.

 

Full-time 4x4 have a centre differential so they can be driven on the road because around corners the front axle travels at slightly different speed than the rear axle and the differential distributes the torque evenly front to rear at varying speeds.

Part-time 4x4 has a transfer case with equal rpm's going to front/rear driveshafts. If you drive it on pavement in the dry you'll feel the whole driveline bind up and eventually a u-joint, axle, driveshaft or other part will SNAP.

As well, full-time 4x4 systems generally have better balanced rotating parts in the front drive system compared to part-time.

Part-time will get better fuel economy as the front axle will not be rotating all the time as with a full-time setup.

There are advantages and disadvantages. You'll really only need 4x4 a for a few uses such as pulling a heavy boat on a slippery boatramp, snow/ice, off-road.

 

Thanks so much for your thorough explanation.

Regards
Habibi

 

Question?
Does the front driveshaft turn at all times? I know the cv axles turn at all time but I am not for sure on the front driveshaft and the front pumpkin?

 

No, the chain sprocket isn't locked. And only the left axle turns all the time, as the right side's side gear is disconnected from the actual right axle shaft until you hit that 4x4. Thus, the ingenious (depending on who you talk to) name "center disconnect." If you look under the truck, it's not really disconnected in the center, as that left axle is short. Maybe it should be called "margin to the left disconnect." Note the center disconnect system is the reason backing up to unlock hubs isn't required anymore, as there aren't any hubs to unlock.

Oh yeah, and just to clarify, the front pumpkin doesn't turn, the left axle side gear spins the pinion shaft gears in reverse, as the right side axle side gear is disconnected..

 

I think I understand.
This was the only part that got me. So your saying that while the left axle is turning the ring and pinon is spinning in reverse. Does this mean that the front driveshaft is free turning in one direction or another? I would say not, just the lext axle spinning the gears, but why in reverse. I have also seen that the front gears are reverse cut. I know what that means I am just having trouble picturing how everything spools up and work. My mind as felt blank all day, must be this cold I have. I had to check three times that I put the drain plug back into the oil pan today.

 

Looks like it's time to open a service manual and see an exploded view of the front axle assembly/hubs/lock-outs to see WTF is going on. Ha ha ha.

 

Yeah it is kind of screwed up. No, I was saying everything was spinning in reverse INSIDE the differential that is bolted to the ring and pinion. The ring and pinion stay still.

Think of it this way, with the rear axle. Normally, you've got a rear driveshaft turning the pinion and ring gear. You've got a box (diff) welded to that ring gear that has two side spider gears and two pinion shaft gears (pinion shaft meaning the pinion shaft inside the differential, has nothing to do with ring and "pinion"). When you take a turn the outer side gear turns faster than the inner (as mandated by the immense torque generated by your truck as it turns) and as allowed by the pinion shaft gears "walking around" the side gears (spinning on their own shafts). Now imagine you are going to push your truck for movement, but you do not want your rear driveshaft turning. Simply use a shifter fork to disconnect your right axle side gear from inside the diff. Now you push your truck, the left axle spins, the right axle spins too. INSIDE your diff, you right axle side gear is out of the picture. Your left axle spins its side gear, but now there is no resistance from the right side gear as there usually is, and the pinion shaft gears just spin and spin and spin INSIDE your diff. Your ring and pinion and driveshaft aren't moving, because they aren't being driven by anything.

I made an error when I said only the left axle spins constantly in the center disconnect system. What I meant was both axles spin constantly, but only one is "engaged" with the diff at all times, which creates the problem (or supposed problem) of installing any kind of limited slip device up front, as some say there would be a "pull" to the left.

 

That makes sense to me. I was looking at it and it's a shift fork that moves a coupler to connect or disconnect the right axle.

Makes sense how a LS carrier would screw with the theory. Are there any LS setups available for our trucks then? An ARB would probably be the only solution otherwise.

They lie a lot when they say "automatic hubs", I always wondered about that.

Theoretically then, for the best fuel economy we need to install manual locking hubs to stop the left and right shafts from turning.

Thanks anyhow for the clarification.

 

p_ferlow

Yes, I agree 100%, but it is truly debateable (and has been debated here) whether a limiting device up front would cause problems. In fact, Powertrax (last I knew that somebody posted here) swears that the pull to the left is not noticeable.

Regarding gas mileage, the proponents of the center disconnect system (meaning all the auto makers) claim that the drag on mileage is not really quantifiable, but who knows. And I have to confess I'm not much of a front wheel drive/CV joint expert, so I presume you know better than me whether you can put manual hubs on a 4 axle shaft (inner and outer) CV front wheel drive system we have on our Fords. For some reason I thought the old hubs were used on some sort of quasi-solid axle front wheel drive system, but like I said I don't know squat about FWD.

I do like not having to back up to unlock the hubs (especially if you are trying to use 4x4 in reverse). Probably the only dislike I have of the center disconnect is the difficulty (or uncertainty) of any slip limitation device up front, coupled with the fact that I hear about the vacuum lines rotting, or being snapped out of place by tall prairie grass and the like. For some reason a mechanical linkage (rather than some air sucking device) seems to me to be more reliabile, but statistically that's probably not the case.

 

I know a few people that could be considered under this statement.
Thanks for the Ford central disconnect 101, I learned a ton of usefull stuff. I tried studying the Haynes and Chilton's manual but neither provide enough or the correct type of info to satisfy me.