
4WD vs. AWD. What's the Difference? - hackuser
http://www.outsideonline.com/2096381/4wd-vs-awd-whats-difference
======
japhyr
Here are the key takeaways for me:

"Thanks to that differential between your axles, an AWD car will send your
engine’s power down the path of least resistance—the wheel with the least
grip. Where a two-wheel drive car can only choose between two wheels, an AWD
system looks for that least resistance across all four wheels."

"4WD works by locking the front and rear axles together, splitting torque
50:50 between them. This provides great traction, but a vehicle locked in 4WD
cannot safely be operated on dry pavement because its front and rear axles are
forced to rotate at the same speeds."

I remember being scared when I bought my first 4wd vehicle, a '91 Toyota
pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and
I was worried about going from snowy roads to dry pavement. When I moved on to
an Outback (AWD, not 4wd) I wondered why I didn't have to worry about
transitioning between slippery roads and dry pavement. Now I understand.

There's also a great video from 1937(!) that shows why we need differentials,
and how they work. Here's a direct link to the video if you're going to skip
the article:

[https://www.youtube.com/watch?v=yYAw79386WI](https://www.youtube.com/watch?v=yYAw79386WI)

~~~
sunstone
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.

~~~
tombrossman
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.

~~~
tiglionabbit
> 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.

~~~
kls
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.

~~~
unusximmortalis
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

------
Animats
There are lots of variations on this theme. There are off-road differentials
which limit the speed difference between the output shafts to a fixed maximum
ratio, usually in the 2:1 to 4:1 range.

It's possible to do some or all of this in software. Tesla has an ordinary
open differential but will apply the brake on an overspeeding wheel. Tesla's
all wheel drive system has a separate motor for the front and rear wheels.
Power distribution between the two during acceleration is mostly equal, but
once speed stabilizes, the rear powertrain takes most of the load. Off-road
slip handling doesn't seem to be well documented, but Tesla cars aren't
intended for off-roading.

The most advanced systems are seen on locomotives. Modern heavy locomotives
(GE Evolution series) use three-phase AC synchronous motors driven by IGBT
inverters from power from a Diesel generator. Each axle has its own motor. All
axles are normally locked together electronically by the control software, so
no wheel can slip ahead of the others. Locomotives can be cabled together so
that the axles on multiple locomotives synchronize. Now that's all-wheel
drive. This is a huge win when starting a heavy freight train. Locomotive
axles that are slipping provide little pulling power and damage the tires and
rails.

~~~
rsync
"It's possible to do some or all of this in software. Tesla has an ordinary
open differential but will apply the brake on an overspeeding wheel. Tesla's
all wheel drive system has a separate motor for the front and rear wheels.
Power distribution between the two during acceleration is mostly equal, but
once speed stabilizes, the rear powertrain takes most of the load. Off-road
slip handling doesn't seem to be well documented, but Tesla cars aren't
intended for off-roading."

I don't think about this a lot, but I _assume_ that all of the interesting
ways to distribute power from a single engine to 1-4 wheels are, all of them,
inferior to having a dedicated motor for each.

That is, I _assume_ that no matter how fancy you made your differentials, two
motors is better than one and four motors (one for each wheel) is better than
two (all else being equal).

Is that correct ? Or is there some scenario(s) wherein mechanical distribution
of torque (with differentials) is superior to (again, all else being equal) a
dedicated motor on that wheel ?

...

If I try to answer my own question, all I can come up with is:

1\. If you have a motor for each wheel, the max output on that wheel is the
max output of that motor, and theoretically, you can distribute _more than 1
/4_ of the single engine to that wheel with differentials, so ... maybe that's
a very big deal ? Do 4WD vehicles often send 70-80-90% of output to one wheel
?

2\. Sending power to a specific wheel via the path of least resistance takes
zero time - it's instantaneous - whereas deciding what to do with each of the
four wheels (in software, presumably) might have a lag ... although that sort
of breaks my "all else being equal" tag, above ...

~~~
mcpherrinm
One more thing to think about, on this line of questioning, is efficiency in
"normal" driving situations.

If you're driving in a straight line, with good traction on all wheels, will a
single larger motor be more efficient than two smaller ones on each wheel? For
many people, I think that's 99% of their driving, so it makes sense to
optimize. Especially on electric vehicles, where increasing battery capacity
is much harder than just adding an extra gas can.

Maybe in that case the complexity of AWD/4WD/Traction control makes sense in
that case.

