If all the wheels are pushing, what’s the difference?
When the SUV craze first began back in the mid 90’s, nearly every single new SUV offering, from every manufacturer, had four-wheel drive (4WD). As customer preferences have evolved, which they can do very, very quickly. That 4WD option that nearly everything was equipped with has changed to what might be more accurately called all-wheel drive (AWD).
AWD and 4WD seem like they are the same thing because vehicles that are equipped with either have the ability to allow all 4 wheels to propel the vehicle down the road. This is true, they are pretty much the same, but there are some fundamental differences that make them what they are, and in the end, these differences make them different in some important ways. One thing that seems certain is that the term 4WD seems to be applied to things that are more truck like, and AWD seems to be applied to things that are more car like. This is one difference, but this is not a rule. Let’s get into this a little deeper.
The drivetrain is the part, or system of parts, that make up all of the rotating assemblies that come inline after the engine. This includes the transmission, the driveshafts, the axles and axle shafts, and the wheels. All of these parts can be laid out in different ways but every vehicle will usually have all of these drivetrain parts somewhere in the system. The drivetrain and the engine together are usually referred to as the powertrain. Front-wheel-drive (FWD) vehicles have a more compact drivetrain with everything essentially located under the hood, or in or near the engine compartment. Small vehicles usually have this kind of drivetrain layout. Rear-wheel-drive (RWD) vehicles have a different layout that supplies the rotational power to the rear wheels. This has all of the same drivetrain components as a vehicle with FWD except it will also have a driveshaft that will come out of the back of the transmission to send power back to the rear wheels. This kind of setup is always found on pickup trucks, on true sports cars, and on some larger cars.
The trucks, SUVs, and a few cars that are equipped with 4WD have a drivetrain layout that is pretty much like a RWD vehicle but there are a bunch of extra things thrown in. Instead of a driveshaft being attached to the back of the transmission, another box of gears known as the transfer case is there between the transmission and driveshaft. The transfer case is responsible for taking some of the torque that is going to the rear wheels and turning that rotation around to send it to the front wheels.
The transfer case usually has several different modes under which it can operate. Some of the more familiar ones include 2 high, 4 high, 4 low, and in many cases Neutral. 2 high is the mode that you would use when roads are good and extra traction is not required. This is essentially the normal mode of operation on a vehicle equipped with 4WD.
4 high will shift the transfer case to direct power to the front wheels and the rear wheels equally. In 4 high the driveshaft to the front wheels and the driveshaft to the rear wheels will turn with an equal amount of force and speed. The transfer case locks these two shafts together. This is the reason that the wheels of a 4WD vehicle seems to hop or bind when the vehicle makes a sharp turn on pavement or hard ground while in 4 high. This mode provides the high traction necessary to maintain control in snow, mud, or loose dirt.
4 low is like 4 high in that power is directed to the front wheels and the rear wheels equally, except now the torque is multiplied through a final, super low gear reduction contained in the transfer case. This means that the engine will turn very quickly but the wheels will turn very slowly. This torque multiplication is used to sacrifice speed for more of the raw twisting force that is torque. This helps a big heavy truck climb straight up a very steep hill with minimal effort. The trade off is that while the truck can now climb the hill that it couldn’t climb in 4 high, it won’t climb very quickly. 4 low is all about maximum torque along with maximum traction.
The last mode is neutral. This disengages the output of the transmission from the output of the transfer case. This is pretty much what neutral in the transmission does; it’s just another way to do it. Some 4WD vehicles do not have neutral in the transfer case; they only have it in the transmission. When towing a truck or SUV with a tow rope, it may be better to put the transfer case into neutral, than to put the transmission into neutral if the vehicle is equipped with an automatic transmission.
All of these different modes of 4WD are usually selected manually as needed. This is why sometimes 4WD is referred to as part-time 4WD. Many newer vehicles with 4WD will have some auto selection capabilities in the transfer case as well. These vehicles might have a transfer case selection that is referred to as AWD or auto 4WD. In this mode the transfer case uses computer controls to look for slippage between the front and rear wheels. If the computer sees any of this slippage it will automatically shift the transfer case from 2 high, to a mode that is essentially AWD.
