4WD vs. AWD -- a primer on the differences

[Jombi]

 

This is just sort of a stream of consciousness essay on my understanding of FWD vs. 4WD vs. AWD.  As Dennis Miller used to say, it's just my opinion--I could be wrong!  With that said I welcome any and all corrections and clarifications.  

 

I seem see a lot of questions and confusion online about the differences between 4WD and AWD, and most of the answers online are vague and sometimes not really accurate.  Many of you know all of this already so I'm putting it here for those who don't or may still have questions.

I’ve dug fairly deep into this subject and tried to distill what I’ve learned from both reading up on the subject and various you-tube videos on the subject.

I highly recommend anything from this site--Engineering Explained https://www.youtube.com/user/EngineeringExplained , they have a great wealth of info and that guy has a very good way of explaining very technical concepts in a way that non tech-nerds can understand.

Even still it’s hard to completely nail down the actual real-world mechanics of it all, but here’s my go at it. 

Again I welcome any corrections, elaborations, etc. I'm not an engineer, just an enthusiast, so my knowledge on the subject is largely informal.

 

Starting from the old days when most cars were 2WD, rear drive:

In America the typical cars from the 60s and 70s were mostly open-rear-diff 2WD, so only when on dry pavement did you have 50% torque split of actual useful energy between left and right rear tire, and fortunately this is most of the time so you get equal wear on the components of each side (bearings, gears, and tires wear differently between being dragged and being under torque).  However, if you tried to pull up your driveway and the left rear tire was on a patch of ice but the right was on dry pavement--the left rear got 100% of the engine’s torque--which, while scientifically may be "some torque" because it's moving the driveline components and physically turning the tire--but for the context of this discussion, it’s getting zero torque because we're not going anywhere.  So the concept of an open differential providing equal torque still stands here--the right rear is providing zero torque as well.  Technically, if the driveline resistance to the spinning tire were large enough, then the right wheel might provide some usable torque and therefore motion (more on this later as that's where ABS/Traction Control come in), but the reality is that you were stuck, period, and had to resort to any number of tricks we had back then to get moving.

 

Around that same time, in the early 70's when I was a teen, the only real high-traction option was true 4WD or (4x4). As a 16 year-old, I drove a tow truck/snowplow for a gas station.  This tow-truck was a true 4WD--this meant it had a limited slip differential in the rear and a solid-locking differential in the front.  These were connected by a transfer case (not a center-differential).  Most of these trucks then, as now, were rear-dominant, meaning that in "normal" driving mode the transfer case was in 2WD driving only the rear driveshaft.  So in normal mode, even if one rear was in the ice, the centrifugal force of the spinning tire would lock up a clutch in the LSD and the other tire would provide roughly equal torque.  Some “4x4” trucks did not have an LSD in the rear (or it wore out over time) so were technically 3x4.

 

However, even with the weight of a tow-hook in the back, this isn't enough to push a big pile of snow, especially when one is sitting in that deep snow to begin with.  The front diff on these 4x4s had an open differential with hard-locking capability in the hubs. So to get into true 4x4 mode, step one was to stop the truck, shift the transfer-case to 4wd (4-Lo for snow plowing, 4-Hi for general off-road or heavy snow/mud driving).  Step two I'd get out of the truck--usually while still on dry pavement so I stay dry, but where my entry to the snow was a straight line (this is very important because once the hubs are locked, making a turn on dry pavement will bind the tires and/or diff gears when locked)--and I'd twist the hub locker to lock the left and right hubs on the front wheels.  The solid locking hubs are much better than an LSD--no waiting for spin to "activate" the other tire--the axle was, for all intents and purposes, a solid shaft from left to right .  So in this situation, we have essentially direct-from-the-engine torque at all 4 wheels--no ifs, ands, or buts--the torque is there even if a tire is aloft. With the weight of the plow and engine up front, and the tow-hook in back, there was so much weight/friction at all 4 tires that I was never getting stuck.  Once finished "plowing" or, because I was a fun-loving teen at the time "just plain having fun on company time" with the boss’s tow-truck in the nearby woods or a snow or mud covered field, I'd pull out onto the dry-pavement area, get out, unlock the hubs, and shift the transfer case back to 2wd and head back to the station.

