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Front Differential Locks in 4WD; Should it?

15K views 35 replies 13 participants last post by  mommymallcrawler 
#1 ·
I am new to the forum and have just obtained my first jeep since the late 1970's. I now have a 2015 Rubicon Unlimited and am still learning how it is supposed to work. I recently tested the 4WD and was surprised to find that the front differential seems to lock when either in 4 high or low. This makes it very hard to take tight turns with the front tires needing to slip through the turn. I asked the service advisor at the local jeep dealer whether or not the differential should always lock when the transmission is placed in 4WD, and they said yes. This makes no sense to me when I already have the axle lock feature that allows me to decide when I want each axle locked. Would someone please clarify for me whether or not the differential should always lock in 4WD, and if so, why even have an axle lock feature? Thanks.
 
#2 · (Edited)
When you put it a n 4wd that connects your transfer case to the front wheels. Think of it as leaving the hubs locked in the old days. If You put the transfer case in 4wd the front wheels would be locked and have power. Take it out and they are free. The locking feature on your dash will lock both front axles to gather like a positrac rear diff but solid. Only works in low with the transfer case in 4wd. Might be something on jeeps website that can explain it better. Just looked there is a video on the website:)
 
#3 ·
If you where on dry pavement in 4hi or 4low.. this is exactly what should happen, as there is so much traction that both tires have power(even with out the locker on).. so this causes binding. 4wheel is part not for drive pavement use. You should however be able to drive straight at slow speed and not have issue for a few feet to make sure its all working. But feel the binding when you turn as the speed between the two tires is different(shorter path for the inside tire and longer for the outer) You don't get his feeling off road as there is slippage in the surface you are on.
 
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#4 ·
Thanks for the response, but I'm not sure that I follow. Even with the transfer case in 4WD Hi or Lo, shouldn't the front differential allow the front wheels to turn at different speeds through a tight turn unless the axle lock feature is activated?
 
#9 ·
Wranglers have what is known as part time four wheel drive. (Not meant to be used all the time).

As mentioned above, when turning you have your front and rear wheels turning at different speeds. When you engage the part time 4WD the transfer case locks the front and rear differentials together. The difference in speeds causes the binding.

Vehicles with full time four wheel drive can remain in 4WD all the time as they have some sort of differential action between the front and rear axles. Usually some sort of slipping clutch arrangement or fluid coupling.

This link has a more detailed explanation with diagrams: part time 4WD systems can not be used on pavement

Even off-road, the wheel speed difference can cause your front tires to 'push' and you won't always go in the direction you want in tight turns... (I don't know if you've locked your front axle yet, but then making sharp turns will get really interesting :) )
 
#14 ·
That will happen naturally in 4wd (no axle lock). Not only the left and right wheels need to turn at different speeds, but the front and rear as well on tight turns. The left and right has a differential. The front and back does not. They are directly connected so the front/rear wheels will bind/slip, bind/slip on tight turns. This is why you should never use part time 4wd on dry pavement. There is NO slippage between the front and rear wheels and it will cause damage if done too much.


If you had the front axle lock on AND 4wd you would for sure know it. Your turning radius is disastrous.
 
#17 ·
Another way of looking at it, and with a picture!! :bop:

1WD means 4WD disengaged on a non-locker vehicle. Only one rear tire has power

You will have actual 2WD when 4WD is engaged, with 1 Front + 1 Rear Tire having power, spinning with the same RPMs, ie, as previously noted, no slippage. This is where the binding comes up if one of the two tires can not "slip/spin" on the surface being travelled. Turning exacerbate the binding because front tire and rear tire are trying to move at different speeds instead of the same speed. The part-time transfer case is giving 50% to the front and 50% to the rear, whereas a AWD provides more RPMs to the front wheels since the combine front tires require more RPMs then the combined rear tires.

So when engaging 4WD with open differentials, it is not locking up the front axle nor the rear axle, it is locking up the rear axle TO the front axle.

The following is what happens when in 4WD with open differentials with slick surfaces.

https://www.artofmanliness.com/articles/gearhead-101-how-part-time-four-wheel-drive-works/

The Problem of Open Differentials on 4WD Vehicles

Open differentials are great in normal driving conditions. But because of the way in which they split power between two wheels, they become a problem in low traction driving conditions. You see, instead of splitting power evenly between both wheels, an open differential distributes power across them following a path of least resistance. This is terrible for traction.

Why?

Let’s explore this on a 2WD vehicle because you’ve likely experienced it.

