Help me understand my new Jeep 4WD

[aldo98229]

Yep, LSD in the rear, lockers both front and rear.

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[macintux]

On the same axle?

discussion: https://www.rubiconownersforum.com/...h-a-lsd-and-a-locker-in-the-rear-d-44.107498/

 

[aldo98229]

discussion: https://www.rubiconownersforum.com/...h-a-lsd-and-a-locker-in-the-rear-d-44.107498/

There are 25 different versions on that thread of what TJ Rubicons had and didn’t have. Half of them contradict the other half.

 

[macintux]

There are 25 different versions on that thread of what TJ Rubicons had and didn’t have. Half of them contradict the other half.

I didn’t say it was coherent discussion.

 

[jeepoch]

This is a very interesting thread.

I have a question on the Rubicon setup. I understand enabling front or rear lockers disables the BLD. But why doesn't Jeep offer an LSD in a Rubicon for the conditions where full lockers can't be used? Like in rain or wet conditions where extra help is needed to keep wheels without slippage. Wouldn't that be a great setup for Rubicons to have all three? BLD, LSD and lockers?

Coupedencal,

One thing that hasn't been mentioned in this thread is the Traction Control System (TCS) that comes standard on every Jeep model. This is implemented much more for on-road safety. But it may come into play (both good and bad) in certain situations.

First of all the Brake Lock Differential (BLD) is basically overridden by TCS when in either 2Hi, 4Hi (or Part-Time 4Hi). Both systems rely on the individual speed sensors, but think of TCS as BLD on steroids.

Traction Control attempts to comprehensively manage all 4 wheels simultaneously during any slip when driving straight while in any high gear mode. Brake Lock Differential is essentially always active but the TCS system additionally also potentially controls the brakes on all wheels on both axles on top of normal BLD which works on just the two wheels of each axle independently.

However in 4Lo, TCS is automatically disabled while the BLD subsystem remains active (always). Recall, on locked axles (either lockers or LSD) both wheels on each axle are mechanically linked so the speed measured on each wheel will be the same. BLD will apply braking action only when the wheels on the same axle are spinning at different rates. So essentially in 4Lo with 'locked' axles, no automatic braking whatsoever should be applied (or expected). So BLD is (naturally) active when running only with open diffs. Note that Rubicon's always run with open diffs until the lockers are enabled. So they have BLD in 4Lo until the lockers are engaged. For Sahara's and certain Sport trims with an LSD, BLD will still always be active on just the front axle. For standard Sports, BLD is always active on both axles.

While you can manually disable TCS in the cockpit via the dash pushbutton, BLD is always active (always). It's only the dynamic operation of the lockers that will keep it from applying any braking action.

But many people find that when TCS is enabled in certain situations, this causes them some grief especially in 4Hi when in deep snow. If the brakes are automatically applied in high drag situations, the driver will need to compensate by adding more throttle. Unfortunately, more power typically results in more slippage (spin) which only exacerbates the problem. By either manually disabling TCS when in 4Hi or explicitly switching into 4Lo should dramatically improve tractive effort when in these corner case conditions.

So while 4wd options are available you still have to be cognizant of the automatic braking systems on your JL as well.

Armed with knowledge of all things under your Jeep and how they all interact with each other will make the probability of getting stuck that much smaller. At least your skill of applying maximum tractive effort will also increase proportionally.

Jeeping can really be a blast when using these systems to successfully navigate through the most difficult of conditions. Especially when others can't. You purchased a very expensive and sophisticated tool, use it appropriately. While the Jeep engineers try to make this technology easy and available to anyone, it's your understanding and intuition in how best to use it comprehensively that will make your experience that much better.

Happy Jeeping
Jay

 

[FINN 2.0]

There is an auto mode option now for some engine choices, "4:1 Rock-Trac® Heavy-Duty Full-Time 4WD System". People buying the rubicon usually like to choose when their lockers are on instead of it automatically happening.

Do you know what the difference is between Rubicon's 4:1 Rock-Trac and Select-Trac?

 

[rcadden]

Jeep says you lose some fuel efficiency that way. It won’t hurt otherwise, but if the pavement is dry there’s no value either.

I have a '21 Sahara Altitude with 4Hauto + Selec-Trac. Just got back from a road trip to/from Colorado/Texas. Coming home, I left it on 4Hauto cause there was ice/snow on the highway. Was at full cruise when the road finally cleared, so just kept driving with 4Hauto on, figuring it would just keep it in 2H.

Finally stopped for gas and switched back to 2H and my MPG went up ~2 MPG over the next ~hour of driving. Same flat highway, same cruising speed, etc. Only difference was shifting from 4Hauto to 2H.

 

[Heimkehr]

I have a '21 Sahara Altitude with 4Hauto + Selec-Trac. Just got back from a road trip to/from Colorado/Texas. Coming home, I left it on 4Hauto cause there was ice/snow on the highway. Was at full cruise when the road finally cleared, so just kept driving with 4Hauto on, figuring it would just keep it in 2H.

Finally stopped for gas and switched back to 2H and my MPG went up ~2 MPG over the next ~hour of driving. Same flat highway, same cruising speed, etc. Only difference was shifting from 4Hauto to 2H.

Not having Selec-Trac in my JLU, I recently installed a set of four studded snow tires to hedge my bet in the white stuff. The new tire/wheel combo is heavier than the OEM wheels/tires (not excessively so, but certainly enough to notice).

Several tanks of fuel have confirmed that the snow tires have imposed a 2.7-3 mpg penalty when compared to the all-seasons. Same roads, same speeds, same conservative right foot. I was expecting, at most, a 2 mpg reduction, but it is what it is.

 

[TX_Ovrlnd]

Do you know what the difference is between Rubicon's 4:1 Rock-Trac and Select-Trac?

Rock-Trac is a full-time 4wd with a 4.0:1 low ratio; in 4H auto, 4wd is automatically engaged when the wrangler senses traction loss (slow crawl speed for overtaking obstacles is better)

Selec-Trac (Sahara) is a full-time 4wd with a 2.72:1 low ratio; in 4H auto it does the same as the Rubicon but a faster crawl speed which is better for traction loss in on-road driving

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