Is Transfer Case also a Differential?

I see people claiming that you can drive in 4WD on dry pavement, as long as you go in a straight line? This doesn't sounds right to me. I know that the front wheels can spin at different speeds from each other, because there is an open differential between them. Same for the rear (LSD not relevant here). But in 4WD, the front wheel set can't turn at a different rate than the rear wheel set, since the transfer case is not an open differential -- it's instead synchronized, right?

I expect that very small differences in tire size, due to wear, would cause the front and back to turn at slightly different speeds. Now, when people actually do drive on dray pavement in 4WD, I suspect the tires must slip a bit on the pavement occasionally to release the tension built up in the drivelines.

 

Here we go



Edit: no, the transfer gear box is not a differential. There are no spider gears, viscous coupler etc. It is fixed. The front and rear output shafts rotate at the same rate (unless you make some modifications to allow them to turn at different, but non-variable rates).

 

You are getting exceedingly technical and if you want to get specific about numbers, you are correct.

However running 4WD on dry pavement in a straight line for a few minutes every 30 days its not enough to cause damage to your truck or tires.

 

I just find it interesting. I read through five pages on the topic in an older discussion, and no one ever brought up my point, which seems to be the key piece of information.

If both your rear tires where significantly more wore down than your front, I expect the difference would build pretty quickly, at 50 MPH.

 

You are correct. That is why you hear a "chirp" or "grind" of the tires slipping on the asphalt since the transfer case maintains the same speed for both axles.

As previously stated, this slippage in on a road surface would have to occur over a long amount of time to cause significant damage.

Accidentally turn on an asphalt road in 4x4? No big deal. Just don't make it a habit

 

Rotate your tires.

You are talking about a substantial difference in tread. I think new tires in the front and bald tires in the rear wouldn’t be substantial enough to make a difference.

 

In theory, you are correct, the front and rear tires are probably not 100% in size and likeness with each other so they will spin at different speeds causing somewhat of a small binding issue. Mostly the difference in size is too small to even notice or make a difference. If that were to occur, you would experience the tires chirping and giving on the street. If these were purely black an white scenario and the tires were not allowed to lose contact and were 100% stuck to the road at full contact then yes the weaker point of the whole system is what will give. Since that is not the case and the tire to road contact is the weak point, it is what gives and will continue to give before any binding in the drivetrain will.

 

Go against the grain, just drive in 4wheel drive at all times. Your truck will adjust to it. Don't listen to what anyone says. Good luck.

 

why dont you try it for a month and report back to us all ok???

 

Define a few minutes. I might do it for 10-15' on the rare occasion and only in a straight line. About 10 miles/month in 4X4 is advised to keep everything lubricated. There aren't any places here where I can drive 10 miles in a straight line. And just because it doesn't break right away doesn't mean you aren't causing damage that will eventually lead to premature breakage.

I hope that is tongue in cheek.

This is what the inside of your transfer case looks like. When you operate in 4X4 on a surface with good traction it stresses the drive chain and it begins to stretch. Over time it will stretch enough to slip on the cogs and need replacing. It doesn't take more than a couple of hundred miles to do this, and the sharper the corners the quicker it will wear out. In extreme conditions you will actually shear off the shafts.

Small differences in tires can matter. A reputable tire store will NOT sell you just 2 tires for a 4X4 vehicle even in the same size. Running new tires on the rear and worn tires on the front can cause problems if you operate in 4X4. This is why it is important to keep the tires rotated so they wear evenly.

Just an example of how much it can matter. My son works in electrical construction and travels quite a bit each day to worksites. He came to me a couple of years ago with a question. Seems he had been driving the same route to and from a construction site for months and it was exactly 100 miles round trip every day. But he noticed that over the last couple of weeks he was now only driving 95 miles round trip. I pointed out that he had just purchased new tires and they new tires, with more tread were enough bigger that they required fewer revolutions to travel the same distance.

 

The difference would have to be so significant that it would be ludicrious. To the point where one tire would have to be completely bald and the other tire brand spanking new. Even at that point I seriously doubt any damage would occur if one were to drive in a straight line.

 

The ONE time I did it in my truck it blew up my CV axle lol. Maybe I'm just that unlucky, or it was already compromised...I dunno. I was in 4hi on an icy mountain road, pulled into a gas station that was covered and the concrete was dry. Turned into the pump and felt the front end bind up so bad it brought the truck to a abrupt stop. Got back on the road after filling up, tried to put it in 4hi again and got the dreaded CLUNK CLUNK CLUNK of a busted rotating CV axle.

Any time I'm on a trail now and I see asphalt coming up, I hit the brakes and take it out of 4wd or back up and do it if I have to, ever since that one experience I'm paranoid about it happening again.

 

Let’s not forget that driving in a perfectly straight line is virtually impossible. There a small deviations from “straight” happening all the time. These deviations allow for differences of rate of rotation between front and rear axles to be lost, relieving whatever minute amounts of binding that might otherwise occur. Still, tires are also not a fixed diameter as they’re mailable and change dimension constantly as they flex under load shift and roll over bumps, pot holes, reflectors etc.

 

The pic of the tcase chain explains it all. Hopefully the chain is stronger than the friction of your tires on the road. I would assume the cv is engineered to be weaker than the chain. Busted chain means 0WD.

 

The answer to the subject question is, of course, NO.
The body of your post is entirely correct in all respects (worth noting is that the transfer case is only synchronized in the 2/4 shift, and not the high/low shift).

When you hear people talking about it going "clunk" when shifting out of 4WD, it is due to the tension you described being suddenly released.

 

Busted chain means 2WD.
If the chain breaks, it will drop to the bottom of the transfer case, and it will leave the main shaft intact. As long as you disconnect the ADD, you should have no issue operating in 2WD.

 

A front-rear tread depth difference of just 1/16th of an inch will result in a driving distance difference of 0.4 inches per revolution.

 

Well, I will hold back from any negative or flaming comments as this statement put a huge bullseye on you...

No, your transfer case is not synchronized but it does sync your front and rear differentials together. You can roll on pavement on 4X4 but straight since it can bind and put undo stress on seals and bearings. Don't confuse this with AWD... 4X4 operates a little differently..

 

Your post was pretty much entirely redundant. OP was asking a question because he *actually does* understand how the system works, but saw people talking about it in a way that suggests that something else was going on there that didn't make any sense in the context of what he understood to be true.

Also note that in the context of transmission components, synchronizers are those parts that are responsible for causing two gears that are about to mesh to spin at the same speed (known as synchronization), allowing them to mesh smoothly instead of grinding. There is a synchronizer in the transfer case that brings the front output shaft up to speed with the main shaft so that it can be safely engaged while the vehicle is in motion.

 

Lol