Let’s say you’re trying to drive your rear-driven 2WD car up your snowy driveway. The left side is covered with snow, but the right side is dry pavement. You’d think this wouldn’t be a problem because your right rear wheel has plenty of traction on the dry pavement to power the car forward. But you’d be wrong.

In a car with an open differential, your right rear wheel isn’t going to get any power. Remember, open differentials distribute power across the axle following the path of least resistance. And in this situation, the wheel with the least resistance is the wheel driving on the snow — the left wheel. So all the torque is going to be sent to your left wheel. But because there’s no traction there, it just spins and spins, while leaving your car stationary.

This same thing happens on 4WD vehicles that utilize open differentials on the front and rear axles. Let’s use the same snowy driveway scenario. You’ve got 4WD engaged so you can make it up the snowy driveway. The transfer case is sending an equal amount of power to the front and rear differentials. You think to yourself “That snow on the left side shouldn’t be a problem at all! I’ve got plenty of traction on the right side and I’ve got both right wheels moving!”

But the differentials on your 4WD are open differentials. And open differentials distribute power across the axle following the path of least resistance. The snow-covered left side has the least amount of resistance. Guess what happens?

All the power goes to the left wheels, causing them to spin in place while your right wheels just sit there like a bunch of lugs leaving your vehicle at the bottom of the driveway. Your 4WD was made impotent by your open differentials.
 
#18 ·
Are you saying that your front Diff is locked? Does it lock automatically every time you put it in 4WD Hi or Low? It should not lock till in 4WD low and only then you push the button to do so.
 
#20 ·
It feels to me like the front differential locks when put into 4WD hi range, but I may be feeling the bind from the front axle to the back axle, not the left front to the right front. When I make a tight turn on some fairly well packed gravel, it is very obvious that every few feet one of the front tires has scuffed the ground as it slides to free up the bind.
 
#21 ·
It is likely that you are feeling the bind between the front and rear axles, and not bind between the left front and right front (as would be if the front axle were locked).
You have a Rubicon, which has the ability to lock the front axle (and the rear axle). As I recall (I could be wrong, I don't have a Rubi) the front axle will only allow it to be locked if the transfer case is in 4L and the rear axle is already locked.
Two easy quick tests. One, lock the front axle and see how it feels, I suspect it will feel massively different to what you are feeling now. You will need to be in 4L and already have the rear axle locked as I recall. If you do this test on dry pavement, I think you will find that it is near impossible to steer the Jeep. That is how it can be with the front axle locked. Turning the Jeep is only possible if the tires can slip / spin. In the dirt it is less noticeable, as the tires are more easily able to slip or spin.
Two, jack up the front end of the Jeep so that both tires are off the ground while in 4H. Spin one side forwards, like it is trying to roll down the road. If the axle is locked, it will not spin. If the diff is open, the tire should spin and the other side should spin in the other direction. It will not spin completely free, because the transfer case is in 4H and so the diff is directly connected to the transfer case and the rear axle. But it should be possible to rotate one of the two tires forwards with some effort. And that effort should also result in the other side spinning the other direction. If it is not possible to rotate the one tire, it may be that your locking differential is stuck in the locked position.
Clearly the first test is less effort.
 
#23 ·
The modern Wrangler transfer case effectively works much like the old Dana 20 case in a late 70's CJ. The guts are completely different (gears vs a chain) but they both do the same thing. 2WD is rear wheel drive. 4HI is 4HI in both. Same with neutral and 4LO.

Modern Wranglers have no lockout hubs on the front axle. They drive around as if you left the hubs locked all the time.

Now, if you ever had a QuadraTrac equipped CJ, that's a different beast. The QuadraTrac was an All Wheel Drive case. You could drive it on the street in 4WD (technically AWD) with no binding since it has a differential in the case. Going around a corner, all 4 tires were free to go the speed they needed to go. Engaging the "E-Drive" mode in the QuadraTrac is the same as 4HI in a modern Wrangler. All that did was make the transfer case lock the front and rear driveshafts to the same speed.

You will feel a little bit of steering push when you're in 4HI depending on how much traction you have. It's a lot more noticeable on pavement but you can tell the difference on dirt if you pay attention. But it's nowhere near as much as you would get if you actually locked the front differential.
 
#25 ·
There is your answer. If the axle were locked you would not be able to rotate one wheel in one direction and the other go in the other direction. Either both wheels would turn together or not at all.
If you did the same test but with the axle locked you would either be able to rotate both in the same direction or, more likely, they would not rotate at all because the rear tires would be on the ground and stop the rotation.
 
#26 ·
With the differentials unlocked, on a tight turn you will still feel slight binding through the steering wheel. In the old days with the hubs locked you would feel the same binding. Of course with the hubs unlocked the wheels would turn freely with no binding.

