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Diffs: The attraction of traction
  |  First Published: September 2007



Why do people buy 4WDs? No doubt there are many varied reasons but the critical factor is that a 4WD vehicle, by its very nature, has four-wheel-drive capability.

The ability, via a transfer case and gears, to engage the front and rear differentials of a vehicle allows four-wheel traction to greatly increase the mobility, safety and flexibility of a vehicle across a variety of terrains.

Such a drive allows a vehicle to move through terrain normally inaccessible to a conventional vehicle. Accordingly, the function and ability of a 4WD is extended for a wide range of functions including military, rural, mining, sport and recreation.

The 4WD has allowed us to extend our driving and exploration horizons in safely and comfort. It’s all about increasing traction, allowing us to gain all of the above benefits as well as a lot of enjoyment in doing so. And enjoyment is all about driving challenges and driving pleasure and getting to go places we can enjoy.

Vehicle traction is a function of a variety of factors.

• The amount of torque produced by the engine and transmission and made available at the input shaft (drive pinion) of the axle.

• The axle gear ratio (ring and pinion ratio).

• The diameter (as well as configuration and design) of the tyres.

• The amount of torque the driving wheels can generate at the ground before slipping – in turn a function of the amount of friction between the ground and each drive wheel as well as the weight on the drive wheels.

•The type of differential.

In this series of articles I will focus on this last factor – the type of differential. The differential is critical to the traction equation and making changes to them can increase traction considerably – and hence vehicle capability and safety.

Differentials are amazing pieces of equipment. They are critical to the effective and efficient operation of wheeled vehicles. The objective of differential gearing in the majority of wheeled vehicles is to transmit power (torque) from the engine to the drive axles and to allow the wheels to run ahead or lag behind, as required, to make turns or overcome obstructions without causing excess tyre scuffing and wear or causing steering difficulties.

CONVENTIONAL DIFFS

The conventional differential is called a standard or open differential. It divides power at the ring gear within the differential equally between the two axle half-shafts. This type of differential operates and functions effectively and efficiently as long as the ground surface is uniform and results in equal traction for both wheels.

If a vehicle with 4WD engaged is driven in a straight line, the conventional differential allows for the equal transfer of engine torque to all four wheels.

When the vehicle turns a corner, the inside wheel must, by necessity, rotate more slowly that the outside wheels. The differential’s design allows torque to be transferred to the wheels that experience the least resistance – that is, the outside wheels, while the inside wheels rotate freely.

Because the conventional differential transfers the torque to the wheels that encounter the least resistance, slippage will occur. As the vehicle encounters varied surface materials such as sand, mud, snow or a surface contaminants such as spilled oil, the open differential can thus be a liability.

The same applies to a situation when a vehicle lifts a wheel – the wheel begins to spin. Spinning wheels reflect dissimilar ground coefficients, such as loose gravel under one wheel and concrete under the other. The vehicle becomes immobile because the driving torque, and hence traction, is limited by the ability of the slipping wheel to transfer or absorb torque at the ground.

Such slippage is damaging to the tyres (excess wear) and transmissions, especially in the long term. The key to this damage is the basic operation of the conventional differential.

If resistance is removed from one wheel (e.g., slippage in a loose surface or if the wheel is off the ground), the differential gear assembly rotates around the stationary axle at twice the crown-wheel speed.

As resistance is re-established, the spinning wheel halves its speed in an instant via friction with the ground. This results in ‘shock loading’ of the differential.

Should the first contact be with a hard, dry surface, a shock is passed from the wheel to the axle and thence the differential to the universal joints, to the drive shaft and hence to the engine components down the line.

If the tyre has an aggressive tread, the greater the friction will be, and hence the shock. This shock load results in general wear and tear while in extreme cases it can result in stripping of the crown-wheel gear teeth and collapse of the universal joints.

Furthermore, axle gear failures can occur if lubricant is forced out of the differential gears. Last week I encountered a prospective customer who came into our store to inquire about a diff locks for his Suzuki Vitara. Our conversation revealed he had lost his cool when bogged on the beach and as a result of a loss of his temper and control, he had shock-loaded his diff a number of times so severely that the crown-wheel teeth had been stripped. His diff was literally a bin job!