~~~
Animats
One motor per wheel, in the wheel, is an old idea. It's been used most notably
on LeTourneau heavy equipment. The usual problem is too much unspring weight,
not a problem for giant earthmovers but bad for fast cars.

Michelin was pushing it for cars, with their "Active Wheel" concept, from
about 2003 to 2012.[1] That seems to have disappeared. Siemens has demoed a
motor-in-wheel unit, and Volvo and Nissan have fooled around with this.
Protean, in Shanghai, is trying to sell their wheel motor. There are others.
Nobody has shipped production cars yet, though.

[1]
[https://news.ycombinator.com/item?id=383607](https://news.ycombinator.com/item?id=383607)

~~~
rsync
"One motor per wheel, in the wheel, is an old idea. It's been used most
notably on LeTourneau heavy equipment. The usual problem is too much unspring
weight, not a problem for giant earthmovers but bad for fast cars."

hmmm ... I wasn't thinking about motors _in_ the wheel, although I am familiar
with that concept.

I was thinking about a more pedestrian motor per wheel configuration wherein
the motor is just behind the spring ... and is thus, sprung weight ... is
there even space for that ?

------
joushou
This definition is broken.

You have 4WD if you have 4 driven wheels (and unless you count the spare,
that's all my wheels). Then, you can have a multitude of differential
configurations on the 3 differentials (open, lsd, locker, diff-less viscous
clutch, diff-less locker).

A Jeep WK uses a locking differential center, and electric engaging LSD's
front and rear. By that article, the car is AWD on road, and sorta 4WD when
off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or
has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A
98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked.
These cars all behave very differently both on and off road, and they don't
fit well in the authors categories.

If you're buying or owning a car that drives all wheels, the only useful
information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and
"Quattro" mean nothing.

Most new 4WD's use open diffs with electrically controlled lockers or LSD's,
as that gives the best combined on/offroad experience. Permanent LSD's/viscous
clutches (Jeep WJ, for example) is a bit more responsive in surprise low-
traction conditions, but it eats your tires when you make tight u-turns, as
the turn will progressively engage a complete diff lock. Center lock without
diff works like a regular 2WD until you lock, but remember that if you have
ANY type of locker on ANY axle (not LSD), then engaging in high traction
conditions might snap your axels in a very loud and explosive manner, even if
you think you're driving straight.

------
justinph
In real life, in most situations, ground clearance, good tires, traction
control and ABS make more difference than driving all four wheels, most of the
time. Personally, I'm more concerned with stopping than starting, and that's
where ABS and good tires matter and what kind of transmission you have
doesn't.

This is a sales pitch video, but it is interesting to illustrate the
differences in AWD approaches, with different vehicles on rollers:
[https://www.youtube.com/watch?v=9cuZYTQLfA0](https://www.youtube.com/watch?v=9cuZYTQLfA0)

I spent a lot of time looking at this several years ago when my family was
planning on taking a vacation to the Outer Banks of North Carolina where you
have to drive on the sand. We ended up renting a Ford Explorer that had a
"sand mode" in it's electronically controlled awd setup. Worked great.

~~~
analog31
Something I've learned is that snow tires make a big difference during the
winter, and I've read that they make a bigger difference than AWD.

~~~
paganel
Have been a driver for only 2 years but I can confirm that. I live in Eastern
Europe where we tend to have at least one or two snow storms per winter, and,
indeed, having winter tires and ABS makes a ton of difference, even on a
1.4-liter FWD vehicle like mine.

And, even more important, you have to keep a level head when you see that
there's snow on the road and not try to do anything crazy, like making sudden
left or right wheel movements. Also try to keep a safe distance from the
vehicle in front of you, and, before you venture on a trip outside of the
city, just check the weather forecasts: if there's a "red code" storm
announced you'd better postpone your journey, only a tank can get you out from
something like this:
[https://www.youtube.com/watch?v=Frf2lV77fl0](https://www.youtube.com/watch?v=Frf2lV77fl0)

~~~
mikestew
Bah, you can still see the road in that video. :-)Not to one-up you, but
rather to point out what's possible, your video is a not-uncommon regular
winter storm in the midwestern U. S. I grew up driving 2WD RWD vehicles in
that stuff, usually got home just fine. I did land my parent's full-sized van
in the ditch once. Should have put 25kg bags of salt in the back end to add
weight, but might have used said for the water softener, don't recall.

