Tech Tip: The Locking Differential
By: Frank Twarog
Many customers will ask - "Why do I need a differential lock or air locker -I thought my Land Rover was a 4-wheel drive?!" It is true that for the most part all Land Rovers are four wheel drive vehicles - trivia buffs will remember the handful of 2 wheel drive Series I's and II's that were produced. However, some models do require the operation of manual levers to achieve maximum traction.
For the sake of this article, we will use a 1995 Land Rover Discovery with an automatic gearbox for the example. This vehicle incorporates LandRovers most "basic" full time four wheel drive system which consists of one transfer box with both low and high range gears as well as a center differential. In addition, there are differentials inside each axle casing - for a total of 3 differentials in the system.
When the main gearbox lever is moved into "drive", power is transferred toward both the front and rear driveshafts through the transfer box differential. This achieves "full time" four wheel drive. However,depending on the traction that each tire has, the percentage of power transferred to the front and back can change. Let's assume optimal traction in a laboratory - brand new tires on a perfectly flat stretch of tarmac going straight. When you accelerate, 50% of the power will go the the front drive shaft and 50% of the power will go toward the rear drive shaft. However, as soon as traction is lessened at only one tire,the percentage can change.
This is when the "path of least resistance" law works its way into the equation. Let's take the same stretch of road, but cover it now with a thin layer of sand. As soon as you accelerate the Land Rover, more power will go toward the tires that are slipping because none of the differentials are resisting that "path of least resistance." In this laboratory situation, if even one tire has no traction the vehicle will be stuck (suddenly, your 4 wheel drive vehicle is reduced to resembling the drivetrain capabilities of a bicycle on ice!)
It is now time to explain the other function of our Discovery's transfer box lever. By moving it toward the left, a horizontal linkage locks the transfer box differential. By doing this, you are now forcing the differential to distribute 50% of the power forward and 50% toward the rear regardless of road conditions. On our stretch of sandy road, theLand Rover is now able to move because that one tire that had no traction before is now being assisted by the tires in the front.Progress can be made.
Now, let's imagine that a landslide has covered our road in wet sticky mud.The Land Rover tries to move but one of the front tires and one of the rear tires has now lost traction - you are now stuck again. Assuming that our Discovery is "bone" stock, we have no option but to get out and seek help. However, there is one other available option besides a winch, jack, or tow truck known as a "locking differential" applied to either or both of your axle differentials.
Similar in concept to the diff lock in the transfer box, an axle differential lock such as ARB's Air Locker forces power to be evenly split between each of the wheels on that axle. Back on our stretch of muddy road, we are still spinning one front tire and one rear tire - but let's now assume that we have equipped our discovery with a front and rear ARBAir Locker. As soon as we activate the Air Locker, compressed air is forced through a line that has been tapped into the axle housing and presses a gear outward, locking the two axles in the casing together -as if there were one long axle. As 50% of the power is distributed from the transfer box along each driveshaft, the power is again split evenly left to right, allowing each tire to have equal power - which will move the vehicle in all but the very worse conditions.
Attentive readers will remember that I referred to the Discovery's system as the most "basic" Land Rover full time system. However, Land Rover has been developing more advanced systems for some time. Beginning with the 1989model year, Range Rovers were fitted with a Borg Warner Transfer box which incorporates a viscous - coupling center differential. This allows the transfer box's center differential to lock only when the unit senses a loss of traction - thus eliminating the need for the drivers to manually activate it. Furthermore, the 1993 model year RangeRovers introduced an electronic traction control device on the rear axle. This unit utilizes the vehicle's WABCO Anti - lock Braking System to apply brake pressure to a rear wheel that has lost traction. This will cause the rear differential to transfer torque to the opposite rear wheel allowing the Range Rover to gain traction.
Keep in mind, manually operated differential locks - either in the transfer box or in the axles - are designed to be activated before traction is likely to be lost. A general rule of thumb approach is to lock the center differential when you have left dry, solid ground - go to low range if necessary. Axle differential lockers such as the ARB should only be used briefly in situations where traction is certain to be lost- unlock the axle differential as soon as possible after the difficult terrain has been crossed.