2H vs 4H

jeepoch

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Lay,

[Edited from peer review feedback.]

One thing not really well understood about 4wd is that it depends on the type of axle, or rather the locking differentials, and whether (or not) you have them. Rubicon's have full lockers on both axles. Most (or probably the majority) of Sahara's have a Limited Slip Differential (LSD) on the rear, which isn't a true full locker. But in some ways it's even better. However like the Sport, the Sahara has an open differential (non-locking) in the front. Finally, most Sports come from the factory with only open diffs on both front and rear axles. Of course there are always some variations, but that's the general rule of thumb when thinking about the various Jeep trims.

Regardless of the trim level however, all axle types are pretty much always open diffs when 'unlocked'. The LSD is the only exception. It's potentially always partially 'locked' or possibly slipping all the time. For the locking diffs, they can only be locked in 4Lo (unless you have special overrides to very specific software calibrations via various third party programmers).

When 'unlocked' or with open diffs one and only one wheel on a particular axle, is getting power (when traction is lost). In fact the power is generally reduced to both wheels during 'slip' such that the overall contribution to motive effort from that particular axle is marginal.

So for every Jeep trim in 2Hi only one wheel (not two) is essentially getting power at the instant when either wheel starts slipping. Either can be powered but only the wheel with the least resistance is the one getting the most energy from the powertrain.

Noting that an open diff axle will distribute power to each wheel equally (50%) but ONLY with adequate traction. Once any wheel starts slipping, power is reduced to both wheels with the slipping wheel getting the most torque. Certainly not advantageous. That is exactly why lockers are so much better.

Also for every trim package when in 4Hi, potentially only two wheels are powered. One wheel in the front and one wheel in the back (if both axles are slipping). Again either wheel on each axle will be powered but only one per axle. So both 2Hi and 4Hi are misconceptions and named improperly. They should be named 1Hi or 2Hi respectively (while slipping). But since either wheel can be powered at any instant it's still considered 2 or 4wd respectively.

Only when in 4Lo do more 'powered' wheels potentially come into play. Only (ONLY) the Rubi's with the full lockers can have all 4 wheels powered (all the time) independent of any slipping wheels. When 'locked' all 4 wheels spin at exactly the same rate and the available torque is equally divided to all four wheels. This makes the Rubicon the only truly possible 4 wheel drive Jeep.

The Sahara with the LSD in the rear and an open diff up front when in 4Lo will have at least 3 powered wheels (during slip). The power to either rear wheels will be variable depending on the clutching action of it's LSD diff. So both rear wheels will have some fraction of the total available torque. However, they will both still be powered. In many situations, the LSD is far superior to the full locker because it will better adapt to give the most power to the non-slipping wheel. However, the Sahara's front axle is exactly like the Sport with an open diff.

With the Sport due to the open diffs both front and back, when in 4Lo it can have a potential of just two powered wheels, one on each axle if they're each slipping. So for the Sport it really can only ever achieve at best 3 but sometimes just 2wd, depending on which wheels on which axle are slipping. Again because either wheel on both axles can potentially be powered it's still considered 4wd.

So in summary (during wheel slip):
Sport:
2Hi -> 1 powered wheel (rear)
4Hi/Lo -> 2 powered wheels, 1 front 1 rear

Noting that most typically only one axle is experiencing slip so effectively there are 3 powered wheels.


Sahara with the LSD:
2Hi -> 2 variably powered wheels (rear)
4Hi/Lo -> 3 powered wheels, 1 front 2 rear

Note that with the LSD, it will always have at least 3 powered wheels during slip.


Rubicon:
2Hi -> 1 powered wheel rear
4Hi -> 2 or 3 powered wheels, same as the Sport

4Lo:
unlocked -> 2 or 3 powered wheels, same as the Sport
locked -> 4 powered wheels, 2 front 2 rear all the time

So advantage Rubicon. It is the only Jeep trim that can have all 4 wheels powered all the time independent of any slip.

I drive a lifted 2019 Sport with 35s. There are fortunately ways to compete with the Rubi's. I've done many trails successfully where some Rubicon's (almost all stock) had to be assisted. The secret is TRACTION. I've added a Sway Bar Quick Disconnect kit for added articulation and I've got a particularly light foot to keep from spinning any wheel. I also invested in really great 35" A/T tires. Regardless of your Jeep, independent of your 4wd selection mode TRACTION is always your best friend!

