shane_th_ee
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Alrighty, here's your primer on four wheel drive systems and modes.
1. What is 4A and what does it do? 4A (A for Auto) is another word for "all wheel drive". In this mode, a viscous coupling* is used in the center differential** to adjust the torque split between the front and rear axles. The viscous coupling divides the engine power between the axles, but still allows the axles to rotate at different speeds. Apparently in the Ford design, all engine power can be sent to the front axle, the rear axle and any division in between. The huge advantage for everyday driving of 4A mode is that, in a turn, the rear axle tracks to the inside of the front axle which means it travels a shorter distance***. 4A allows the rear axle to make fewer rotations in a turn which means the axles don't bind and the wheels don't hop. The downside to 4A is that the system will transfer power from an axle with slipping wheels to the axle without slipping wheels. This increase in power applied to the non-slipping axle is the same as if you were to increase power by stomping on the gas. And in really slippery situations, you never stomp on the gas, because the sudden increase in torque will cause the non-slipping wheels to slip as well.
2. What is 4H and what does it do? 4H (H for High) is where the center differential is mechanically "locked" and the two axles are forced to rotate at exactly the same speed. The center differential gearing in 4H is such that the vehicle can travel at "normal" (non-highway) speeds. This is awesome for getting going and not prone to the same problems as discussed above in the 4A description. But it has two big downsides: first, as mentioned above, it can cause binding in turns. This is really bad and can cause things to break. The other downside is that torque is always applied to both axles and net torque on the vehicle from the two axles is in opposite directions. What am I talking about: think torque steer in a front wheel drive vehicle which is where the front wheels want to turn (always the same direction) when you stomp on the gas or in a rear wheel drive vehicle, how the rear end is also biased to kick out to one side. The front axle wants to move to one side and the rear axle wants to move to the other. If you hit a patch of black ice (or compact snow and ice) where no wheel has traction and hit the gas with the system in 4H, the vehicle WILL spin out. This is exactly why you see so many 4x4s spun out in the ditch next to the highway when it's really icy/snowy.
3. What is 4L and what does it do? 4L is exactly like 4H, except the gear ratios within the transfer are much lower such that the vehicle is pretty much limited to walking speed. Most systems will give the vehicle a maximum speed of, at most, ~20mph. Why is this useful? First, it's like adding a smaller gear to the front of your bicycle, so it makes it easier for the vehicle to climb much steeper inclines. Second, it allows the vehicle to be slowed by the engine rather than the brakes. (Try downshifting on a steep hill sometime, and you'll notice the car will slow without you touching the brakes...) This is fantastic when you're trying to descend really steep and slippery hills because you put the transfer case in 4L, put the transmission in 1st gear and just let the vehicle crawl (walking speed or less) down the hill without having to touch the brakes. Because if it's that slippery, touching the brakes will accomplish nothing but causing the wheels to lock up and an uncontrolled slide down the hill. The third use-case for 4L is when you're using your vehicle as a tractor and need a ton of torque, like recovering a stuck vehicle or pulling a stump.
4. N or Neutral. In this mode, the drive shafts are disconnected from the engine. This mode is used to tow the vehicle with all four wheels on the ground. It can be used to recover a stuck vehicle or to tow it, say, behind an RV without a dolly.
*Usually a viscous coupling is used. There are a couple of other designs and I haven't been able to find the actual details of the Ford design. But "4A" is always an unlocked differential where the front and rear axles are allowed to rotate at separate speeds.
**For the truly uninitiated, a 4x4/AWD vehicle has three differentials: the center differential which divides the engine's power between the front and rear axles and another on each axle each of which divides the engines power between the two wheels on that axle.
***Think about a running track and why the runners start from staggered positions instead of all in a line like they do when starting a marathon.
1. What is 4A and what does it do? 4A (A for Auto) is another word for "all wheel drive". In this mode, a viscous coupling* is used in the center differential** to adjust the torque split between the front and rear axles. The viscous coupling divides the engine power between the axles, but still allows the axles to rotate at different speeds. Apparently in the Ford design, all engine power can be sent to the front axle, the rear axle and any division in between. The huge advantage for everyday driving of 4A mode is that, in a turn, the rear axle tracks to the inside of the front axle which means it travels a shorter distance***. 4A allows the rear axle to make fewer rotations in a turn which means the axles don't bind and the wheels don't hop. The downside to 4A is that the system will transfer power from an axle with slipping wheels to the axle without slipping wheels. This increase in power applied to the non-slipping axle is the same as if you were to increase power by stomping on the gas. And in really slippery situations, you never stomp on the gas, because the sudden increase in torque will cause the non-slipping wheels to slip as well.
2. What is 4H and what does it do? 4H (H for High) is where the center differential is mechanically "locked" and the two axles are forced to rotate at exactly the same speed. The center differential gearing in 4H is such that the vehicle can travel at "normal" (non-highway) speeds. This is awesome for getting going and not prone to the same problems as discussed above in the 4A description. But it has two big downsides: first, as mentioned above, it can cause binding in turns. This is really bad and can cause things to break. The other downside is that torque is always applied to both axles and net torque on the vehicle from the two axles is in opposite directions. What am I talking about: think torque steer in a front wheel drive vehicle which is where the front wheels want to turn (always the same direction) when you stomp on the gas or in a rear wheel drive vehicle, how the rear end is also biased to kick out to one side. The front axle wants to move to one side and the rear axle wants to move to the other. If you hit a patch of black ice (or compact snow and ice) where no wheel has traction and hit the gas with the system in 4H, the vehicle WILL spin out. This is exactly why you see so many 4x4s spun out in the ditch next to the highway when it's really icy/snowy.
3. What is 4L and what does it do? 4L is exactly like 4H, except the gear ratios within the transfer are much lower such that the vehicle is pretty much limited to walking speed. Most systems will give the vehicle a maximum speed of, at most, ~20mph. Why is this useful? First, it's like adding a smaller gear to the front of your bicycle, so it makes it easier for the vehicle to climb much steeper inclines. Second, it allows the vehicle to be slowed by the engine rather than the brakes. (Try downshifting on a steep hill sometime, and you'll notice the car will slow without you touching the brakes...) This is fantastic when you're trying to descend really steep and slippery hills because you put the transfer case in 4L, put the transmission in 1st gear and just let the vehicle crawl (walking speed or less) down the hill without having to touch the brakes. Because if it's that slippery, touching the brakes will accomplish nothing but causing the wheels to lock up and an uncontrolled slide down the hill. The third use-case for 4L is when you're using your vehicle as a tractor and need a ton of torque, like recovering a stuck vehicle or pulling a stump.
4. N or Neutral. In this mode, the drive shafts are disconnected from the engine. This mode is used to tow the vehicle with all four wheels on the ground. It can be used to recover a stuck vehicle or to tow it, say, behind an RV without a dolly.
*Usually a viscous coupling is used. There are a couple of other designs and I haven't been able to find the actual details of the Ford design. But "4A" is always an unlocked differential where the front and rear axles are allowed to rotate at separate speeds.
**For the truly uninitiated, a 4x4/AWD vehicle has three differentials: the center differential which divides the engine's power between the front and rear axles and another on each axle each of which divides the engines power between the two wheels on that axle.
***Think about a running track and why the runners start from staggered positions instead of all in a line like they do when starting a marathon.
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