So you don't _need_ 4WD (let alone a tank) if you have some sense. Which is
why a lot of people buy 4WD vehicles, I guess.

------
bitL
Great! Finally somebody explained this to me, I thought it was just a
marketing term related to different patents of similar ideas.

I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley
on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I
thought they were the same, just different names, so I argued with the ranger
that I do in fact have 4WD, which is in my case just called AWD and in other
cases 4x4 as well. Well, he was still pretty upset but the "I am a European
and we have different cars" finally disarmed him...

~~~
blackguardx
Generally a sign saying you need 4wd in the US really means you need a lot of
ground clearance. Your car probably would have high-centered, that is stuck
resting on the middle of the car with the tires in the air.

The annoyed ranger was probably just tired of pulling stuck tourists out.

~~~
tunap
Normally I would 2nd this, but it does not apply on this particular 4x4 road.
It really sucks when opposing traffic appears as it's barely wide enough for 1
vehicle in most parts:

 _The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25%
average grade, with steeper grades in sections. This is a paved public road
but it is open only to 4 wheel drive vehicles. It is the steepest road of its
length in the United States._

[https://en.wikipedia.org/wiki/Waipio_Valley](https://en.wikipedia.org/wiki/Waipio_Valley)

~~~
anexprogrammer
That doesn't seem that bad for normal traffic, conditions permitting - unless
the steeper gradients are markedly so. I'm sure they have their reasons for
4wd only though - picking too many tourists out of the ditches or whatever.

There's quite a few roads in the UK with 1:4 gradient, and one just down the
road from me at 1:3 whose only restriction is no trucks and vans. Going
downhill on the motorbike makes it feel like you're falling off! :)

[https://en.wikipedia.org/wiki/Hardknott_Pass](https://en.wikipedia.org/wiki/Hardknott_Pass)

~~~
abraae
Here in New Zealand we lay claim to the steepest street in the world at 1:2.86
gradient
[https://en.wikipedia.org/wiki/Baldwin_Street](https://en.wikipedia.org/wiki/Baldwin_Street).

There's an annual charity event that involves the rolling of over 30,000
Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.

~~~
markdown
How have I not heard of this event? Sounds similar to (and less dangerous
than) cheese rolling.

------
mtreis86
This article is good but goes overboard in explanation. AWD and 4wd by all
accounts are just buzz words for having front and rear differentials. In
actual use, a 4x4 (a term I mean to refer to both 4x4s and awds as its just a
buzz word) has three differentials. A center diff splits power from the
transmission out to the other diffs, which split power to the wheels. There
are three general types of diffs, whether centered or front or back:

Open (just gears, sends power the the least resistant path)

Locked (a spindle connects the input directly to the output and all three spin
proportionally)

Posi (multiple ways of actuating this, can be viscous clutch, electric or air
powered actuation, or mecanical geared torsion style - regardless the point is
to keep power at all wheels, some with variable speeds allowing for turns -
others act like a spindle)

Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in
the front and center and an air locker in the rear (not the stock design). My
old BMW 325ix has a viscous clutch in the center and in the back, and open in
the front. My dad's audi has open gears all around and uses the brakes to
actuate were the power is delivered. At the end of the day tho, its three
differentials.

~~~
sliverstorm
There is a non-buzzword practical difference; one system cannot be driven on
dry pavement (part-time 4x4). One system can be driven on dry pavement (full-
time AWD).

------
elihu
It'll be interesting to see what happens when off-road vehicles start becoming
electrified. Separate front/rear motors eliminate the need for that middle
differential or transfer case, and if we eventually transition to a separate
motor for each wheel, we'll be rid of all of the differentials and have much
better ground clearance as well as better traction.

