Page 311 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 0. 1 Tyres and wheels

Fig. 10.1-40 If, during cornering, a f ~ a r , the handling of a vehicle Fig. 10.1-42 If there is a greater slip angle a r on the rear wheels
can be described as neutral. than on the front (a f ), the vehicle oversteers. The positive angle
describes the angle between the vehicle longitudinal axis and its
speed at the centre of gravity.
(a f ¼ a r , Fig. 10.1-40), one speaks of neutral handling
characteristics. Over-steering behaviour is present if the
tail of the vehicle moves outwards during cornering and Since in greater bend radii the average steering angle

the slip angle on the rear axle is greater than on the front d m is less than 5 , it can be assumed that the sine and
axle (a f < a r , Fig. 10.1-42). The driver must respond to radius values of the angle are equal, and the angles d o and
this by reducing the steering angle. d i correspond to this:
As understeering behaviour is consistent with the
expectations and experience of the driver, it is this which sin d m zd m zd o zd i ðradÞ
needs to be aimed for. In normal driving conditions
(anti-skid roadway, lateral acceleration of less than 6 m/s), It is now possible to determine the relationship between
all vehicles, therefore, are now designed to understeer. steering angle, turning circle diameter D S (Fig. 8.1-69)
With increasing lateral acceleration, the under-steering and slip angles at a constant cornering speed:
behaviour should be as linear as possible and then, also as
a warning to the driver that the stability limit is about to 2 =
f
be reached, increase progressively. If the handling char- d m ¼ þ a a r (10.1.10)
D S
acteristics change to oversteer at the stability limit, for
instance with very high acceleration, this is an un- The Kingpin offset at ground r s is so negligable in
predictable driving situation which the untrained driver comparision to Ds that it can be ignored.
can only control with difficulty. For active riding safety,
the predictability of self-steering properties in all kinds
of conditions (vehicle loading, the distribution of driving 10.1.8.3 Coefficients of friction and slip
torque in four-wheel drive vehicles, different coefficients
of friction, acceleration or braking procedures, changes in To determine the cornering behaviour, the chassis engi-
tyre pressure, etc.) is of paramount importance. neer needs the lateral forces (or the coefficient of fric-
For a simplified representation of the relationships tion) based on the slip angle and the parameters:
described, the so-called single-track model is used, in
which the wheels of the vehicle are drawn together in the vertical force (or wheel load) in the centre of tyre
middle of the vehicle, without taking into account the contact
height of the centre of gravity (flat model). tyre pressure
wheel camber
tyre type.
The measurements are generally taken on test rigs, up

to slip angles of a ¼ 10 . The drum surface with its
friction values of m 0 ¼ 0.8–0.9 sets limits here, and
larger angles hardly give increasing lateral coefficients of
friction:

m Y;W ¼ F Y;W =F Z;W (10.1.11)

Conditions on the road are very different from those on
Fig. 10.1-41 If there is a greater slip angle a f on the front wheels the test rig; the type of road surface and its condition play
than a r on the rear, the vehicle understeers. a role here. As can be seen in Fig. 10.1-43, the coefficient

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