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And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. "

So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.

Well both tactics work in different scenarios. When power goes to the slipping wheels, it helps keep the car drive strait. For example, in the rain or light snow. If one wheel slips, then suddenly power goes to the wheel with traction, you'll veer off-course.

When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem.

Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.

Well, not quite; the viscous drive unit really just ensures that every path has some resistance floor, and thus that not all torque goes to one wheel.

At one point I remember there was a workaround mentioned by the Hummer engineers (I believe) to apply the brake simultaneously with the gas if you ran into the "one wheel spinning in the air" problem. This is a simply a manual application of the same physical solution of creating at least some resistance everywhere.

The trick with the hummers was slightly different.

Hummers have torsen diffs, which have a pair of worm gears where a conventional open diff would have single star gear. As a result, the torsen diff sends some fixed multiple (changes with model) of the torque used by the easier to spin side, to the harder to spin side.

Of course if one wheel is spinning in air, the easy side uses effectively 0 torque, and 0 times anything is still 0, hence the brakes.

Applying the brakes increases the torque requirement of all wheels by a fixed amount, but the hard to spin wheels get More than that amount of extra torque.

That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation.

I had an old Toyota Tercel 4WD station wagon and it was very difficult to steer sharply at low speed with 4WD engaged because of the lack of a differential. It would lurch and I had to press harder of the accelerator to get straightened out and pop it back into 2WD. It was my first car so I didn't know any better. Fun fact: In 4WD, you got an 'extra low' gear below 1st and that car could very nearly climb trees. The gear ratio was insane. No ground clearance but I used to scare the crap out of friends by going up very steep inclines in extra low gear with 4WD engaged. They would have to park their pickup trucks at the bottom of the hill and ride with me, and this caused them some embarrassment. Good times.

I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often.

One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)

I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned.

One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors vechicles now, but I feel the 80's to 90's vechicles will see a huge boost in interest.

I had an 87 pickup and 87 tercel. The engines will last forever, but rust is the killer on the truck. Currently have a 2005 Tacoma and it is rust free (plastic wheel wells) but even it has a recall for rusty suspension.

> That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation.

I think you just said the opposite of the guys above you while saying you agree. 4WD should not have a problem on slippery terrain.

A typical 4WD without locking diffs will still spin wheels. You have to have differentials at each end to go around corners on pavement.

An AWD car has _3_ diffs, one in the middle that splits power F/R, and then one at each axle that splits it L/R.

Highend 4wd setups can lock the diffs so all wheels spin at the same speed all the time - essentially on slippery ground, but very counterproductive when you actually have traction.

It actually has nothing to do with 4WD or AWD, the article is poorly written and confuses several terms. At the surface 4WD is manually selected and AWD is 4WD all the time. Thats it they are the same thing, it's just 4WD means you opt to put the vehicle in 4WD.

The term the are talking about when traction is applied to the wheel with the most binding to the ground and away from the one that has the least traction is called a Limited Slip Differential and by virtue of the slipping of the low traction wheel it will spool the high traction wheel and bind it transferring torque to it. This can also be done by what is called a locker, either electronic or manual but manual lockers can be dangerous at high speed if you are not used to their manor. You generally only see manual lockers and welded differentials in offroad trucks. Limited Slip differentials and electronic lockers are available in both 4WD and AWD vehicles.

The term they are talking about to apply power to both the front and rear wheels is not a differential but rather a transfer case and it is not always at a 50:50 ration man times they run 60:40 or 70:30. Both 4WD and AWD have a transfer case, 4WD's allow you to select whether they are engaged or not.

The term they are talking about when they talk about adjusting power front to back is called a Traction Control Unit and is handled by a Body Control Module that may or may not also communicate with the Engine Control Unit or the Transmission Control Unit to defuel or detorque the powertrain, as well as adjust torque via electronic lockers in the differential to the wheels that need it and away from the ones that do not. As well as engage or disengage front and back axles via the transfer case. Many newer AWD systems have this but it is also available in most newer 4WD vehicles.

TLDR is 4WD and AWD can be virtually identical depending on the subsystems but the article confuses a lot of those subsystems as being unique to one or the other. Though the upper end of 4wd's have more "hardcore" options not generally found or offered in AWD vehicles.

I found the article very good explaining the basics and history of awd vs 4wd. What you explain is the status of today's 4wd vs awd, where indeed the features in awd vs 4wd cars can overlap a lot and because of the big variety of implementations quite hard to understand by regular Joe

That's the nature of 4wd -- take a modern jeep, but it in 4wd on dry pavement and feel it lurch and moan because it can't slip. Put it in 4lo and on dry pavement and it'll lurch so hard your face will go through the windshield.

4wd without lockers means you can still get stuck (been there) because front and rear each have their own diffs. One wheel in each diff will be spinning and then you need to get a tug from a friend. The rule of thumb is always have 3 points of contact if you don't have a locker -- then one diff can spin while the other pulls you along.

