May 1, 2002
by Jim Kerr
Sometime near the end of May, Ford’s redesigned 2003 Expedition will be in the showrooms. While its independent front and rear suspension give it a great ride, it is the Expedition’s new transfer case, front axle, and traction control design that are the hidden features helping take this SUV a step ahead of the competition.
Ford has had electric shift automatic transfer cases as an option in their SUV’s and trucks for some time, but the Expedition uses a design that Ford calls ControlTracT. On the dashboard, you find a rotary selector knob that enables the driver to select two-wheel drive, 4-Hi, 4-Lo, or Automatic 4wd. Shifting into or out of 4-Lo requires the vehicle to be stopped and the transmission placed in neutral. Other ranges can be shifted on the go.
In Automatic mode, torque is transferred rear to front as required. Normally, the torque goes to the rear wheels, but as soon as the ABS wheelspeed sensors indicate a difference in speed, clutches inside the transfer case send torque to the front. Clutch application is continuously variable and computer controlled. The computer varies the strength of an electromagnet that attracts a ramped apply plate. A second rotating ramped plate is separated from the first one by some apply balls. When the electromagnet holds the first plate, the balls roll up the ramps, creating end thrust that squeezes the clutch plates together, and torque goes to the front wheels. It sounds a little complicated, but works instantly to balance front/rear power.
Shifting to 4-Hi or 4-Lo bypasses the clutch operation. Instead of placing high torque loads through the clutch, an electric motor slides a gear into direct engagement with the transfer case clutch hub. This provides a solid connection between the front and rear wheels.
An integral part of the 4×4 system is the engagement of the front axle. A few 4×4′s use manual locking front hubs to engage the front wheels with the front axle. Some use automatic hubs that engage if the vehicle is placed in 4×4. Many front axles are engaged internally in the differential assembly. The 2003 Expedition does it differently.
Ford’s new system uses a sliding collar on the inside of each front wheel hub to engage the axle. Vacuum applied to each wheel assembly holds the collars in the released position. In 2WD mode, the wheels are free to turn without the front axles or differential turning. This eliminates wear and improves fuel economy.
Shift to automatic or a 4WD mode and the vacuum is released at the front hubs. The spring-loaded collars move out, engaging the wheels with the axle shafts. The entire assembly is compact and sealed against dirt and water.
One more feature optimises the Expedition’s traction. Ford’s option name is AdvanceTrac. AdvanceTrac uses four-wheel traction control to put the power where it is needed. If many 4×4 vehicles lose traction on one front and one rear wheel, they are stuck. The differentials in the axles allow low traction wheels to spin and the wheels with traction to sit still. So does the vehicle.
Limited-slip, locking, or anti-spin differentials help, but only at the rear. These units are not used on the front axle because they affect steering during normal driving. Even with limited slip rear axles, a 4×4 can be stopped dead if both rear wheels and one front wheel lose traction. That is where AdvanceTrac takes over.
AdvanceTrac applies the brakes on a spinning wheel to equalise power from side to side. With only one wheel having traction, AdvanceTrac enables the vehicle to pull itself to a location hopefully with better traction.
With ControlTracT providing front and rear traction, and AdvanceTrac providing side to side traction, Ford has all the bases covered. Computers have enhanced vehicle safety and operation in many ways. Enhanced on and off-road traction is one more way