Last Updated 17/10/2016
Diff lockers and traction control are traction aids that help send torque to the wheels with the most traction. They are a massive benefit when offroading on steep and uneven terrain. But which is better?
Traction control utilizes:
- wheel speed sensors on each wheel
- brake actuators for each set of brakes
- something to provide the energy to activate the brakes (provide hydraulic pressure)
- electronic controller (a computer processor) to interpret the sensors and control the actuators
Traction control applies the brakes to the wheels that have the least traction. This transfers torque to the wheel with more traction.
A diff locker mechanically locks together the left and right drive shafts of an axle. The left and right wheels always rotate at the same speed when a diff locker is engaged. This forces whichever wheel that has more traction to take more load and prevents the wheel with less traction from spinning freely. Diff lockers can be selectable (turned on or off by the user) or automatic. I have a selectable diff locker and this article relates to this type of diff locker when discussing actuation.
Which is Simpler
Traction control is an active system. It takes inputs from sensors, interprets the data from the sensors through the software running in the electronic controller, and sets outputs that activate the actuators. It does this continuously in real time as traction conditions change at each wheel, constantly interpreting the data at each moment in time and deciding the required state of the outputs. It takes a lot of effort to actively control something in this manner. It’s a sophisticated control system.
A diff locker is a passive system. Once it is locked, any wheel with more traction will immediately get more torque as the load naturally distributes across the axle. There is no sensing or interpretation of data. The are no actuators to control. The torque is distributed perfectly according to the traction at each wheel due to the physics of the mechanical arrangement.
Diff locker is simpler. It’s passive. There is no intelligence to the system. It relies on the physics of how loads propagate through rotating wheels to perfectly distribute the torque.
Which is Faster
Traction control is reactive. It requires loss of traction to be detected and then brake actuators to be set. This needs to be interpreted from the data coming from the sensors and therefore takes time to process. The actuators rely on physically opening and closing of switches and valves which also takes time. There needs to be some safety margin to ensure noise from the sensors and inaccuracies in the software model do not accidentally trigger the traction control to be activated when it shouldn’t have. This makes the system even slower.
Diff lockers distribute torque immediately as a result of the traction available at each wheel. It is inherently very fast. There is practically no delay. Load from the wheel with more traction immediately transfers through the axle and more torque is the result.
Which is More Accurate
When a wheel loses traction in a traction control system this does not indicate how much traction is available at the other wheel. How hard should the system apply the brakes? Only after the brake actuators are activated and the results are measured does the processor know whether too much or too little braking was applied, by checking the relative speeds between the two wheels when the brakes were applied. This system may initially not brake hard enough and under utilise the wheel with more traction. In this case the vehicle does not advance as well as it could have and loses some momentum. The system may brake too much and under utilise the wheel that had less traction, stopping it from spinning. Although having less traction, that wheel can still contribute to the motive force of the vehicle. Again the vehicle does not advance as well as it could have and loses some momentum.
What if one wheel has massive traction available and the other wheel has completely no traction? In this case the actuators on the traction control system may saturate (reach the limit of their braking capability). The wheel with massive traction might never receive sufficient torque and the vehicle is stuck.
A diff locker will perfectly distribute torque at all times. It happens instantaneously as a result of forcing each wheel to move at the same speed. No matter what the traction distribution is, a diff locker will always distribute torque with great accuracy. If one wheel has massive traction then that wheel will immediately receive massive torque simply by ensuring it rotates. The available traction will transfer load to the axle and the result is massive torque.
Which is Smoother
For traction control to activate it requires a loss of traction. Once this is detected, the actuators are activated and torque is sent to wheel with more traction. This means a momentary loss of forward drive must occur for traction control to activate. Then a sudden burst of forward drive occurs when the traction control activates. Then the actuator releases and the process repeats. This is particularly a problem in low traction conditions where all wheels rapidly lose and regain traction. The traction control system goes berserk, violently shuddering for extended periods.
Diff lockers are perfectly smooth. No loss of traction is required. No sudden pulse of forward drive occurs. When a wheel has more traction it will load up the axle and receive more torque. When a wheel has less traction it will unload the axle and receive less torque. It occurs seamlessly and instantaneously.
Which Allows More Stability / Better Control
Traction control may or may not activate depending on relative wheel speeds and throttle input. It may activate but not with sufficient force. You cannot predict what the traction control will do. You may be attempting to advance up an obstacle expecting the vehicle to effortlessly clear it but the traction control does not provide enough torque. To combat this more throttle is required to generate more wheel spin and more torque transfer. Then once the obstacle is cleared you need to rapidly reduce throttle to avoid going too fast and hurling the vehicle into a heavy impact further down the track. The result is unpredictable and yields large variations in vehicle speed.
Another source of variability in traction control performance is loss of the accumulated energy that provides the hydraulic force to the brakes. When traction control activates it depletes the accumulator that stores the energy to activate the brakes. The traction control system will not operate adequately if the accumulator is not given time to regenerate.
With diff lockers all wheels always rotate. This yields very predictable vehicle performance and extremely stable vehicle speed. There is no requirement to adjust throttle. You can chug along slowly, safety and smoothly at a constant speed.
