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Brake Lock Differentials (BLD) Explained

85556 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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Nice thread! The place where BLD/LSD let you down (versus a locker) is in an off camber situation where one tire has zero traction. The place where a locker will let you down is in situations that have intermittent slippery surfaces. As the surface goes from wet to dry, you have poor steering ability.
Green machine, Great job with that. I do have one question about your explanation. My understanding of and experience with open diffs (principally on Porsches) includes a subtle difference from your explanation. When one wheel has zero traction it receives 100% of the motor's torque. In other words, all torque is still made by the motor, and that torque seeks the path of least resistance; if one wheel (speaking of a 2WD axle now) has less traction than the other but still some traction the torque will go in inverse proportion to the low-traction axle. So that if that wheel has zero traction, 100% of the torque will go to that wheel. To see this in action, get your open diff Jeep or car stuck in snow. Then put a traction block of some sort under one wheel, giving it adequate traction to back the car up. When you apply torque to the rear axle, one wheel stays perfectly stationary, and one spins furiously. This works in a manual gearbox as well as auto, and there is no torque converter in the manual to absorb a small amount of torque (such as when you sit at a light with an automatic, the engine is turning but the car is not moving; the torque at idle is being absorbed by the torque converter. But a manual doesn't do that; it would stall the motor if even a tiny amount of torque goes through the gearbox but is unable to spin a wheel.) So what I'm saying is, the "small amount of torque, let's say 15 ft/lbs" that in your example was making its way to the wheel with traction, couldn't be doing that in a manual Jeep -- that 15 ft. pounds has to spin something, there's no slop in the system to absorb it. So if it were going to the wheel with traction, and that wheel weren't spinning, the motor would stall. Another way of looking at it is this: the motor will make whatever its rated torque at a certain RPM is. It cannot make less torque just because a spinning wheel doesn't require much torque. So let's that in trying to unstick it you run the motor up to 3000 rpm, and it makes 240 lbs-ft at that point. I has to send that torque out through the gearbox to the diff, and in so doing, to the wheels. The open differential chooses to send 100% (240 lbs-ft.) all to the spinning wheel, since that is the path of least resistance. But with BLD, the fact that you begin "adding traction" to the spinning wheel (at least in the "mind" of the open differential) means that it begins handing some torque back to the wheel with traction, since it's suddenly equally hard to spin the spinning wheel. And so it goes back to 50/50, as long as the BLD is willing to apply brakes hard enough to even it back up. It still may not get you unstuck, of course, if the wheel that used to have traction finds that it doesn't really have enough traction to handle the 120 lbs-ft of torque it's now getting, and starts spinning also. When that happens, whether you have open diff, LSD, or lockers you're SOL, cuz none of them manufacture traction. :)
I believe that the situation you describe above is correct. An LSD can shift it back to ~3:1 towards one wheel. The BLD should further perpetuate it so long as the spinning wheel has ground contact.
This is the main reason I got a Willys with LSD instead of a Rubicon, which is what I had assumed I would get when I started shopping. When I learned that the Rubicon is reduced to open diff status unless you have lockers engaged, I realized I would have far more situations where an LSD would save my bacon than situations where I needed E-lockers. (plus, for my world, I've found the 4:1 and 4.10 combination just too dang low for me. 2.72:1 and 3.73 works perfect around my part of the world. Well, plus the Willys is just so dang cool looking. :D)
I did the same. I don't live in rock crawler land (though I enjoy it when I get the chance). The LSD is exceptionally capable. I've been in a couple of places where a locker would have made things easier, but I've yet to find something I couldn't do w/o them.

To be fair to the Rubicon folk, they do in fact have BLD, so it's not a totally open diff.
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.
Thanks for the clarification, and to stop me from perpetuating bad info on the net!
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.
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.
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