 

As an aside, in the snow when you're plowing (and really in almost any snow situation unless the snow is much deeper than your cars ground-clearance) the goal is to get the tires dug down to the pavement, and this particular truck had tall thin snow tires—perfect for that purpose.  I’d see other 4x4s with “mudders”—big wide tires—that could get stuck in snow even in full 4x4 mode because they were basically skiing or floating on top of the snow.   However, If that heavy tow truck and thin tires were in very wet mud then it has no pavement to dig down to and risks digging itself till it’s frame is settled on the mud and getting stuck, so there is a good case for mudder tires.  At one point in the mud I almost needed to get out and wrap the tow-hook around a tree to get me out, but some gentle rocking got me free.

 

FWD—a little better but not the complete solution. 

In the mid/late 70s, Front-wheel drive came on the market.  This was mostly in economy cars made to combat the ongoing gas-crisis.  Cars such as the Dodge Omni were typical—these gave a noticeable advantage in traction due to the simple fact that instead of the driving wheels being under the (normally empty) rear trunk of the car, they were under the engine up front, which helped a great deal.  ABS/Traction Control were still well over a decade away, but taking advantage of physics this way was a no-brainer and a welcome advance.  It also introduced us to understeer and torque-steer, and sadly--unless you had a center lever parking brake in your FWD car--it took away the great fun found in RWD oversteer in big empty parking lots after a fresh snow!  Gradually FWD made its way into mainstream sedans and the of course mini-vans, which were coming just over the Omni…er…Horizon (probably only those who remember those Dodge/Plymouth FWD cars models will get that arcane reference!).

 

Fast forward to the days of AWD. 

AS FWD proved an incremental advantage, more was needed to really overcome the traction issues faced by those driving in snow, ice,and even rain, so in came AWD.

 

The simplest and most generic implementations of AWD at first consisted of three open diffs--front, center, and rear.  (this discussion excludes a small variety of makes, like Jeeps, Subarus, Audis, BMW X-drive and a few others that have nearly always had some sort of transfer-case or locking/LSD center differential.)  Most of these initial AWD implementations were in SUV's that started life as FWD SUVs.   So for the majority of AWD SUVs, the dominant axle is the front, which has shorter and beefier axles, and the AWD option threw in an open center diff with a little prop shaft to the back to turn a couple of pencil-thin rear axles to assist if you hit ice or snow.  It’s important to note that most of these front-dominant AWD vehicles even today will only do a max 50/50 split front/rear, because the thinner, longer axles/half-shafts in the rear simply couldn’t handle 100% of the torque if it needed to move close to 100% of the vehicle’s maximum load.  They will do their job admirably by helping in snow, even hard-pull 1/4 mile launches on dry pavement, and to help cornering, etc---but in the case of the typical grocery-getter AWD SUV, the rear driveline just isn’t beefy enough to tow a huge boat uphill all by itself--and the carmakers wouldn’t (shouldn't) allow that 0/100 torque split to happen if it didn’t beef up the components.

 

As an example, in my Edge, the front axles are relatively short, thick and stubby with the fat part of the shaft about 1 ¾” diameter while the most tapered part of the shaft has a diameter of 1 3/16”.   The narrow part of the shaft is only a couple of inches in length at each end not counting the splines.  Contrast that to the rear half-shafts, which are only 7/8” diameter end-to-end, and around two feet long.  So if you were stuck crawling up a rocky hill in your AWD SUV with all the 2-plus tons on the back wheels and all the torque is also happened to be at the rear diff—you risk twisting and snapping an axle.  If you were able to turn around and go up the rocky hill in reverse using the front dominant axle (thicker shafts), you could make it without damage.  But AWD SUVs were never made for rock crawling, so hopefully nobody puts themselves in that situation with a "typical" AWD vehicle.

