Single vs. Two-Speed Transfer Cases

Ford’s Control Trac II...        read full caption Ford’s Control Trac II four-wheel-drive system in the Escape is operated by a switch on the instrument panel. In “auto” mode, engine power flows to the front wheels, and when the front wheels start to slip or spin, progressive amounts of torque are instantly redirected from the front to the rear until traction is regained. This allows the Escape to be a light off-roader. We’ve seen it hold up well in washouts and on minor sand hills. The “on” mode is comparable to the 4-Hi of a two-speed transfer case.

Trends indicate that automakers...        read full caption Trends indicate that automakers are little by little moving away from standard transfer cases with floor-mounted shift levers and manual locking hubs to automatic sensor-actuated systems that engage without driver input, such as Ford’s Control Trac.

Tested at the front and the...        read full caption Tested at the front and the rear, the Escape Control Trac II splits power to the wheels that need traction. With a rotary blade coupler (RBC), power is transferred and sent from the front to the rear. The RBC is located at the end of a two-piece driveshaft mounted in front of the rear differential.

In order to have good wheel...        read full caption In order to have good wheel travel and traction while on a rough road or trail, a vehicle’s suspension needs to be soft and flexible. The goal is to allow the tires to contact the ground as much as possible to gain as much traction as possible, and to get the power from the power source to the appropriate wheel. In terms of suspension design, good performance on pavement is often contradictory to good performance off-road. What’s more, having an off-road suspension isn’t the best for on-road feel. Most small SUVs have fully independent suspensions creating a stable, more comfortable, carlike ride. A side effect of independent suspension, though, is that wheel travel is restricted. This is one reason solid-axle suspensions, both front and rear, have such good reputations for off-road performance.

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.

One-Track Mind

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.