The Crux of the Matter

In Articles, Technical by Glyn DemmerLeave a Comment

At a recent training session the instructor explained the workings of a differential as well as the locking interventions available, in this case they were a lockable differential and a differential fitted with a Limited Slip Differential.

It was apparent that this was possibly the most difficult section of the day given the bemused looks at the time so I thought it would be worth touching on!

The Differential

This is the bulbous casing generally on the centre of an axle although in some cases they may be off-centre on front axles. Each differential contains a set of gears (twogears are set on the axle shafts that are driven by two or four gears powered by the drive shaft held in place by a pin) the gears allow the driven wheels to rotate and transfer the rotating motion of the driveshaft splitting power to each driven axle shaft- in doing this the differential serves three purposes;

  • Directs power to the wheels
  • It reduces the final rotational speed of the transmission and transfer case before the power is delivered to the wheels
  • And transfers power to the wheels allowing them to operate at different speeds (i.e. when cornering an inner rear wheel will need to turn a lot slower than the outer wheel as it has to travel a shorter distance).

But this also has a drawback; if one wheel slips off-road (and this happens a lot) more torque is directed to that wheel resulting in a loss of forward movement. This often takes place in a “cross axle situation”.

So now we need a traction intervention to combat this loss of torque.

The limited slip differential

This differential will send equal power to the wheels when the vehicle travels in a straight line, however it is able to “limit” the action of an open diff to direct power to a wheel that lacks traction, so when slip is sensed the differential is able to redirect a limited amount of torque to the wheel that still has traction. This is not however the maximum available torque but generally is enough to assist one to regain traction. This system suits vehicles that are driven on a daily basis by multiple drivers where damaged may be caused to the vehicle should locked diffs not be disengaged. It’s action also makes it a passive safety system when on-road.


The locking differential

Commonly referred to as a locker it uses a mechanism or intervention, which may be manual or electronic to lock both wheels ensuring that 50% of available power is directed to each wheel, thus the wheel with traction will more than likely have enough power to allow you to regain forward momentum.

What is however important here is one’s driving technique, try to maintain traction by keeping all four wheels in contact with the ground and if you have a locking differential rather engage it before you get stuck and disengage it after you are through the obstacle. Learning to “read a line” is an important technique as it allows you to correctly assess an obstacle and traverse it safely and maintain traction as much as possible.

In the sport of offroading almost inevitably the conversation turns to improving performance.  When offroad an important keyword is traction.  Better traction generally boils down to four components, your tires.  Quality tires designed for offroad make a world of difference in your ability to find traction and have fun.  But if a tire is not turning due to a lack of power being sent to that tire, the best tires in the world won’t help you.  That’s where the right differential makes a difference.   This article will attempt to explain what role your differential plays in traction and the different types of differentials available on the market and how they will affect your vehicle’s traction and handling characteristics.


The Differential

The differential in a vehicle is located in what is sometimes called the “pumpkin”, or that center section of the front or rear axle that intersects with the drive shaft.  Within that center part of the axle is contained the differential.

In a vehicle, the differential usually consisting of a set of gears, that allows each of the driving wheels to rotate.  The gears convert the rotating motion of the driveshaft or drive train and split power to each of the driving axle shafts of that axle.  In 4 wheel drive vehicles there are two differentials, one in the rear axle and one in the front axle.

The differential has three jobs.  It directs engine power to the wheels.  It acts as the final gear reduction in the vehicle, slowing the rotational speed of the transmission (and transfer case of 4 wheel drive vehicles) before it hits the wheels.  The differential also transmits the power to the wheels while allowing them to rotate at different speeds, thus the term “differential”.

The main purpose of the differential is to allow each half of the axle (each tire) to spin at different speeds, while supplying an equal amount of force to each wheel in that axle.  The need for the wheels to rotate at different speeds is especially apparent when turning corners.  When cornering the inner wheel travels a shorter distance than the outer wheel.  With an open differential they both propel the vehicle forward with equal force, so long as both wheels remain in contact with the road and have traction.  However if one wheel slips, for instance on ice, more torque is sent to the wheel that spins. If that slipping wheel completely looses traction, all power will be sent to that wheel and you have no forward momentum.   When offroad, this is where the common open differential fails to remain effective.  When offroad there are many situations where a wheel will spin free.  In most stock 4×4 vehicles the common Open Differential can be found in both the front and rear axles.  When a wheel in the front AND a wheel in the back are allowed to spin free due to the Open Differentials, that 4×4 is essentially a 2 wheel drive vehicle.  One front wheel, and one back wheel.  This is where other types of differentials will make drastic improvements to traction.


Types of Differentials

Differentials can be generally classified into 4 categories.  Open Differentials, Limited Slip Differentials, Locking Differentials and Spools.  Spools are really just the elimination of the differential, so really, there are three categories.

Beyond the open differential, the various types of “non-open” differentials will provide varying degrees of limiting of the spin or slip of an open differential.  What also varies is the feel of these differentials, which translates into varying degrees of handling characteristics on road and offroad.


Open / Standard Carrier Differential 

The standard differential, or what is referred to as an open carrier, is what comes with most OEM vehicles.  The open carrier holds the ring gear in place and within the open carrier is generally a set of gears called spider gears.  These spider gears are responsible for allowing a vehicle to negotiate a turn and allow the outside wheel to travel farther and turn faster than the inside wheel.  This type of open design works great for most of vehicles on the road today.  However when a vehicle with an open differential meets a lack of traction, it directs power to the wheel with the least amount of resistance. The result is the wheel on the traction-less surface spins free, while the opposite wheel of that axle on the better traction surface provides little or no power.


Limited Slip Differentials, Posi-Traction (Posi, Posis)

Limited Slip and positraction (posi) differentials are designed to “limit” the tendency of open differential to send power to a wheel that lacks traction and redirect the power to a degree to the other wheel of the axle.  The Limited Slip and Positraction differential will send power to both wheels equally when traveling straight, however when one wheel spins due to a lack of traction, the differential will automatically provide torque to the other wheel with traction.  Limited Slip and Positraction (posi) differentials limit the loss of torque to a slipping wheel through various mechanisms such as clutches, gears cones, and other methods dependant on the unit.  The limited slip and positraction will not provide 100% lock up of the differential in extreme situations such as when a wheel completely looses traction.  Limited Slip and Positraction (posi) differentials are recommended for daily driven vehicles and are used in many applications where traction is sometimes needed as in emergency vehicles.  They are also ideal for front axles of 4×4 vehicles that are not equipped with front hubs that can be disengaged.  The term “positraction” (“posi” for short) was used by General Motors years ago for their limited slip differential and has been used to refer to limited slips since.



Lockers, Locking Differentials

A locking differential or “Locker” uses a mechanism that allows left and right wheels to “lock” relative to each other and turn at the same speed regardless of which axle has traction and regardless of how little traction a slipping wheel has.  In this state, the axle acts more as a “Spool”.  This means traction can be sent to a wheel that may be planted firmly on the ground while the other wheel of the axle is completely off the ground.  In this situation an open differential will spin the free (lifted) wheel sending absolutely no torque to the wheel in the ground.  A limited slip in this situation will send some torque to the wheel on the ground but possibly no enough to provide any forward momentum.

Lockers use various mechanisms to provide lock-up and can be divided into two categories, Automatic Lockers and On-Command, or selectable Lockers.

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