772 Understeer and oversteer characteristics

In general, tractive or braking effort will reduce the cornering force (lateral force) that can be generated

Tyre Tire Slip Angles
Fig. 7.30(a and b) The influence of front and rear tyre slip angles on steering characteristics

for a given slip angle by the tyre. In other words the presence of tractive or braking effort requires larger slip angles to be produced for the same cornering force; it reduces the cornering stiffness of the tyres. The ratio of the slip angle generated at the front and rear wheels largely determines the vehicle's tendency to oversteer or understeer (Fig. 7.30).

The ratio of the front to rear slip angles when greater than unity produces understeer, t, • ©f , i.e. Ratio —— < 1.

When the ratios of the front to rear slip angles are less than unity oversteer is produced,

If the slip angle of the rear tyres is greater than the front tyres the vehicle will tend to oversteer, but if the front tyres generate a greater slip angle than the rear tyres the vehicle will have a bias to understeer.

Armed with the previous knowledge of tyre behaviour when tractive effort is present during cornering, it can readily be seen that with a rear wheel drive (RWD) vehicle the tractive effort applied to propel the vehicle round a bend increases the slip angle of the rear tyres, thus introducing an oversteer effect. Conversely with a front wheel drive (FWD) vehicle, the tractive effort input during a turn increases the slip angle of the front tyres so producing an understeering effect.

Experimental results (Fig. 7.31) have shown that rear wheel drive (RWD) inherently tends to give oversteering by a small slightly increasing amount, but front and four wheel drives tend to understeer by amounts which increase progressively with speed, this tendency being slightly greater for the front wheel drive (FWD) than for the four wheel drive (4WD).

7.7.3 Power loss (Figs 7.32 and 7.33) Tyre losses become greater with increasing tractive force caused partially by tyre to surface slippage. This means that if the total propulsion power is shared out with more driving wheels less tractive force will be generated per wheel and therefore less overall power will be consumed. The tractive force per wheel generated for a four wheel drive compared to a two wheel drive vehicle will only be half as great for each wheel, so that the overall tyre to road slippage will be far less. It has been found that the power consumed (Fig. 7.32) is least for the front wheel drive and greatest for the rear wheel drive, while the four wheel drive loss is somewhere in between the other two extremes.

The general relationship between the limiting tractive power delivered per wheel with either propulsion or retardation and the power loss at the wheels is shown to be a rapidly increasing loss as the power delivered to each wheel approaches the limiting adhesion condition of the road surface. Thus with a dry road the power loss is relatively small with

Ov$r$teer

Constant 1 radius of curvature

Neutral steer

Vehicle speed (km/h)

0 20 40 BO

Vehicle speed (km/h)

Fig. 7.31 Comparison of the over- and understeer tendency of RWD, FWD and 4WD cars on a curved track

tRWD

-

/ , 4WD

/ ,FWD

j

Vehicte speed (km/h)

Vehicte speed (km/h)

Fig. 7.32 Comparison of the power required to drive RWD, FWD and 4WD cars on a curved track at various speeds

10

Traction limit

Braking limit

iW = 0 3

0 3

- f1 =

0 6 /

\ 0 6

Ofy-*

V s W

J

y

<10 20 0 20 10 Tractive power (kW)

Fig. 7.33 Relationship of tractive power and power loss for different road conditions

<10 20 0 20 10 Tractive power (kW)

Fig. 7.33 Relationship of tractive power and power loss for different road conditions

Fig. 7.34 Comparison of the adhesive traction available to Drive, RWD, FWD and 4WD cars on a curved track at various speeds

increasing tractive power because the tyre grip on the road is nowhere near its limiting value. With semi-wet or wet road surface conditions the tyre's ability to maintain full grip deteriorates and therefore the power loss increases at a very fast rate (Fig. 7.33).

If friction between the tyre and road sets the limit to the maximum stable speed of a car on a bend, then the increasing centrifugal force will raise the cornering force (lateral force) and reduce the effective tractive effort which can be applied with rising speed (Fig. 7.34). The maximum stable speed a vehicle is capable of on a curved track is highest with four wheel drive followed in order by the front wheel drive and rear wheel drive.

7.7.5 Permanent four wheel drive transfer box (Land and Range Rover) (Fig. 7.35)

Transfer gearboxes are used to transmit power from the gearbox via a step down gear train to a central differential, where it is equally divided between the front and rear output shafts (Fig. 7.35). Power then passes through the front and rear propellor shafts to their respective axles and road wheels. Both front and rear coaxial output shafts are offset from the gearbox input to output shafts centres by 230 mm.

The transfer box has a low ratio of 3.32:1 which has been found to suit all vehicle applications. The high ratio uses alternative 1.003:1 and 1.667:1 ratios to match the Range Rover and Land Rover requirements respectively. This two stage reduction unit incorporates a three shaft six gear layout inside an aluminium housing. The first stage reduction from the input shaft to the central intermediate gear provides a 1.577:1 step down. The two outer intermediate cluster gears mesh with low and high range output gears mounted on an extension of the differential cage.

Drive is engaged by sliding an internally splined sleeve to the left or right over dog teeth formed on both low and high range output gears respectively. Power is transferred from either the low or high range gears to the differential cage and the bevel planet pinions then divide the torque between the front and rear bevel sun gears and their respective output shafts. Any variation in relative speeds between front and rear axles is automatically compensated by permitting the planet pinions to revolve on their pins so that speed lost by one output shaft will be equal to that gained by the other output shaft relative to the differential cage input speed.

A differential lock-out dog clutch is provided which, when engaged, locks the differential cage directly to the front output shaft so that the bevel gears are unable to revolve within the differential cage. Consequently the front and rear output shafts are compelled to revolve under these conditions at the same speed.

Fig. 7.35 Permanent 4WD Land and Range Rover type of transfer box

A power take-off coupling point can be taken from the rear of the integral input gear and shaft. There is also a central drum parking brake which locks both front and rear axles when applied.

It is interesting that the low range provides an overall ratio down to 40:1, which means that the gearbox, transfer box and crownwheel and pinion combined produce a gear reduction for gradient ability up to 45°.

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