Many trucks, SUVs, and cars come with all-wheel-drive or four-wheel-drive. Most of the times trucks and Jeeps are equipped with four-wheel-drive. Cars such as Subarus are equipped with all-wheel-drive. The only problem is most people think they’re the same thing. Let’s dive in and explain the difference between AWD and 4WD.
All-wheel drives system are very common in cars like the Subaru Impreza. Sometimes this system is used in larger trucks and SUVs like the Cadillac Escalade or the Ford Raptor. Like the name implies, the all-wheel drive system powers all four wheels.
Unfortunately, splitting the power 50/50 between the two axles won’t work in high grip situations such as on the street. When taking a turn the wheels on the front axle will move at a slower than the wheels on the rear axle. If the power is locked 50/50 then binding will occur mid corner. This will put the transfer-case, axles, differentials, and driveshafts under extreme load causing major damage over time.
The AWD system fixes this issue by allowing the power split in between the front and rear axle to change as needed. This works by simply adding a differential in the transfer-case which works exactly like a differential in an axle. During a turn more drive speed can be sent to the front or rear axle which eliminates the binding issue we mentioned.
Many AWD systems utilize this feature to increase fuel economy. Front wheel drive cars inherently use less energy to accelerate. By sending all the power to the front axle unless wheel slip is detected you can increase fuel economy while also maintaining high levels of grip when needed.
You would think since cars have four wheels that all-wheel drive would mean the same thing as four-wheel drive, but it doesn’t. The main problem with all-wheel drive revolves around the differential in the transfer case. In high grip situations such as driving on the road, wheel slip isn’t likely to happen. Off-road, however, wheel slip is very common.
The differential in the transfer-case is considering an open differential. When an axle has an open differential, the wheel with the least traction will get all the drive force. This same problem also affects center differentials in transfer-cases. The axle with the least traction will get the most drive force. This can become an issue when climbing up a steep hill because the front axle will begin to slip and inevitably receive all the drive power.
Four-wheel drive combats this issue by removing the center differential and locking power at a 50/50 split. This means one axle will never receive more drive power than another which is very important for off-road situations. This design also introduces the ability to have multiple ratios in the transfer-case. One ratio can be much lower which multiplies torque, but limits drive speed which is perfect for “crawling” situations. It should be noted that some AWD systems also have this high/low gear ratio feature.
Pros and Cons
When rain or snow is introduced to the road you want more wheels to have drive power to increase grip. A four-wheel drive system would work fine in this situation if the road was completely snowy, but often times there are patches of a dry road during snow. If you go from snow to dry mid corner with four-wheel drive engages you will cause sudden binding in the drivetrain. This is why places where snow is common such as Flagstaff, Arizona many people use all-wheel drive cars.
As we mentioned before the problem with AWD is in extreme off-roading situations. The issue of one axle receiving all the drive power can be solved, however, with a center differential locker and/or traction control. We will discuss this a little bit further down in the article.
One of the best features of an AWD system is the ability to torque vector. Torque vectoring helps cars corner even faster by sending power the outside wheel during a corner. Sometimes the rear axle needs more power to help to car rotate more. Sometimes the front axle needs more power to help the car recovery from a mid-corner slide. A system like this can make a car extremely fast on a track which Nissan has leveraged to make the GTR one of the best performing track cars in history.
- 4WD: Best for Extreme Off-Roading Situations
- AWD: Best for Low Grip On-Road Situations
Center Differential Locker and Traction Control
How can you get the benefits of both AWD and 4WD? Well, the easiest solution is to add a center different locker. This will allow power to be locked 50/50 between the two axles when needed. Land Rover used this in a few of their vehicles in the past and it works fantastic. The on-road grip of an AWD setup with the off-road grip of a 4WD setup is truly the best way to go.
Combined with an advanced traction control system which when setup correctly can perform as well as actually differential lockers, the result is truly amazing. Land Rover also uses a multi-speed transfer-case with some of their AWD systems. This allows for the low gearing necessary for “crawling” plus the locked center differential, plus the traction control.
Having a multi-speed locking differential does increase complexity which can mean parts need to be stronger. This system also increases cost which is why a system this complex is generally reserved for expensive SUVs and trucks.
To summarize all this information I’ve just thrown at you; AWD is great for on-road environments and 4WD is great in an off-road situation. The AWD system uses a differential in the transfer-case to manage the power split and the 4WD system simply locks power 50/50 between the two axles.