4WD vs. AWD (and stuff about differentials)

January 31, 2017

I find myself answering a lot of questions like ‘what is 4WD’ and ‘how are 4WD and AWD different’ to friends/family/coworkers (sometimes even strangers) – so I thought it pertinent to dive a little deeper and write about this subject.

 

 

I’ll also start by saying there are quite a lot of complicating and confusing factors in this discussion – so it’s not surprising that these kind of questions continually arise, even among people who consider themselves very knowledgeable and experienced vehicle operators.  Afterall, the common car/truck has four wheels – so if all of them are driven, isn’t is four-wheel-drive?  Semantics would require the answer to be yes – but practically speaking, there is a very real difference between the two systems.  Unfortunately though, there is no uniform standard among automobile manufacturers that requires them to use specific terminology when marketing their products.  There also are systems that combine features of both AWD and 4WD – so it absolutely makes sense that confusion abounds.

 

That said, I’m not purporting to be a complete expert here – I am not a professional engineer and I will happily engage anyone who thinks I’m mistaken, or just wants to discuss further.  However, I am someone who has built robots and vehicles (out of legos – including: manual transmissions, automatic transmissions, automatic locking differentials, selectable locking differentials, planetary gearsets, etc.) and have studied drive-train technologies as a hobby – so I know a few things about this subject.

 

First and foremost, in this discussion it is important to understand what a differential is and how they work.  There are lots and lots of articles and videos about this – but one of the best and simplest I have seen is from 1937 and was put forth the General Motors Corporation:

 

https://www.youtube.com/watch?v=yYAw79386WI (start paying attention around the 2:00 mark and through the 8:00 mark or so)

 

In short, a differential takes rotational power from one input and sends it to two outputs, dynamically.

 

FWD or RWD (front wheel drive/rear wheel drive) vehicles have one differential; it exists in between the driven wheels (either the front or rear) and receives power from the engine.  Other types of vehicles (classic ‘part-time’ 4WDs) have two differentials – one in the  front axle and one in the rear axle; usually the rear is always sent power from the engine and the driver can engage/disengage the front to the same power source.  AWD vehicles have three differentials – the same as a classic 4×4 setup, plus a center differential that divides the front and rear axles, and is where engine power is first sent.

 

That is the bare-bones difference between a 4WD and an AWD vehicle – the existence or absence of a center differential.

 

However, if it really were that simple, I’d be writing about something else…

 

Let’s back up for a second and think about what a differential accomplishes and why it’s necessary.  As the video pointed out, it allows two wheels to spin at different speeds; it mechanically sends rotational power where there is less resistance, and that action can be dynamic based on the resistance changing.  An example of this being necessary is when a vehicle turns a corner; in order for both wheels to maintain traction and not skid or slide, the differential takes the engine power (input) and divides it dynamically between the left and right wheels (outputs) and that allows the outside wheel to spin faster than the inside wheel because there is ‘less resistance’ at the outer wheel.  The same thing happens in a slightly less-pronounced way in between the front and rear.  An easy way to measure this is to get your vehicle on a sandy surface; mark a starting point and then drive slowly forward for a short bit, then make a turn and then stop.  Get out and go measure the length of the four tire tracks left by each wheel from the starting point to the stopping point.  All four wheels will have traveled a different distance (this will be even more noticeable if the arc of your turn changes throughout the turn – e.g. you start turning with the steering at about 15% and then by the end of the turn the steering is about 25-30%, for example).  This difference in distance traveled between the front and rear is what the center differential allows.

 

 

So, in an AWD vehicle power can be dynamically sent to the front and rear as well as dynamically to the left and right; in a 4WD vehicle, power is equally sent to the front and rear and then dynamically sent to the left and right.

 

Now, it would be logical to think that AWD is superior because it can be more dynamic and responsive; if surfaces on which we drove were always the same, that would be true.  In fact, it is true (that AWD is superior) when driving on a dry, wet or icy road.  This is the case because the road surface is generally the same for each of the four wheels.

 

In practice however, not all road surfaces are the same; in off-road situations 4WD is most certainly superior.  Remember to consider the surface on which the vehicle drives; when off-road, the surface is often low-traction (dirt/sand/mud etc.) and at the same time, usually uneven (rocks/ruts/ditches).  It is this uneven nature that off-road surfaces usually exhibit which makes 4WD superior.  It is also easier to get ‘unstuck’ in a 4WD vehicle.  All of this said, it’s also important to note these are the exact same reasons why it’s not a good idea to drive in 4WD on surfaces where there is good, constant traction (e.g. a dry road) – because the lack of a center differential would mean a lot of extra stress on other components of the drive-train (driveshafts, transmission, etc.) and stress is no good.