 

Anyway, from a traction standpoint, AWD at first blush might be seen as a potentially worse situation than a 2wd vehicle.  Because now--while you've got the "opportunity" to have four total chances at traction (remembering the concept of the open-differential)--it seems instead that we really just gave ourselves more chances to have a tire happen to be on a patch of ice.  Now it's only *one* of four wheels that will really get the torque if you’re in an icy patch--and it'll be the one with the least traction.  

 

Fortunately, this is where traction-control comes in.  It’s basically implemented the same as it had been implemented in the FWD sedans and SUVs, but now just includes all 4 wheels in the logic.   The Trac systems use ABS to clamp the spinning tire, and continues to do so on other tires as they spin until it "finds" the one with the most traction; they also typically throttle-down the engine.  So in reality, AWD does in fact provide the advantage of having one tire out of four getting good traction instead of two tires of four--but as anyone who's driven them knows, this comes at the price of a very slow start-off from stop when you’re in snow.  With Trac’s throttle-down and the clamping/unclamping different wheels dozens if not hundreds of times per second, you’re just kind of easing your way forward slowly.  You do get going but it is nowhere near what a true 4x4 can do.  I’d also guess that a 2WD with a Torsen (say, a Mustang GT with a few bags of sand in the trunk) might even do better than an AWD with all open diffs because it will have a positive torque split at two wheels instead of one.  Put snow tires on that Mustang against summer tires on the AWD SUV and the Mustang with the Torsen (and don’t forget the bags of sand in the trunk!) has a very clear advantage.   In the snow, true snow-tires with 2WD vs. summer tires and AWD is a no brainer—I’ll take the snow tires over virtual slicks any on any vehicle regardless of powertrain.  And not just because of the launch traction--you still need to corner and you still need to stop.  Waaaay too many people think AWD helps them stop better and corner better in the snow--I've seen too many rear-enders and AWD newbies in the ditch to know that's still the case and there should be a warning on that with a new AWD vehicle--especially with the cheap "good-enough-to-drive-off-the-lot" tires that some of these cars come stock with.

 

So the next iterations of AWD started introducing some sort of LSD for the center-diff, whether it be a viscous clutch, a dry clutch, electronically locking mechanical drum or brake, a Torsen, or some other way of guaranteeing that some torque will go to both front and rear regardless of the traction situation back there.  So now instead of 1-of-4 chances at traction, with a locking center diff you have 2 positive chances at traction now--and that is a huge step forward, literally and figuratively. 

For example I have an AWD Tucson which has an electronically locking center diff, and the difference that center locker makes in snow is very significant and noticeable.  The Tucson’s AWD on its own without engaging the center lock was a big leap in snow traction over my old Sonata FWD sedan--but when I hit that locker button that’s a whole different feeling--it’s a real game changer in the snow and even rain.  That is until I hit 18 mph when the safety system unlocks it because too many people would leave that locked and end up on dry pavement, making a turn, and tearing up the tires and possibly gears.  It’s important to note that the reason for the clunking on dry pavement when the center-diff is locked is not due to the left vs. right tire speed rpm difference in a turn, but instead due to the front vs. rear propshaft rpm difference, because both pairs of outer and inner tires will turn at slightly different speeds—for similar reasons as the left vs. right but it’s a distinction that should be noted.   All types of unlocked center diffs compensate for this front/rear rpm difference the same as the front and rear diffs deal with it—the center-diff may have slightly different technology and gear-sets but they accomplish the same goal.

 

Finally...