For now, I suppose electric trucks/jeeps are more affected by "range anxiety"
than daily-driver sedans, since people tend to imagine themselves driving to
some remote location when they buy them. But as technology improves, I would
love to see something like a modern reincarnation of a 1960's era CJ-5 (but
with better safety features) -- simple, easy to modify, nothing that doesn't
need to be there, and reasonably priced.

~~~
tajen
The army gets interested in electric vehicles for troop transport _because_ of
their range. It happens that 1. A hybrid can basically provide the same range,
2. A hybrid can regenerate energy when the driver slows down, providing much
better range, and 3. They can approach a target in silence for the last 15
minutes, thanks to the electric motor.

The drawback is, they struggle with on-field repairs.

------
schwede
The video from the 30s was fascinating. It gave a perfect progression of
complexity while explaining the concept of a differential. I'd like to see
more of those kind of videos (from now our from clear back then).

~~~
LyndsySimon
You might like Sheaffer's "The Twenty-Six Old Characters", c. 1947. It touches
a bit on the progression of language, but it's really about writing
instruments. It culminates in explaining Sheaffer's "Snorkel" mechanism, which
was probably the most novel and complex pen system ever designed. I have
several and love them :)

[https://youtu.be/1xUDehNvbrE](https://youtu.be/1xUDehNvbrE)

------
dsfyu404ed
The performance of either system is completely dominated by the traction aid
at the axles. The article is otherwise good because most AWD vs 4WD stuff
completely igores the axles (because it's a heck of a lot to explain if you
really want to compare the various systems) but kinda misses the main point.

Open, bad traction control (cuts throttle, applies a little brake), good
traction control (all brake no throttle), the many varieties of LSDs,
automatic lockers, manual lockers, Lincoln lockers and spools all have a huge
affect on traction.

Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's
full time and you'll have something that can out wheel a $50k Jeep but trades
off a tiny bit of grocery getting capability.

~~~
rsync
Thank you for introducing me to the concept of lincoln-locking a differential.
Appreciated.

------
giardini
4WD/AWD are almost never necessary in USA and Europe today, to the extent
that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk
(for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has
shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in
bad weather.

But sales of 4WD and AWD cars have been a boondoggle for the auto
manufacturers. Cost, complexity and maintenance are significantly higher:
often you must "fiddle" with them, woe to you should you damage them due to
bad judgement. They reduce mileage, increase car weight, and reduce
reliability.

I prefer a RWD car for most driving, since it handles better and I'm never
climbing bare hills or plunging through swamps, but I was raised on
oversteering go-karts.

"All Wheel Drive Does Not Make You Safer":
[http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-
do...](http://www.mrmoneymustache.com/2014/12/01/all-wheel-drive-does-not-
make-you-safer/)

"The Myth of the All-Powerful All-Wheel Drive:"
[http://www.popularmechanics.com/cars/a3091/the-myth-of-
the-a...](http://www.popularmechanics.com/cars/a3091/the-myth-of-the-all-
powerful-all-wheel-drive-15202862/)

4WD/AWD safety - Is 4WD/AWD safer on snow and ice?
[http://www.4x4abc.com/jeep101/safe.html](http://www.4x4abc.com/jeep101/safe.html)

~~~
elliotec
This is absolutely false.

I live in Utah where we regularly get snow measuring feet deep. Driving up the
canyon to go skiing, or doing anything in the state during winter or in all
the wilderness, it is required by law to have 4WD.

All over the west there are roads that are impassable without 4WD. I use it
all the time.

~~~
wil421
The parent and other people commenting about trashing 4WD systems are from
Europe. I'm not sure if there are as many remote places in Europe anymore
especially compared to Utah.

I go trout fishing a lot in North Georgia and many places I go would be
accessible in a small fwd car. In fact if people saw you driving an incapable
vehicle they would stop and warm you about proceeding further.

~~~
lb1lf
Try Norway for size. I live in a rural area and the everyday car is a Land
Cruiser - in summer, just about anything goes - but come winter, and big,
studded tyres, low range and diff locks all over (well, center and rear in my
case) are what gets you from A to B and back.

