Brake Lock Differentials (BLD) Explained

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!