there is an option to also lock the front axle, but that is fairly uncommon. In this case ALL 4 wheels MUST turn at the same rate. Not good when turning a corner on dry pavement since the inside and outside wheels must turn at different rates and therefore either inside or outside MUST slip.
since there is no center differential, 4wd allows a 100%/0% torque split between the front and rear axles. If the axles have a limited slip each axle can split the torque to some upper limit, usually near 60/40 to 80/20. If it has a true locking differential, then the torque split in the axle can be as high as 100%/0%. In other words, all the torque goes to the wheel with traction, and none to the wheel with no traction.
In a normal "open" differntial, torque is split equally between the wheels 50/50 at all times. so whatever torque the spinning wheel can apply to the ground is the same torque the non spinning wheel can apply to the ground. If the spinning wheel is on ice, not much torque is going to the ground, so the other wheel isn't going to get you moving.
AFAIK AWD NEVER has a full time lock on the center differential. In most/all cases AWD has some kind of differential with some form of torque splitting, such as a viscous clutch between the front and rear drivelines. This allows you to use AWD on dry roads w/o the attendent issues of 4wd, and basically allows the AWD function to be "automatic" to the user. In some vehicles there is an option to lock the center differential, making it a "4wd" with the advantages/disadvantages of a locked center differential. E.G. with a locked center differential you really don't want to be driving much on dry pavement, especially with turns or you can bind up the driveline and will wear the tires faster since the will scrub on turns.
AWD with an unlocked center differential, typically will not allow a 100%/0% torque split between the axles, often it is 60/40 or maybe 80/20. since no axle or differential is locked, all 4 wheels can turn at a different rate. nice for dry pavement and works well for many/most situations on pavement with bad road conditions.
which is better??? really depends on what your needs are. Personally I have 4wd with an eaton autolocker on the rear diff on my silverado. For what we do, I'd really prefer AWD, still with the eaton autolocker on the rear diff, and an option to lock the center differential. Would make it much more usefull on roads alternating between wet/snowy/icy and dry. I'd even go for AWD if I couldn't lock the center differential.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2004.5 Silverado 4x4 CC/SB Duramax/Allison
I have had two vehicles with AWD and one with real, push a button to engage, 4WD. I got stuck in the mud with my AWD van and stayed stuck. The front wheels would not spin if the steering wheel was turned and I got no traction to get out. My 4wd would have gotten me out. The AWD is good on that van in rain, ice, and snow driving as it gives it a much more stable ride with less rear swing that a similar model van I had with just 2wd. On all the AWD vehicles that I have had, the front tires only engaged if the rear tires were slipping and did not spin if the car was driving normally.
It seems like every manufacturer has a different idea of what AWD and 4WD means. We have a 4WD Honda SUV that is really just AWD with a button to push that is supposed to get you out of being stuck which then automatically disengages if you get up to 15 mph. The real 4WD SUV that I had before this, had two buttons - one to engage all four wheels all of the time and another to engage the front wheels with low power to pull out of a hole. Without any button pushed it was 2WD. Find out exactly what the manufacturer is calling AWD and 4WD.
Our AWD ML320 is MUCH better in snow, or on ice, than our 4-wheel drive Ford.
2010 Ford Expedition TV
2010 Outback 230RS Toybox, 5390# UVW, 6800# Loaded Not yet camped in Hawaii, 2 Canada Provinces, & 2 Territories I can't be lost because I don't care where this lovely road is going
My 2011 and 2002 RAV4's are both labeled "4WD". The '02 has a switchable electronic traction control system, while the '11 has the traction control system plus a "Lock" switch that sends equal traction to all wheels at speeds up to 25 MPH.