The theme for this issue is transfer cases. And why not? Without a transfer case, your 4x4 would be a 2x4. More than any other part of your truck, the transfer cases defines it as a four-wheel drive. What exactly is a transfer case, though? And how does it work? Glad you asked.
The entire purpose of a transfer case can be summed up by saying that it splits engine power and sends it to all four tires via the front and rear axles. Transfer cases have been around nearly as long as the automobile. It wasn't too long after people started driving horseless carriages that they got stuck. The tow truck usually looked a lot like a pair of horses wrangled by a person who mocked the horseless transportation. Not wanting to withstand mocking, things like pavement, rubber tires, and, yes, the transfer case were invented.
Just like the car itself, transfer cases have changed dramatically since their early days. We'll depart from the history lesson at this point and dive into the details of the transfer cases that you are most likely to run into in your off-road adventures.
Steve Roberts and Vic Carroll of Advance Adapters provided many of the images in this article and gave us a brief history lesson on transfer cases. In addition to making adapters to mate popular transfer cases, transmissions and engines together, Advance Adapters also makes their own gear-driven transfer case, the Atlas. And it offers the Atlas as a conventional two-speed transfer case or an extremely cool four-speed unit.
The transfer case is the center of the drivetrain in a 4x4. Mounted to the back of the tra
Anatomy
If you dissect a transfer case, you'll find a few common components. There's an input shaft which is spun by the transmission. That input shaft is connected to two output shafts: One that spins the front driveshaft and one that spins the rear. Many transfer cases also have a differential. Just like the differential in the axles, this allows one output to spin at a different rate than the other to avoid driveline bind on hard surfaces. More on that in a minute....
You might say that the only transfer cases worth talking about are two-speed transfer cases. That means that there is either a second set of gears or a planetary gear set that produces low-range gearing when engaged. This essentially gives you the opposite effect of the gearing in your transmission, slowing the speed of the driveshafts relative to the speed of the transfer case input shaft. This is an important aspect of 4x4s, as the gear reduction multiplies the power delivered to the tires. It also slows the wheel speed. The result is more power delivered to the ground at a slower wheel speed, giving you more control.
There are a bunch of names for various types of four-wheel drive, most of which depict a slightly different differential within the transfer case. Here's the who's who and what's what in the name game.
One of the most revered factory transfer cases is the NP205. These work horses have a cast
Part-Time 4WD: This is the most common type of transfer case. It allows you to operate the vehicle in two-wheel-drive, four-wheel-drive high-range (4Hi) and four-wheel-drive low-range (4Lo). The 4Hi operation utilizes a differential. Because the front driveshaft and axle can be cut off completely from the power, these systems offer smoother operation on pavement and better fuel economy. The strongest transfer cases are generally part-time systems because they were designed for real off-road use, often in a truck application.
Full-Time 4WD: This is the simplest type of transfer case. It sends power to the front and rear axles all the time. To eliminate, or at least diminish, driveline bind on hard surfaces, these use an open T-case differential in high range. Some offer a 4Hi Lock position which locks the differential.
Active 4WD: Over the last decade or so, a variety of full-time and part-time systems have been developed that use electronic or mechanical means to adjust the amount of power delivered to the front axle according to wheel slip. They have a variety of names and levels of performance, but they provide some of the benefits of a part-time system without the owner every having to switch anything.
Because active 4WD was designed for smooth operation without any input from the driver, they can be found on everything from BMWs to Dodge 3500s.