Hope this helps.
Jay
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TheRaven

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When 'unlocked' or with open diffs one and only one wheel, is getting power (ever). So for every Jeep trim in 2Hi only one wheel (not two) is getting power at any instant. Both can be powered but only the wheel with the least resistance is the one getting the energy from the powertrain.
I dunno if you perhaps aren't saying what you mean, but the above is not correct as written. As long as both wheels have equal traction, in an open diff setup, they get equal power...both wheels ARE powered. The problem is when one wheel spins and the other maintains traction. But again, as long as both wheels have traction, they are both powered.

See for more details:

https://www.carthrottle.com/post/en...differential-and-whats-most-suitable-for-you/

Open differentials are the most basic form of a differential. The purpose is to allow for different speeds between the two wheels, while torque split is held constant at 50/50. A common misconception with open differentials is that when one wheel is lifted, 100 per cent of the torque is sent to it. This is not true, however the amount of torque sent to the wheel with traction is very low because the amount of torque required to spin a wheel is also low. Remember, both wheels always receive equal torque, but if one has no resistance (eg. if it’s in the air), the amount of torque sent to the drive axle as a result is very low.
 

txj2go

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I dunno if you perhaps aren't saying what you mean, but the above is not correct as written. As long as both wheels have equal traction, in an open diff setup, they get equal power...both wheels ARE powered. The problem is when one wheel spins and the other maintains traction. But again, as long as both wheels have traction, they are both powered.
The differential is a simple mechanical device- the spider gears transmit equal torque to each axle shaft as long as both tires maintain traction. If one tire loses traction then that side spins and short circuits the system. It is perhaps misleading to say that all of the torque goes to the spinning tire because by definition the tire has lost traction and isn't resisting any force. It is almost like you have shifted the transmission to neutral- nothing is resisting torque from the engine and it revs freely. I'm thinking ice and snow where the spinning tire is not spinning against much resistance. On a gravel trail there could a good amount of resistance. And I suppose except for some internal friction as the spider gears are spinning, the tire that is not spinning is still receiving the same torque as the spinning tire, that torque just isn't very high.

In vehicles with open diffs it is not unusual to smoke one of the rear tires and the vehicle sits still with the other rear tire not turning. A Wrangler Sport in 4HI at least gets the resistance from trying to spin 2 tires instead of one. But it still means that if half of your lane is dry and the other half if covered in ice, you will have a hard time moving.
 

Northern88

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Lay,

One thing not really well understood about 4wd is that it depends on the type of axle (or rather the locking differentials) and whether (or not) you have them. Rubicon's have full lockers on both axles. Most (or probably the majority) of Sahara's have a Limited Slip Differential (LSD) on the rear, which isn't a true full locker. But in some ways it's even better. However like the Sport the Sahara has an open differential (non-locking) in the front. Sports come from the factory with only open diffs on both front and rear axles. Of course there are always some variations, but that's the general rule of thumb when thinking about the various Jeep trims.

Regardless of the trim level however, all axle types are pretty much always open diffs when 'unlocked'. The LSD is the only exception. It's potentially always partially 'locked' or possibly slipping all the time. For the locking diffs, they can only be locked in 4Lo (unless you have special overrides to very specific software calibrations via various third party programmers).

When 'unlocked' or with open diffs one and only one wheel, is getting power (ever). So for every Jeep trim in 2Hi only one wheel (not two) is getting power at any instant. Both can be powered but only the wheel with the least resistance is the one getting the energy from the powertrain.

Also for every trim package when in 4Hi, two (and only two) wheels are powered. One wheel in the front and one wheel in the back. Again either wheel will be powered but only one per axle. So both 2Hi and 4Hi are misconceptions and named improperly. They should be named 1Hi or 2Hi respectively. But since either wheel can be powered it's still considered 2 or 4wd.

Only when in 4Lo do more 'powered' wheels potentially come into play. Only (ONLY) the Rubi's with the full lockers can have all 4 wheels powered. When 'locked' all 4 wheels spin at exactly the same rate and the available torque is equally divided to all four wheels. This makes the Rubicon the only truly possible 4 wheel drive Jeep.

The Sahara with the LSD in the rear and an open diff up front when in 4Lo can only ever have 3 powered wheels. The power to either rear wheels will be variable depending on the clutching action of it's LSD diff. So both wheels will have some fraction of the total available torque. However, they will both still be powered. In many situations, the LSD is far superior to the full locker because it will better adapt to give the most power to the non-spinning wheel. However, the Sahara's front axle is exactly like the Sport.