So nowadays most modern AWD SUVs have some sort of LSD in the center to ensure positive propshaft torque to the rear without having to rely on Trac to fake it.   However, many still have open diffs in the front and rear and use Trac to simulate an LSD both front and rear, which, while not optimal, is still something.   Notable exceptions to this as mentioned at the top would be Jeeps, Subarus, G-wagons, and Land Rovers—most of those come standard with some form of a transfer case and/or locking or LSD for the center-diff.   Often they have the option for LSD both front and rear, as well.   So for example, a Jeep with a transfer case will always have an advantage over a typical AWD SUV even if it has open front and rear diffs.  Not only can they put positive torque on more wheels, the drivelines are also made so both front and rear can carry the entire load if called upon to do so.  The minor disadvantage of course is that you still only want to fully lock the front/rear axles when you’re in slushy situations, and unlock it when dry.  There are a lot of ways they accomplish this—many are electronic and very convenient.  It’s been many, many years since I’ve seen anyone step out of their truck to lock the front hubs!

 

So that's my story and I'm sticking to it but like I said--I could be wrong and welcome corrections and elaborations because I've seen so much confusion over this issue and I'd like to do what I can to clear it up for everyone--including me.

Edited January 25, 2020 by Jombi
remove unclear info on shaft length & torsion

[akirby]

Ford’s AWD can send almost 100% of the power to the rear wheels for short periods.

 

Also Ford doesn’t use a center diff - it electronically engages the rear diff via an electronic clutch.

 

Otherwise pretty much correct.

[Jombi]

1 hour ago, akirby said:

Ford’s AWD can send almost 100% of the power to the rear wheels for short periods.

 

Also Ford doesn’t use a center diff - it electronically engages the rear diff via an electronic clutch.

 

Otherwise pretty much correct.

Thanks! That’s exactly the type of info I’m looking for to fill the gaps in my knowledge.

[Waldo]

12 hours ago, Jombi said:

 

T

The simplest and most generic implementations of AWD at first consisted of three open diffs--front, center, and rear.  (this discussion excludes a small variety of makes, like Jeeps, Subarus, Audis, BMW X-drive and a few others that have nearly always had some sort of transfer-case or locking/LSD center differential.)  Most of these initial AWD implementations were in SUV's that started life as FWD SUVs.   So for the majority of AWD SUVs, the dominant axle is the front, which has shorter and beefier axles, and the AWD option threw in an open center diff with a little prop shaft to the back to turn a couple of pencil-thin rear axles to assist if you hit ice or snow.  It’s important to note that most of these front-dominant AWD vehicles even today will only do a max 50/50 split front/rear, because the thinner, longer axles/half-shafts in the rear simply couldn’t handle 100% of the torque if it needed to move close to 100% of the vehicle’s maximum load.  They will do their job admirably by helping in snow, even hard-pull 1/4 mile launches on dry pavement, and to help cornering, etc---but in the case of the typical grocery-getter AWD SUV, the rear driveline just isn’t beefy enough to tow a boat uphill all by itself--and the carmakers wouldn’t (shouldn't) allow that 0/100 torque split to happen if it didn’t beef up the components.

 

As an example, in my Edge, the front axles are relatively short, thick and stubby (this matters), with the fat part of the shaft about 1 ¾” diameter while the most tapered part of the shaft has a diameter of 1 3/16”.   The narrow part of the shaft is only a couple of inches in length at each end not counting the splines.  Contrast that to the rear half-shafts, which are only 7/8” diameter end-to-end, and around two feet long.  So not only are the rear shafts much thinner, they are much longer too.  Even if they were the same diameter as the front, the length makes a difference and effectively lessens the absolute load it can handle.  Consider that it’s much easier to twist and snap, say a 10-foot bar that’s an 1/2 diameter (it's already sagging at a length of 10 feet!) than a 2” long bar of same diameter.  

 

It's true that the rear shafts are designed to less than the max torque of the vehicle, but in slippery low speed situations, they can still handle 100% of the available torque if that available torque is less than half of the max torque.  That's how Ford can say they can deliver 100% to the rear on an Edge.

 

And you've got your physics backwards.  A shorter axle will be able to handle much less torque (not load, torque) than a longer axle of equal diameter.  That's how they can be so thin, because they are fairly long.