Anecdotal evidence makes me claim that electronic traction control systems are
excellent, but still no match for good ol' manual systems - probably in part
because the automagic systems are easily taken for granted, making drivers
forget what icy conditions are like - and using the electronic systems to
drive faster, not safer.

~~~
sliverstorm
Electronic systems also work best on the road. The looser conditions get, the
worse they often perform.

------
tzs
The problem of the outer wheel going farther during a turn also occurs with
trains, and trains do not have differentials to let the wheels have different
angular velocity. On a train, the inner and outer wheels have the same angular
velocity regardless of turns.

How trains accomplish this is interesting: [http://jalopnik.com/5820296/how-
trains-make-turns-without-di...](http://jalopnik.com/5820296/how-trains-make-
turns-without-differentials)

I'm linking to Jalopnik, which embeds Richard Feynman's video on train
turning, rather than directly to the video because there are some interesting
comments on the site. In particular, see the comment from railroad employee
"Gonemad" about how they have a system that automatically greases the wheels
before turns, which seems rather counterintuitive at first.

------
examancer
Kind of an odd post to see on hacker news. As somewhat of a car buff my take
away is that this article gives an adequate high level explanation but leaves
out a lot of useful details, especially to anyone evaluating 4WD/AWD vehicles,
like the tremendous differences between modern AWD systems.

------
ebbv
One thing that this article only touches on briefly is that more and more AWD
systems today are just open differentials plus the TCS instead of using more
expensive and heavier limited slip differentials. These modern systems are
lighter and cheaper but they have disadvantages; not least of which is that
using the brakes to force power to move around wastes torque, and it's only
going to be as good as the sensors. If the ABS/TCS system has any issues your
AWD goes out too.

I've owned cars with every type of AWD system (Audi, Subaru and currently a
Honda) and day to day they all work just as well. Given that, I suppose the
manufacturers are ultimately right to go with the cheaper and weight saving
system.

~~~
dsfyu404ed
The brake based system is great if the traction control is programmed for "get
me up this snowbank" and not "prevent me from being an idiot"

There was an episode of Dirt Every Day where Fred and co. took a Jeep
Renegade(?), or whichever the small one is, off-roading there were some great
scenes of the traction control in action. Basically by spinning the dial to
rocks the traction control knows to use ABS to approximate how one would work
the brakes with a torsen.

------
repler
The newer Jeeps actually have both! It's an improvement over AWD and they call
it "Active Drive".

It engages/disengages the center differential (4WD) as needed automatically,
but can also transfer power to be greater in the front or rear as desired -
with presets for 60/40 and 40/60 rear/front power. (Sport mode, Mud/Snow)

It's also got traction control w/ ABS, and it uses the brake locking
differential technique on each axle as the article describes in advanced AWD
systems.

Of course they have different variations, one of which also has locking
differentials on top of everything else.

It's amazing to drive. It's all fully automatic too.

------
userbinator
No mention of the "Detroit Locker", IMHO one of the more clever automatic-
locking differential designs --- it uses a pair of dog clutches which are
normally engaged, one for each side, and the wheel that spins faster is
allowed to. It's self-contained and requires no additional controls, just like
a regular differential. I think the only reasons why they didn't become more
popular are due to their cost and the noise they make --- a very noticeable
clunking on tight turns as the outer wheel tries to go faster and unlocks,
then locks again as the vehicle straightens out.

[http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen...](http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differentials/detroit-
locker/index.htm)

Having driven a "4WD" with all _three_ differentials of this type, I now
wonder if it would be classified as 4WD or AWD.

~~~
linsomniac
Detroit locker is effective, but I wouldn't really call it clever. For that I
think of Torsen, the Torque Sensing differential that uses planetary gears.
The problem with the Detroit is primarily noise, as you mention. The problem
with Torsen is that it doesn't work in the presence of near 0 traction on one
of the outputs, so people will drag the hand brake in that case.

------
ynniv
Of course, this only matters when you are driving four wheels with one engine.
Now that we're going electric you can have one engine per wheel, or one front
engine and one back engine and use ABS across single axles. It's kind of
amazing how much progress we have made with simple internal combustion
engines.

------
linsomniac
I don't really buy this article. The terms AWD and 4WD don't really define a
set of functions, but the article says they do then waves hands at all the
exceptions.

Really, from what I've seen, current usage is AWD is a 4WD system marketed for
on road use, and 4WD is an AWD system targeted to off road use. The biggest
indicator seems to be if a vehicle has a low range transfer case, it will
likely be marketed as 4WD. Also, if it is marketed as 4WD, _HOPEFULLY_ it will
have under body armor.

Remember, not all AWD/4WD systems are equal. Last winter our neighbor's Subaru
Impreza with Symmetric AWD (which Subaru claims is super good) couldn't make
it up a small incline by our houses. Both my Audi and Land Cruiser on all
season tires had no problem.