With the Sport due to the open diffs both front and back, when in 4Lo it can only have a maximum of two powered wheels, one on each axle. So for the Sport it really can only achieve 2wd in 4Lo only. Again because either wheel on both axles can potentially be powered it's still considered 4wd.

Of course with open diffs, power can be applied to either wheel but unfortunately the power goes to the wheel with the least resistance, in other words the spinning wheel (yuck).

So in summary:
Sport:
2Hi -> 1 powered wheel (rear)
4Hi/Lo -> 2 powered wheels, 1 front 1 rear

Sahara with the LSD:
2Hi -> 2 variably powered wheels (rear)
4Hi/Lo -> 3 powered wheels, 1 front 2 rear

Rubicon:
2Hi -> 1 powered wheel rear
4Hi -> 2 powered wheels, 1 front 1 rear
4Lo:
unlocked -> 2 powered wheels, 1 front 1 rear
locked -> 4 powered wheels, 2 front 2 rear

So advantage Rubicon.

I drive a lifted 2019 Sport with 35s. There are fortunately ways to compete with the Rubi's. I've done many trails successfully where some Rubicon's (almost all stock) had to be assisted. The secret is TRACTION. I've added a Sway Bar Quick Disconnect kit for added articulation and I've got a particularly light foot to keep from spinning any wheel. I also invested in really great 35" A/T tires. Regardless of your Jeep, independent of your 4wd selection mode TRACTION is always your best friend!

Hope this helps.
Jay
So a good set of all terrains is the secret eh... I kid I kid
 
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jeepoch

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Remember, both wheels always receive equal torque, but if one has no resistance (eg. if it’s in the air), the amount of torque sent to the drive axle as a result is very low.

Kevin,

You are of course correct. Hell I felt like I was writing a damn encyclopedia as it was. I was also typing this on my phone while away from home.

My point was to highlight the fact that on an open diff with a spinning wheel, due to a loss of traction, is pretty much useless. You are correct in that both wheels do get half the power when both are grabbing and I could have worded that better. However, on-trail, ice, snow or whatever, traction is very important up to the point where you loose it and get into trouble. When you are at that point some wheel is loose and spinning putting the axles precisely in the conditions I was describing.

Thanks for the peer review. Accuracy is indeed a good thing.

[Edit]
I have since updated my original post with your feedback. Thank you.

Jay
 
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Heimkehr

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Sports come from the factory with only open diffs on both front and rear axles.
A Limited Slip Differential for the rear axle was a $595.00 option when I configured and ordered my 2021 Sport.
 

jeepoch

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So a good set of all terrains is the secret eh... I kid I kid
They certainly do help. Not the only factor, but yes for me they are a very large part. Anything that helps improve traction is the secret sauce. My Sway Bar Quick Disconnects I would rate right up there.

Jay
 

jeepoch

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Sports come from the factory with only open diffs on both front and rear axles. Of course there are always some variations, but that's the general rule of thumb when thinking about the various Jeep trims.
James,

You must be a professional journalist. Taking the entire sentence out of context.

Thanks, tough crowd tonight.
Jay
 
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TheRaven

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Kevin,

You are of course correct. Hell I felt like I was writing a damn encyclopedia as it was. I was also typing this on my phone while away from home.

My point was to highlight the fact that on an open diff with a spinning wheel, due to a loss of traction, is pretty much useless.
Yes this is what makes it so tough for people to grasp...it seems completely counter-intuitive. Since open diffs are designed to divide up torque evenly between the wheels, and since torque requires resistance, once one wheel slips, you go nowhere (because the other wheel gets the same amount of torque as the spinning wheel, which is almost zero). The LSD is designed for the opposite of this, which is to make sure that both wheels get the amount of torque that the non-slipping wheel gets. But this creates stability problems in turns, because both wheels are on the ground and getting traction, but spinning at different speeds. So an open diff is the best option for traction in a turn and a locker is the best option for traction in a straight line, while an LSD is the best option for rapidly changing traction situations. They all run into big problems when they are outside of their best case scenarios.

