Page 313 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 0. 1 Tyres and wheels

Fig. 10.1-44 Lateral cornering forces of the 155 R 13 78 S ‘82 series’ steel radial tyre, measured on a dry drum at p T ¼ 1.8 bar. The load

capacity at this pressure is around 360 kg, corresponding to a vertical force F Z,W ¼ 3.53 kN. Also shown are the forces at a ¼ 10 and
P T ¼ 1.4 bar and 2.0 bar to indicate the influence of the tyre pressure on the lateral cornering properties.

Thus, the actual load factor k m at 210 km/h becomes: lateral forces at the centres of tyre contact on both
wheels when the vehicle is moving unimpeded in
front k m;f ¼ð375=450Þ 100 ¼ 83% (10.1.14) a straight line. This effect is magnified by an increase in
back k m;r ¼ð425=505Þ 100 ¼ 84% slip rigidity, as, for example, in wide tyres.

10.1.8.5 Influencing variables 10.1.8.5.4 Variations in vertical force
During cornering, vertical force variations DF Z,W in the
10.1.8.5.1 Cross-section ratio H/W
centre of tyre contact cause a reduction in the transfer-
The 185/65 R 15 88H size used as an example in the able lateral forces F Y,W as the tyre requires a certain
00
previous section is a 65 series wide tyre; the 15 diameter amount of time and distance for the build-up of lateral
also allows a good sized brake disc diameter(Fig.10.1-10). forces. The loss of lateral force DF Y,W,4 depends on the
In contrast to the 82 series standard tyre, the sizes of
effectiveness of the shock absorbers, the tyre pressure p T
the 70 series and wide tyres (H/W ¼ 0.65 and below) (which can enhance the ‘springing’ of the wheels) and the
generate higher lateral cornering forces at the same slip type of wheel suspension link mountings. Further in-
angles (Figs. 10.1-9, 10.1-45 and 10.1-46). As can be fluences are wheel load and driving speed. To calculate
seen in Fig. 8.1-6, these, as F Y,W,o ¼ m Y,w (F Z,W þ DF Z,W ), cornering behaviour, an average loss of lateral force
are all the greater, the faster the vehicle takes a bend. DF Y,W,4 due to variations in vertical force and dependent
only on tyre design and slip angle a, should be
10.1.8.5.2 Road condition considered:
The force transmission ratios between the tyres and road DF (10.1.15)
are determined by the state of the road (see construc- Y;W;4 z40 N per degree a
tion, surface roughness and condition; Figs. 10.1-43 and
10.1-47). 10.1.8.5.5 Camber change
Wheels that incline with the body during cornering have
10.1.8.5.3 Track width change a similar, detrimental influence on the transferability of
The track width change that exists, in particular on lateral forces. As can be seen from Fig. 8.1-6, positive
independent wheel suspensions, causes undesirable angle (þ3 w ) camber changes occur on the outside of the

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