 

Time for a thought experiment!  Imagine an AWD vehicle in a position where three wheels are on high-traction surfaces and one (say, the left-rear wheel) is on a very low-traction surface (ice patch, ditch, etc.).  Without momentum to assist, if you tried to drive anywhere the three differentials would work together and send power to the wheel(s) with the least resistance; in this instance, the left-rear wheel (which is on an ice patch or in the ditch) would have MUCH less resistance than the other three wheels – and thus all the power would be sent there, it would spin but you’d go nowhere – and be stuck.  If you were in a 4WD vehicle, the left-rear wheel would still slip and spin and take all of the power being sent to the rear (so the right-rear wheel would be still) but the front would still be getting power (since there is no center differential) and since those wheels have traction – they would pull the vehicle out and you’d be on your way.

That thought experiment leads the next point about combined AWD/4WD systems (a.k.a. ‘full-time’ 4WD) and ‘lockable’ differentials.

 

If we take the same situation as before (stuck vehicle with the left-rear wheel on an ice patch) but add another patch of ice underneath the right-front wheel, having 4WD will not make a difference – we’d still be stuck.  Because now, one of the front wheels AND one of the rear wheels has little/no traction – so all of the power being sent to both the front and rear will then be sent to the wheel at that end with no traction.  We either need a tow truck to pull us out, or the ability to ‘lock’ one of the differentials.  Locking the differential can occur in a number of ways (automatic, selectable, via clutches, via pins, via brakes…the list really gets quite long) but essentially it forces the unit to send equal power to the left and right outputs.  If our stuck vehicle has ‘lockers’ then we’d be able to free ourselves and be on our way quite easily.

 

A system where a center differential exists, but can be locked (thus forcing equal power be sent to the front and rear) is essentially a combined AWD and 4WD system.

 

Now, locking differentials can be taken a step further.  One of the most common features that are either highly-desirable as a ‘factory option’ or as a common aftermarket modification, are rear and/or front differential lockers.  This can really help in off-road situations where traction and wheel resistance get tested at their extremes.

 

One more thought experiment: imagine you are driving off-road, and there is a large rock in front of you, roughly the size of one of your wheels.  It’s too big to fit underneath and between the wheels, so you need to literally point one of your wheels at it, and drive over.  Once your wheel (say, the left-front) comes in contact with that rock, resistance is going to skyrocket; it literally has to climb over and help lift part of the vehicle along the way.  If you had a regular ‘open’ differential in the front axle, this would be difficult because at the instant your left-front wheel contacts the rock, the differential would start sending more power to the right-front wheel since there is less resistance there; the wheel that has to do a lot more work (the left-front) would in actuality be having power taken away from it and sent to the right-front wheel…that is not climbing anything.  This is an instance where having a front locking differential would come in very handy; with the front differential locked, when your left-front wheel contacts the rock it would continue to be sent equal power and climb over the rock quite easily.  In this scenario, the differential action would be a limitation, and temporarily eliminating that action is what needs to happen in order to proceed most seamlessly.

 

So – now that you hopefully have a better understanding of the differences between AWD and 4WD systems, perhaps you’re wondering what vehicles fall into which categories, and what is good to consider when buying a new or used vehicle that can take you to cool places.  Quite an understandable question – but honestly, the answer is going to depend on too many things to discuss in this entry (though, please stay tuned for future posts and drop a comment with any specific questions!) so I’ll just offer up what I think the two ‘best’ vehicles are – in that they offer the best possible drive-train configuration straight from the factory AND are/were sold in the U.S. market.  Given those two qualifications, it’s quite simple:

 

Jeep Rubicon (2007 – present)
– Classic 4×4 system (no center differential)
– STANDARD front and rear differential lockers

 

Mercedes Gelandewagen (2002 – present)
– AWD system with lockable center differential
– STANDARD front and rear differential lockers

 

Toyota Land Cruiser FZJ80 (1993 – 1997)
– AWD system with lockable center differential
– OPTIONAL front and rear differential lockers

 

Hope you enjoyed reading this post as much as I’ve enjoyed writing it! Stay tuned to see what the Zoverlander team is up to!

 

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