Page 309 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 0. 1 Tyres and wheels

conditions occur; compression of the surface can lead
0.8 Water level (mm) to the formation of water which has a lubricating effect
and reduces the coefficient of friction to m 0:08
0.2 x;w

0.7 (Fig. 10.1-36). At 25 C, a temperature that is by no
means rare in the Nordic countries, values of around
0.5
m x;w ¼ 0:6 can be reached. At low temperatures, co-
X,W 0.6 1.0 efficients of friction and sliding friction are further apart:
Coefficient of friction µ 0.4 2.0 10.1.8 Lateral force and friction
(10.1.7)
m
0.5
w 2m
X;W
X;W;lo
0.3

0.2 coefficients
10.1.8.1 Lateral forces, slip angle and
3.0 coefficient of friction
0.1
Lateral forces on a rolling tyre can be caused by the tyre
rolling diagonal to the direction of travel (so-called slip),
60 80 100 km h −1 140 the tendency of a tyre to move from its position vertical
Speed to the road, camber or conical effects. The build-up of
lateral forces as a result of slip will be discussed next.
Fig. 10.1-35 Coefficients of friction m X,W of a summer tyre with an If a disturbing force F c,V acts at the centre of gravity of
8-mm deep profile dependent on speed at different water levels.
1
Hardly any influence can be detected under 60 km h ; at higher the vehicle (e.g. a wind or side negative lift force), lateral
speeds and 3-mm water depth, the curve shows a lowering of wheel forces F Y,W,f,o , F Y,W,f,i , F Y,W,r,o and F Y,W,r,i are
m X,W which indicates the aquaplaning effect. needed to balance the forces (Fig. 10.1-37). To build up
these forces, the vehicle must alter its direction of travel
about the angle a, the slip angle. The size of the slip angle
10.1.7.3.3 Snow and ice depends on the force transmission properties of the tyre
Similar to aquaplaning, low coefficients of friction occur and the disturbing force (Fig. 10.1-38).
on icy roads, although these are highly dependent on When cornering, the interference force should be equal

the temperature of the ice. At close to 0 C, special to the centrifugal force F c , V , which results from the speed
0.7
Speed
10
0.6
20
40 −1
0.5
Coefficient of friction µ X,W 0.4
km h

0.3

0.2

0.1

0
−5 −10 −15 −20 °C −25
Ice temperature

Fig. 10.1-36 Influence of ice temperature and car speed on the coefficient of friction m X,W of an 82 series winter tyre; the extremely low

values at 0 C can be seen clearly.

310