In the Spotlight Technical

Axle Differentials … put simply – what, why and how


Differentials or “diffs” as they are commonly called are an integral component in the drivetrain of any vehicle. For the off roader they are a key piece of kit that needs to be understood, as this will affect your driving and the vehicles ability to keep momentum and traction.
In this article, you’ll learn why your car needs a differential, how it works and what its shortcomings are. We also look at several types of variations of the differential, known as limited slip differentials, and locked differentials.
Power from the engine is passed through the transmission box to the propeller shaft, then to the differential where the power, in most vehicles, makes its last stop before spinning the wheels.
On a vehicle’s axle, the differential is usually near centrally positioned and consists 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.
Why You Need a Differential


Car wheels spin at different speeds, especially when turning. You can see from the animation that each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.


The Differential has three jobs:
  • To aim the engine power at the wheels
  • To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
  • To transmit the power to the wheels while allowing them to rotate at different speeds. This is where the differential gained it’s name.



Differential Types:
Differentials can be generally classified into 4 categories.
I,   Open Differentials
II, Limited Slip Differentials
III, Locking Differentials
IV, Spools.
Spools are basically the elimination of the differential component, so in reality truthfully speaking, there are only 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.


Standard/Open Diff;differential-diff
The standard differential, or what is referred to as an open differential, 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 Diff;
Limited Slip Differentials (LSD) are designed to “limit” the tendency of an open differential to send power to a wheel that lacks traction and redirect the power to the other wheel of the axle. LIMITED-SLIP-DIFF The Limited Slip Differential will send power to both wheels equally when traveling straight, however when one wheel spins due to a lack of traction, the LSD will automatically provide torque to the other wheel with traction.  LSD’s limit the loss of torque to a slipping wheel through various mechanisms such as clutches, gears cones, and other methods dependent on the units components.  The limited slip diff will not provide 100% lock up of the differential in extreme situations such as when a wheel completely looses traction, these units are recommended for daily driven vehicles and are used in many applications where traction is ‘sometimes’ needed as in wet field crossings or muddy tracks or occasional snow and ice.
You may hear the term “positraction” or “posi” for short mentioned, this was used by General Motors years ago for their limited slip differential and has been used to refer to LSD’s in some countries since.


Locking Diffs or “Lockers”
A Locking Differential or “Locker” uses a mechanism to lock the diff gearing therefore locking left and right wheels into position relative to each other, allowing rotation at the same speed regardless of which axle has traction and regardless of how little traction a slipping wheel has. LOCKING-DIFF
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.
Automatic locking differentials are designed to lock both wheels of an axle automatically when torque is applied so that both wheels are providing power.  When torque is not being applied such as when the clutch is press down, the differential is allowed to unlock, permitting a variance in wheel speed while negotiating turns.  Automatic lockers do tend to create odd handling characteristics on the road as they can lock and unlock frequently taking some getting used to.


Did you know that the first historically verifiable differential was used in china around 30 BC?
Take a look at these video clips demonstrating the actions of various diffs, the first clips is literally a classic but the still the best tutorial for fact todate …





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