PHASE 8

2.3.4 OVERALL BIAS CONTROL

The differential may be designed with different bias ratios ranging from approximately '2.5:1' to '6:1' or higher. This may be accomplished by varying the side gear helix angles, or by altering the friction characteristics for the primary components. An increase in helix angle increases the thrust component of the side gear meshes along the axis of the side gears so that smaller portions of the loads communicated by the side gear meshes are related to rotation of the side gears.

In addition, the higher thrust component along the axis of the side gears increases frictional resistance at the end faces of the side gears which opposes side gear rotation and thereby further contributes to an increase in bias ratio.

2.3.5 BIAS RATIOS BETWEEN DRIVE AND COAST MODES

It is also an important design freedom to provide for different effective bias ratios between vehicle driving and coasting modes. Since the Torsen differential is designed to have little or no effect on vehicle performance unless torque is being transferred by the differential, it should be understood that what is meant by the coasting mode is actually vehicle deceleration caused by engine braking. This mode is most evident with standard shift vehicles engaged in downshifting.

Index gearing also makes possible this important design alternative. The side gears within the differential are designed with the same hand of helix angle. When engine power is applied to the differential (i.e., drive mode), both side gears are thrust against the same end of the differential housing. Alternatively, when the engine is used to brake the drive wheels (i.e., coast mode), the side gears are thrust against the opposite end of the housing. This feature provides an opportunity to vary frictional characteristics between opposite ends of the housing to vary bias ratios between the opposite directions of power transfer through the differential. The possibilities for independently varying bias ratios between the two directions of power transfer enables the differential to be designed with one bias ratio to compensate for undesirable steering effects associated with downshifting and a second bias ratio which is selected for most  other purposes.

CHAPTER - 3

COMPONENTS AND DESCRIPTION

3.1. DIFFERENTIAL:

WHAT'S A DIFFERENTIAL?

When a vehicle is negotiating a corner, the outside wheel has to travel a grater distance than the inside wheel. Therefore, the outside wheel must turn faster than the inside wheel. The differential is the device within the axle assembly which, in addition to transmitting the power to each axle shaft/wheel, allows one wheel to turn at a different speed than the other. A conventional open differential sends equal amounts of torque to both axle shafts (top). If one wheel spins because of lost traction, it is sustaining zero engine torque, so zero engine torque is also going to the wheel with traction. Adding a locking differential—in this case a No Spin locker (bottom)—mechanically links the two shafts so that power will be delivered to both axles in all circumstances.

WHY YOU NEED A DIFFERENTIAL

Car wheels spin at different speeds, especially when turning. You can see from the animation below 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. For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue.

There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.