4WD Silverado, Ray's current column: Center differential & turning

I didn’t think the center differential in a 4WD vehicle was for dealing with wheel speed differences when turning. I thought it was for wheel speed differences front wheels vs rear wheels.

My 4WD truck doesn’t use a center differential. I use 2WD mode on hard surfaces. I’ve never considered the technical details about this issue. Is the center differential in a 4WD vehicle involved with wheel speed differences when turning?

Modern AWD systems are so complex and varied that the answer to your question might depend on the make and model. As far as I can tell, the only people who really understand contemporary AWD designs are the guys who invent them.

I’d like to see that article. There is no llnk to it. 4WD doesn’t use a center differential, those are for AWD.

The Q & A is on the Car Talk site;
When Deciding Between 4WD and 2WD, Ignore the Internet - Car Talk

The truck mentioned has a locking center differential, therefore Auto (AWD) and four-wheel drive high/low. The Jeep Grand Cherokee has had a locking center differential for 30 years.

After tutoring so many people with drive train problems you are not familiar with four wheel drive? Engauge your trucks transfer case in four-wheel drive and drive around the block. You might understand the need for a center differential.

The front wheels will go farther during a turn than the rear wheels so the center differential should come into play.

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Disclaimer, I did not read the article.
George’s truck and my 4X4 truck have ‘conventional’ transfer cases, no center differential. Therefore should not be used on dry, paved roads. Mine also has an e-locker rear differential, again only for low traction situations.
I rarely have the opportunity to go off roading, occasionally will go to a dirt road just to exercise the transfer case. Must say, I prefer the old, manually operated shift for the transfer case, had one problems with the electronically controlled shift, fixed under warranty.
Many different 4WD and AWD drive systems on the market under various names. Haven’t read the information in many years, I think in the mid 70s read the various systems Jeep had available, even that was confusing.

Why can you run your Chevy/Ford/Dodge/Jeep in 4WD on hard pavement? They all use the same transfer basic cases (different model numbers) and they all have a slip yoke in them. Bear in mind that if you drive in 4WD on hard pavement too much, the slip yoke will wear out so it is not a good idea to make a practice out of this.

I had a Toyota Tercel 4WD Wagon once. Its transfer case did not have a slip yoke and driving just a few feet on hard pavement would lock it up. You could not steer it and you could not take it out of 4WD until you jacked up one rear tire off the ground. Then you could get it back into FWD and drive on. Some hard core off roaders will remove the slip yoke and replace it with a non slip yoke.

AWD uses a center differential with a viscous clutch or something like that. That basically makes the center differential a limited slip, but it does allow tires to turn at different speeds for steering.


I think of the terms this way.

AWD: Front/Rear differences solved by viscous coupling. Good for daily drivers, but not for hard-core 4WD purposes.
4WD: One of 3 types
type 1: Xfer case has no differential, 4WD is only mode, only for use on dirt surfaces…
type 2: Xfer case has no differential, but 2WD mode available for hard pavement using free-running hubs. This type of transfer case is similar to type 1, uses gears only, no chain. (My truck is type 2.)
type 3: Xfer case contains differential (ring and pinion gear type), can be use in 4WD mode on any surface. This type of xfer case (on some Fords anyway) uses an internal chain drive.

That has nothing to do with a slip yoke.

A slip yoke is so the length of the drive shaft can lengthen or shorten because of suspension movement.

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True AWD will have a center differential. Either an open diff or some variety of limited slip. Usually a Torsen gear type, viscous or electronically controlled types. It must be there to prevent binding caused by varying wheelspeeds while turning. These can be put in serious offroad machines… like Range Rovers.

Trucks using a NP246 transfer case uses a selectable overrunning clutch that only engages the front axle if the rear slips. Then it can select a locked center for 4WD.

My truck uses a slip yoke (on the rear driveshaft at least), and that’s what I thought it was it’s main purpose. It could I suppose also help with preventing all four wheels from completely binding & locking up if driving on pavement in 4WD mode. I’ve never experienced any drivability symptoms w/my truck driving short distances on pavement in 4WD-Hi mode & hubs engaged. Maybe the slip-yoke helps, don’t know.

It was in the video I linked to.

Once you shear the splines on a slip yoke there will be no front/rear binding, but then you will have a 2WD vehicle.
More likely to break the u joint first, then a drive shaft will be on the pavement.

On a serious note: Interesting the NP 246.