Differentials and Diff Locks
What is it that makes a differential work? How is it possible for torque to be transmitted to both wheels on each end of a differential if the two (mostly internal on 4×4 vehicles) drive shafts are not connected to each other and can be rotated independently, and in opposing directions when both wheels are lifted off the ground?
Sadly, even some experienced mechanics are less than clear on the mechanics of differentials and how the internal arrangement of gears makes it possible for the wheels on one differential to rotate at differing speeds, except to agree that this arrangement makes it possible for cars, and off-road vehicles of course, to be steered around corners.
Going around a corner means that the inside wheel(s) on vehicles in 4WD mode, have a shorter distance to travel and needs to be slowed down, while the outside wheels need to be speeded up by the same amount, but with a corresponding increase in torque. This increase in torque, which is a function of the difference in the rotational speeds of the wheels, have the happy effect of helping to steer cars around corners by applying a directional force to the axle, which is transmitted to the structure of the vehicle through the suspension. While this effect is not very noticeable at low speeds, it becomes very much so when cornering at higher speeds where under steering tendencies are more pronounced and can be corrected by applying power. This power, delivered to the wheel(s) opposite the direction of turning through the difference of wheel rotation speeds, and thus the increased torque to the outside wheels, forces the vehicle into the turn.
However, applying too much power can cause a vehicle, off-road vehicles included, to lose traction and start sliding, but by using a differentials’ ability to deliver differing amounts of torque to individual wheels, traction control and/or EBD (Electronic Brake Distribution) systems prevent and even correct this by applying braking force to the outside wheel(s) only, slowing them down, thus causing an increase in torque to the inside wheel(s), forcing the vehicle out of the turn. There are of course many factors that are taken into account when this corrective action is initiated by an EBD or traction control system such as the steering angle, throttle opening, road speed, selected gear in the case of automatic transmissions, and others but it is a differential’s ability to deliver differentiated torque and rotational speeds to the wheels that make the effective functioning of traction control systems possible. In fact, modern systems are so effective at using the differentiating characteristics of differentials that they can (and often do) prevent, and correct potentially dangerous and unsafe conditions even before the driver of an off-road vehicle realises that a condition exists.
Loss of traction and differential locks
Every off-road driver knows what happens when one wheel on a driving axle loses traction; the other wheel stops working because the wheel that is spinning or off the ground has all the torque diverted to it because it rotates faster than the stationary wheel. The ideal situation is of course to have both wheels in contact with the ground; however, this can also lead to trouble if both wheels start spinning in for instance, deep mud, although it is more often a case of one wheel spinning, then gaining traction, causing the other to start spinning until eventually the vehicle is entirely bogged down.
One way to prevent this happening is to provide equal amounts of torque to both wheels, irrespective of driving conditions, which is what a traction control system does by selectively applying braking force to individual wheels, like ABS in reverse, in a manner of speaking.
However, drivers of older off-road vehicles do not have the benefit of electronically controlled systems to assist in maintaining traction, which is where devices called diff locks come in. These devices have the function of locking the two separate drive shafts together; creating a solid axle that divides the available torque equally between the wheels, making it impossible for only one wheel to lose traction, even if one wheel should not be in contact with the ground.
But there are limitations to these systems: since the axle is now solid, it is not possible to execute turns on hard surfaces where tyre scrub cannot absorb the difference in rotational speeds that would have resulted had the differential not been locked. Nevertheless, the advantage of having torque on both wheels simultaneously more than makes up for this, in fact so much so that in many cases, the use of the low range gears and even the 4WD system becomes unnecessary. So, how do diff locks work, and which type is best?
The mechanics of diff locks
While design specifics on the actual locking mechanisms differ, the two main types are dog-clutches, which operate on the tongue-and –groove principle, and electro-magnetic clutches, which work on much the same principle as the clutch that drives the vehicle. The important distinctions from the operators point of view, is how these locks are engaged and then disengaged again.
• Air Locker
These systems are not generally found on the small off-road vehicles mere mortals can afford: being air operated, they are fitted to vehicles with air brake systems, where there will always be a supply of compressed air to actuate the locks. On vehicles with a third differential, one that divides the torque between the front and rear axles, this third differential will also be provided with a lock.
• Electrically operated diff locks
Regardless of the specifics of the actual lock, these systems rely on an electrical current to either activate an electro-magnet to engage the lock, or a solenoid-type actuator to engage a mechanical lock. The advantage of this is that the driver can operate the system via a switch on the dashboard. The disadvantage of course, is that these control mechanisms can be damaged by water, or worse, flying rocks and other debris that could conceivably cut the electrical supply to the differential.
• Control cables
Some older systems are operated by control cables via a lever in the cabin; however, while control is positive, these cables need constant maintenance to prevent them rusting and binding, making operation difficult, if not impossible.
• Automatic systems
Electronically controlled, these systems measure individual wheel speeds and based on pre-defined operational parameters, automatically activates if it is determined that one wheel is losing traction. Although activation is automatic and immediate, the operation of these systems do not go unnoticed; the intermittent operation of the system causes extremely disconcerting handling characteristics and it takes some time to get used to. In some cases these systems are inter-linked with other driver assist systems and many experienced (and purist) off-road drivers are of the opinion that these setups are overly complicated.
In the 4×4 context, especially as it relates to long, extremely difficult overland expeditions through for instance Africa on a Cape to Cairo trip, where simplicity is more often than not directly related to reliability, the over-reliance on electronics may be counter intuitive: a system like this cannot be repaired in the bush, where all systems need to be as dependable as they could possibly be.
Another feature of automatic systems is the fact that some systems keep the diff locks engaged permanently, only releasing the lock when it senses that damage to the system could occur if it remains engaged- immediately re-engaging the lock as soon as conditions allow. Such a system may have advantages in luxury SUV’s on a Saturday morning trail, but the real-world conditions of Africa or the Outback may prove to be too demanding. Use your best judgement when deciding on diff locks but always bear in mind that in the 4×4 universe, simplicity is often best.
• LSD or limited slip differentials
Although not a true differential lock, this type of mechanism does not divert all the torque to the wheel that has lost traction: by using various means that are mostly proprietary and thus unit specific, some torque is directed to the wheel opposite the one that has lost traction. This makes it possible for a vehicle to maintain momentum in for instance deep mud, where one or either wheel could alternatively lose and regain traction.
The major advantage of a limited slip differential is that it makes negotiating corners possible in conditions that are not entirely dependent on tyre scrub to absorb differing wheel speeds.
Traction control vs. Diff locks
The debate over which is best, brake based traction control to prevent wheel spin or a solid, positive mechanical diff lock is far from over, at least as far as the debate relates to soft roaders, or SUV type vehicles.
To the squire visiting outlying points on his estate, an electronically controlled brake system based traction control setup might get him out of the occasional mud puddle but then only for about half a minute or so: these systems rely on energy storage (most often an accumulator connected to the master cylinder) to function and once that energy is gone, the system ceases to function until the accumulator is recharged.
On the other hand, a positive mechanical diff lock will work for as long as it is required to work: provided some basic precautions and good sense are employed, by for instance, not trying to go around corners, or using the system on hard, paved surfaces, or using diff locks at excessive speeds, there is nothing as good as diff locks on all differentials to get a 4WD across a long stretch of mud or sand, of which much of Africa consists, especially during the wet season.
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