782Operating conditions

Normal differential action (Fig. 7.41) With good road wheel grip the multiplate clutch is disengaged by closing the delivery solenoid valve and releasing fluid to the reservoir tank via the open return solenoid valve. Under these conditions when there is a difference in speed between the inner and outer road wheels, the bevel-planet pinions are free to revolve on their axes and hence permit each sun (side) gear to rotate at the same speed as its adjacent road wheel thereby eliminating any final drive transmission windup and tyre scrub.

One wheel on the threshold of spinning (Fig. 7.41) If one wheel commences to spin due to loss of traction the wheel speed sensor instantly detects the wheel's acceleration and signals the ECU; the computer then processes this information and taking into account that a small amount of slip improves the tyre to ground traction will then energize and de-energize the delivery and return solenoid valves respectively. Fluid will now be pumped from the power assistant steering systems pump to the servo-piston, the pressure build up against the piston will engage and clamp the multiplate clutch via the thrust-pins and plate so that the differential

Multi-

Left hand plate bearing Differential clutch support cage -n. t flange e gv Pin Thrust

Right hand! jSldUX planet \ Plate output \ geJ\ pinion drive shaft

Wheel speed sensor

Thrust Crown pins wheel and gear plate

Servo piston and seals

Right hand bearing and piston support flange

Right hand output drive shaft / coupling flange

Bolt and clamp plate

Thrust Crown pins wheel and gear plate

Servo piston and seals

Right hand bearing and piston support flange

Bolt and clamp plate

Oil seal pinion splines

Flanged universal joint coupling

Electro-hydraulic control valves

Electronic Control Unit

Fig. 7.41 Electro-hydraulic limited-slip (differential in locked position)

Oil seal

Differential cage taper bearing Differential housing

Bevel pinion pinion inner taper bearing

Spacer sleeve pinion outer taper bearing

Oil seal pinion splines

Flanged universal joint coupling

Electro-hydraulic control valves

Electronic Control Unit

Fig. 7.41 Electro-hydraulic limited-slip (differential in locked position)

cage now is able to provide the reaction torque for the other wheel still delivering traction to the ground.

The ECU is able to take into account the speed of the vehicle and if the vehicle is turning gently or sharply which is monitored by the individual brake speed sensors and the steering wheel accelerator sensor. These two sensors therefore indirectly control the degree of lock-up which would be severe when pulling away from a standstill but would ease up with increased vehicle speed and when negotiating a bend.

7.9 Tyre grip when braking and accelerating with good and poor road surfaces (Fig. 7.42) The function of the tyre and tread is to transfer the accelerating and decelerating forces from the wheels to the road. The optimum tyre grip is achieved when there is about 15-25% slip between the tyre tread and road under both accelerating and decelerating driving conditions, see Fig. 7.42.

Tyre grip is a measure of the coefficient of friction (u) generated between the tyre and road surface at any instant, this may be defined as u = F/ W where F is the frictional force and W is the perpendicular force between the tyre and road. If the frictional and perpendicular forces are equal (u = 1 -0) the tyre tread is producing its maximum grip, whereas if u = 0 then the grip between the tyre and road is zero, that is, it is frictionless. Typical tyre to road coefficient for a good tarmac dry and wet surface would be 1.0 and 0.7 respectively, conversely for poor surfaces such as soft snow covered roads the coefficient of friction would be as low as 0.2.

Wheel slip for accelerating and decelerating is usually measured as the slip ratio or the percentage of slip and may be defined as follows:

accelerating slip ratio = road speed/tyre speed decelerating (braking) = tyre speed/road slip ratio speed where the tyre speed is the linear periphery speed.

Note the percentage of slip may be taken as the slip ratio x 100. There is no slippage or very little that takes place between the tyre and road surface when a vehicle is driven at a constant speed along a dry road, under these conditions the slip ratio is zero (slip ratio = 0). Conversely heavy acceleration or braking may make the wheels spin or lock respectively thus causing the slip ratio to approach unity (slip ratio = 1.0).

If the intensity of acceleration or deceleration is increased the slip ratio tends to increase since during acceleration the wheels tend to slip and in the

1.0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1.0

Slip ratio (braking) Slip ratio (accelerating)

Fig. 7.42 Tyre grip as a function of slip ratio for various driving conditions extreme spin, whereas during deceleration (braking) the wheels tend to move slower than the vehicle speed and under very heavy braking will lock, that is stop rotating and just slide along the surface. When considering the relationship between tyre slip and grip it should be observed that the tyre grip measurements are in two forms, longitudinal (lengthways) forces and lateral (sideways) forces, see Fig. 7.42. In both acceleration and deceleration in the longitudinal direction mode a tyre tends to produce its maximum grip (high with a slip ratio of about 0.2 and as the slip ratio decreases towards zero, the tyre grip falls sharply; however, if the slip ratio increases beyond the optimum slip ratio of 0.2 the tyre grip will tend to decrease but at a much slower rate. With lateral direction grip in terms of sideways force coefficient of friction, the value of ^ (grip) is much lower than for the forward rolling frictional grip and the maximum grip (high is now produced with zero tyre slip. Traction control systems (TCS) respond to wheel acceleration caused by a wheel spinning as its tyre loses its grip with the road surface, as opposed to antilock braking systems (ABS) which respond to wheel deceleration caused by a wheel braking and preventing the wheel from turning and in the limit making it completely lock.

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