We've developed many "helper" systems for this though - any modern vehicle with an open diff comes with brake limiting (or stability control which takes it several steps further), which makes sure that no wheel is ever without resistance. Going back 20 years, my 5th gen T/A had an electronically controlled LSD - I had a button on the console that when pressed would turn the normally open diff into an LSD. So with the button off I could do a one-wheel burnout, and then turn it on to lay two strips of rubber. I think they did away with that system though because the diff was known to explode when presented with a highly-modified motor...and almost no one had a stock example of that car.

It remains very complicated to explain the reasons behind each type of diff but the important point to get across is that there is no "best" diff overall. Each one excels in different situations which is why all three types still exist on new vehicles even today.
 

jeep-v

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Here is the torque graphs for 3.6 vs. 2.0T. It is very clear - 3.6 lags behind starting from 1500 rpm. Car normally starts moving at 2500 rpm from stop on auto transmission, so 3.6 is like 40% behind at 2500 and it stays behind up to max rpm.
 

jeepoch

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It remains very complicated to explain the reasons behind each type of diff but the important point to get across is that there is no "best" diff overall. Each one excels in different situations which is why all three types still exist on new vehicles even today.
Kevin,

Thanks again for the very good explanation of the various types of differentials. For the OP's original inquiry of when to use the Wrangler's various 4wd modes I felt this answer needed a little more depth than just the knobs and levers in the cockpit.

However, it's clear that different type of differentials have applications that go way beyond the problem domain of just Jeep's. However, I think it may be safe to say that for an off-roading application the LSD's and the lockers shine in this regard.

Still, with the knowledge of how your rig works under the covers is what allows better on and off-road decisions in exactly which 4wd mode to apply in every situation. For most it's obvious, for some not so much. However for a noob this seems like very relevant information in how best to get started.

I was only intending to highlight some of this subtle 4 wheeling detail. You've provided even more valuable information than I had originally intended. For the uninitiated, the continuing idea of traction and stability control using the braking system to emulate the functions of an LSD still adds yet a whole other dimension to this subject.

However, these stability control systems are generally disabled during 4Lo. So this leaves the driver to best control traction on whatever equipment they may have when using it. Hopefully these past flew blurbs have helped.

Jay
 

TheRaven

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Here is the torque graphs for 3.6 vs. 2.0T. It is very clear - 3.6 lags behind starting from 1500 rpm. Car normally starts moving at 2500 rpm from stop on auto transmission, so 3.6 is like 40% behind at 2500 and it stays behind up to max rpm.
I don't see where you are getting ANY of that out of that link.

Here's a REAL test of REAL engines:

https://www.motor1.com/news/276660/2019-jeep-wrangler-dyno-test-video/

The 3.6l bests the 2.0l in power and torque until about 2800rpm. Now I grant that this is only one graph, but i've seen many, and NONE have the 2.0l anywhere near the 3.6l at 1500rpm. Ain't happening. Best i've ever seen is about 2300rpm. The 2.0l makes somewhere in the area of 50lbft more than the 3.6l at peak, but it comes on late and quits early, while the 3.6l's torque comes up REALLY fast and is dead even until the bitter end.

Also if your JL starts moving at 2500rpm you've got a transmission issue. Mine starts moving around 1000rpm. Even my wifes 2.0l Cherokee starts moving around 1300rpm...BARELY, but there's movement.
 

Wabujitsu

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Here is the torque graphs for 3.6 vs. 2.0T. It is very clear - 3.6 lags behind starting from 1500 rpm. Car normally starts moving at 2500 rpm from stop on auto transmission, so 3.6 is like 40% behind at 2500 and it stays behind up to max rpm.
The graphs you linked to are not actual dyno graphs. They are a guess only; the thread was posted before the first independent dyno graph was published, in 2018.
 

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However, these stability control systems are generally disabled during 4Lo.
"ESC On" is the default operating mode when the vehicle is running. The driver may disable it manually using the Partial Off or Full Off features, which are exclusively accessed via the ESC button itself.

A full explanation of the ESC system and how it works is evident beginning on, for example, p. 152 of the 2020 Owner's Manual. It is not stated, there or elsewhere, that the act of shifting in and out of 4WD Low will have any effect on the functioning of the ESC system. If there's proof that this does in fact occur, we'll be happy to review it here.
 

TheRaven

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However, these stability control systems are generally disabled during 4Lo. So this leaves the driver to best control traction on whatever equipment they may have when using it. Hopefully these past flew blurbs have helped.
Just wanted to add a note - BLD (the system that brakes the wheel without traction) cannot be turned off. So even with stability control off it's still active.

Otherwise, all good.
 
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