Purpose of 4WD axle actuator?

Forgive my naiveté, but can someone please explain to me why the 4WD front axle actuator is necessary?

If the front axle disconnect did not exist (and both front half shafts were permanently connected to the front diff.), I would think there would be no issue with binding in 2WD since the front driveshaft will spin freely as long as 4WD hi is not engaged?

What am I missing here?

Thanks,

Alex

It would be a lot of unnecessary wear on the moving parts. Trucks that have lock in hubs can have the hubs locked in, but if the shifter is not in 4WD it will still be 2WD, but the axles are still engaged. The extra drag can have a negative effect on fuel economy too.

Add-in some additional effects on steering feel when the axle actuator is engaged, at high steering angles.

I never figured out how a FWD vehicle can steer smoothly with the steering wheel against the lock, but an '88-up 4WD pickup can't, even though they use the same kind of U-joints on the front axle.

Add-in some additional effects on steering feel when the axle actuator is engaged, at high steering angles.

I never figured out how a FWD vehicle can steer smoothly with the steering wheel against the lock, but an '88-up 4WD pickup can't, even though they use the same kind of U-joints on the front axle.

Hmmmm good point, yes that is odd. I'll have to do some noodling on that one.

Add-in some additional effects on steering feel when the axle actuator is engaged, at high steering angles.

I never figured out how a FWD vehicle can steer smoothly with the steering wheel against the lock, but an '88-up 4WD pickup can't, even though they use the same kind of U-joints on the front axle.

Schurk - It's not the differential travel between the two front wheels causing the "porpoise" effect. As you point out, that problem is easily resolved with the open differential and CV axle/joint arrangement.

The problem is that when turning the tires on the steering axle move farther than the tires on the fixed axle. When only a little turn is involved the difference is small and allowed by tire slip. At larger steering angles the front tires may travel more than 2 times the distance of the rear wheels. As a matter of fact, in very tight turns the rear axle may travel a net of almost zero distance (pivoting on the axle) with one tire moving forward and one backward. At the same time the front tires must roll the full circumference of the arc of the turn.

The point where all this motion comes together is the transfer case. A differential is a form of mechanical summing device where the pinion speed is the average of the two axle speeds through the differential gears. The transfer case must also be the "summing junction" between the front and rear axles. However, there is no geared differential mechanism in the case, and if there were the result would be that the "loose" axle would get all the power to spin while the "traction" axle would get no power - Just like an open differential axle. This would completely negate any benefit of 4WD.

GM addressed this with a slip clutch in the NPG transfer cases, but only to a point. The common steering shudder in GM vehicles can occur when the case fluid is old and the friction modifier is no longer effective, i.e., the clutch doesn't slip - Just like an old Auburn style rear axle without friction modifier which shudders on turns.

The disconnection of the RF axle via the Blazer-style shot pin lets the front axle do whatever it wants independent of the rear axle.

the actuator locks the front axles . this is on in auto 4WD. the encoder is what connects the front to rear axles together . when a slip is sensed the encoder connects the rear to front axles in auto 4WD. then drops out .

when in auto 4wd you can feel slight vibs in the steering wheel in auto 4WD.
in bad weather conditions I always use auto 4wd and you cannot tell when it locks in to 4wd it is so fast. I rarely use full on 4WD. AUTO works much better when turning/maneuvers .

The only time I do use full on 4WD is like when I do a tire rotations. much easier no rolling issues . vehicle will not move.

In case the written explanation was inadequate, a diagram might make it more obvious. This is what happens when you drive down to your favorite taco stand and swing into a parking place:

http://www.wwdsltd.com/files/TurningDynamics.jpg

Wow thanks for all the informative commentary! However, I think Shurkey's point is still valid for when the truck is NOT in 4WD mode. I understand the mechanism for wheel shudder when 4WD is engaged. However think I recall that even when the tcase is in 2WD but the front axle actuator is activated (such as when a malfunctioning encoder does not listen to 4WD command) there is wheel shudder.

To shurkeys point, this is basically a FWD setup, i.e. open front diff plus CV axles. In this situation the chain in the tcase is not engaged so there should be slip between front and rear propeller shafts. So still I wonder, why the shudder?

Even if my memory is incOrrect, then my original question stands - why the need for the front axle disconnect? If the tcase controls the engagement between front and rear propeller shafts, then in 2WD mode there should be no need to disconnect front axle since tcase effectively does that for you. I'm confused?