If all the wheels are pushing, what’s the difference?
|Typical FWD layout|
|Most 4WD and some AWD systems|
will be laid out like this.
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. This means that the engine will turn very quickly but the wheels will turn very slowly, in all of the gears. 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.
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. They are allowed to turn at different speeds if needed.
Because of this center differential, 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 does have 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 4WD or AWD.
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 type 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
|AWD cannot compete with something like this SUV|