Do you know what helps your four-wheel-drive vehicle gain traction in slippery situations or gives you enough torque to crawl over rocks at a slow speed? In large part, the transfer case is responsible. So, what is a transfer case?
Transfer cases, sometimes referred to as T-cases, can be found on four-wheel-drive vehicles and are responsible for dividing the power to all four wheels. Without transfer cases, off-roading would be much more difficult. Transfer cases are what help give our rigs more traction, so we can get out of a tricky situation. Dividing power from the transmission to both the front and rear wheels, the transfer case disperses engine power in order to enhance traction or provide more torque. Although all-wheel-drive vehicles also have transfer cases, we are mainly going to focus on their application in 4WD vehicles.
How Do Transfer Cases Work?
We’ve established that transfer cases distribute power to both the front and rear axles. But how do transfer cases work? The engine powers the transmission, which in turn sends that power to the transfer case. Sending power from the output shaft either through a chain or a set of gears, torque is sent to the front and rear driveshafts, which then delivers power to the axles.
Chains are the most common method of powering a transfer case just because they are quieter and lighter than gears. Although chains are mostly found in modern transfer cases, enthusiasts and old-school off-roaders typically prefer gears just for their reliability and ability to handle greater amounts of torque in comparison to chain-driven transfer cases. When four-wheel-drive is not engaged for part-time 4WD systems, then 100% of the power is being sent to the rear wheels. However, when four-wheel-drive is engaged, then the transfer case will split that power up evenly between the front and rear, powering all four wheels. Transfer cases work in coordination with the drive axles, differentials, driveshafts, transmission, and wheels in order to evenly disperse power.
There are typically two gear settings within the transfer case: High and low. Low gears provide more torque at slower speeds and are geared toward intense off-road situations like rock crawling and hill climbing. High gears are for more basic off-roading situations such as ice, dirt, heavy rain, and other situations where you really just need additional traction. These are typically referred to as 4-Hi and 4-Lo, or 4H and 4L. Some transfer cases will have a lever that will need to be manually engaged but many newer vehicles now can be engaged electronically with the simple push of a button.
Part-Time vs Full-Time 4WD
There are two main types of four-wheel-drive systems — part-time and full-time. As expected, part-time four-wheel-drive indicates that the vehicle remains in two-wheel-drive, typically rear-wheel drive, for normal driving conditions. Not only does this help improve fuel economy in relation to full-time 4WD vehicles, but it also helps put less stress on the drivetrain. When four-wheel drive is needed, you simply have to engage it. Most four-wheel-drive systems will come with several modes to choose from, the most common being auto, 4-Hi, and 4-Lo. Some will have terrain modes, so you can select the proper gearing for the corresponding road conditions. Other part-time 4WD systems are automatic and will engage all four wheels when the system senses a loss of traction.
Full-time four-wheel-drive systems operate with all four wheels at all times, even on the highway and driving on paved roads. Full-time four-wheel drive also has the low gear and high gear settings, but you usually cannot switch it to two-wheel-drive since it is a full-time 4WD system. The high and low gears are extremely helpful in off-roading situations where you might need the lower gear to crawl over some rocks or get over a steep hill in addition to the higher gear setting that will give you that added traction when necessary. Locking differentials also help distribute power evenly to the wheels and enhance off-road performance.
Jeep Transfer Cases & 4x4 Systems
For Jeep Wranglers, there are several different 4x4 systems available, depending on which model that you have. The Rubicon, Jeep’s rugged off-roading Wrangler, is going to be equipped with the most capable transfer case in comparison to the less expensive models. Some part-time four-wheel-drive systems are manual, but many modern vehicles feature automatic 4x4 systems that will automatically engage when they sense the need.
Command-Trac Part-Time 4WD System
Standard on both the Sport and Sahara Wrangler models, the Command-Trac System is a part-time four-wheel-drive system. Featuring an NV241 G11 transfer case, Command-Trac is a shift-on-the-fly system, which means the transfer case can shift from 2WD to 4WD at speeds up to 45 miles per hour. The low range ratio is 2.72:1, which will give you that torque needed to slowly crawl over rocks. It is best not to operate the Command-Trac system on dry pavement or good weather conditions as it is mainly made for off-roading situations or when you may need some extra traction in icy or snowy weather. When not engaged, the vehicle will operate in rear-wheel drive.
Selec-Trac Full-Time 4WD System
Although the Selec-Trac 4WD System has been used for other Jeeps, the system was revamped and introduced as an upgrade for the fourth-generation Wrangler. Only an optional upgrade for the Sahara, the Selec-Trac is an additional $595. The Selec-Trac includes a 2.72:1 low range, which is the same ratio offered in the Command-Trac System. However, the Selec-Trac is a two-speed active full-time 4WD system that can be operated in 4H Auto, which automatically engages and disengages depending on driving conditions. Traction control sensors and internal wheel speed help the system determine when to switch into 4WD mode. Although it may seem like an inexpensive upgrade at $595, the system also requires the 8-speed automatic transmission, which will cost you a hefty $2,000 more, and the anti-spin differential rear axle that is an additional $595. So, if you’re thinking about upgrading to the Selec-Trac 4WD System, it’ll cost you an extra $3,190.
Rock-Trac Part-Time 4WD System
Equipped with an NV241 Off-Road Transfer Case, the Rock-Trac System can be found on the Wrangler’s most capable off-roading model, the Rubicon. This 4WD system is heavy-duty for superior off-road performance and provides a higher amount of torque at slower speeds. With a 4:1 low-gear ratio and an 84:1 crawl ratio with the manual transmission, the Rock-Trac system helps the Rubicon maintain a slow and steady rock-crawling pace with enough torque to conquer nearly any obstacle. Tru-Lok electronic locking front and rear differentials plus an electronic front sway bar disconnect enhance the Rubicon’s off-roading capabilities even further. If you plan on doing any extreme wheeling, the Rubicon’s transfer case will help you grip and rip down the trail!
Source: Jeep, Four Wheeler Network, Popular Mechanics | Image Credit: Jeep, Popular Mechanics
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