Brake Lock Differential (BLD) techniques for open diff (non-locker & non-LSD) JLs

[Traveller128]

My '21 JL has the factory limited slip in the rear axle. If I've taken advantage of the BLD in off road situations, I haven't been aware of it but that's not to say it hasn't helped to get me out of some tough situations. I wish the Jeep LSD was the Detroit Tru-Trac, but whatever it is works pretty well in my experience.

It's a clutch style, and it's not a particularly robust one. If you have the LSD you have the D44 rear, and There's a Tru-Trac out for it now. When yours wears out, you can upgrade instead of replacing the broken clutch setup.

That's what we're going to do, then we'll have Tru-Tracs on both ends.

Only issue is, you'll start doing the math on just replacing the gears, and before you know it, you've spent real money!

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[Bzinsky]

Using TFL as an example (your example), they also have their roller test that proves these systems work.

I’ve seen that too, they don’t seem to work nearly as well in the real world where traction on each tire is constantly changing.

 

[Htfan]

Appreciate all of the comments and input. From what I gather, I was in a situation outside the capabilities of the BLD or my JLUs BLD is not functioning as designed.

 

[stumblinhorse]

@jimcoffey62 excellent write up. Very many people believe that LSD is a magic differential that allows wheels that have traction to always have power. That isn’t how it works. When you explain it to people they think you are describing an open diff. But yes ladies and gentlemen, if you have a front and a rear tire in the air, your LSD only vehicle will be stuck. BLD is what gets you moving, granted you know what you are doing.

 

[Ratbert]

Using TFL as an example (your example), they also have their roller test that proves these systems work.

Have they done that test with a Wrangler? I've only seen them do that with other vehicles.

 

[Capt. Don]

The vehicle senses wheel slipping and applies brakes to transfer power. If you apply brakes it's to both sides, negating wheel slippage, making the vehicle think there isn't a need to apply it to the one slipping. The driver should apply throttle not brake.

ETA: BLD tries to get the tires to turn at the same speed. If you apply the brakes, you limit that, and the torque. That results in you overriding BLD, and torque not being transferred. Here is an explanation: https://blog.stellantisnorthamerica.com/2008/02/11/jeep-brake-traction-control-explained/#:~:text=BLD will apply brake pressure,the wheel on the ground.

Key points:
Using the vehicle’s wheel speed sensors, BLD knows when one wheel on a driven axle is turning and the other is not. BLD will apply brake pressure to the wheel that is turning. (emphasis added)

In order to get the most out of BLD, the driver must adapt their driving style to characteristics of BLD. For example, when in a situation where one or more wheels loose traction and the vehicle will not continue in the desired direction, the driver should carefully and smoothly apply the throttle to allow more torque to go the wheels with traction as the brake(s) are applied. (emphasis added)

BLD looks at individual driven axles and tries to keep the wheels turning at the same speed.

Great explanation and attached article. I tried a steep muddy trail in my JLUHTXR to get a feel for BLD. In 4-Low I was amazed that with mudded up BFG All-Terrains it was still climbing on a slope steep enough that if I let off the throttle and applied brakes it slid back down the slope 20 yards. But with light throttle it climbed back to the same spot with no detectable wheel slip and very little slinging of mud up the side of the Jeep. Looking at how mudded up the tires were, I had difficulty believing it could climb that muddy slope slowly, not using previous momentum. Just climbed, slid back down the slope with brake on, and then climbed again. Not taking a run at it, just slowly climbing using BLD and LSD (rear) in 4-Low. Very impressive and no drama. One thing that I did not like in my 2008 JKR with front and rear lockers engaged was its tendency to go sideways on an off-camber snowy trail. Not fun on an off-camber snowy shelf road in Colorado. This system using BLD/LSD in 4-Low is much more planted as the camber changes on a slippery trail, much better directional stability.
So, on the new Rubicons does engaging the lockers front and rear turn off the BLD? Seems that would be a step in the wrong direction for directional control but understandable for maximum traction at all four wheels.

 

[Capt. Don]

I think you think it isn't engaging because it probably isn't. You're probably not applying steady, even throttle because it's slipping, thinking you need to let off the throttle, or apply brake. The opposite is true. That's when you need to increase throttle slightly, until it engages. You'll hear it, as you maintain throttle it'll start to make nasty braking sounds, and then the other wheel will start to turn.

