Originally Posted by troverman

The Ford manual recommends many things which may not be necessary. For example, on Super Duty's with manual hubs / manual 4x4 shift, they recommend shifting in and out of 4x4 at speeds less than 5mph. In fact, with the hubs already locked in, and the rear wheels not spinning, one can shift in and out of 4x4 High at virtually any speed. To avoid damaging anything, it is best to do this with the front wheels turned straight and your foot off the throttle. To shift into Low Range, the transmission should be in neutral and the vehicle speed should be 5 mph or less. This is the same procedure for probably any vehicle with low, electric or manual shift, or permanent 4x4 such as my Range Rover. The reason is the reduction gear, of course...you cannot 'shift on the fly' into low or the sudden reduction in the drivetrain would most likely cause damage to any number of torque-stressed components, or perhaps over-rev the engine, or cause loss of vehicle control, or all of the above.

Another 4x4 basic knowledge (as somewhat already covered here) is knowing that these Super Duty trucks have a Part-Time system. The part-time system only means that there is no method of varying speed between the axles. If we all only drove in a straight line, a part-time 4x4 is no worse for wear than a full time 4x4. You can engage your truck's 4x4 Hi or Low at any time on dry pavement without fear of damage so long as you don't turn. Of course, that is not practical, so 'Part Time' refers to only engaging 4x4 when driving on low traction surfaces, such as snow, ice, or off-road.

A brief explanation of why a part-time system cannot be used all the time, but a full-time or permanent system can:

When a vehicle steers, the wheel on the outside of the turning radius (the corner you are going around) must actually spin more times than the wheel on the inside of the turning radius. This is because the outside wheel is covering more distance. To allow this to happen, vehicle manufacturers use a differential (the pumpkin hanging down on the front and rear axles) However, when the same vehicle is steering around this same corner, there is also a difference in speed between the front and rear axles. This is because the rear wheels do not actually steer, so they 'follow' the front wheels, but not in exactly the same path. Notice when you drive your vehicle in fresh snow on a corner and then look behind you, there are not just two tire tracks...there are four. This is all totally fine on a two-wheel drive vehicle...the front and rear axles are not connected, and the axle differential will handle any difference in speed between inner / outer wheels. The problem is when 4x4 is engaged on a part-time system, the transfer case spins the drive shaft to the front and rear axles at exactly the same speed, at all times. This forces the front and rear wheels to turn at exactly the same speed. While the front and rear axle differentials will compensate for wheel speed variations on their respective axles, they cannot compensate for the fact that the rear wheels together are spinning slightly slower or faster than the front wheels, together, during a turn. Thus, by engaging 4x4 on a part-time system, you are locking the axles together. When used on a lower-traction surface, such as mud, sand, dirt, snow, etc...there is no problem...the front or rear wheels actually slip a little on their own to compensate for differing axle speeds. However, on dry pavement or other tractive surface, the wheels cannot easily slip...resulting in "hop" and driveline stress.

Now consider a vehicle with 'full-time' or 'permanent' 4x4: a third differential is included into the system, typically installed in the transfer case (center differential). This allows the front and rear driveshaft speed to vary between the axles...and the result is very smooth 4x4 operation that will never hop, even on the most high-traction surface.

Keep in mind, however, that any normal, open differential (front, center, or rear) is a double-edged sword. It makes for smooth operation, but by its very design it will always send power to the path of least resistance. This is fine for a high-traction surface, but now consider this scenario: your truck is parked on a street, facing uphill. You are in 2WD, and water running down the hill in the gutter has frozen. Your right wheels are, in fact, setting completely on ice while the left wheels are on dry pavement. You try to pull away, but only the right rear wheel spins! You go nowhere. So you engage 4x4 and try to pull away. Now, both right-side wheels in the gutter on shiny ice spin, but both left side wheels on pavement don't turn at all...and your truck goes nowhere. The reason is that power flowing through an open differential always goes to the path of least resistance...in this case it is easier for the diff to spin the wheel(s) on ice than to shove the truck forward against the traction of the dry pavement.

The vehicle with full-time 4x4 parked just up the street from you does no better. In fact, he does even worse. In his case, only his right front wheel is on ice, and the other three wheels are on dry pavement. It is true his vehicle is always in 4x4, but now there are more differentials in the equation. In this case, the center diff sends power to the path of least resistance, which is the front axle; the front axle diff then sends the power to continue along the path of least resistance which is the front right wheel, which spins uselessly on ice. So, despite being in 4x4, this vehicle can actually only spin one tire.

