Comparing Single-Speed and Two-Speed Transfer Cases - Power Splitters
Single and Two Speed Transfer Cases
Well-dressed marketing gurus want you to believe that owning a rugged SUV with a 4x4 option will allow you to be young and in vogue with an active lifestyle. The "live young" marketing scheme has worked because consumers are buying the idea--the prospect--of being able to go off-road even if they never actually do. A 4x4 is rarely purchased with the intention of being used seriously off-road. So if you're wondering why most automakers got on that "more carlike" train, the reason is not surprising--if they make products that look like 4x4s but ride and drive like cars, then consumers will consume.
Because only 10 percent of 4x4 owners demand that 4x4s be capable off-road as well as on, well...you can follow the supply-and-demand theory of economics. Purchasing trends have forced a movement toward single-speed transfer case options such as in the Ford Escape. And because two-speed transfer cases have been around since the early 1900s we thought we'd take a closer look at the differences between old and new, single- and two-speed 4x4 options. Graciously, Ford let us examine the differences between two of its more popular models--the 4x4 single-speed Escape and two-speed Borg Warner 1354-ed Explorer Sport Trac--to examine their differences.
Single-Speed Transfer Caseolgy The best way to describe how a single-speed transfer case works is to first know how a two-speed works. If you don't already know how one works, go to the "Power Splitter Basics" sidebar. Single-speeds are similar to two-speeds in that they offer a 4-Hi. What that means is that there is no low-range gear reduction inside the single-speed case. It also means that there is no locking mechanism in the 4-Hi position to lock the differential and rear driveshafts together. With most single-speed cases there is a differential action that occurs between the front and rear driveshafts.
The Escape's Control Trac II four-wheel-drive system uses a rotary blade coupler (RBC) to split the power between the front and rear axles. The RBC is located at the end of a two-piece driveshaft mounted in front of the rear differential. Even when equipped with the 4WD system, the Escape's design, capability, and suspension is inclined to provide better handling and traction control on paved roads than off-road. It's clear that the Escape isn't designed to be a hard-core off-roader. Knowing the Escape's limited purpose, Ford and other cute/ute designers decided the extra expense and weight of a two-speed transfer case were not worth the off-road performance gains. Given this decision and the soccer-mom craze, it's very likely that single-speed will be seen more and more on concrete mountains.
Two Can Play That Game
Two-Speed (Sport Trac Borg Warner 1354) Low range or 4-Lo offers an additional set of gears in the transfer case, which allows for a lower gear ratio. This lower ratio transfers extra torque and a slow output speed to the wheels. A part-time transfer case locks the front axle driveshaft to the rear axle driveshaft. This forces all of the wheels to turn at the same speed--Ta-duh! That's the 4x4 magic.
In the Sport Trac, as in many other two-speed transfer-cased vehicles, there are three settings: 2WD for normal driving; 4WD-Hi for slippery surfaces at higher speeds; and 4WD-Lo for slow speed heavy snow, and off-road terrain. Explorer's Control Trac four-wheel-drive system is designed to improve traction where traction is limited. Via a rotary knob, the system's "auto" setting operates with sensors that measure and compare front and rear driveshaft speeds and throttle position, and the system computes the mathematical formula to determine how much torque to send to each axle. These calculations are made 50 times per second, to signal an electromagnetic clutch to adjust the transfer case bias and redirect torque. If the rear wheels lose traction, the optimal amount of torque for the situation is transferred to the front axle--up to 500 lb-ft. In 4-Hi the system locks the front and rear driveshafts together to provide traction at all four wheels. In 4-Lo the system locks the front and rear driveshafts together but adds a 2.48:1 gear reduction, adding the advantages of a much lower gear ratio, greater torque, and more traction control.
Power Splitter Basics
By virtue of its design, a chaindrive transfer case transfers power coming from the engine and transmission to the differential gears in the axles, which then redirect the power to the four wheels of the vehicle under perfect conditions. Each of the four wheels is responsible for 25 percent of the torque created by the engine, transmission, transfer case, and axles. Because each tire has only 25 percent, as opposed to 50 percent in a 2WD, it is much less likely for the tires to break loose. That's why a 4WD can climb much steeper grades than a 2WD.
Power comes in through the output of the transmission through the gear reducing planetary gears to turn the rear driveshaft. When 4-Hi or 4-Lo is engaged, the shaft turns the chaindrive (note that there are varieties of this design) and forces the front driveshaft to turn allowing power to be transferred to the front wheels. A four-wheel-drive system will always have more traction than a two-wheel-drive system because it applies power to all four wheels. This is very beneficial for vehicles that venture off-road.