I will say, it seems counter-intuitive, but don't let up on the throttle or apply brake, because that defeats BLD
[/QUOTE

Yes, I agree. It is smooth, steady and fairly light throttle that slowly works my JLUHTXR up some impressively steep muddy trails.

 

[Wabujitsu]

Great explanation and attached article. I tried a steep muddy trail in my JLUHTXR to get a feel for BLD. In 4-Low I was amazed that with mudded up BFG All-Terrains it was still climbing on a slope steep enough that if I let off the throttle and applied brakes it slid back down the slope 20 yards. But with light throttle it climbed back to the same spot with no detectable wheel slip and very little slinging of mud up the side of the Jeep. Looking at how mudded up the tires were, I had difficulty believing it could climb that muddy slope slowly, not using previous momentum. Just climbed, slid back down the slope with brake on, and then climbed again. Not taking a run at it, just slowly climbing using BLD and LSD (rear) in 4-Low. Very impressive and no drama. One thing that I did not like in my 2008 JKR with front and rear lockers engaged was its tendency to go sideways on an off-camber snowy trail. Not fun on an off-camber snowy shelf road in Colorado. This system using BLD/LSD in 4-Low is much more planted as the camber changes on a slippery trail, much better directional stability.
So, on the new Rubicons does engaging the lockers front and rear turn off the BLD? Seems that would be a step in the wrong direction for directional control but understandable for maximum traction at all four wheels.

Capt., my guess is BLD just won’t engage with both axles locked, because the wheels on each axle are moving at the same speed regardless of traction, or lack thereof.

 

[Ratbert]

One thing that I did not like in my 2008 JKR with front and rear lockers engaged was its tendency to go sideways on an off-camber snowy trail. Not fun on an off-camber snowy shelf road in Colorado. This system using BLD/LSD in 4-Low is much more planted as the camber changes on a slippery trail, much better directional stability.
So, on the new Rubicons does engaging the lockers front and rear turn off the BLD? Seems that would be a step in the wrong direction for directional control but understandable for maximum traction at all four wheels.

You probably shouldn't have both lockers engaged in that scenario. Locking the fronts severely impacts your ability to control where it goes.

BLD can't do anything useful with a locked axle. Of the two tires there's not one that's getting less traction.

 

[AcesandEights]

Great explanation and attached article. I tried a steep muddy trail in my JLUHTXR to get a feel for BLD. In 4-Low I was amazed that with mudded up BFG All-Terrains it was still climbing on a slope steep enough that if I let off the throttle and applied brakes it slid back down the slope 20 yards. But with light throttle it climbed back to the same spot with no detectable wheel slip and very little slinging of mud up the side of the Jeep. Looking at how mudded up the tires were, I had difficulty believing it could climb that muddy slope slowly, not using previous momentum. Just climbed, slid back down the slope with brake on, and then climbed again. Not taking a run at it, just slowly climbing using BLD and LSD (rear) in 4-Low. Very impressive and no drama. One thing that I did not like in my 2008 JKR with front and rear lockers engaged was its tendency to go sideways on an off-camber snowy trail. Not fun on an off-camber snowy shelf road in Colorado. This system using BLD/LSD in 4-Low is much more planted as the camber changes on a slippery trail, much better directional stability.
So, on the new Rubicons does engaging the lockers front and rear turn off the BLD? Seems that would be a step in the wrong direction for directional control but understandable for maximum traction at all four wheels.

I have found, like off camber snow, that the BLD works better than engaging front and rear lockers, some times. I've always condered lockers to be "low side finders" and when I recently got pretty sideways, off camber in deep snow it worked exactly that way. I tried digging out, and using lockers. I found not engaging the lockers worked better trying to back out of that off camber ditch. To me, BLD, and lockers, are different tools in the tool box. They're different, but overlap.

 

[AcesandEights]

Have they done that test with a Wrangler? I've only seen them do that with other vehicles.

I emailed them maybe three weeks ago and asked, when I saw Tommy was selling his Jeep. I never received a response, but within a couple days they did it with their First Edition Bronco.

 

[diesel_dave]

Have they done that test with a Wrangler? I've only seen them do that with other vehicles.

I too would like to see them do this test with a new Wrangler. I expect that would do incredibly well given my experiences with it.