~~~
djsumdog
I owned a WRX that I never got stuck in snow. I once came up to a hill with a
Sabb and Civic both spinning; and a semi-truck that couldn't really move when
they were in the way. I had to wait for both of them to give up, left off my
clutch, and crawled up the hill.

------
kinofcain
There's a lot of blurring and different labeling of 4WD systems based on the
market segment.

Generally it's more helpful to think of 4WD/AWD as fitting into three
categories:

1: A system with a _transfer case_ that predominantly drives one axle (almost
always the rear, and almost always labeled "4WD"). This is your typical pickup
truck 4wd system. When it is engaged, the two axles are locked together and
may as well be welded and they'll turn at the same speed.

2: A system with a _center differential_ , in which both axles are driven but
may turn at different speeds (Almost always labeled AWD). The differential may
allow full or progressive locking and/or torque vectoring. It's rare to find
one of these that can fully lock (Subaru WRX STI is the only one I can think
of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or
that can partially lock in response to slipping of one axle (often with the
brand name Torsen). These are almost always found on passenger cars, and with
few exceptions they're found on cars with longitudinally mounted engines
(Mitsubishi EVO being the only transverse layout car with a center diff I can
think of), that means they're usually found on higher end cars that are based
on front-engine rear-wheel drive models.

3: A system with a _power-take-off unit_ (often brand name Haldex), in which
one axle is driven and a clutch pack can partially or fully engage the other
axle. (this is often labeled AWD when put on a passenger car, and 4WD when
placed on a "truck") The axles can operate at different speeds but can also
fully lock in the way a transfer case can. Typically you find these on "AWD"
cars that are based on transverse-mounted front-drive models as it's easier to
add such a system and get partial engagement of the rear axle, but you'll also
find them on performance AWD applications based on rear-drive mid/rear-engine
models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled
as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford
Raptor. These tend to operate more like an automatic transfer case, as the
only way to partially engage the secondary axle is to slip the clutch, which
you don't want to do 100% of the time. (Ford actually calls this system a
transfer case on the Raptor).

You'll see all sorts of spurious claims about systems "sending 100% of the
torque to the rear axle" (often just means that the system can fully lock, and
100% torque to the rears means the fronts are in the air). You'll also see
things like "rear biased", which sometimes means you have a center
differential that overdrives the rear axle by default, which is great, but is
often misleadingly applied to PTO/Haldex systems where the rears are also
overdriven, but only during the rare times when the system is engaged.

Basically if you're looking for traction in low speed settings where you might
not have grip at all on one axle, like offroading or being stuck in the snow,
you want as many of your axles locked as possible, which technically you can
get with any of these systems, but is usually seen on transfer cases or
haldex/PTO systems.

If you're driving quickly on pavement with some traction, you usually want
something more like a center differential, preferably one that can partially
or entirely lock when you lose some grip at one end.

~~~
rconti
Right. I've got a Golf R with Haldex; it allows 30+mpg under cruising
conditions but can "lock 100%" in the sense that the fronts and rears are
locked together. It's created a lot of forum wars because some people read the
marketing materials saying "100% of torque to the rear wheels" or "max 50/50
split" and don't understand the difference. If it's fully locked, on dry
pavement, of course you're getting 50/50\. If the front wheels are on a magic
0 friction surface, of course, all of your torque is going to the rears
because the fronts have nothing to grip on, but they're all spinning at the
same speed (theoretically).

~~~
kinofcain
Great car, and yeah, it's even more fun when you explain that the Haldex
systems often pull off either the left or right half shaft...

------
mschuster91
I wonder why car manufacturers still use direct force transmission with hugely
complex (and in case of ignoring the instructions, outright dangerous) like
4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on
locomotives.

