From:                 Self 
To:               RAV4-L@midrange.com
Subject:          Re: AWD, 4WD, full time 4WD
Send reply to:    w.j.markerink@a1.nl
Date sent:        Thu, 20 Mar 1997 22:36:03

I was hoping someone else would step in, but alas....;-))

On 19 Mar 97 at 8:25, Guido Salvucci wrote:

> I have got what I think is a simple question, but
> maybe not....

The question is quite simple indeed, yet the answers get complicated, 
simply because the terminology is spoiled by markedroids, and the 
techno-geeks (me?!? nahh...) start splitting hairs....;-))

> What is the difference between all wheel drive and
> four wheel drive, which one does the rav4 have, and
> what (mechanically) happens when you lock the
> center differential ?

Short, and not without bias:

4WD refers to part time systems, which run in 2WD on pavement, and 
on which you only engage the other axle (mostly front axle, but there are 
exceptions) when extra traction/stability is needed. Most (but not 
all) of these are heavy offroad vehicles with a split gear T-case 
(high/low range). Most (but not all) have a separate frame. Most (but 
not all) have solid axles.

Full Time 4WD [RAV4] are those systems which send torque to all
wheels all the time, without user interference or axle-engaging
automagic systems to achieve that. *Additional* user interference
(center diff lock as the most common [RAV4 5sp manual]) and
*additional* automagic systems (viscous LSD on center diff mostly
[RAV4 auto]) can be present. No real distinction between road-going
vehicles and heavy offroad vehicles; both often have full time 4wd 
(but again only the latter will have a split gear T-case).

AWD (All Wheel Drive) is the least defined category; often it includes 
the full time 4wd category, but I tend to define it more strictly as 
those vehicles that *only* send (a significant amount of) torque to 
the other axle (mostly rear axles, only a few front axles) *after* 
slip occurs. This behaviour is in contrast to full time, where all 
wheels get torque all the time, and at least with a fairly even 
balanced split (50/50% up to 33/66%, but much more bias than that 
makes it rather AWD than full-time. 

(warning: this is excellent flame-bait in a newsgroup like 
rec.autos.4x4....;-))

A down side of some of the more agressive visco AWD systems is that
they tend to bind in tight corners. VW Syncro's are known for this,
yet even the Jeep Grand Cherokee Quadratrac (96+ system) is not free
of this binding. Not good on ice. Other systems, like on some of the
Ford Explorer models, only know two modes: locked or unlocked. Far
from perfect as well.

There are only a few heavy offroad vehicles with this AWD, most are
road going vehicles.

From a technical/driving point of view you would prefer full time
4wd, as this is the system that gives you the most predictable
onroad handling, (corner) stability at high speeds, in cross winds
and when towing a trailer. The shifting AWD systems only act after 
slip has occurred, and only act as long this slip occurs, and you 
might be ages in the ditch already before that happens....8-))

Basic summary: 

(part time) 4WD: passive, on demand

Full Time 4WD: active

AWD: reactive, automatic

Note that he term 4x4 has not been addressed; I just saw a recent
hair split thread that 4WD is considered more work-horse than the
'fancy' 4x4. Does make some sense, as 4WD was put on the very first
basic offroad vehicles, yet 4x4 was used to promote them in a more
recreational way of use. Nice subjective hair split indeed....:-)) 

As to your question what locking the center diff on your RAV4 will 
do: note that this manual lock only comes on the 5sp manual; the auto 
version comes with an additional visco LSD instead. 

Locking this center diff will mean that both front and rear drive
shaft rotate at the same speed, regardless of traction on either
axle. 

This will prevent you getting stuck with one axle (or even one
single wheel) spinning on ice, while the other wheels/axle still
have sufficient grip (the wheel with the least traction determines
your overall traction, and although all wheels get the same amount
of torque, it might not be enough to move forward, either uphill or
through mud/snow).

Willem (I think I just created another FAQ document) Jan










FROM: "Robert Miller" 
SUBJECT: AWD vs. 4WD
DATE: Sun, 23 Jan 2000 23:29:01 GMT
NEWSGROUPS: rec.autos.4x4

What's the difference between all wheel drive and four wheel drive?