CONTROLLING TRACTION

Over the years, engineers have experimented with a variety of designs to limit the disadvantages of the open differential and improve traction control. Varying degrees of success have been achieved.

Methods include:

• Removing the differential, but this causes major steering and tyre wear problems.

• Manually locking the differential – this has resulted in driver misuse as well as accidents.

• Introducing a ‘clutch pack’ limited-slip differential – but these result in constant wear, contamination, chatter and shorter life.

• Providing a gear-type limited-slip diff without friction plates or springs.

• Providing a locking differential. These differentials generally fall into two main groups, automatic mechanical NoSpin differentials such as the NoSpin Detroit Locker and non-automatic, manually-controlled lockers such as the air or pneumatic locker, electric locker (e.g., the current Toyota lockers), and hydraulic lockers. Locking differentials are designed to deliver 100% of the available power (torque) to both the drive wheels.

There is a lot of energetic and sometimes uninformed debate and argument over the benefits and disadvantages of these two groups of lockers. In fact, the passion exhibited by the two camps at competitive meets can be intense and at times embarrassing to observe because ignorance is often bliss.

In reality, both lockers work extremely well in achieving their goal of increased traction. They result in better traction, which also has the side effect of less damage to the environment. By providing greater control, driver and vehicle safety is enhanced.

All in all, the driver is able to tackle difficult terrain more efficiently, safely and with greater confidence. They both have pros and cons and it is up to the discerning and informed buyer to make a careful choice. It really boils down to choice.

AUTO LOCKERS

I will cite two examples of automatic mechanical lockers. We have installed and used these over the years and have found them to be very effective and efficient if installed and loaded properly. They rarely exhibit the extreme sounds and effects claimed by the air-locker camp in particular. Let’s focus on the facts and take it from there.

The Lock-Right (as against the Locker-Right – a cheap imitation) automatic differential has been around for some time and represents excellent lighter-duty locking differentials.

These differentials operate in the opposite way to the open differential. When travelling in a straight line they operate the same as a standard diff, allowing equal transfer of torque to all wheels.

However, when cornering, the differential lock applies the torque to the outside wheels which are experiencing the greatest resistance or, conversely, the greater traction with the ground. These are the inside wheels when travelling on a hard surface.

If one wheel loses traction due to encountering a loose or slippery surface, equal torque is provided to all wheels, allowing the vehicle to maintain traction and momentum in the direction of travel.

The Lock-Right automatic mechanical differential has four major components – two driver gears and two coupler gears. Combined with several spacers, springs and pins, these components replace the original spider gears inside the original diff housing. This locker locks and unlocks automatically simply and efficiently while driving and turning.

When driving in a straight line, it is locked. When the vehicle turns, the outside wheels rotate faster than the inside wheels, causing the coupler gear (attached to the axle) to ‘ramp up’ and separate from the driver gear. This allows the outside wheels to travel freely while torque is still applied to the inside wheels.

Once the turn is completed, all the wheels rotate at the same speed. The springs in the locker now force the driver gear and coupler gear back together to again lock the differential.

The two forces which cause the differential to lock and unlock are opposite to each other. One is internal and produced by the engine’s torque, which causes the differential to lock – the more torque, the greater the locking effect. The other is external and is the result of the force generated by the ground over which the vehicle is travelling.

When one wheel is forced to rotate faster than the other, as occurs in a turn, the outer wheels unlock. This avoids the problem of ‘wind up’ which, if not released, will cause severe differential or CV damage.

However, when a loose or slippery surface is encountered, or when one wheel loses contact with the ground, the Lock-Right remains locked, providing equal and 100% traction to both wheels.

Figures 1 to 3 illustrate the operation of a conventional open differential, while Figures 4 to 6 which illustrate the operation of a Lock-Right Diff lock. Figure 7 illustrates the construction of a Lock-Right diff lock. Figure 8 illustrates how the diff centre (spider gears) are replaced with the locker.

The Detroit automatic mechanical NoSpin Differential Locker has had a long history of achievement, performance and dependability. These NoSpin positive-locking differentials are designed to deliver 100% of available power to both the drive wheels, yet unlock as required, especially when cornering on hard surfaces to allow speed differentiation.

