My apologies if this has been done before recently – I searched back in the forums and didn’t see it, but I might have missed it.
What’s the difference?
4WD is a vehicle that is normally driven with power to two wheels. The driver manually engages the other two wheels when it is needed.
AWD is power to all four wheels all the time.
Yep, and AWD vehicles don’t have the option of a Low Range (super low gearing for off road work).
There are some differences, but the lines are really blurry. There are AWD vehicals with low range, but may not be available in your area (i.e. Subarus have a low range in their Australia divisions, and that is unquestionably AWD).
Another way to look at it is AWD is a all purpose vehicle for most roads, 4WD is for the really bad roads, but again that’s another generalization with it’s exceptions.
Not really. My GMC Envoy has 2WD, AWD, 4WD and 4WD low. You can switch between them at any time.
But I’ll guess that AWD is OK to use on dry roads, and any of the 4WD options are not.
Most (All?, Some? Many?) AWD systems drive primarly two wheels and switch power as needed to the others. For example the Audi, Volvo and Landrover all use a Haldex coupling that under normal conditions puts 95% of the power to the front wheels. If slippage of the front wheels occurs, the system can change from 95F/5R to 5F/95R in an instant.
kanicbird nailed it, the lines are very blurred.
I think the best way to put it is they are marketing terms more then the technology behind them.
AWD is used for a vehicle when they want to portray a image of:
convenience
limited off road use
Excellent bad condition traction
and in some cases better performance
4WD when they want to present a image of:
true off road capabilities
Ruggedness
ultimate drive it anywhere
control
4wd is mostly reserved for use describing a system which should only be used in slippery conditions. 4wd is mostly used to describe systems that are actually dangerous to use on dry surfaces and are a hazzard to themselves when it comes to manuevering, as opposite wheels are going to rotate at the same speed. 4wd is rarely used to describe a safety feature. It’s a traction feature for loose road conditions (think truck climbing gravelly hill).
AWD is usually refered to a full-time or hands free, no-thought/input from driver system. The power is actually alternated with some purpose between axles and/or wheels. These systems are true safety features, and can provide balanced feel on dry roads. Wheels can operate indepently, etc.
If a company insists on calling all the systems ‘4wd’, they will distinguish their better type as "real time’ or ‘full time’ 4wd.
Basic 4wd is incredibly limited and actually a danger at times.
Seek AWD, Full time 4wd or Real Time 4wd. All refer to some type of active safety feature capable of enhanced handling and dynamic traction to all four wheels. These are systems that are light years ahead of 4wd.
4wd has merits off road, or if on-road conditions detroiate to the point that it might as well be off road.
The most general difference is that locking hub or shift-on-the-fly 4WD vehicles are almost invariably intended to be driven in this mode only off-pavement or on slick pavement, whereas AWD vehicles are intended to be driven on dry or slick pavement, but not for heavy off-road applications. This is because most 4WD vehicles lack a center differential which permits the front and rear drive axles to rotate at different speeds, which is imperative when cornering at high (<30 mph) speeds on dry pavement because the wheels move at differnt rates. This is good for a true offroad vehicle, because it means that even one set of axles has no traction and is free spinning, the other will still get torque from the engine. With a nonlocking diff, the spinning wheel(s) will take all the torque and leave the other wheels motionless.
Note that all AWD systems, even from the same manufacturer, are not equivilent in either function or behavior. Most modern AWD vehicles utilize a limited slip or autolocking differential which uses various mechanisms (viscous coupling, computer controlled clutches, traction control via brakes, et cetera) to distribute at least some torque to the wheels that have traction; some systems work better than others. Audi has marketed at least for essentially independent systems under their Quattro label, and Subaru has three different systems (viscous clutch, computer controlled lockup on planetary diff, multi clutch pack) under their Symmetrical All-Wheel Drive label.
While most commuter automobiles with AWD are not off-road vehicles (despite their SUV-like or crossover appearance) it’s not true that all AWD-equipped cars are not capable of off-road use, and indeed many of the more advanced AWD SUVs like the Toyota Land Cruiser and the and the Land Rover Defender (not the cheaper, on-road-only Discovery) are exemplary off-road vehicles. For off-road use, you generally need an extra low range gearing shift in the transfer case coupled to an engine that develops a lot of low end torque for doing things like slogging through a heavily potted trail or crawling over rocks. Commuter vehicles don’t need this and generally don’t have the kind of low end performance that can make use of this anyway, being optimized for cruising at highway speeds.
So, most 4WD vehicles are part-time, intended to be shifted into this mode only for trail use, and have very torquey engines with a low range transfer gearset. Of AWD vehicles, most are intended for performance (some very high performance) on smooth, sealed pavement at highway cruising speeds, but AWD systems can be designed for off-road applications. I hope that makes it all as clear as the mud splattered on the brush guards of a Series FJ-40 Land Cruiser in the Kenyan lowlands.
Wikipedia has a reasonably good article on the topic which gives a general precis on the different types of AWD systems and vehicles they’re used on.
Stranger
Could someone shed a bit of light on what “Low Range” means and why it’s good for off road driving?
low range refers to a lower gear - usually in the transer case of a 4WD vehicle.
Basically it means the the wheels turn slower at a given engine speed. This results in more torque to the wheels and more controllable power.
On 4x4’s instead of transferring power directly by a drive shaft from the transmission, the transmission connects to a transfer case that that splits the power to two drive shafts. One for the front axle, one for the rear. It is in this transfer case that also allows you to shift between 2wd, 4wd, 4hi and 4 low (range).
Low range essential changes the gear ratio of the vehicle giving it a much lower top speed (say 30mph) but much more power. Since most off road driving is done at slow speeds, this is very useful.
Climbing over stuff.
Pulling others out.
I even use it when I have to back my trailer up my steep gravel drive.
Low range is an extra gearset that gives you a somewhat lower gearing, and thus higher torque. It’s kind of like an extra gearbox between the engine and the main gearbox that multiplies the nominal gear ratios of the latter by some fraction, analogous to the crank ring sprocket on a ten speed bicycle.
You’d use this when you want to go really slow but develop a lot of torque, like when driving down a bouldered trail or driving up a steep incline. When you get in a truck or a jeep and see an extra level (usually near the gearshift, or in the same place a manual gearshift would be) which says “2H-4H-4L” the 4L is the low range transfer. A few cars have also been built with just an extra low gear built into the main gearset, and you normally start on dry pavement in gear 2 (I believe that the vaunted Porsche 959 supercar was set up like this with gear 1 being a “reduction gear” to be used over rough ground) but this adds complexity and size to a standard design, especially with a planetary gear automatic transmission, so it’s generally just easier and offers more flexibility to add on the low range gears to the 4WD transfer system.
Stranger