Jargon Buster

A differential is a collection of gears that splits engine torque from the central tailshaft of a vehicle to the two perpendicular axles that lead to the wheels, allowing each to rotate at different speeds.

A differential can be found between the driving wheels on almost all modern vehicles, with many all-wheel drive and four wheel drive vehicles employing up to three (front, centre and rear).

There are many mechanical variations on the differential, but all deliver the same basic operation.

For more on differentials, see Limited Slip Differentials.

A limited slip differential is a collection of gears, clutches and springs or a viscous coupling that splits engine power from the central tailshaft of a vehicle to the two perpendicular axles driving the wheels, allowing each to rotate at different speeds.

If a vehicle was only ever to drive in a straight line on a flat road and never lose traction, then there would be no need for anything but a fixed axle assembly. The reality is however that roads are not always straight, flat or even, surface conditions change and the stresses imposed on a vehicle change by the second, according to the forces of acceleration, braking, cornering and more.

In their most common form, limited slip diffs employ mechanically driven clutches and gears to slow the rotation of wheels under less load (eg. those that have lost traction) transferring that power to those wheels under more load (eg. those that have maintained traction) thus ensuring all available power is being maximised.

A commonly found and easily overlooked problem in modified performance vehicles is a damaged diff housing. While the sharp and sudden impact from a collision will send owners crawling all over their vehicles looking for macro damage, it's the silent, near invisible micro damage that results from excessive weight, heat or stress on your housing that can easily be missed. When damage invisible to the eye can result in leaking seals, shortened bearing life, pinched axle shafts, misalignment and more, it's easy to go spending your hard earned on fixing the symptoms rather than the cause.

Ozdiff employs specialised state-of-the-art diff straightening machinery to accurately measure and straighten damaged diff housings; in many cases returning them to their optimal shape without the use of heat. If you suspect your housing may be need straightening, contact us to arrange an inspection from one of our performance diff experts.

During cornering, a vehicle outer wheels must travel further than its inner wheels, while bearing the increased load imposed by the vehicle's body roll (away from the corner). Without a LSD, the reduced load on the inner wheels may result in inner wheel spin. With a LSD the inner wheel's rotation is retarded and power transferred to the outer wheel driving it around the corner.

Gear lapping is a fine grade finishing process in which a gear set is reshaped and meshed together with an abrasive compound, whilst under pressure. As the lapping process truly mates the gear set's profile by removing and refining its contact surface, lapped gears generally exhibit a high quality finish, increased accuracy, increased strength, a longer life and an improved performance when compared to non-lapped gears.

Gear lapping can also be used to restore and give new life to worn gear sets.

Positrac LSD is the brand name given to Chevrolet's own LSD. The popularity of this unit was such that the Positrac name is now commonly accepted as a synonym for generic LSDs. Specifically, a positrac employs carbon disc clutch packs and preloaded springs to retard each differential side gear. As power and torque are increased, the forces created increase the clamping load of the clutch packs which in turn increases its retarding ability.

The Trutrac LSD is yet another brand and variant on the LSD principle. In a Trutrac LSD helical gears are employed to automatically retard rotational power. The shape of these gears allows the diff to run freely until loaded, at which point they are drawn automatically into operation. With no wearable parts the Trutrac is a popular, maintenance free, LSD option.

The Detroit locker is patented variant of the LSD created to most closely resemble the action and performance characteristics of a true rigid/fixed axle assembly (ie. 100% of torque to both wheels) as often as possible, whilst permitting wheel speed differentiation when required (i.e. around corners). With its emphasis of fixed and equal drive to both wheels, the Detroit Locker is the preferred locker for many off road and 4WD applications.

Constant Velocity Joints are devices for transmitting rotary action between non-aligned shafts. They allow uniform transmission of the rotary motion, which means both the input and output shafts revolve with the same angular velocity at any given time, and at angles up to 45°. The outer joint (wheel-end) transmits the torque through six equally spaced balls located in matching curved tracks in the housing and race. The balls are in compression under load, and roll within the tracks. The cage helps to maintain this alignment by holding the six balls snugly in its windows. These design features allow angular deflection up to 45° which is required for steering purposes.

The inner joint (gearbox end) transmits the torque through three basic designs: Double Offset (six balls and a cage), Tripod (three trunions and needle rollers), and cross groove (helical grooves and six balls). During angular movement of the joint, the balls or trunions roll in their respective straight tracks, while allowing a plunging movement (when the front wheel travels up or down through suspension movement). These design features allow 22° to 25° of angularity and up to 60mm in plunge.

The tailshaft is the primary component of the drive train responsible for delivering engine power and torque to your drive wheels.

In its simplest form a tailshaft is comprised of two yokes, two universal joints a slip/stub, and a length of steel tube. More complex tailshaft assemblies join two or more of these systems via a centre bearing, a double cardan joint, a rubber flange or in various other ways.

As it joins a vehicle's gearbox to its rear differential and ultimately its wheels, a vehicle's tailshaft should be sized to reflect engine power, rpm and torque as well as wheel size, whilst not compromising the operating angles of the tailshaft's critical components.

See Operating Angles and Shaft Critical Speed.

As your vehicle bumps and bounces down the road, the angle of your wheels and the driveline that comes after them is constantly adapting to the varying angles. While your u-joints absorb the angular rotation, your tailshaft must also stretch and compress to cater for the varying transfer case-diff lengths these angles create. While a fixed length shaft will operate successfully (ie. At a constant velocity) in a single plain, any lateral or vertical movement of this shaft (such as those caused by the angular change that results from wheel travel) would compress /stretch the shaft beyond its tolerances resulting in catastrophic failure.

A slip spline / slip yoke / spline yoke coupled with a slip stub, is a splined male-female join that divides a fixed length shaft into two solid-state components, allowing each of these components to slip/slide in relation to each other and stretch/compress as needed to cater the varying lengths caused by angular variations in the driveline.

The tailshaft arrangement present in most vehicles is set as zero degrees phasing or parallel. To minimise power loss from the operation of the universal joint, two things must be considered with regards to the operating angles of those joints.

1. Make sure shaft centre lines are parallel, ie. The centreline of the transmission or output shaft is parallel with the pinion shaft of the differential whilst the vehicle is at ride height.
2. Ensure both universal joints are operating at the same angle - see diagram. Angle "1" should equal angle "2" and each angle should be less than 3 degrees. Some racing applications may need the pinion angle to be set differently to normal street applications.

A single piece tail shaft is limited by the centrifugal forces created by the shaft's maximum revs per minute - or critical speed. While all rotating shafts become dynamically unstable at certain speeds, resulting in vibration, most of these instabilities are fleeting and inconsequential.

When a shaft's critical speed is exceeded the resultant instability creates severe and violent horizontal displacement, similar to that of a rotating skipping rope, which in turns results in tail shaft failure, diff failure and also possible gearbox damage. For this reason all tail shafts must operate within their critical speed limitations determined by the stiffness of the tube, the diameter of the tube and the shaft's length. As a general rule; larger diameter shafts = higher critical speeds.