Part time 4wd

[jacal]

Part time 4wd, in my eyes, is the most common system used on pickup trucks. I completely understand the rear differential "side" of the equation. But I don't fully understand the "front" half on trucks without front-axels (which are most light-duty trucks).

So, let's take an example, say a Ford F-150 or Chev silverado. Both are part time 4x4, with the chev having optional 'on-demand' system. Without a front axel, how does part time 4x4 handle the 2 front wheels? In other words, are they locked? I have a chevy driving friend who swears that in 4x4 mode both fronts are locked. Is power split evenly to both CVs? If 1 front wheel is spinning, can the other still get meaninful power?

From what I can gather, only the Nissan Titan offers a system of limited slip on the front wheels. Do any of the others?



J

 

[trijicon]

From what I've heard, with locked fronts, both front wheels spin with same amount of power and torque. My understanding is, if the truck is tilted to the right, more pressure is placed on the front right wheel. Slip diffs will placed most of the torque on the right. But if the front left wheel is hooked on a rock, it needs the locked diff to pull the truck up, the slip diff won't allow the left wheel to do this.

 

[fourwd1]

If I read your question right, I think you may be confusing part time/full time 4WD with open/limited slip/locked differentials.


Part time vs full time is self explanatory for the most part (although there are many different full time systems).

Most 4WDs, part or full time, come with open diffs.
With an open diff power goes to the wheel with the least traction (if one wheel is in mud and the other is on solid ground, the wheel in the mud gets the power). So basically your 4WD is really 2WD.

Next step up is a limited slip diff. Here the diff attempts to send some of the power to the wheel with some traction. It works under some circumsatnces, but if one wheel has no traction at all, you're still stuck.

The next step is a locker, of which there are several types.
Basically, a locker locks the diff so equal power goes to both wheels. That way the wheel with some traction can pull you out.
Most lockers come on and off automatically, ie, they're unlocked under normal straight line driving, and lock when a wheel looses traction. Like I said, there are different types of lockers, and they work and react differently, Some are better on the street, some are better off road (and can be unpredictable on the street under some conditions). The best would be a selectable locker like the ARB or the electric locker. These allow you to go from full open to fully locked at the touch of a switch.


The Jeep Wrangler Rubicon comes with selectable lockers front and rear, which also act as limited slips when the locker is off.

 

[jacal]

Most 4WDs, part or full time, come with open diffs.
With an open diff power goes to the wheel with the least traction (if one wheel is in mud and the other is on solid ground, the wheel in the mud gets the power). So basically your 4WD is really 2WD.


This answers my question! So the front, even if it is a "Real" axel or not (i.e. some IFS) will be open unless otherwise stated (one of the traction control methods.

I guess I was having trouble understanding the concepts for IFS.


J

 

[jwyshrek]

Wait, now I'm confused.

Let's take my other vehicle for instance...2000 Jeep Cherokee, with NP231 transfer case...part time 4wd.

I always believed (and heard) that when the transfer case is engaged in 4wd (hi or lo), the front and rear drive shafts are 'locked'. So, both drive shafts have to be turning at the same speed...all the time. In a sense, there is no 'differential' action between the front and rear drive shafts. This is why you should never drive a part time 4wd vehicle on dry pavement in 4wd mode...you need the front wheels and rear wheel turning at different speeds for turns and such.

So, lets say the both rear wheels have traction and only one front wheel has traction...the other front is either in mud, up in the air, whatever.

If both the front diff and rear diff have to be turning at the same speed, then that front wheel with no traction can't be just 'free spinning'...it would be going 'faster' than the rear wheels.

Does this make sense to anyone?

My Cherokee does have a front diff (solid axel)...but I think the diff is only there for 2wd mode so that the left and right (front) wheels can turn at different speeds for turns and such.

But in 4wd mode, the front driveshaft and rear driveshaft are 'locked' so both front wheels have to turn as the same speed as the rear...so, in a sense, they are 'locked'...the two front wheels.

Now, if you had a center diff or a transfer case with a diff...and it can't be locked, then I would see why you would want a diff lock or some sort of traction control device in the front diff.

Isn't this why pickups (like tacoma) and suvs only have diff locks available for rear?

Hmmm...still confused...

isn't there something about the new FJ...that if you turn on the diff lock, it disables the ATRAC???

J

 

[JB.]

But in 4wd mode, the front driveshaft and rear driveshaft are 'locked' so both front wheels have to turn as the same speed as the rear...so, in a sense, they are 'locked'...the two front wheels.

You're right. When on a traction surface the open front is essentially locked by being connected to the locked center and rear.
Thai pointed out to me elsewhere, though, that in mud, for instance, as soon as all the wheels start spinning, the open front shows up and you end up with just 3 wheels spinning; two in back, one in front.

 

[jwyshrek]

Ah, then I see the need for a front diff locker.

Basically, the inverse of the scenerio I described previously...

thanks,
J

 

[fourwd1]

Wait, now I'm confused.

Let's take my other vehicle for instance...2000 Jeep Cherokee, with NP231 transfer case...part time 4wd.

I always believed (and heard) that when the transfer case is engaged in 4wd (hi or lo), the front and rear drive shafts are 'locked'. So, both drive shafts have to be turning at the same speed...all the time. In a sense, there is no 'differential' action between the front and rear drive shafts. This is why you should never drive a part time 4wd vehicle on dry pavement in 4wd mode...you need the front wheels and rear wheel turning at different speeds for turns and such.

If I remember correctly from my Jeepin days, the 231 is not like that. The 241 case can do that in the hi-loc or lo-loc positions.

My Cherokee does have a front diff (solid axel)...but I think the diff is only there for 2wd mode so that the left and right (front) wheels can turn at different speeds for turns and such.

The diff is there so your front axle can be a drive axle (it converts the drive shaft rotations to wheel rotations).
It is open so on a turn the outside wheel can rotate at a faster RPM than the inner wheel (required for turning). If the diff were locked, the vehicle would want to go straight even though you were steering into a turn.

This is also why a rear diff with a locker should be unlocked on a turn, but its not as critical as the front. Sometimes in slippery on-road conditions an auto locker in the rear can lock up (it senses the slip) and can send the rear end of the vehicle spinning out of control. That is why on the few vehicles with a factory rear locker (which is selectable, not auto, there are many interlocks to prevent you from engaging the locker when you are in 2WD or above 5 MPH.


Nowadays there are many kinds of part time and full time 4WD systems that use high tech ways to improve traction. They use viscous couplings, ABS sensors, and other stuff to stretch the envelope in traction.

For example, the system on my Isuzu Trooper has 2WD mode, a 4WD high/full time mode (where I can hit 60mph on dry pavement with out hurting it) that senses front wheel slippage (utilizing the ABS sensors I think) and only sends power to the front when needed. Then it also has the traditional 4WD low range that sends power front and rear equally. Plus the 4 wheel ABS works in 4WD too (not just 2WD like most) and has a factory LSD.

 

[jwyshrek]

The NP231 is part time with 2h-4h(part time)-N-4l(part time)