The differential locks are a totally different concept. As you know, with open diffs, you really only have power to one wheel on an axle. So, standard "two wheel drive" is in effect one wheel drive and open diff "four wheel drive" is in effect really two wheel drive (one driven wheel per axle).

The electrically activated lockers on on a Rubicon lock both axle shafts together so both wheels turn at the same rate. You really only want to do this to the front axle when traveling in a straight or near straight direction as it is extremely difficult to turn the Jeep even though the wheels are turned. Normally the inside wheel turns slower than the outside wheel. This is not possible with the diffs locked. Even the rear wheels want to turn at a different rate in a turn.

Locking diffs are far different than limited slip diffs. A locking diff will transmit the power to one wheel and if that wheel slips, then it transmits the power instead to the other wheel.

If you are in 4Lo and have both diffs locked while on dirt (or grass) and exceed the traction of the tires, you will dig 4 holes. I have done it a few times in my Rubicon when trying to pull a heavy load.

When you engage 4WD, you will get a light on the instrument console. If you lock the rear axle when in 4LO a second light on the instrument console will be lit and when you lock the front axle you should get a third light. While they are locking, the lights will flash and will go solid when locked. When unlocking they will again flash until they go out indicating that axle (diff) is no longer locked.

It is recommended that you periodically exercise the transfer case to 4HI and 4LO but the front and rear lockers as well. You do not need to be off pavement to do this as you don't need to travel to exercise the features. You may need to rock the Jeep back and forth a bit to activate both 4HI and 4LO as well as the lockers and deactivate them.
 
#27 ·
The Rubi also has a light to tell you the front or rear axle is locked side to side if the light is not ON then your axle should not be locked side to side.

Try putting it in 4Low and push the axle lock buttons, You might have to drive a few feet forward but then the lights should come ON telling you the axles are locked side to side

If they come ON try turning like that and you should see a even bigger bind in the steering than you are feeling with just the front axle locked to the rear in 4 High

When I'm plowing snow in my pickup even though it has an Open front differential I can still feel the bind if I'm driving on bare pavement and not on snow anymore.

By the way when you feel that bind you should not try to force it to turn any more as you may damage the U joint on the front hubs, which is doing the binding.

.
 
#29 ·
What you are experiencing on the steering wheel is NOT due to differential locking. It is an inherent design limitation in the axle shafts u-joints connecting the differential to the wheels. Once the Jeep (Rubicon or Non-Rubicon) is in 4WD, the axle shafts are no longer driven, rather driving the wheels, thus causing so called u-joint binding.

I think this video demonstrates very well, what every Jeep driver experience turning the wheels in 4WD (HI or LOW) ... (Note this demonstration is for a transfer-case driveshaft with two u-joints connections on both ends, yet the principle is the same)

 
#35 ·
Bob is absolutely correct. On tight turns, even in 2HI you can feel the slight binding in tight turns such as a parking slot in a parking lot. After a while you get so used to it, you tend to ignore it. When I make a tight turn out back on grass, you can see the effect on generally the outside wheel as it leaves scuff marks in the dirt/grass.

Back in the day of manual hubs, we didn't have this issue for not only was the front drive shaft disconnected in 2H, but the wheels were disconnected from each front axle shaft. Of course if you forgot to lock your hubs, putting the short stick in 4H or 4L had absolutely no impact. Just one of the reasons they did away with manual hubs as 4WD became more prevalent and the general public did not become educated on how to use manual hubs.

My '90 Dodge W250 came from the factory without manual hubs and just a simple cover over the end of the hubs. But it had central axle disconnect on the front axle so when in 2H you didn't have the axle binding in tight turns. A friend and I put manual hubs on the axle anyway. I am not sure, but I think the YJs had that as well. CJs had manual hubs. The TJ and JK do not have a CAD, but I believe it is back on the JLs for just the reason that caused the OP posted.
 
#36 ·
The first few times you are on a run offroad and watch someone drive with locked axles, it's sort of cool watching as you can very visibly see what "locking" the differential does.

There of course is a solution to turning a locked front axle even with big tires with no steering bind - beef up the steering. PSC big bore steering box and hydraulic assist steering. Add in RCV front axle shafts to eliminate bind. And you literally can turn steering stop to steering stop with fully locked axles using one finger. Of course it's going to cost you another $3k-ish (plus install if you cant do it yourself) for the PSC plus another $1400-ish for the RCV (plus install if you cant do it yourself). So for nearly $5k-ish, theres a solution.
 
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