It will even lurch and moan on slippery surfaces too if you turn a corner too tight.

I have a Dodge 2500 4x4 diesel and even with all that torque (about ~600 ft-lbs) it still bucks if I turn too tight even on ice.

I'm guessing it's due to the transfer case my truck doesn't have a center differential.

Here I am in snow no buck I guess no tight turns https://www.youtube.com/watch?v=zdK75UmoRMY

Yeah, it probably has full-time 4WD with a viscous clutch at center - when you turn, your axles lock, and 3 out of 4 wheels will have to slip to let you get anywhere. This will cause jumping and tire noise. Engine torque won't change much.

LSD's front and rear makes this worse, but center is the most important when it comes to on-road maneuverability.

A well engineered differential [0].

[0] https://www.youtube.com/watch?v=JEiSTzK-A2A

Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.

just got a 2nd hand impreza, can confirm more then a couple flogged cars in the market!

>> "...sends power away from the slipping wheel(s)..."

Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American.

In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.

Audi Quattro is great, although IIRC, it also eats a bit tires on tight turns like anything but an open diff would. I also like the viscous clutch on the jeep, although it definitely is less on-road friendly (A tight 90 degree turn is enough to hear the tires, 180 and you can almost feel the car get lower). My worst system was my 98 explorer - it engages the clutch completely when front axle falls behind, waits a second, disengages to check if axle is still behind, if so, reengages... It's jumpy as hell when turning, and sucked no matter what. You could mod it for pure 2WD operation by switch, but stock was awful.

However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.

In the famous Saudi videos (there are hundreds of them), the wheels that lose traction completely stop. See https://www.youtube.com/watch?v=MQm5BnhTBEQ

This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.

Very hard to read because you bunched everything into one huge paragraph but if I read it right then you are absolutely correct.

hah, I'll edit after a bit :) . Wrote while re-enacting the battle of hoth in lego form with my son...hard to do paragraphs on iphone.

I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only if the system detected slippage in the rear, and would receive about 35% of the engine's total power output).

The funny thing was, even though you were supposedly able to drive in full time 4WD on dry pavement, it would sometimes engage the front diff in parking lots and when taking tight turns at low speed, causing the front wheels to hop. I ended up leaving it in 2WD unless I was on a dirt road or it was raining, figuring full time 4WD was a marketing gimmick to compete with Subarus.

sounds like maybe the locking clutch pads were malfunctioning (pretty sure they used clutch packs vs. gear type). the hopping is never fun.

From what I remember when I had to replace the transfer case a year after I bought it (it was a 1996 that I bought in 2006 and the previous owner abused it), it was more or less a "traditional" Jeep 4WD setup but depended on the transfer case to keep the front and rear axles independent until slippage was detected. I think it was pretty much fully mechanical, except for the auto locking front hubs. I believe Jeep's marketing term for it was Select Trac or something like that.

I think the issue was with the auto locking hubs; when it malfunctioned it would lock the front hubs together and in turn the transfer case would lock the front and rear diffs together, causing the wheel hop. This defect probably contributed to the death of the original transfer case, now that I think about it.

I learned about limited slip differentials from Marisa Tomei's great courtroom scene in My Cousin Vinny: https://www.youtube.com/watch?v=CFdJza0AbeA

Locking the center differential in a 4WD vehicle is fine on pavement as long as you don't turn, so forgetting for a small straight of pavement in the middle of the trail is no big deal. Some trails are half dirt road, half long-forgotten pavement, so constantly reaching down can get tiring if one follows that rule verbatim. Snow transitions are similar; your truck would have been fine and I'd just reach down for a dry corner if you can disengage in motion. If not, yeah, bigger concern.

Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)

If locking the transfer case and doing U turns on pavement is going to find a weak link in your drive-train then you're not doing something right...

Well, yeah. Mine will bark pretty loudly if you do that but it did survive the one time it happened. Said experience is also why nobody but me sits left seat.

I'd have to find my paperwork but I think such maneuvers are cause to void drivetrain warranty. Chaining the tires is, too, for unrelated reasons.

woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).

I would add that in addition to locking between the front and rear axles, many 4x4s allow for the wheels to be locked across single axles. Right and left tires then turn at the same rate. This is horrific on dry pavement as one wheel will drag in every corner, but an enormous advantage in rough terrain.

Some of these can be locked manually/physically, but more often these days they are controlled electronically. Of course this defeats any traction control systems, normally leading to a warning light telling you that you are on your own in that regard. Antilocks also often disabled in this mode.

Nice video (well, I'd say motion picture). Reminds me of the Master Hands:

https://www.youtube.com/watch?v=8bT6txm4RpA

Wow, that video does a really great job building up the explanation. I already knew how a differential worked (thanks to Legos), but I had never thought about it that way before.