Like the Lock-Right, this is all done automatically. Unlike the Lock-Right, these differentials are heavy-duty and are designed for tough applications and conditions. The NoSpin differential powers both wheels, yet freely permits wheel speed differentiation when required. The Detroit Locker:

• Assures 100% of the available torque is delivered to both wheels. When towing, it effectively increases drawbar pull.

• Prevents wheel spin and loss of power when one wheel loses traction.

• Compensates for differences in wheel travel when turning or operating on uneven surfaces.

Figure 9 illustrates a drive axle equipped with a NoSpin locking differential. The Detroit Lockers technical manual notes: There are no spider gears but rather two drive members called clutch assemblies. They mate with the spider assembly, which is driven by the ring gear through the differential support case.

As long as the vehicle is operated in a straight direction (forward or reverse) over a smooth surface, the drive clutch assemblies remain locked to the spider assembly. The NoSpin differential allows the vehicle to perform as if the axle half-shafts have been welded together – the axle is completely locked.

This means both wheels turn at the same speed. If one wheel loses traction or leaves the ground, the opposite wheel, which still has traction, continues to drive the vehicle until traction is regained by both wheels. There can be no one-wheel spin-out – See Figure 10.

When the vehicle turns a corner, or when one wheel passes over an obstruction, the outside wheel, or the wheel passing over the obstruction, must travel a greater distance and therefore faster than the other wheel. When this occurs, the NoSpin differential automatically allows for the necessary difference in wheel speed.

During the turn (Figure 9), the inside-driven clutch remains completely engaged with the spider and continues to drive the vehicle. The outside-driven clutch automatically disengages from the spider, allowing the outer wheel to turn freely in the turn. When the vehicle completes the turn, the outside driven clutch automatically re-engages the spider, as both wheels again travel at the same speed.

This automatic ability to engage and disengage is arguably the major advantage of the Detroit Locker. As a mechanical locker, it is constantly at work doing its job. As an automatic locker, it continually engages and disengages, maintaining traction while allowing steerage (although heavier than normal steering from our experience).

These lockers provide driveline protection, as do all lockers. If both wheels on the driving axle turn at the same speed, even when a wheel gets airborne it maintains the same speed – in contrast to an open diff where it spins faster). When it regains the surface, it gets a soft landing because it is turning at the right speed for the ground speed. There is no heavy tyre friction or shock-loading to the differential or transmission and differential function is maintained at all times.

These lockers can be noisy when disengaging, although we have found that this is usually the result of incorrect fitment and loading. They can also alter the driving characteristics of your vehicle on the bitumen. We have found that they can cause erratic steering (although minor) and some oversteer.

Thus, during normal driving conditions, vehicle responsiveness can become less predictable. In wet and icy conditions, driver concentration needs to be focused, to say the least.

However, overall, these lockers are doing their job continuously and this is a feature that many drivers come to value and appreciate.

A few years ago I had my only crash in a vehicle – an 80 Series LandCruiser. We were returning from one of our escorted expeditions and travelling east of Bourke towards Brewarrina at about 100kmh. My co-driver was driving.

Suddenly, the rear RHS tyre of the vehicle blew out. The aluminium wheel disintegrated and the vehicle continued on its hub. The rear shock tower gouged into the ground, spinning the vehicle sideways to the right.

The vehicle continued sideways at speed and out of control. We held on.

It hit a culvert at about 90kmh, spun front to rear, simultaneously rolled and landed on its roof. By the grace of God we survived without a scratch although the rest of the group thought that we were dead for sure.

The 80 Series is a tough truck and so were the Stratos seats. The cargo barrier kept all the gear in place while the window hammer allowed us to release ourselves from the safety belts (we were suspended upside down) and break out the front windscreen.

What caused the wheel to shatter? The tyre was found nearly 600m across the plain where it had hit a tree. It appears that the rear Detroit Locker, when faced with a sudden increase in speed of the LHS rear wheel with its sudden loss in diameter caused an instantaneous shock to the wheel, shattering it.

We have mulled over this incident again and again and discussed it with other engineers, coming up with the same conclusion: Certainly, it would appear, it’s a downside to automatic diffs.

However this cannot be said with certainty although what was apparent was that the wheel showed no signs of fault or damage that would have caused it to shatter due to structural failure. We live and learn.

• Next: Pneumatic or air-locking differentials. These were pioneered by the ARB Corporation in Australia and represent a great success story for ARB and for 4WD technology in general.

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