4 X 4 X Ford - Special Operating Characteristics
In order to understand customer complaints and properly service the 4 x 4 system on Ford 4WD trucks, you must be familiar with the special operating characteristics of these systems. If you are not, you may find yourself trying to fix something when it's actually working properly.
Figure 12:
What do we mean by "special operating characteristics?" To begin with, let's take a look at the basic functioning of wheels and axles. Figure 12 is a typical 2WD truck tracking straight down the road. The power is delivered to the rear wheels, and the front wheels are turning freely.
For reasons that we will explain later, we have checked carefully to make sure that all four tires are the same size, brand and tread design, are worn the same amount and are inflated to the specified pressure. In addition, the vehicle is not overloaded. As long as this vehicle continues in a straight line and the driver doesn't "burn rubber," then all four wheels will turn at the same rate and travel the same distance.
Figure 13:
Figure 14:
To illustrate this better, let's stop the truck and paint a mark at the same place on each tire (Figure 13). Now let's drive straight forward some distance and stop again. Under our conditions of equal tire size, proper inflation and no overloading, all four marks should be nearly, if not exactly, in the same position on all four tires (Figure 14).
Figure 15:
Now let's go into a turn and see how the situation changes. Figure 15 shows that each wheel travels in a different arc. The outside front wheel travels the greatest distance, and the inside rear wheel travels the shortest. Each front wheel travels further (along a larger arc) than the rear wheel on the same side.
Figure 13:
Figure 16:
If we started with the marks on the tires at the same place (as in Figure 13), they might look like this after a turn (Figure 16). Because the front wheels travel along a greater arc, they travel further. Because they travel further, they have rotated more. Therefore, their marks would be ahead of the marks on the rear wheels.
Is it a problem that the front tires travel further than the rear ones? No, because the front wheels in this 2WD example are free to turn independently of the rear wheels. Also, the different distances traveled by each of the rear wheels is compensated for by the rear differential. But, what happens if the front wheels are linked to the rear wheels and can't turn freely as is the case in 4WD in a 4 x 4?
Operating In 4WD
In 4WD, the front hubs are locked (either automatically or manually). As a result, the front wheels are no longer free to turn independently. The front and rear driveshafts are also now locked together and must turn as one. What's going to happen when the vehicle turns? That's a very important question.
Figure 17:
In 4WD, just as in 2WD, each front wheel travels further than the rear wheel on its side. But this time, the front wheels aren't free to turn independently of the rear wheels. They are linked to the drivetrain by the transfer case and front drive shaft (Figure 17).
Even though differences between the inner and outer front wheels are compensated for by a front differential, torsional windup is still going to occur. Anytime one driveline component travels further than another, there will be torsional windup that must be released. Before we explain how this happens, we need to point out some very important facts about 4WD in a 4 x 4.
4WD in a 4 x 4 provides increased drive traction and performance in off-road situations and when driving on snow or ice. It is not designed for operation on dry surfaces and when traction is good. It is not suitable on paved roads and in shopping mall parking lots. It is this difference between where 4WD should and shouldn't be used that gives us the answer to what releases the torsional windup.
When a 4 x 4 is driven off-road in 4WD, the wheels are in contact with dirt, sand and sometimes mud. The vehicle is also likely to bounce over bumps and ruts. In turns under these conditions, torsional windup will occur. If it does, the loose dirt or sand or the slippery mud will allow wheels to slip a little, limiting the windup. Tension is also relieved when the wheel bounces in the air going over a bump. Snow and ice will also allow the wheels to slip and limit the windup.
When operating in 4WD on hard pavement, even more torsional windup occurs, and it isn't easily released. When the windup becomes great enough, the driver will feel the vehicle hop, skip or bounce. This is caused by the front or rear wheels slipping as the windup is released. Whichever wheels have the least traction are the ones that will slip. The tighter the turn, or the greater the traction, the greater the hop, skip or bounce. It is this torsional driveline windup that is the cause of many owner complaints concerning the performance of their 4 x 4.
One final point. You will recall that at the beginning of this section, we mentioned that we had made sure that all four tires on our truck were the same size. We also specified that the tires were truly the same size, were worn the same amount, were inflated to the specified pressure and were subjected to reasonable loading. Avoiding driveline windup was the reason for this.
Figure 18:
Figure 18A shows that tires of even slightly different size roll different distances every revolution. Figure 18B shows that the same effect is true for tires that are the same size but inflated or loaded differently. Operating a 4 x 4 in 4WD with tires of different size or inflation will produce driveline windup, even when driving straight ahead! If the vehicle is being driven in 4WD on dry, hard pavement, the driver will notice drag and may experience the hop, skip, bounce effect as the front or rear wheels release the windup.
Aerostar All-Wheel Drive
The problem of driveline windup does not apply to the Aerostar All-Wheel Drive. Using a differential built into the transfer case, the all-wheel drive system distributes more torque to the rear wheels than to the front under normal conditions. This differential action offsets any windup. It is only when traction is poor, such as on snow or ice, that the Aerostar system locks up and equal rotation is directed front and back. In slippery conditions, windup is released, just as it is in a 4 x 4. The Aerostar system continually senses road conditions and switches out of lockup as soon as there is sufficient traction.