In addition to the previous answers, AWD and 4WD systems have important technical differences. A 4WD system uses a gearbox to mechanically lock the front and rear driveshafts together, such that they always have to spin at the same speed. AWD systems use a viscous clutch, allowing the driveshafts to turn at different speeds when some tires lose grip, at the cost of some wear on that friction surface.
That’s why replacing the tires evenly is important on AWD, but not necessarily 4WD: different diameter tires spin at different speeds forcing the driveshafts to turn at different speeds all the time, wearing out that clutch prematurely.
Edit for completeness: on a 4WD system with differently-sized tires, the wheels would still be forced to turn at the same speed which means the tires themselves would be where the slip and wear occurs. That’s why 4WD systems aren’t supposed to be used on paved roads: having that slip is no big deal on a loose surface, but doesn’t work well when all four tires have grip. (This also applies when all four tires are the same size, when the differential is locked and you’re making a turn. The wheel on the outside of the turn has to spin faster the inside wheel, but can’t, and so the tires scrub instead.)
In my experience, that’s rare. Most AWD systems that I’ve seen use an open differential with no viscous coupler or locker of any kind. The '94 - '97 Toyota Land Cruiser (w/ ABS) in the US market used a viscous coupler, and Toyota called it “full-time 4wd”. It also locked the transfer case in low range. To my knowledge, this was a really unusual configuration. They discontinued the viscous coupler after that, instead switching to their ATRAC traction management that used individual application of brakes to prevent wheel spin. Most of the Jeep SUVs in the last ~30 years, to my knowledge, used an entirely open center differential in AWD models.
It seems like modern, CUV AWD systems are usually FWD with computer controlled engagement of rear tires.
AWD and 4WD are also marketing terms rather than regulated in any way. In general you’re right, but to be safe, always check what type of transfer case you’re actually dealing with. Some “4WD” cars these days are AWD with super clever electronical controls for off road usage.
I suppose these days there’s also the case where some cars just have electric motors attached to separate [groups of] wheels with no shafts connecting between them at all and don’t really even fit in either of the two previous categories.
The exact meaning of “4WD” seems to depend on manufacturer. Toyota’s 4WD works fine on hard surfaces because the differentials are normally open, which makes it a lot more like AWD. The center differential needs to be locked for it to behave like your described.
Yes but you have no choice other than maybe pulling a fuse. Most AWD systems, at least in budget cars, work like that, though often via a clutch or a viscous coupling or something rather than an electric motor. Premium cars get you torsen and other cool solutions.
Although also AWD vs 4WD are marketing terms and not strictly regulated.
Eh, oversimplification. AWD systems can and do get used offroad pretty often. They’re not quite as good as 4WD in some specific situations, but AWD is still better than 2WD in basically every offroad situation, and it’s good enough to get the job done most of the time, for mild offroading. If you’re doing serious rock crawling or going through very deep soft sand, mud, or snow, 4WD will likely be better than AWD. But for tackling your average trail or poorly maintained dirt road, AWD will do fine.
AWD systems can and do get used offroad pretty often. They’re not quite as good as 4WD in some specific situations
The moment that you have one wheel spinning with AWD, you are stuck. One wheel in the air due to a heavily rutted fire road? Stuck. One wheel on wet ice on the road? Stuck.
AWD is great for transient traction conditions, like roads that are partially snowy with patches of exposed asphalt.
If you have an AWD system with fully open differentials all the way around, yes. But many of them (most of the modern ones?) are more advanced than that, with a limited slip differential, at least for the center diff. And pretty much all the modern ones, as far as I know of, can do tricks like automatically applying brakes on the slipping wheel to direct more torque to the other three.
A decent AWD system definitely will get you through more stuff than simple 2WD. Not as good as true 4WD in some situations, but AWD can still be very useful offroad.
Iirc, all wheel drive means it’s always all 4 wheels, while 4 wheel drive is when an off-road vehicle has the ability to switch between 4 wheel drive and the normal 2 wheel drive
Why is it all wheel drive and not four wheel drive? Offtopic but I’ve never thought about it before
In addition to the previous answers, AWD and 4WD systems have important technical differences. A 4WD system uses a gearbox to mechanically lock the front and rear driveshafts together, such that they always have to spin at the same speed. AWD systems use a viscous clutch, allowing the driveshafts to turn at different speeds when some tires lose grip, at the cost of some wear on that friction surface.