Of course, most vehicles with full-time or permanent 4x4 include a solution to this problem, with some sort of limited-slip or locker device on the center differential. In terms of traction, this now puts them on equals with a stock "part-time" system. Why are vehicles still made with part-time systems? Easy, a transfer case with a center diff and some additional type of locker or anti-slip mechanism costs considerably more than a simple part-time system. Additionally, a part-time system is lighter weight, and has the advantage of improving fuel economy when 4x4 is not engaged. It is also interesting to note that a vehicle with full-time / permanent 4x4 with a locked center differential will also suffer the same 'hop' and driveline stress as a part-time 4x4 vehicle with the front axle engaged.

Finally, back to the icy street scenario: at the very least, traction control will get the truck moving with the right-side wheels spinning. The ABS detects the right wheel spinning, but not the left. The ABS then pulses the brake rapidly on and off (on the spinning wheel only) which creates resistance, sending pulses of power over to the left wheel with traction. This is usually enough to get a vehicle moving and on to better traction, but the traction control itself cannot be used continuously or it will overheat the brakes and shut down. This is where a locking rear diff, or even better locking front and rear diffs, come in. Some Super Dutys have the factory rear locker...toggle it on, and the rear diff locks together, meaning both wheels are forced to turn at exactly the same speed. In this scenario, our example truck easily drives away. Once again, however, note that locking axle diffs cause even more difficulty with turning than locking transfer cases, due to not allowing any variation in speed between the inner / outer wheels. Very few vehicles offer locking front differentials as factory due to turning being almost impossible. Two vehicles that come to mind include the Jeep Wrangler Rubicon (locking front and rear diffs, part time 4x4) and the Mercedes G-wagen (locking front, center, and rear diffs, permanent 4x4).

Sorry for the long post, hope it is helpful to provide some basic understanding of 4x4 systems and differentials.

Originally Posted by BCM

When you miss the 'f'.

Originally Posted by PSDlongbox

there are exceptions to your all time 4x4 scenario...Take a subaru for example. An STi with DCCD I could lock the center diff at 50/50 torque split. This coupled with front and rear LSD make for about the most impressive 4x4 driving scenario on the planet. Many other companies even the old Land Cruisers that were all time AWD allowed for a locking center differential and had optional front and rear locking differentials.

I would take a locking front/center/rear diff setup in AWD format over any traditional part time 4WD such as the super duty.

Originally Posted by A/Ox4

Ive never been in a situation where I thought I had insignificant traction and required 4x4 yet thought it was safe to travel at 70mph or higher.

But thats just me.

Sent from my Galaxy S4

Originally Posted by troverman

Ford Limited Slip rear ends use clutches which when one wheel begins to slip, force a set amount of the torque over to the other side which presumable has more traction. The downsides of these are that the clutches wear out, and when they do, the differential becomes a normal, open diff with no control of side-side wheel slip. Additionally, the clutches cannot direct very much torque away from the side which is slipping; sometimes as low as 15%...depends on how aggressive the clutches are set from the factory. On the upside, they are fully automatic, better than nothing, and can be rebuilt when they fail.

Originally Posted by BCM

Is binding normal on a regular right hand turn? Pulling out from a stop sign today, inch of snow on the ground (and still snowing), and the truck hopped around the turn. Next stop, I took it real wide and it didn't hop.

Also, when I got to the parking lot, I shut off the 4H, drove about ten feet, and it felt like it was still engaged (hopped as I turned into a parking spot). I stopped and backed up to try again and it felt normal then. The 4x4 indicator was off on the dash at the time. Will these trucks not disengage fully sometimes, or is something going on?

Originally Posted by EXv10

Not set at the factory and the force goes to the clutches attached to the wheel with the traction.

Originally Posted by troverman

If its only binding when you turn right, chances are one of the automatic hubs is not disengaging on its own. Trip to the dealer, rightfully so.

Originally Posted by BCM

Is binding normal on a regular right hand turn? Pulling out from a stop sign today, inch of snow on the ground (and still snowing), and the truck hopped around the turn. Next stop, I took it real wide and it didn't hop.

Also, when I got to the parking lot, I shut off the 4H, drove about ten feet, and it felt like it was still engaged (hopped as I turned into a parking spot). I stopped and backed up to try again and it felt normal then. The 4x4 indicator was off on the dash at the time. Will these trucks not disengage fully sometimes, or is something going on?

Originally Posted by offroadnt

Should do exactly the same in left hand turns.

Originally Posted by BCM

It does. Just made a quick trip and found that out. lol

troverman... Didn't see your post up there before I started whining again. Guess it makes sense. Just never noticed it as much as I do now.