So what's the difference between AWD, 4WD and Full Time 4WD?

> 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 off-road 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.

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).



Sadly these terms are not subject to rationality/mechanics, but to market droids, 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 market droids have messed up the technical classification, and there is nothing one can do about it.