That’s why replacing the tires evenly is important on AWD, but not necessarily 4WD: different diameter tires spin at different speeds forcing the driveshafts to turn at different speeds all the time, wearing out that clutch prematurely.
Edit for completeness: on a 4WD system with differently-sized tires, the wheels would still be forced to turn at the same speed which means the tires themselves would be where the slip and wear occurs. That’s why 4WD systems aren’t supposed to be used on paved roads: having that slip is no big deal on a loose surface, but doesn’t work well when all four tires have grip. (This also applies when all four tires are the same size, when the differential is locked and you’re making a turn. The wheel on the outside of the turn has to spin faster the inside wheel, but can’t, and so the tires scrub instead.)
In my experience, that’s rare. Most AWD systems that I’ve seen use an open differential with no viscous coupler or locker of any kind. The '94 - '97 Toyota Land Cruiser (w/ ABS) in the US market used a viscous coupler, and Toyota called it “full-time 4wd”. It also locked the transfer case in low range. To my knowledge, this was a really unusual configuration. They discontinued the viscous coupler after that, instead switching to their ATRAC traction management that used individual application of brakes to prevent wheel spin. Most of the Jeep SUVs in the last ~30 years, to my knowledge, used an entirely open center differential in AWD models.
It seems like modern, CUV AWD systems are usually FWD with computer controlled engagement of rear tires.
AWD and 4WD are also marketing terms rather than regulated in any way. In general you’re right, but to be safe, always check what type of transfer case you’re actually dealing with. Some “4WD” cars these days are AWD with super clever electronical controls for off road usage.
I suppose these days there’s also the case where some cars just have electric motors attached to separate [groups of] wheels with no shafts connecting between them at all and don’t really even fit in either of the two previous categories.
The exact meaning of “4WD” seems to depend on manufacturer. Toyota’s 4WD works fine on hard surfaces because the differentials are normally open, which makes it a lot more like AWD. The center differential needs to be locked for it to behave like your described.
4 wheel drive is selective meaning you can run in 2wd. AWD is you have no choice its always on.
Or in my case off until needed.
Car’s a hybrid so runs in FWD until it thinks you need the rear electric motors to keep you on the road then it’s AWD.
Yes but you have no choice other than maybe pulling a fuse. Most AWD systems, at least in budget cars, work like that, though often via a clutch or a viscous coupling or something rather than an electric motor. Premium cars get you torsen and other cool solutions.
Although also AWD vs 4WD are marketing terms and not strictly regulated.
4 wheel drive is to drive offroad, AWD is to keep you on the road.
Eh, oversimplification. AWD systems can and do get used offroad pretty often. They’re not quite as good as 4WD in some specific situations, but AWD is still better than 2WD in basically every offroad situation, and it’s good enough to get the job done most of the time, for mild offroading. If you’re doing serious rock crawling or going through very deep soft sand, mud, or snow, 4WD will likely be better than AWD. But for tackling your average trail or poorly maintained dirt road, AWD will do fine.
The moment that you have one wheel spinning with AWD, you are stuck. One wheel in the air due to a heavily rutted fire road? Stuck. One wheel on wet ice on the road? Stuck.
AWD is great for transient traction conditions, like roads that are partially snowy with patches of exposed asphalt.
If you have an AWD system with fully open differentials all the way around, yes. But many of them (most of the modern ones?) are more advanced than that, with a limited slip differential, at least for the center diff. And pretty much all the modern ones, as far as I know of, can do tricks like automatically applying brakes on the slipping wheel to direct more torque to the other three.
A decent AWD system definitely will get you through more stuff than simple 2WD. Not as good as true 4WD in some situations, but AWD can still be very useful offroad.
Iirc, all wheel drive means it’s always all 4 wheels, while 4 wheel drive is when an off-road vehicle has the ability to switch between 4 wheel drive and the normal 2 wheel drive