Mike











FROM: w.j.markerink@a1.nl (Willem-Jan Markerink)
SUBJECT: Re: AWD vs. 4WD
DATE: Mon, 24 Jan 00 12:56:53 GMT
ORGANIZATION: A1 Internet news-server
NEWSGROUPS: rec.autos.4x4

In article <19Mi4.979$H74.60052@typhoon1.rdc-detw.rr.com>,
   "Robert Miller"  wrote:
>What's the difference between all wheel drive and four wheel drive?
>
>Mike


Sadly these terms are not subject to rationality/mechanics, but to 
markedroids, many of whom have no idea what they are talking about 
(especially at dealer level, when pointing to their own, or worse, 
competitive brands (I still haven't heard the correct explanation about 
the Mercedes ML-series for example, and that was at two main 
dealerships....horrifying crap, even from good mechanics).

However, assuming the manufacturer uses the term, it comes down to 3/4 
categories:

AWD: 
*can* power all 4 wheels, but mostly only does so when, after, and 
as-long-as slip occurs. In my terminology these are called 'reactive 
systems', in contrast to 'active' fulltime.
Refers mostly to lighter vehicles, without low range T-case. Hardly found 
on vehicles that are meant for bush work.

4WD: 
mostly refering to parttime 4wd, as the 'old system', but IMNSHO it refers 
to both parttime and fulltime....pointing at more serious/heavy 4x4's, with 
mostly a low-range T-case (but, since both fulltime and parttime also comes 
on lighter cars without T-case (heck, even the WWII jeeps didn't have a low 
range, only a very low 1st gear), that's not a key element).

Parttime-4WD: 
normally 2wd, can be manually shifted into 4wd.

Fulltime-4WD: 
always 4wd, powering the wheels all the time (unlike most AWD)


As you can see, the markedroids have messed up the technical 
classification, and there is nothing one can do about it.

More ponderings about the difference in behaviour of the various systems 
can be found on my homepage:

http://www.a1.nl/phomepag/markerink/main_4x4.htm

(posted & mailed)

--
Bye,

Willem-Jan Markerink

      The desire to understand
is sometimes far less intelligent than
     the inability to understand


[note: 'a-one' & 'en-el'!]










FROM: dazza@deadspam.com (DaZZa)
SUBJECT: Re: Help understanding Prado 4x4
DATE: 16 Dec 2001 10:27:28 GMT
ORGANIZATION: You gotta be kidding. Organised? Me!
NEWSGROUPS: aus.cars.offroad

Willem-Jan Markerink  mumbled:
>In article ,
>   dazza@deadspam.com (DaZZa) wrote:
>>thesnowbaron  mumbled:
>>>To make it simple!
>>>If you have a central differential you have a permanent four wheel drive
>>>vehicle and there is no need to know how to put it into four wheel drive; it
>>>just will always be in four wheel drive. High range or low range, it will
>>>always be in four wheel drive.
>>>If you like to know more detail contact me.
>
>>Bzzzzt. Thanks for playing anyway.
>>I have a Pajero. It has a central diff. It is *not* a permanent 4WD.
>
>It *is*, at least in the context of 'it can be' (since some Pajero's have 
>the option of 2wd as well, on top of 4wd-high-open, 4wd-low-closed (most 
>Japanese 4x4's lock the center diff in low range; most do this 
>electrically, and can be bypassed, some however have a too mechanical 
>nature for this trick (TLC-90/Prado auto or manual, can't remember, the 
>other one is electric).

"Permanent" implies it can't be turned off. Mine can - I drive in 2WD mode
most of the time {I get better fuel mileage that way} unless the conditions
are crappy, in which case I drive in 4WD with the center diff unlocked.

When I hit the dirt, I lock the center diff and run in 4WD high locked, or
I hit 4WD low with no option to unlock the diff.

DaZZa

-- 
Address in header is spamblocked. ROT13 the following for email replies
			qnmmn@mvc.pbz.nh 









FROM: w.j.markerink@a1.nl (Willem-Jan Markerink)
SUBJECT: Re: Help understanding Prado 4x4
DATE: Sun, 16 Dec 01 11:49:35 GMT
NEWSGROUPS: aus.cars.offroad

