Turning wheel all the way while in 4WD

[AggieJeep]

Just throwing this out there but this is the greatest f&*#ing differential video in the history of mankind and it's over 80 years old at this point. Good part starts around 3:00 but it gives an excellent base of knowledge as to why 4WD binds.

4WD locks your differentials together that the pinion (input) gear spins at the same rate front and rear all the time. If you watch the above video, you'll see that on any given axle, the faster of the two wheels always spins with the pinion gear (and therefore the rest of the upstream powertrain) at the rate determined by the gearing.

So when you lock 4WD, that means there are one front tire and one rear tire that must spin at the same rate. Since your rear tires track straight and your front are able to turn, the rate of rotation is different between all 4 wheels through a turn. The sharper you turn, the larger the difference between wheel speed is.

But since one front and one rear have to spin together, there is conflict because when both have grip, they both want to spin at different rates. So, you get binding and hop as the one with less friction breaks free to match the one with more friction.

In 2wd, the front differential isn't engaged and will freewheel so there's only one rear tire that needs to spin at the same rate as the powertrain.


For OP's issue, I tend to agree with @ShadowsPapa that it's most likely u-joint binding if OP truly went full lock and was actually driving on snow. Snow itself isn't grippy enough to cause 4wd binding but "snow covered road" doesn't really differentiate between a dusting or a foot so it could be either. With light snow there's still more than enough traction to cause binding with good tires.

This^^^

This Jeep part time 4WD system locks one rear and one front wheel to the same speed. In a turn, those wheel speeds must differ so something has to slip. Don’t underestimate the road surface traction. If you are able to drive, that same level of traction that allows to get moving and stop will resist different wheel speeds in a turn.

CV joints do smooth things a bit over U joints when the drive angle moves off centerline but the differing front/rear wheel speeds are still the root issue. The stresses on the driveline are more apparent with U joints but stresses do still exist with CVs when driveline is off center. The physics of a CV are cool and worth digging into if you haven’t yet.

All that is just to say this is normal behavior. Go easy when the truck gives you feedback that it is working hard.

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[ShadowsPapa]

Here's the perfect test to show what's going on -
2020 JT in 4H making a u-turn
2021 JT in full-time 4H making a u-turn.
The former will likely jump,
the latter should make a smoother turn.

 

[Mac]

My Tundra did the exact same thing and it had CV joints in the front...

 

[21RG]

Lets make this a little more clear as mud.

1. It is normal.
2. Read #1

Lets clear up a couple of things.

The jeep has a transfer case, it the piece that is connected to the 4wd shift lever. It is not a center diff. A center diff is normally a part of the transfer case that has full time 4wd capability and it will allow the front and rear to operate independently just as your axle allows each tire to rotate independantly.

When in 2wd the power is transmitted to the rear axle only.

When engaged in 4wd, the transfer case is engaged to drive the front axle.

When in 4wd and you make a sharp turn, the front and rear are binding due to the traction that both the front and rear had.

The binding goes away as you straighten the front tires. This is completely normal.

When in 4wd, think of the front tire/axle pulling the vehicle and the rear tire/axle pushing. When you change the direction of one of them, they will bind against each other as they are connected thru the transfer case.

The reason they don't bind in low traction is because the front and rear are able to slip in say.... mud. So there is no bind transmitted and you don't feel it.

Full time 4wd has a center diff located inside the transfer case, they allow power to be transmitted to the path of least resistance, this is normally the side that has less traction than the other. A center locking button or lever is used to fully engage the 4wd and you will then transmit power to each axle 100% of the time.

Again, the bucking the driver was experiencing is normal while in 4wd and when both the front and rear have solid traction and the front tires are turned past a particular point. Completely normal.

Ways to prevent this bind,
- Do not turn as sharp
- Take out of 4wd; only engage when traction is limited.

 

[Kevin_D]

The Jeep AWD transfer case uses clutches, not a differential.
Just FYI.

Kevin

 

[ShadowsPapa]

The Jeep AWD transfer case uses clutches, not a differential.
Just FYI.

Kevin

I'd want to see the full schematic, but even the NP129 viscous coupling was called a differential.
I know in the case of the 2021 Wrangler and Gladiator transfer case the 4H full time mode is technically "automatic" meaning it applies power to the front when slippage is detected - but is it completely controlled by clutches or is there still what they call a differential in there like the New Process t-cases used.
Part time they are locked together - perhaps it's actually not a direct locking of the shafts so much as it is locking via a clutch pack and full-time allows the clutches to relax when there's no slippage.
My diagrams and tech docs on transfer cases ends with about 1996 Jeeps. Those are the last I've personally been into or dealt with.

