Whether electronic traction control or a mechanical difflock system is “best“ is a great question and a very tricky one to answer, because the variety of systems is so large and because “best“ can be defined in so many different ways.
Why did the motor industry embrace electronic traction control? There are a few good reasons:
* Cost. The system allows the control of spin-out by mostly using hardware that is already on board (for the ABS system)
* Idiot-proofing. Traction control readily integrates with vehicle electronic systems and allows the vehicle and indeed the driver/occupants to be protected from driver mistakes (e.g. generating wind-up by leaving diffs locked on high grip surfaces or skidding off a track due to steering interference from locked diffs).
Absolute vehicle mobility was not the reason for the rise in popularity of traction control. If the goal is mobility over terrain where spin-out is likely (whether that is due to terrain undulations or variability in underfoot conditions), there is no substitute for a system that uses mechanical differential locks at all inter-axle locations and in all axles under full control of a driver that knows what he is doing.
As a gross generalisation, but based on the current state of the art of electronic traction control, I would judge a system with traction control at all four wheels and a center diff lock (if full time 4x4) to have approximately equal mobility to a system with mechanical rear and center diff locks only, all else being equal. Each of these systems will have the upper hand under certain specific conditions, though: traction control on all four wheels might come out on top in slow rock crawling where the traction control system has the time to react, whereas the rear diff lock system will win in the sand or on a loose hill climb where momentum has to be used and where things happen to fast for the electronics. The moment another axle diff lock is added to the latter system, the traction control system is blown out of the water.
The primary advantages of mechanical diff locks are:
* Nothing can beat the speed at which they can alter the torque split between the diff's output shafts.
* They can transfer 100% of the torque via one output shaft, i.e. they have the largest possible torque bias ratio.
* All of the engine's power is always available for propulsion.
* Torque is transmitted in a smooth manner without interfering with gear ratios.
The primary disadvantages of mechanical locks are:
* They can lead to driveline abuse in the hands of the inexperienced.
* They can lead to steering interference.
* They are expensive to fit, because they require dedicated hardware. This also adds mass.
* They are expensive to fully integrate. into an electronic control system
* They can generate large loads in driveline components due to their large torque bias ability.
The primary advantages of electronic traction control have been alluded to:
* Easy integration into control and protection schemes.
* Ability to act on all wheels
* Ability to prevent steering interference by electronic means
The primary disadvantages of electronic traction control are:
* The system is reactive, because it has to determine that slip has already occurred by means of wheel speed readings instead of preventing slip from ocurring in the first place. Until traction control systems can use wheel torque readings to determine what is about to happen, this disadvantage will remain.
* It is fundamentally flawed to apply brakes when propulsion is the objective. In a perfect traction control system, the wheel speeds on an axle will be regulated to be identical and equal torque will be transmitted via each side shaft. This means that 50% of the power transmitted via that axle is dissipated as heat. Brake wear is also accelerated.
* Real traction control systems are not able to equalise wheel speeds perfectly and pulse the application of the brakes at the slipping wheel. If that wheel is actually brought to rest, the gear ratio of that axle is halved due to diff action, which plays havoc with the tractive effort available. In practice the wheel is not brought to a complete standstill, but the effective gear ratio still pulsates.
* Traction control systems depend on a hydraulic brake booster for their actuation energy. These components were developed for ABS and typically have a short activation duty cycle capability. If they are called upon to operate continuously for a longer time (long difficult climb) they generate excessive heat and switch themselves off in the interest of self preservation, leaving the vehicle with open diffs.
So what is the ultimate system? That is not easy to answer, but I think it would be very hard to beat a fulltime 4x4 system with center and rear mechanical diff locks in conjunction with electronic traction control that can operated simultaneously with the diff locks...
2003 Toyota Land Cruiser 100 VX TD
2003 Mitsubishi Pajero 3.2 DiD LWB A/T
Gone & missed
1999 Nissan Patrol 4.5E GRX M/T
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1996 Toyota Land Cruiser 80 VX 4.5 EFI A/T