How a 4WD Differential Works
Most of us don't need to know the inner workings of components such as transmissions, engines and diff's but a basic understanding can give us some clues as to why our vehicles behave the way they do.
The Spin on 4WD Differentials
The basic differential performs 3 functions
- To receive power from the transmission and driveshaft and transfer it 90° to the wheels.
- To provide an overall reduction gear, slowing the rotational speed of the transmission, before delivery to the wheels.
- To transfer power to the wheels while allowing them to rotate at different speeds while cornering.
The differential is simply a combination of gears or cogs which combine to make the wheels turn. In it's simplest form the 4WD diff receives input power from a single source (the tail-shaft or drive-shaft) and splits that power into two outputs (the left and right axles).
Because of the nature and size of the gears in the differential, the rotating speed of the tail-shaft is reduced, creating a manageable power delivery to the wheels.
When a car negotiates a turn the inside wheels have less distance to travel than the outside wheels. The outside wheels must rotate faster to 'keep up' with the turn. It is the reason track and field athletes get a head start when they race in the outside lanes of a circular track - the outside lanes are longer than the inside.
If the wheels couldn't rotate at different speeds while cornering one of the tyres would have to 'slip' or skid to accommodate the mismatched revolutions of the two wheels.
The Open Differential
Open differentials are found in most passenger cars and light-duty four wheel drive vehicles. They are a reasonably simple affair that work well in most given situations. Their single drawback is their characteristic to favour the wheel with the least amount of traction.
In the diagram on the right we can see the Ring Gear and Carrier are driven by the input of the drive shaft. The Carrier holds the Planetry Gear which can spin independently as it revolves around the axis of the carrier. In turn the Planetry Gear drives the Side Gears of both axles.
If the resistance on the Axle Side Gears is equal then both wheels rotate at the same rate.
When the resistance of one axle is greater than the other the Planetry Gear begins to spin as it revolves around the Axle Side Gears. This occurs if one tyre is on loose gravel while the other tyre is on hard bitumen. It also occurs when turning the vehicle, as the outside wheel needs to travel further and faster than the inside wheel.
The resistance of one wheel causes the Planetary Gear to spin. This spinning action transfers an accelerated rotational force to the opposite axle which in turn drives this wheel faster.
The drawback arises in racing type situations while accelerating out of corners. The outside wheel encounters greater resistance during the turn and greater drive is applied to the inside wheel resulting in wheel-spin.
The same principle affects a four wheel drive on a loose surface. The tyre with the most resistance (i.e.: traction) will remain stationary while the the tyre with the least traction spins aimlessly.
Limited Slip Differentials were developed to overcome the inherent tendency for Open Differentials to take the path of least resistance.
The Limited Slip Differential.
The Limited Slip Differential or LSD is often factory fitted to performance, off-road and commercial vehicles (utes for instance).
Two main types of LSD are used - 'Torque Sensitive' or a 'Viscous Coupling'.
Torque Sensitive diffs rely on clutch packs or helical gears to create additional resistance against the axles and consequently reduce the tendency for unwanted wheel-spin. The clutches or gears respond to driveshaft torque and generate more internal pressure as driveshaft torque increases. Very simply, as the axle with the least load (traction), tries to 'break out' and spin, the forces on the clutch pack increase - keeping both wheels driving. The system is still flexible enough to allow some 'slippage' to occur at the clutch plates, thus allowing the car to turn.
A Viscous or Speed Sensitive LSD is a simpler unit that uses hydrodynamic friction to accomplish a similiar result. Vanes on each axles motivate special differential fluid and generate heat and friction, increasing the resistance on the axles. Viscous Differentials tend to be less efficient than Clutch type LSD's with occasional problems and power loss due to overheated oil.
Limited slip diff's don't lock the wheels together to create a single, equal driving unit. They must 'slip' to allow the wheels to rotate at differing speeds. They do make a considerable difference to vehicles that require greater traction. A 2WD car fitted with a LSD is much better at recovery on slippery terrain than a vehicle fitted with an open differential. Many four wheel drives are sold with a limited slip diff fitted to the rear and an open diff fitted to the front wheels.
'Diff Lockers' are an additional component that allows the driver to lock both axles together to gain maximum traction in off-road situations (both axles rotate at the same rate regardless of load). They are virtually unmanageable when 'locked' and propelled along hard surfaces.
The article on Transmission Wind-Up examines what can happen to driveline components like differentials when they are unable to rotate independently.
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