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Brake Lock Differentials (BLD) Explained

85781 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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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. Which means 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: You indicate that the torque will always be 50:50, and that if the spinning wheel only requires a small amount of torque, the other one will also only get a small amount. But the reason that can't be is 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 say that in trying to unstick it you run the motor up to 3000 rpm, and it makes 240 lbs-ft at that point. It 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. :)
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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.
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)
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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.
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.

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.
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.

When the motor spins, it first produces torque in the crankshaft. That is connected to the flywheel, which is connected (or not, depending on where your clutch pedal is) to the clutch plate, which is connected to the transmission, which is connected to the driveshaft, which is connected to the rear differential, which is connected to the two rear wheels. All of those connections are "positive" and don't allow torque to escape. It has to be accounted for. You can't make 240 and only send 30 to the wheels.

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.
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 to that wheel only, by means of the BLD, 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 wheel with traction 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.

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