 

[five9dak]

A limited slip uses either gears or a clutch to control how much torque is sent to each wheel. This is called the BIAS. Typically it is 3:1 or 4:1.

For example: Suppose your jeep is on a small hill and it needs 10 ft lb of torque to get it to move forward.

If one wheel is free spinning with 1 ft lb of torque, an open diff will apply the same 1 ft lb to the other wheel. This is less than 10 so you don't move. While a 3:1 LSD will apply 3 ft lb to the other wheel and since 3<10 you still don't move.

BEFORE BLD, the solution was to manually apply some brake (which goes to both wheels). Let's say you apply brake force of 5 ft lbs to both wheels and then give it some throttle. The LSD applies 3x=15 ft lbs of engine torque to the non spinning wheel. so the non spinning wheel now has 15 lbs of forward torque, but it also has -5 lbs of brake torque to overcome - which leaves you with 10 ft lbs of usable torque and that's enough to get you rolling.

Without BSD, the disadvantage is that when you apply the brake it goes to both wheels, so you need to overcome not only the free spin, but also overcome the brake force on the good wheel.

BLD works really well with a LSD. BLD will sense that a wheel is spinning and apply braking force JUST TO THE SPINNING WHEEL.

So consider a LSD with BSD and you need 10 ft lbs of torque to move forward.
BSD only needs to apply 3.3333 ft lb of brake force to the spinning wheel and the 3:1 LSD will apply 3x=10 ft lb to the good wheel and voila' you start moving.

Without the LSD ... the BSD must apply 10 ft lb of brake force to the spinning wheel because my open diff has the EXACT SAME TORQUE to both wheels.

Now imaging you are trying to climb a really steep hill and your jeep is loaded up with gear so it weighs a bunch. Instead of 10 ft lbs to get moving you need 100 ft lbs to get moving.... do the math.

Nothing you said is technically wrong, but it is important to note for most readers that the Jeep Wrangler JL LSD is not a torque biasing limited slip differential. It is a clutch pack based differential which has some minimum torque between the wheels it maintains, before it allows differentiation. This torque is based on the preload in the clutch pack and friction.

BLD probably will work a lot better with a torsen than it does with a clutch pack style LSD.

 

[Heimkehr]

To test your BLD:
If you are in 2wd you can jack up one rear wheel just enough that the wheel will spin.

Turn OFF electronic stability control (ESC) squiggly line button center console, bottom row of buttons, just to the left of your RED TRIANGLE hazard light button.

Edited to acknowledge the varying placement of the ESC button based on the vehicle's trim. Also because the button is on the dashboard and not the center console.

 

[6.2Blazer]

Nothing you said is technically wrong, but it is important to note for most readers that the Jeep Wrangler JL LSD is not a torque biasing limited slip differential. It is a clutch pack based differential which has some minimum torque between the wheels it maintains, before it allows differentiation. This torque is based on the preload in the clutch pack and friction.

BLD probably will work a lot better with a torsen than it does with a clutch pack style LSD.

Might just be differences in terminology, but any limited slip differential I have heard of are rated the same in regards to the torque bias. I think I know what you are saying but any LSD is technically torque biasing, and I think you mean the difference between a clutch style unit and a gear driven/torsen style like a True Trac. I'm not saying you are really wrong or anything, and the explanation of using the brakes to get a torsen style "working" versus a preloaded clutch setup is accurate. How aggressive, or how much power it can transfer to the the other wheel, is based on the bias ratio. A lot of factory LSD's are pretty mild and have lower bias ratios like 2:1 or 2.5:1. Basically they can only keep both tires on a axle spinning if there is a relatively small traction difference between them. For example the difference between one tire being on dry pavement and one on wet pavement where both will "spin". However if one side in in deep slick mud and the other on pavement there is a good chance only the side in the mud will spin and you are stuck. Good aftermarket LSD's typically have higher bias ratios meaning that there can be a much greater difference in traction between the tires and they can keep them spinning. However there is always a limit on a LSD where the traction difference is so great it can't bias enough torque to the other side to get the vehicle moving. Generally if one tire is in the air and requires almost 0 torque to spin you won't get enough torque to the other side to get going.....after all, zero times any number is still zero..... That's when full locking differentials come into play.

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