Which is More Efficient
Traction control applies the brakes. This is wasting energy. Sometimes the energy wasted is significant enough to noticeably feel the vehicle is being sapped of power, particularly in low traction environments when wheels rapidly lose and regain traction with the system effectively braking all four wheels simultaneously once you take an average of the constantly pulsating brakes.
Diff lockers waste no energy. All energy provided by the engine is transferred to the wheels and distributed perfectly.
Which is More Reliable
Passive systems are usually more reliable than active systems. There’s more points of failure in an active system. For a traction control system there’s many sensors and actuators, there’s the electronic controller, the accumulator and there’s all the wiring and brake lines in between. A failure at any of these points will render the system inoperable. I have heard about wheel speed sensors and associated wiring being damaged in offroad situations.
For diff lockers there is a single, slow moving, strong, reliable actuator that locks it in. Once locked it almost cannot fail unless there’s a major mechanical failure within the differential. There is still the potential for the actuator to fail but it is a single point of failure. There are no sensors, processors or other actuators.
Which Causes More Wear
Immediately we know that, when activated, traction control wears out the brakes. It also wears out the switches and valves that operate it. These switch rapidly during operation and so can quickly accumulate wear due to huge number of operations. Not only do the brakes and actuators wear out, but the vehicle’s drivetrain also suffers. The sudden and rapid application of the brakes presents as a shock load to the drive train. In isolation this shock load may not be significant but over thousands of operations it may accumulate to significant wear and an eventual failure.
I’ve done a few defensive driving courses over the years and part of the course usually involves hard braking and manoeuvring on a skid pan. I recall on one occasion an instructor explaining that the CV joints on the vehicles used in the courses suffer regular failures due to shuddering loads caused by the ABS and traction control systems. The wear caused by these systems is real and will contribute to a failure if activated often enough.
Diff lockers do not cause shock loads. The torque transfer is smooth and continuous at all times as governed by the traction available to each wheel. Of course shock loads can still occur if the driver applies excessive throttle when a wheel leaves the ground or bounces off an obstacle. When returning to the ground the wheel will experience a severe shock load. Repeated events can cause failure, particularly on the front CVs of a vehicle fitted with front diff lockers and even more so if the vehicle as been lifted. Traction control systems equally suffer from this type of abuse.
Which is More Automatic
Wouldn’t it be great if you could get a robot to drive your four wheel drive on these difficult trails and you could sit at home on your couch in perfect comfort and watch a video recording of the trip that the robot posts onto facebook for you? A fully automated car. Actually no. That would be crap.
Traction control happens automatically. There is no driver intervention required. It is the more modern, fancy, automated option. Like most modern gadgets that creep into vehicles, to me it’s more of a con to try to keep you trapped in the consumer cycle of buying new cars rather than any real enhancements that make you happier.
With diff lockers you need to decide when you want to use them. It’s part of the experience of controlling the vehicle. You need to turn them on when approaching a tricky section of track. You need to remember to turn them off once your’re on a smooth, high traction surface. You also need to turn a front diff locker off when negotiating sharp turns since an engaged front diff locker makes it difficult to steer.
For me, automating something I enjoy doing and that is by no means arduous to do myself, is not an advantage. Some people might like automatic features. In any case this is not a significant performance measure. Either way it’s pretty damn easy.
Which is Easier to Implement in a Modern Vehicle
Modern vehicles are bulging with sensors and actuators and electronic controllers. So traction control capability is usually already there due to the requirements of other systems. Any additional electronics are cheap. So it’s very easy for modern vehicles to have traction control.
Diff lockers requires an extra actuator and modification to the differential which no other system would share. It’s dedicated. This means more effort and cost is involved in having a diff locker.
What Do Vehicle Manufacturers Think?
Practically every current four wheel drive vehicle has traction control. Modern implementations are fast and accurate and offer good offroad performance. However some new vehicles with traction control, including the new Toyota Hilux, come with a factory fitted rear diff locker. The original manufacturers understand the advantages of a diff locker relative to traction control.
Here is a video demonstrating the performance difference between traction control and diff lockers. The first vehicle is my Hilux with a rear diff locker. It makes it through smoothly and easily whilst the Landrover hesitates due to traction control, rolls back and gets a nice bit of panel damage on the right rear. To add salt to the wounds it’s unable to drive out on its own and must be pulled out by the Hilux.
Note that the Hilux only has a rear diff locker whilst the Landrover has 4 wheel traction control. The one diff locker is so awesome that it can immediately and smoothly send massive torque to the one back wheel that had the traction. Imagine what combined front and rear diff lockers can do!
Is traction control another crappy electronic gizmo that unnecessarily complicates our lives whilst performing worse than the original mechanical method? In my opinion – YES. Traction control performs worse in every performance measure except for ease of implementation. Give me a diff locker any day. I’m a simple man with simple tastes!
This doesn’t mean, if your vehicle already has traction control, that you should get a diff locker. Traction control is pretty good and is more than adequate for most applications. The point of this article is not to instruct people to get diff lockers. Mainly it explains the operation of diff lockers and traction control. Additionally this article deals with consumer culture. Maybe we don’t need complicated and expensive cars. Maybe we don’t need complicated and expensive lifestyles. Consider what you’re trying to achieve before opting for the fancy, technologically advanced, complicated, expensive solution.