In article ,
   dazza@deadspam.com (DaZZa) wrote:
>Willem-Jan Markerink  mumbled:
>>In article ,
>>   dazza@deadspam.com (DaZZa) wrote:
>>>thesnowbaron  mumbled:
>>>>To make it simple!
>>>>If you have a central differential you have a permanent four wheel drive
>>>>vehicle and there is no need to know how to put it into four wheel drive; it
>>>>just will always be in four wheel drive. High range or low range, it will
>>>>always be in four wheel drive.
>>>>If you like to know more detail contact me.
>>>
>>>Bzzzzt. Thanks for playing anyway.
>>>I have a Pajero. It has a central diff. It is *not* a permanent 4WD.
>>
>>It *is*, at least in the context of 'it can be' (since some Pajero's have 
>>the option of 2wd as well, on top of 4wd-high-open, 4wd-low-closed (most 
>>Japanese 4x4's lock the center diff in low range; most do this 
>>electrically, and can be bypassed, some however have a too mechanical 
>>nature for this trick (TLC-90/Prado auto or manual, can't remember, the 
>>other one is electric).
>
>"Permanent" implies it can't be turned off. Mine can - I drive in 2WD mode
>most of the time {I get better fuel mileage that way} unless the conditions
>are crappy, in which case I drive in 4WD with the center diff unlocked.
>
>When I hit the dirt, I lock the center diff and run in 4WD high locked, or
>I hit 4WD low with no option to unlock the diff.

De definition for 'permanent' being 'always switched on' is outdated 
IMNSHO....it used to be the only incarnation of a center diff, since 
neither visco-clutches nor optional 2wd T-cases were around at that time.

The danger with that definition today is that it would also apply to 
visco-clutches, which do *not* fit my stricter definition of 'transferring 
torque to all wheels all the time' (whatever clutch there is in the center, 
it will always only engage the other axle *when*, *after* and _as long as_ 
slip occurs)

And in that context your 2wd-option falls partly outside the definition 
too....;))

And yes, all this *is* theoretical, but just as with the true definitions 
of torque & power, one can only understand reality correctly with some 
solid theory. Otherwise one will be ran over by the very first markedroid 
who simply repeats the blabla-brochures, and doesn't know diddly squat 
about 4wd systems. 

There is a large Mercedes dealership nearby, covering an area of 
500.000-750.000 people, and where they talk such incroyable bullshit about 
4wd, from mechanic to salesman, combined with an attitude & arrogance 
towards their infallibility, that your hair turns grey in an instant.
And I know a guy who organizes their trainings, including 4wd 
systems....there is *no* reason for them to talk this bullshit, but they 
simply do....flabberghasting mindboggling.

F*cking unbelievable, but a reality for the customer.


--
Bye,

Willem-Jan Markerink

      The desire to understand
is sometimes far less intelligent than
     the inability to understand


[note: 'a-one' & 'en-el'!]











From: Jepolla@aol.com
Date: Sun, 25 Apr 2004 11:28:24 EDT
Subject: Definition of Real-Time 4WD
To: w.j.markerink@a1.nl

I understand "part-time" and "full-time", but what is "real-time 4WD?  Is it 
the same as the "on-demand" systems which engage the passive axle only when 
the need arises?

BTW, that was a great explanation you gave in this webpage:
http://www.a1.nl/phomepag/markerink/awd_4wd.htm

Thanks in advance.












From: Willem-Jan Markerink 
To: Jepolla@aol.com
Subject: Re: Definition of Real-Time 4WD
Date: Sun, 25 Apr 2004 21:14:29 +0200

On 25 Apr 2004 at 11:28, Jepolla@aol.com wrote:

> I understand "part-time" and "full-time", but what is "real-time 4WD?  Is it 
> the same as the "on-demand" systems which engage the passive axle only when 
> the need arises?

Yes, that has become the more distinctive name for what sleazy 
markedroids would call 'full-time', in the old days/context ('can be 
left engaged all the time').
The 'real-time' would then also suggest a lightning-fast reaction, 
but alas, it will still only enage 4wd *when*, *after*, and as-long-
as slip occurs....it's a traction aid for light conditions, but not a 
stability aid, nor for heavy conditions (or a fragile surface like 
ice & sand, where traction might break any second (spinning or 
digging)).
 
> BTW, that was a great explanation you gave in this webpage:
> http://www.a1.nl/phomepag/markerink/awd_4wd.htm

Thank you for your kind words Sir (or Milady, if applicable....:)).

--
Bye,

Willem-Jan Markerink

      The desire to understand
is sometimes far less intelligent than
     the inability to understand


[note: 'a-one' & 'en-el'!]

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl