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Brake Lock Differentials (BLD) Explained

85562 Views 82 Replies 37 Participants Last post by  WillB
Why would I need BLD?
In order to understand the true benefit of the BLD system (or any locking axle differentials) it is important to understand first what an open differential is.

An open differential is what most passenger vehicles are equipped with (excluding four wheel drive and all wheel drive vehicles). An open differential is beneficial because it allows the outside wheels to turn faster than the inside wheels when negotiating a turn. The vehicle would have a very difficult time turning without the ability for opposite tires on the axles to turn at different speeds.

The open differential is perfect for dry, level, on road conditions. In these conditions all the driven wheels are receiving equal amounts of torque. So in this circumstance when the Wrangler is in 2H, both rear wheels are receiving 50% of the available input torque. Under the same road conditions, a Wrangler in 4H or 4L will send 50% to the front and the other 50% to the rear wheels. Life is good; the Jeep has no problems moving forward.

The problem with the open differential is that torque is always split 50/50. Let’s imagine the road conditions change in such a way that one tire no longer has traction. This could happen if the tire is not in contact with ground or if it’s on a very slippery surface such as snow, ice or mud. The slipping wheel in this situation takes very little torque to spin it, let’s say 15 ft-lb. This means the other wheel, which does have traction, can also only get 15 ft-lb of torque. In many cases this would not be enough torque to keep the vehicle moving. Even in 4H or 4L a situation could arise where one front wheel and one rear wheel are slipping thus effectively stopping the open differential vehicle in its tracks.

The way to overcome this is to “lock” the differentials together, effectively making them on unit, so that the slipping wheel receives the same amount of torque as the wheel with traction. The Rubicon comes standard with selectable lockers that do exactly that! However, they’re only available in 4L mode unless you’ve done some hacking to enable them in other modes. As mentioned previously, while the axles are locked, steering becomes much more difficult and “binding” can occur causing large amounts of stress on the driveline.

If only there were a way to overcome this open differential drawback without the fuss of having to manually lock and unlock your axles but yet still have the dry, on road benefits…

What is the purpose of BLD?

The purpose of BLD is to simulate true locking differentials, described above, in order to provide additional torque to the wheels with traction. A Wrangler equipped with BLD will navigate many obstacles that a similar vehicle with true locking differentials will.

How does BLD work?

The Wrangler has speed sensors on each wheel and therefore it is able to know when one wheel is spinning faster than its opposite. When it senses one wheel spinning and the other not, it automatically applies the brake to the spinning wheel. This means that more torque is now required in order to get the “braked” wheel to spin. Ah! Remember, in an open differential the torque is split 50/50 and now that the braked wheel is receiving more torque so is the wheel with traction! In many cases the extra torque is enough to keep or get the vehicle moving. The BLD feature does not care how fast the wheels are turning, nor does it try to limit how fast they’re turning, so long as they are turning at the same speed.

This sounds good so far, but of course, there is always a negative side too. The negative is that the input torque must be double the amount required because of the brake being applied. However, this is usually not a problem, especially in 4L where plenty of torque is generated.

How do I use BLD?

The BLD feature is an automatic process and is active when in 4H or 4L modes. It is part of the Electronic Stability Control (ESC) / Traction Control systems.

To effectively use the BLD system the driver should slowly and gradually apply more throttle when they enter a situation where a wheel is slipping. This will allow the sensors to determine the need to apply the brake to the slipping wheels while at the same time generating the necessary power and torque to send to the wheels with traction.

Which models have it?

All Jeep Wranglers (JKs) have BLD. This includes: Sport, Sport S, Willys, Sahara, and Rubicon.

Are there any other things to consider?

Can using these brakes to overcome obstacles cause them to overheat? Jeep engineers thought of this and implemented a checking system that monitors the temperature of the brakes, if the temps exceed the set threshold then the BLD system is automatically disabled until the temp drops below the threshold.

According to a Jeep Engineer: “Since BLD is only trying to keep both wheels on a driven axle turning at the same speed and not control overall wheel speed, the actual energy input to the brakes is relatively low. In all of the testing done at Moab, I have never seen brake temperatures reach a point where the thermal model turned off traction control.”

I hope this proves useful for those that are new to Jeep and/or new to off-roading. As always, I tried to be as accurate as possible. Please correct me if I made an error!
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Great explanation. I put Truetracs front and back in my jeep. Will the BLD compliment the truetrac operation if one wheel is off the ground?
Great question! The truetrac, and any limited slip differential (LSD), will actually kick in prior to the BLD system. This means that if a wheel looses traction the LSD will trigger first and if it is enough to overcome the obstacle then the BLD will not be needed. However if the LSD is not enough to overcome, then the BLD will go into affect.

So in short, yes the BLD will compliment your LSD.
NoGaBiker. The freely spinning wheel will will not require zero torque to spin because it still have some weight to the actual wheel and probably some resistance if it's just on snow or mud.

The engine produces it's max torque and you can hear it as the RPMs skyrocket but the spinning wheel will only required 15 ft-lbs of that available torque to get it moving and therefore the other wheel can only get the same amount.

The BLD applies the brake to the spinning wheel causing it to need to use more of the available torque to spin it. It keeps applying the brake until the other wheel with traction is able to spin at the same speed. Essentially it's allowing the jeep to use the available torque that it's making.

Cornfed. The BLD is actually designed to overcome obstacles where one tire or more tires are in the air.
Check out this off camber situation with BLD in action.
I believe you misunderstood me, GreenMachine. I am saying that it does only require a small amount of torque (not zero) to spin a tractionless wheel, but the ENGINE has to make whatever it makes -- say 240 lb-ft. So that torque HAS to go somewhere. Other than the amount that is consumed at all times by driveline losses (say 40 lb-ft) the rest HAS to go to the wheel or wheels. In a manual, ZERO torque can go to a stationary wheel, or you will stall the engine. But with an open diff, ZERO torque does go to the wheel with traction, and 100% of the torque gets sent to the spinning wheel. So where does the remaining 210 lb-foot of torque go, if only 30 of it is going to the two wheels in your illustration? You see my point? It ALL goes to the spinning wheel, and none of it goes to the wheel with traction. The results are as you say, but not for those reasons. The spinning wheel is getting 100% of torque because the torque wants to go the path of least resistance in an open diff. Now you begin applying the brake, which tricks the diff into thinking you've gotten traction over there. So it begrudgingly begins to send less torque to the spinning wheel, and more to the stuck wheel until one of two things happens: 1) the increased amount of torque going to the wheel with traction overcomes that traction before the Jeep has enough traction to move, and now you have two wheels spinning with equal amounts of torque being sent to them. Or 2) the increased amount of torque is sufficient to move the Jeep on that one wheel that has traction, until the other wheel regains traction and all is well. But the takeaway here is that all of your torque (as determined by throttle position and engine rpms) WILL go to your two driven wheels -- nowhere else for it to go. The ratio will be determined by the diff. Cheers!

The rest of the produced torque is wasted on the spinning wheel. So think of it as the spinning wheel getting the majority of the torque but only needing a very small amount to actually spin it. The rest is wasted and that's why the RPMs shoot up so fast. That small amount that was required to spin the wheel will also be applied to the opposite wheel but it's not enough to spin it because the other wheel is turning so freely. Enter BLD...

I think we are on the same page, though. Just looking at it in reverse so to speak.
The spinning wheel is getting 100% of the available torque whether it needs it or not. Period. Engine revs do not shoot up when you spin a wheel unless you push the throttle further.
I read several articles while doing my research. I think my description is fairly on par.

Google brake lock differentials and see what you find.

Not sure if I can link them here. (Legally)
I think I found the wording that caused the confusion.

"Remember, in an open differential the torque is split 50/50 and now that the braked wheel is RECEIVING more torque so is the wheel with traction!"

That RECEIVING should read as NEEDING.

I'd reword it like this for more clarification.

Remember, in an open differential the torque is split 50/50 and now that the braked wheel is needing to use more of the applied torque to spin the wheel the other wheel with traction also gets the same amount of increased torque thus (hopefully) moving the jeep in the desired direction.
BLD only works in 4wd? Also when you are in 4wd lo or hi if you turn off traction control does that turn off BLD?
BLD cannot be turned off, it remains on in 4H and 4L even if you turn off traction control.
Rubicon have lockers and BLD Sahara have no lockers but BLD. Please read the description under the movie. Here it is "Jeep Wrangler JK's going through a grade 5 obstacle at Moegatle. Jeeps & mods (to best of my knowledge) in order of appearance: JK Unlimited Rubicon Auto - stock JK Unlimited Rubicon Auto - 3" Zone Lift w/ BFG MT 285/70 JK Unlimited Sahara CRD Manual - 3" Old Man Emu HD Lift w/ BFG AT 285/70 JK Sahara Auto - 3" Old Man Emu HD Lift w/ BFG AT 285/70 JK Unlimited Sahara Hemi 6.4L Auto - 4" Lift w/ BFG MT 35/12.50, Air Lockers Apologies for clip ending abruptly. Used a trial version dvd ripper and could only get 5 mins of footage from the dvd. I must add the last JK did make it through after locking both Air Lockers. The 2nd last JK eventually went around after sustaining damages to the front axle." There are offcourse different drivers with different skills, but in this movie the Rubicon made it and Sahara did not exept the last sahara that did have airlockers.
This demonstrates that the BLD is not a substitute for true lockers. The BLD must expend twice the torque to negotiate the obstacle because it needs to overcome the brake and the obstacle. In most cases the jeep will have enough torque unless you're gearing is too high (numerically low).

That Sahara may have had 3.21 gears and then added larger tire reducing his gear ratio even more. This would negatively affect the crawl ratio and could be a factor in its ability to climb this obstacle.

This is why it's not always beneficial to add larger tires unless you also adjust all the other components to work in the new system.

Another thing to consider is the weight of the vehicle. If it's too heavy from modifications and the gearing isn't right, it will struggle to climb.
I was personally trying to better understand the BLD system. So I began to research.

This article, along with several others, and a multitude of videos as well as reading the manual is what led me to make this post.

I figured I might as well share what I learned. I attempted to summarize everything I learned and make it easily understandable and provide some examples.
So what about the Limited Slip, How does that work into the equation. I know the Rubicons with lockers don't have it but do all the other Jeeps have it.
From my research I found that the LSD compliments the BLD.

It will increase the affect of BLD by the torque bias ratio. This means the BLD will not have to apply as much brake pressure on the spinning wheel whereas a wrangler without the LSD the BLD would have to apply more brake pressure. Essentially the less brake pressure that needs to be applied the better and the LSD helps to reduce that.

Edit: second question answer. All JKs have BLD, even the Rubicon (but Rubicon also has true lockers as well)
LSD will help, but it is an option for the rear axle on Sahara's only. A front selectable locker with rear LSD is an excellent set-up for those that want to DD their vehicle and conquer something less than a grade 5 obstacle (or so I'm told). I think the 4:1 TCase was also exceptionally helpful for that obstacle.
Actually it's an option on the Sport S too... Because I have it on my Sport S. :)
There is one phenomenon that we are not accounting for here: Torque cannot exist without resistance. If an engine can produce 240 lbs/ft it does not mean that it always is. If you were to place the whole jeep on blocks and place it in drive you will only have the amount of torque necessary to rotate the mass(drivetrain, tires) . Thats why on a Dyno there are rollers that apply "resistance" and then you can measure the amount of resistance that can be overcome(Torque). Don't confuse rotational speed as being the same thing. On an open diff,lets say one wheel on glare ice, the other on dry pavement- both wheels will see the same amount of torque. If it only requires 15 lbs/ft to spin the tire on ice, the tire that is not spinning is also receiving 15 lbs/ft, even though it may look like its doing nothing it is applying effort to the ground. Although it is likely no wheres near enough to move the vehicle. When you apply some amount of additional resistance to the spinning tire(LSD, BLD) the non spinning tire will receive a commensurate amount of torque and hopefully be enough to move the vehicle. If its an LSD it may still not be enough. The BLD can completely stop the spinning tire if needed and send enough torque to propel the vehicle GreenMachine13s explanation is accurate in stating how the torque "NEED" plays out. I guess you can tell we are all loving our Jeeps. :awesome:
Thanks, Odamo. You described it really well.

I've learned so much about not only jeeps but physics and mechanical engineering all thanks to my wrangler. :)
Good video showing the BLD in action as well as true lockers.

The black JK has been upgraded with front and rear ARB Air Lockers.

The white JK has open diffs with just the BLD system.

You can see the advantage of having true lockers but you can also see that the BLD system is pretty capable as well.
Yes, I know I am bringing back a necro-thread. But I believe there is valuable information here for those who are completely new to the Jeep world.

And I have a question.

From reading through the thread and watching videos on Youtube, it would appear that the BLD is completely automatic and engages after a few seconds of wheel slippage, is that correct?

I went on a trail last weekend with the local club and one of the big hills was kinda blown out by the time I got to it in my Sport (no lockers). It took me 5 attempts to get up it and I think I now know why.

I was in 4 Low the whole time, the first attempt was slow and I backed off when a wheel started slipping. The final try was accomplished in 4 Low, 1st gear, revving the piss out of the engine (which I didnt really like).

Afterwards, I was told by one guy that I could have tapped the brakes to engage the BLD, another guy said I could have rode the brakes to slow the spinning wheel, and yet another guy said I could have pulled the parking brake to stop the spinning wheels.

However, according to this thread and the videos, I should have maintained the low and cautious approach not letting off the throttle and the BLD would have taken care of itself.

Is this correct?

That is correct. Once a wheel starts spinning, gradually increase throttle and the BLD will "sense" the spinning wheel and automatically brake it thus sending torque across the axle to the other wheel.

The key is having finesse with the throttle... don't usually need to redline it but just give it enough to overcome.

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I think this BLD is a load of "sh$t". I use to have an Hummer H1 and they have Brake throttle modulation, once a tire started to spin you apply the brakes and it would LOCK all 4 wheels.

In my jeep in an off camber situaltion in my yard KO2's on wet lawn left from and right rear spinning and the jeep was stuck BLD did nothing!!!

NOW the kicker, same spot wifes honda pilot with all seasons tires, climbed the hill with no issues AND i didn't have to lock the differentials. (yes the pilot has them)

If you're not in 4H or 4L or if the wheel speed sensors are not working than BLD will not work.

I have seen unlocked Jeeps do some pretty crazy stuff because I was in the driver's seat. :)

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I really think you just need to learn how to interact with the BLD system. Steady gas does the trick, and you don't have smash the throttle either, it works great.

Fast forward from 1:41min to about 2:30min or so. That will be my Jeep BLD in action, off camber, over a slick boulder.

I'd like to see a Honda Pilot try to navigate that terrain. ;)

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Thanks for the quick response! I see you have a Sport. Have you seen the BLD in action?

The only version that does not have it is the 2 wheel drive model but hardly any dealers sell those.

A base sport with 4x4 definitely had BLD. It works pretty darn well too. I've seen it used to crawl over all sorts of obstacles because I was in the driver's seat of my sport. :)

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How does the trutrac compare to the BLD? They seem to be the same when ready the discriptions?

Trutrac is an actual mechanical device (gears) that reside in the differential housing that when a wheel loses grip the gears automatically mesh thereby making both wheels on the axle spin at the same speed.

BLD is a computer program that uses sensors to apply the brakes to free spinning wheels thereby making both wheels on the axle spin at the same speed.

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