A computer could instantly shift torque with both of the hybrid solutions, you
wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-
drive including the trailer in trucks. And a fuel-electric drivetrain could
easily plug in a battery or supercaps for braking energy recuperation, and the
fuel engine could always run at peak efficiency/exhaust gas condition...

~~~
samwillis
The Jaguar C-X75 [1] concept car uses exactly this idea, it uses a tiny and
very efficient jet engine that is charging the batteries. Very neat idea.

1:
[https://en.wikipedia.org/wiki/Jaguar_C-X75](https://en.wikipedia.org/wiki/Jaguar_C-X75)

~~~
mschuster91
What the f..k. Thanks for that link, a jet engine?! That's crazy. But it's the
"awesome" kind of crazy.

And the CO2 values... wow, how is that possible? Burning a defined amount of
fuel should always produce the same amount of CO2?

~~~
dredmorbius
Gas turbines operate efficiently at constant output, which a hybrid-drive
system with storage can manage. They only need to kick out enough power, on
average, to keep the storage system fully charged. Batteries and eletric
motors can handle both high-demand accelleration, and regenerative braking.
Since the turbines are running at a very constant range, and can be rated to
average out power load inclusive of idle times, they can be designed for
optimum efficiency within that range. Or that's the theory.

There've been other gas-turbine automobiles, including some prototypes built
in the 1960s. A friend of mine test drove one, claimed it could lay a patch
(spin the tires) at highway speeds. Though my understanding is that in direct-
drive applications (such as that), turbine lag is an issue.

The exhaust also runs quite hot, and turbines typically emit a lot of NOx
(nitrous oxides), what you get when you run atmospheric nitrogen through a
high-temperature field.

Much anecdata here, apply salt liberally.

~~~
mikeash
For anyone interested in turbines and cars, there's a delightful episode of
Jay Leno's Garage about a Chrysler turbine car:

[https://www.youtube.com/watch?v=b2A5ijU3Ivs](https://www.youtube.com/watch?v=b2A5ijU3Ivs)

He talks extensively about how it works, what it's like to drive, and the
various difficulties that resulted in it never entering mass production.

~~~
fnj
It sounded like a gigantic loud vacuum cleaner even just idling, and gave the
impression of straining mightily to achieve even mild acceleration. Fuel
consumption was TERRIBLE.

------
lsllc
Alright, time to bust a myth. As an AWD owner, every time I have a tire issue,
they tell me I need to replace all 4 even if the others are good ... because
AWD. "Won't someone think of the tires ... the tires need to to be the same
size so they can rotate at the same rate."

Clearly, with AWD the car is designed to allow wheels to rotate at different
rates (hence the diffs). Otherwise you would't be able to make a turn. Once
you dispel that myth, they then tell you to replace the pair ...

Why do dealerships, repair shops and tire places keep insisting on replacing
all 4? ... good for tire sales I guess.

~~~
mariusc23
The reasoning I've heard is that your differential will get worn out.

~~~
lsllc
That makes no sense to me. Unless you are driving in straight lines, the 4
wheels will always be turning at different rates (and consequently, the front
and back axles will be rotating at different rates).

------
ledriveby
Subaru's higher end AWD system has separate output shafts from the engine for
the front and the rear, with electronically-controlled clutches modifying the
torque. Default is 60/40 front biased, but it can send 100% of the torque to
either end of the car. Excellent performance with no manual mode switching
needed, also manages good fuel economy at the same time. Best system in the
business.

~~~
bluedino
However, if you don't have locking differentials you can still get into the
situation where one wheel slips and the wheel that grips doesn't get any power
- as shown by the Subaru getting stuck on the hill in this video:
[https://youtu.be/hrdb_UVTa20?t=543](https://youtu.be/hrdb_UVTa20?t=543)

~~~
post_break
There is a trick that seems idiotic but if you have a mechanical hand brake
you can stop the wheel from free spinning. I'm not sure how well it would work
for a front wheel, maybe you could hold the brake slightly while on the
throttle. Again, seems ass backwards but if you think about what you're doing
you're basically "locking" the open diff.

------
slr555
All this talk of traction and moving forward is great but one of the most
important things to remember about 4WD or AWD especially in snow and ice is
that is doesn't enhance your ability to brake. It is easy to feel fearless
being able to accelerate on a slippery road but a bummer to discover you can't
stop (yes, I get that engine braking plays a role...).

