Differentiating Power Transmission
After the engine and gearbox, power is transmitted to at least two driven wheels. In a straight line, the torque is distributed equally between them. The moment it negotiates a turn or a curve, it is necessary for the outer wheel to rotate faster (greater circumference) than the inner one.
But in something like a go-kart which has a relatively narrow track, small tyres and not much power, both driven wheels can rotate at the same speed in the corner with the difference in circumferential distance being accommodated by slipping on the track surface.
On road-going vehicles, the loss of traction due to tyre slip will result in extreme understeer or oversteer situations as the two tyres fight to maintain the necessary speed.
Hence, the purpose of the device we call the differential. Located between the left and right driven wheels (we will stick with 2-wheel drive for the moment), it transmits power from the gearbox to the wheels and is designed to allow the rotating speed differential between the two wheels in a curve or turn.
In a straight line, both driveshafts rotate at the same speed, but once a curve is encountered, the inner wheel rotates at a slightly lower speed while the outer wheel increases its rotational speed by exactly the same amount. In other words, assuming road speed remains constant, the mathematical sum ofleft and right rotational speeds is always the same.
Because it allows one wheel to turn faster than the other in conditions when one wheel is on a frictionless surface or has lifted off the ground (as in hard cornering), the differential will transmit all the torque to the less loaded wheel. This limits the car’s ability to accelerate.
This was the main reason differentials with some form of limitation in “slip” was developed. Early limited slip differentials, or LSDs, relied on a multi¬-plate clutch that would engage gradually as the speed differential increased. Being purely mechanical, clutch-type LSDs require some degree of pre-loading and are fine for competition, but it can make driving in the wet tricky.
Technology has moved along quite a bit since, but curiously, inside most of today’s LSDs, you will still find a clutch pack because it is still the most efficient way to transmit torque.
BMWs variable M differential lock on the M3 consists of a clutch set that is actuated by hydraulic pressure. A built-in pump begins to pressurize hydraulic fluid when one wheel begins to spin, causing the clutch to engage, transmitting torque to the more laden outside wheel and thus maintaining traction. Although there is no direct electronic control, the “M-diff’ works brilliantly and helps make the M3 one of the finest handling rear-wheel drive cars in existence.
Which brings us to the issue of 4- wheel drive (4WD) cars? In addition to a differential between the wheels at either axle, such cars also require a third differential that allows the front-to-rear speed variance. The most basic versions of some 4 WD cars make do with simple “open” (non-LSD) differentials, but the more sophisticated lot have a LSD set in the middle. In addition to traction on all four wheels, the inclusion of a centre LSD offers the flexibility for engineers to define the car’s handling balance. Firstly, the centre differential can be geared to provide a torque bias (front or rear), thus altering the car’s dynamic behavior, and secondly transfer torque to the end with a higher level of traction.
The recently announced Golf R comes standard with an active Haldex centre differential. In addition to its differential functions, the integration of electronic controls and an electric pump enable the Haldex to continuously vary torque transfer between front and rear depending on the car’s dynamic behavior. Sounds like an extremely agile car, but we will have to wait till early 2010 to find out for ourselves.
Whether it’s front- or rear-wheel drive, 2- or 4-wheel drive, the uncanny differential is a mandatory element in the powertrain. Its use as a component to allow motor vehicles to negotiate curves is now probably its simplest task. With state-of-the-art engineering, both mechanical and electrical, the differential has been carefully developed to optimize driving performance.
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