 

[Kevin_D]

I'd want to see the full schematic, but even the NP129 viscous coupling was called a differential.
I know in the case of the 2021 Wrangler and Gladiator transfer case the 4H full time mode is technically "automatic" meaning it applies power to the front when slippage is detected - but is it completely controlled by clutches or is there still what they call a differential in there like the New Process t-cases used.
Part time they are locked together - perhaps it's actually not a direct locking of the shafts so much as it is locking via a clutch pack and full-time allows the clutches to relax when there's no slippage.
My diagrams and tech docs on transfer cases ends with about 1996 Jeeps. Those are the last I've personally been into or dealt with.

I just read a short article saying that the Jeep AWD/4WD transfer case used an electronically-controlled clutch pack rather than a differential.

Kevin

 

[21RG]

I just read a short article saying that the Jeep AWD/4WD transfer case used an electronically-controlled clutch pack rather than a differential.

Kevin

Either way, the AWD mode allows the front to be connected, but still turn or rotate at a slightly "different" speed to prevent binding. Once locked, it can and will bind creating the effect the original poster brought up.

Whether it is through clutch packs, gears, or electronically controlled system using a clutch pack, it results in the same action.

 

[gt3mike]

What @21RG said is correct. Binding has nothing to do with CV joints or universals. Binding has nothing to do with the front and rear axle differentials. Binding is all about the transfer case.

The part time 4WD transfer cases in Wranglers and Gladiators do not allow for different rotational speeds between the front vs rear driveshafts. When vehicles turn (the ones that don't have 4 wheel steering anyway), the front axle's track/radius is wider than the rear axle's track/radius. It's simple geometry. This is where the binding comes from. The sharper the turn, the bigger the front vs rear radius difference. The longer the wheelbase, the worse it gets. This is why Gladiators bind worse than a 4dr Wrangler, and the 4dr Wranglers bind worse than a 2dr Wrangler.

Full time 4WD transfer cases allow different rotational speeds between the front and rear driveshafts (when they aren't locked). This is why full time 4WD vehicles don't bind during turns.

 

[ShadowsPapa]

OK, here ya go...........

 

[CerOf]

NP249 downs in the ZJ grand cherokees used a viscous coupling. 96-98 could actually lock when in 4lo. The earlier didn’t.
Beginning on 99 the WJ used a gerotor to pump fluid to engage the front to rear when in fu

OK, here ya go...........

-b+~square root of bsquared. Minus 4ac all divided by 2a.

ya had to go get all scientific-y on us!

LOL.

 

[ShadowsPapa]

NP249 downs in the ZJ grand cherokees used a viscous coupling. 96-98 could actually lock when in 4lo. The earlier didn’t.
Beginning on 99 the WJ used a gerotor to pump fluid to engage the front to rear when in fu


-b+~square root of bsquared. Minus 4ac all divided by 2a.

ya had to go get all scientific-y on us!

LOL.

Just showing I had gotten in a hurry and not thoroughly thought it through.
I knew better, truly did.

The modern t-cases seem to work on the principal that the front drive shaft isn't powered until there's slippage detected then the clutches lock together. Putting it in 4H full-time basically locks the FAD and preps the system to go into 4 if needed, otherwise it's not truly powered.

 

[syreeves]

Agree best explanation video ever.

"Just throwing this out there but this is the greatest f&*#ing differential video in the history of mankind and it's over 80 years old at this point. Good part starts around 3:00 but it gives an excellent base of knowledge as to why 4WD binds. "

 

[brianinca]

CV's and Birfields will bind up as well, they just don't have the u-joint kickback.

Hey i man i just wanted to let you know that what your experiencing is completely "normal",Why? its because you got u joints in the front like all of us and that bucking/crow hop you got is them getting binding,but if you had some cv shafts from rcv for the front that would eliminate that and its a awesome upgrade if you wheel alot,but yeah man i hope i cleared any confusion

 

[brianinca]

Again, the U-Joint will give a cyclical kickback as angular velocity increases and decreases through the rotation across the union.

That is in addition to the binding of the whole drivetrain as wheel velocity differences get evened out with the tires skidding on a surface with traction. CV = constant velocity, and Birfields have the same characteristic.

well, im certainly up to learn something.

-if its that, why doesnt it do it on 2wd? The front diff isnt locked in 4wd...

-why did a previous truck with cv joints not do it while the center diff was open it would uturn but not when the center diff was locked?

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