~~~
lathiat
which is precisely why tyre choice is more or less equally important on your
little buzz box as it is your high end sports car.

------
kilovoltaire
Wow, early cars connected the engine to only one wheel, to completely avoid
issues of differential.

(According to the cool old video linked in the article.)

~~~
mod
My cousin had a firebird, I think a '67, that (seemingly?) had 1wd. He could
do donuts that left one trail.

Not sure if by design or some kind of failure...

------
pkulak
And then there are the dual motor cars like Teslas and Toyotas that have no
mechanical link between front and rear axle at all. As you'd imagine, this can
allow for a heck of a lot of flexibility.

------
esaym
You don't need 4 tires to get traction. Just need a 2 cylinder John Deere!
[https://youtu.be/sZBjNKhcWtY](https://youtu.be/sZBjNKhcWtY)

------
outworlder
Can't we do away with all the complexity by just having 4 electric motors and
a gas engine as a generator? Then each wheel can be computer controlled
independently.

------
Avernar
In my Jeep Wrangler, I can instantly tell I forgot to switch from 4WD to 2WD
as I turn onto pavement from a dirt road.

------
dghughes
>Torque goes to whichever of the four wheels has the least grip.

That should be the tire with the most grip. If torque goes to the wheel where
the tire on it has the least grip the tire still has no traction torque
doesn't make grip appear out of nowhere.

In four-wheel or all-wheel drive one wheel where a tire has no grip the other
three wheel with tire that have grip move the vehicle isn't that the entire
point of 4WD and AWD?

~~~
johngalt
Mechanically a drivetrain has no means of differentiating between a tire that
is slipping vs a tire at that's on the outside of a turn. A 4wd car with three
open differentials will absolutely send all of the engine power to one
slipping wheel.

The author is describing the tricks used to get around that fact. Namely
locking differentials or limited slip differentials. The end result is a
fraction of the torque makes it to other Wheels depending on the design of the
drivetrain. The author spent a lot of time on locking differentials when those
are rare. Limited slip differentials are more common. Most awd cars will have
a LSD in the center diff.

There are torsen diffs which do what you describe but they are rare.

~~~
kinofcain
Even a torsen diff, or similar helical/gear type limited slip diffs are
"torque multiplying", they need a load on one wheel in order to send the
"multiplied" load to the wheel with traction. Modern stability/traction
systems equipped with helical diffs will apply the brake to the "free" wheel
in order to provide a load to the mechanical diff. You can also apply the
brakes yourself if you have a torsen diff but no electronics.

A clutch-based diff or some other type of locking diff would otherwise be
required to solve the "wheel in the air" problem without applying an
artificial load to the free wheel.

~~~
lathiat
There is at least one mechanical design which (claims to?) solve this problem,
Wavetrac.

Don't want to embarrass myself trying to explain it, so click "how it works"
on this page:
[http://www.wavetrac.net/technical.htm](http://www.wavetrac.net/technical.htm)

------
libso
AWD should be redefined as Automated Wheel Drive or Auto all Wheel Drive.

------
rascul
This article falsely assumes certain differential setups based on 4WD or AWD.

------
dan_m2k
Great explanation

------
spacerace
Jesus what a SHIT article It gets so many things wrong I don't even know where
to start.

Why is this garbage spam on Hacker News?

~~~
UnoriginalGuy
I always find it suspect when people claim that something is so bad that they
cannot find a single concrete thing to criticise it on.

I'd suggest you start with the thing you believe they got "most wrong" and go
from there. Otherwise this comment seems to lack substance.

~~~
spacerace
Oh there's an absurd amount of concrete things to criticize about this
article.

I just don't see why I should be giving free editorial services and in effect
encouraging what I consider to be blog spam infecting a site I love.

~~~
UnoriginalGuy
Then your comment isn't constructive.

This is a discussion site, meaning we discuss things, telling us a potential
problem with the source is a great discussion topic. Taking unqualified
potshots at the article doesn't provide a jump-off point to any discuss
worthwhile.