So then what exactly makes this auto mode AWD, and not 4 high? When the vehicle is running with AWD, the transfer case is splitting power between the front and rear wheels, but the 2 drive shafts, meaning the front and rear driveshafts, are not locked together in the transfer case. They are allowed to turn at different speeds if needed.
Any transfer case capable of running in this kind of AWD mode contains some kind of differential assembly. This is the set of gears that allows the front and read driveshafts to turn at different speeds while both accepting torque from the transmission. The differential allows a rotational speed differentiation to occur when needed. The time that this is needed is usually while the vehicle is turning. Remember the SUV in 4 high, and how the wheels bind and skip when turning on dry pavement? This is because there is nothing in the transfer case to allow one driveshaft to turn at a different speed from the other while the vehicle is going around a corner. Each drive axle on the vehicle will also have differential located between each axle shaft that turn each wheel. This allows the outside wheel to turn faster when going around a corner than the inside wheel.
Because of this center differential in the transfer case, AWD is not as effective in extremely low traction situations as 4 high. This is because if the front or rear axle has no traction whatsoever, and the wheels are slipping, the differential will actually take all of the torque from the wheels or axle that has traction. The slipping wheels literally take all the power from the non-slipping wheels and the vehicle quits moving. This happens because of the way the differential works. It must allow for faster rotation of one wheel or axle if that’s what that wheel or axle what’s to do. On normal dry pavement the wheel spinning faster is allowed to do so in order to go smoothly around corners, so in low traction situations the wheel that wants to spin faster (the one with no traction that is spinning on the ice) is allowed to spin as fast as it can.
Many AWD and 4WD vehicles will have some kind of mechanical or electromechanical device that will prevent this from getting out of control to the point where you are totally stuck. If your vehicle doesn’t have this kind of torque limiting device then it is very frustrating to get stuck and see just one wheel spinning, even if you have AWD. Some of the electronic control systems found on some AWD vehicles are very advanced. The famous Subaru AWD system or the Audi Quattro AWD system watch each individual wheel to make sure they are all spinning at the same speed. These systems work very well.
Some vehicles are equipped with AWD for normal driving situations on dry pavement, and can automatically lock a center differential in the transfer case when encountering a low traction situation off-road. This makes 4 high automatically engage, but they also have a manually selectable 4 low. The number of variations when you start looking at AWD systems is endless.
In Plain English
So let’s sum it all up. AWD has a center differential and 4WD does not. AWD usually requires no action on the part of the driver to make all 4 wheels turn, and 4WD usually does require the pull of a lever or the push of a button in order to engage. AWD works well for all driving situations, and 4WD is only for low traction situations. AWD vehicles almost never have a low range, and 4WD vehicles almost always have a low range. AWD is good in snow, a little mud, and gravel. 4WD is better in each of these situations and will also climb steep hills, as well as go down steep hills and remain in perfect control (most of the time).
Nearly all 4WD vehicles have the exact same drivetrain layout, but when it comes to AWD the layout has many variations. Many AWD vehicles will have a drivetrain layout that is essentially the same as any FWD vehicle. This is because they usually run around with only the front wheels moving the vehicle down the road. They will of course have a driveshaft coming out of some sort of transfer case that goes to the rear wheels, but this rear driveshaft, and the rear wheels will only engage when the front wheels slip. This is the type of AWD system that you typically find on all of the cross over vehicles that are on the road today. Cars such as the Chevy Equinox, Ford Escape, Honda CRV, and Toyota RAV4 fall into this category. Sometimes you will see some kind of emblem that says 4WD on the back of a vehicle with this type of FWD bias AWD system. Don’t be fooled, they do not have true 4WD.
Which One Should I Buy?
If you want good traction in the snow then get AWD. If you want good traction in the snow but spend a fair amount of time off-road then get 4WD. If you want to drive something more like a normal car then get AWD. Trucks with 4WD will require a bit more maintenance, are usually heavier, and usually don’t get very good gas mileage. 4WD is also more complex and might cost more to manufacture. Trucks with AWD are usually not really that great off-road, and cannot come close to getting to the places that 4WD can go. Because most people are not into climbing steep mountains and slogging through deep mud, they usually go for the AWD. That is why today’s crossovers usually have AWD and not 4WD.