Diff Locks
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Why would we need to fit a diff lock to a 4WD - a vehicle supposedly equipped to humble the toughest off-road challenge?

The answer lies in the way normal differentials behave in situations of less than ideal traction.

The majority of four wheel drives are equipped with a 'limited slip' rear differential and an 'open' front diff. Differentials are, by nature, flawed devices when it comes to finding traction. Their design inherently allows the wheel with the least traction to receive the most torque or driving force. What we are really seeking in a 4WD is for the wheels with the most traction to receive the most torque.

Limited slip diffs overcome this problem to a degree, by directing some torque to the tractable wheel of a diff but in certain situations they also fail to deliver drive.

Diff Locks overcome the shortfalls or regular differentials by coupling the two axles or output shafts together. This locking action essentially delivers equal torque to both wheels so they rotate at the same rate regardless of traction.

Full-time full wheel drives are fitted with a centre diff lock. The centre diff lock only couples the front and rear differentials so the car gains equal torque between front and rear. When the centre diff lock is selected on a full-time 4WD it behaves in the same manner as a part-time 4WD.

There are a number of diff lock (or diff locker) systems available for various purposes (tow trucks, tractors, race cars, earthmoving equipment etc.). When we refer to diff locks for 4x4 we are looking at a system of locking the front or rear or both differentials to gain maximum traction. Our most common choices are automatic diff locks or manual (selectable) diff locks.

Automatic Diff Locks

Automatic lockers are relatively simple devices with a comparatively attractive price tag.

The concept has been around for a long time and brands like 'Detroit Lockers' have built their reputations on their effectiveness and reliability.

In their simplest form an automatic positive diff lock consists of installing a pair of cam gears - one for each axle of the differential. Some manufacturers claim that any mechanically minded owner with basic tools can successfully install a set themselves.

The diff is 'locked' by default and works by 'unlocking' as required - for instance, when making a turn, where one wheel needs to rotate faster than the other. Neither wheel is able to rotate at a lesser rate than the differential carrier, meaning the wheel requiring the fastest rotation is 'overdriven'. The driver has no input and isn't required to manually engage the automatic diff lock. It is present and working at all times.

Automatic diff locks are reputed to have a couple of idiosyncrasies. The constant locking and unlocking action can manifest itself as a clicking noise during operation while some vehicles develop symptoms of mild understeer or twitchiness that may require a slightly adjusted driving style. While this is true of older style auto lockers, modern devices are mostly properly sorted bits of kit that engage and disengage with barely a twitch or murmur.

Part-time four wheel drives, where the front axles can be disengaged via the locking hubs, respond well to the inclusion of an automatic diff lock in the front differential. The front differential has no influence on a 4WD's on-road handling when the locking hubs are set to the 'Free' position'.

Sadly automatic diff locks can't be used in a full-time 4WD because the additional forces create steering anomalies and drive-train binding.

Common brands: Detroit, Lokka, Lock Right

Manual or Selectable Diff Locks

Manual diff locks offer the driver full control over available traction at the push of a button. They are, however, unforgiving beasts that should only be used as required.

A manually locked front differential offers limited steering when engaged. The tyres must skip or slide to accommodate direction changes because both wheels are driven at the same rate. This means that manually operated diff locks should never be engaged on bitumen or hard surfaces.

Most manual or selectable lockers engage dog-clutches that lock the side and centre differential gears. An onboard compressor drives a piston to facilitate the process while other options involve locking the gears electromagnetically.

Most four wheel drives are fitted with a Limited Slip Differential in the rear and an Open Differential in the front. A limited slip differential reverts to 'open' differential operation with the fitting of a manual diff lock. The ability for the diff to compensate the wheels with poor traction is gone unless the diff lock is engaged.

Manually operated diff locks are generally much more complex and consequently much more expensive than their automatic counterparts. The fitting of the locker in the diff housing is a substantial mechanical process and ancillary items like compressors, switches and wiring are required.

For many 4x4 drivers the outlay and complexity is easily outweighed by the convenience and additional off-road capability.

Common brands: ARB, TJM, Eaton

Who Needs Diff Lock?

Competitive rock climbers and motor racers need them. Aid workers in dangerous third world countries (who need every bit of self-reliance they can get) need them. But does the average four wheel driver or weekend-warrior require diff locks? Well, the answer is yes and no.

Drivers who mostly traverse loose or slippery surfaces like sand, gravel or ice may not reap the full benefits of diff-lock. Level, low traction surfaces like these tend to present a fairly even resistance to the wheels. In these situations all four wheels are usually spinning at the same rate.

Diff lock really comes into it's own on gnarly or steep terrain. On steep hills gravity is the constant enemy and diff-locked 4WD's can climb hills only regular 4WD's can dream about.

Rutted, rocky, uneven ground often sees the wheel of the vehicle lifted completely off the ground. In this situation regular differentials are rendered useless - an open diff delivering all available torque to the airborne wheel while a LSD will deliver 'limited' torque to the grounded wheel.

Diff locks overcome the shortcomings of differentials by rotating the wheels at the same rate regardless of traction.

Front or Rear Diff Lock?

Front or rear, auto or manual? A lot comes down to personal choice and cost.

As we said most modern 4WD's come factory equipped with an LSD rear and an Open front differential. The limited slip rear is already doing a much better job of finding traction than it's brother up the front. (providing the diff is in good condition - LSD's wear and lose efficiency). A diff lock fitted to the front differential is a great way to increase the crawling abilities of a four wheel drive.

If you have a part-time 4x4 then the option of an automatic front locker provides real bang-for-buck potential.

Full-time 4WD's require a manually operated system. Air operated diff-locks are more expensive than automatic but thankfully, a single compressor can run both front and rear differentials.

Summary

Automatic Diff Lock Pro's.

  • Cheap
  • Simple set and forget system.
  • Easy Installation.

Automatic Diff Lock Con's

  • Can be noisy.
  • May alter steering characteristics.
  • Can't be used in full-time 4WD's

Manual Diff Lock Pro's

  • Fully controllable.
  • Positive locking system.

Manual Lock Con's

  • Requires driver input to select.
  • Drive-train at risk if driven on hard-top.
  • Costly and Complex

In The Spin on 4WD Differentials we examine the workings of open and limited slip differentials and in Transmission Wind-Up we look at what happens to the drive-train of a 4WD on the bitumen.

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