Where would the hard-core off-road community be without the auxiliary transmission known as a transfer case? Probably buried axle deep in sand or mud, that's where. And, what about rockcrawling? Ha! Without the T-case's ability to send engine power to the front and rear differentials and driving axles, the sport of 'crawling simply wouldn't exist. In fact, much of what we enjoy about 'wheeling, such as the challenge of negotiating a tough trail, would be lost without the all-wheel traction and the Low-range torque multiplication provided by a transfer case.
However, this article is not about what we don't have; rather, it's about what our 4x4s do have: the two-speed T-case and how it sets our machines apart from a garden-variety vehicle. Although the study of transfer cases is complex, the fundamentals of transfer case function and the basics of transfer case design are straightforward and worth studying. The transfer cases used on 4x4s during the '50s, '60s, '70s, and '80s are comparable in overall concept to today's state-of-the-art T-cases, but details vary between old and new, especially in the manner by which 4WD is engaged.
Whether a 4x4 is equipped with a traditional or an electronically controlled late-model T-case, there's no denying that a transfer case plays a vital role in a 'wheeler's off-road capabilities. Without the four-wheel-drive traits provided by a transfer case, an off-road vehicle would be limited in function, relegated to finding an easy way around difficult off-road obstacles - and what fun would that be? Let's take a closer look at 4x4 transfer case systems.
Why Use A Transfer Case?The need for a transfer case is twofold. First, under conditions of limited traction, a 4x4 must have power sent to the front and rear driving wheels, which is a T-case's primary function. Second, adverse conditions require that a transfer case provide additional gear reduction, which boosts the amount of torque sent to the tires, which is necessary in order to power through soft terrain or to climb over severe obstacles.
That, of course, is a basic overview of the need for a transfer case, but it serves to illustrate the concept behind a transfer case. Delving deeper into the need for torque multiplication, let's look at the theory behind a transfer case's Low-range gearset. To set - and keep - a 4x4 in motion requires a lot of torque. Even powerful V-8 engines don't develop enough low-speed grunt to efficiently move a 4x4 through deep mud or sand, and rockcrawling places a tremendous load on an engine, so a torque-multiplying device - a gearbox - is installed between the engine and the driving axles. The gearbox - either a manual or automatic - is known as a transmission. A transfer case functions in much the same way as a standard transmission. By selecting a transfer case's Low-range gear, a notable increase in torque multiplication is achieved, although the engine isn't producing additional power. The additional power is sent to the front and rear wheels, and a 4x4's performance during situations of low traction is greatly improved.
Sending enhanced power to the front driving wheels - in addition to the rear wheels - is the reason a transfer case is equipped with front and rear output shafts. Without a front output shaft, a transfer case would simply be an additional gear-reduction unit, sending Low-range power to the rear wheels. To this point, we've only mentioned the additional low gear reduction a T-case provides. However, most transfer cases are of the two-speed variety, with a Low-range in the 2.0:1 ratio and a high range of 1:1. What the high range does is simply send power to the front and rear diffs; there is no torque multiplication involved other than what is provided by the transmission.
Transfer Case FunctionIn the old days - prior to the mid-'90s - there were basically two types of transfer cases: gear-driven and chain-driven. Today, there are T-cases with internal differentials, sophisticated electronically managed transfer cases, and T-cases equipped with all manner of automatic controls and actuating devices. However, the overwhelming majority of transfer cases still in use rely on output shafts that are either chain- or gear-driven.
In function, a transfer case takes power from the transmission in through its input shaft. On a gear-driven T-case, a gear mounted to the input shaft sends power to a gear on the rear output shaft as well as to gears on an intermediate idler shaft, which, in turn, routes power to the gears attached to the front output shaft. The power sent to the output shafts spins the front and rear driveshafts that are attached to the front and rear differentials, creating four-wheel drive.
A chain-driven transfer case is more complex than a gear-driven T-case, but the concept is fundamentally the same. Power from the transmission is sent to the transfer case's input shaft, which sends torque to the rear output shaft, but is also fitted with a drive sprocket. A large multilinked Hy-Vo chain, spun by this drive sprocket, sends power to a driven sprocket that's fitted to the front output shaft. Additionally, there are gearsets connected to the input shaft that provide a low gear ratio. As with the gear-driven transfer case, power is ultimately sent to the driveshafts via the output shafts.
Control over a transfer case is accomplished via manual shift lever, vacuum-operated lever, or electronic shift lever. Whichever is used, a true off-road transfer case functions in multiple modes, including 4-Hi, 4-Lo, Neutral, and 2-Hi. Some transfer cases, such the New Process 203, are full-time cases, and thus use N, 4-Lo Lock, 4-L, 4-Hi Lock, and 4-Hi. We say a true off-road transfer case functions in the aforementioned modes since some late-model transfer cases only operate in a 2WD/4WD mode, where 2WD sends power to the rear diff, and 4WD sends power to the front and rear diffs, but only in a 1:1 ratio, without the benefit of gear reduction/torque multiplication. On a traditional two-speed T-case, 4-Hi uses a gear ratio of 1:1, which doesn't multiply engine output, but sends power to the front and rear diffs. When shifted into 4-Lo, a two-speed transfer case adds gear reduction - usually from 2:1 to 2.8:1 - which multiplies the engine's output through the magic of torque multiplication.
Building A Better T-CaseAs would be expected, the transfer case is fertile ground for aftermarket parts suppliers. The development of components focused on the enhancement of a T-case's function has been impressive. Some hot items are the aftermarket gearsets available from Tera Manufacturing, which, once installed, create a 3.15 Low-range in Dana Model 18 and 20 T-cases. According to Tera, changing out the OE 2.03 ratio gearset and swapping in the 3.15 cogs has the same effect as changing the rear differential gear ratio from 3.54 to 5.38. Tera Flex and Marlin Automotive also manufacture T-case cogs with radically lower gear ratios for a variety of transfer cases.
Beyond tricking out a T-case with aftermarket internals, there's the add-on transfer case/auxiliary gearbox concept. The Marlin Crawler system is basically two Toyota transfer cases that are mated; when Low-range on both boxes are engaged, the resulting gear ratio is an astounding 22.09.
Taking another path toward a super-low transfer case gear ratio is Klune-V's Extreme Underdrive, which is a gearbox that installs between the vehicle's transmission and the T-case. The Klune-V's dual internal gear ranges mean that the T-case's existing high range now has three selectable ratios; the same is true for the transfer case's Low-range, as the Klune-V provides Low (the T-case's original Low-range), Super-Low, and Unbelievably Low ratios.