 

And the only difference between 4WD and AWD is which marketing department decided to put the badge on the vehicle.  There is no technical delineation between the two.  Consider that the 2011-2019 Explorer had a 4WD badge but the AWD badged Edge and Flex used exactly the same system.

Edited January 14, 2020 by Waldo

[Jombi]

8 hours ago, Waldo said:

 

It's true that the rear shafts are designed to less than the max torque of the vehicle, but in slippery low speed situations, they can still handle 100% of the available torque if that available torque is less than half of the max torque.  That's how Ford can say they can deliver 100% to the rear on an Edge.

 

And you've got your physics backwards.  A shorter axle will be able to handle much less torque (not load, torque) than a longer axle of equal diameter.  That's how they can be so thin, because they are fairly long.

 

And the only difference between 4WD and AWD is which marketing department decided to put the badge on the vehicle.  There is no technical delineation between the two.  Consider that the 2011-2019 Explorer had a 4WD badge but the AWD badged Edge and Flex used exactly the same system.

Yes I agree that for some Mfrs the 4WD/AWD may be intermixed these days from a marketing standpoint.  I guess it's the 4x4 that is the designation that really stands separate--that should mean "this vehicle has a transfer case" at the very least, meaning no messing around with some computer or logic to decide when the other propshaft is turning, and really I think that 4x4 should also mean it has at least one locking or LSD front or rear differential, preferably both.

 

But 'm not sure I follow you on the physics of the shorter axle handling less torque.  The shorter the axle (and in my case it's also much thicker) will be less prone to torsion/twisting, and is also stronger and thicker, so I'm not seeing how it would handle less torque than a longer axle, even of the same diameter.  For me the distinction between torque and load is still a little fuzzy to me, but if I had a power source--whatever it may be--and it had some sort of power-take-off shaft to turn whatever--a generator, a water pump, or a car's wheel, I would think a short thick shaft would be the most direct way to transmit as much torque as possible to the power source with the least loss through torsion, etc.  The longer shaft (given the same material) is always going to be prone to distortion through torsion--even if it's supported by bushings along the way to keep it from sagging.   I'd welcome any elaboration on that, thanks!

 

Edited January 14, 2020 by Jombi

[akirby]

I think the longer axle can twist more and absorb the initial torque without breaking.  Just a guess though.

[Jombi]

On 1/14/2020 at 2:39 PM, akirby said:

I think the longer axle can twist more and absorb the initial torque without breaking.  Just a guess though.

I checked with an engineer from Mark Williams Enterprises, a manufacturer of axles for dragsters and other race cars.  He said the failure torque would be exactly the same--but the longer axle would take a split second longer to fail because (as you suggest) it would twist first then snap, while the shorter one would just snap.    

 

But back to the Edge itself--the rear shafts are still significantly thinner (and I have to assume the same grade of steel) so they definitely won't handle what the front will.  That's kind of what got me down this path--I'd been used to working on friends trucks and Wranglers and I'd see super beefy drivelines both front and rear.  It was actually when I got my Tucson a couple years ago, which is smaller and over 800 lbs lighter than the Edge, and seeing the relative pencil-thin rear axles on that thing it was a real shock.

[akirby]

Also remember that the rear isn’t directly connected to the transmission.  I’m sure the engagement of the rear diff solenoid buffers at least some of the torque.

[F4Gary]

66 Olds Toronado was beginning of Domestic FWD.  Great car, Dad owned 2 of them.

67 Eldorado followed suit as it was built on the same platform.

Edited March 4, 2020 by F4Gary

[PaulSchott]

On ‎3‎/‎3‎/‎2020 at 11:43 PM, F4Gary said:

66 Olds Toronado was beginning of Domestic FWD.  Great car, Dad owned 2 of them.

67 Eldorado followed suit as it was built on the same platform.

My dad had an Eldorado.  The CV joints cost him a fortune to replace.  A real beauty though.  He went back to rear wheel drive after the Caddy.  That's when GM actually made cars that people wanted.