Page 308 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Tyres and wheels C HAPTER 10.1

1.0
1 Dry
Dry asphalt
0.8 0.8 Damp
Wet asphalt 0.6 Wet
0.6
Coefficient of friction 0.4 Loose snow Coefficient of sliding friction 0.4
Loose gravel

0.2
0.2
Ice
0
Slip 0
20 40 60 km h –1 100
Fig. 10.1-33 Coefficient of friction m X,W of a summer tyre with Speed
80–90% deep profile, measured at around 60 km/h and shown
in relation to the slip on road surfaces in different conditions (see also Fig. 10.1-34 Dependency of the coefficient of sliding friction
Fig.8.1-64).Widetyresin the‘65series’and belowhavethegreatest m X,W,Io on speed on different road conditions.
friction at around 10% slip, which is important for the ABS function.

when it relates to the maximum value, and the coefficient 10.1.7.3.2 Aquaplaning
of sliding friction, also called sliding friction factor
The higher the water level, the greater the risk of aqua-
m X;W;lo ¼ F X;W =F Z;W (10.1.5a) planing. Three principal factors influence when this
occurs:
when it is the minimal value (100% slip) (Fig. 10.1-33). road
F x is designated F X,W,b during braking and F X,W,a during tyres
traction. speed.
In all cases m x,w is greater than m x,w,lo ; in general it can
be said that
With regard to the road, the water level is the critical
on a dry road m x;w z1:2 m x;w;lo (10.1.6) factor (Fig. 10.1-35). As the level rises, there is a dis-
proportionate increase in the tendency towards aqua-
on a wet road m x;w z1:3 m x;w;lo (10.1.6a) planing. When the level is low, the road surface continues
to play a role because the coarseness of the surface ab-
10.1.7.3 Road influences sorbs a large part of the volume of water and carries it to
the edge of the road. Following rainfall, the water levels
on roads are generally up to 2 mm; greater depths can
10.1.7.3.1 Dry and wet roads
also be found where it has been raining for a long time,
On a dry road, the coefficient of friction is relatively during storms or in puddles.
independent of the speed (Fig. 10.1-34), but a slight On the tyre, the tread depth has the greatest influence
increase can be determined below 20 km/h. The reason (Fig. 10.1-47). There can be up to a 25 km h 1 difference
lies in the transition from dynamic to static rolling radius in speed between a full tread and the legal minimum
(see the example in Section 10.1.2.5.4) and is therefore tread depth of 1.4 mm. High tyre pressure and low
linked to an increasing area of tyre contact. At speeds running surface radius r (Fig. 10.1-5) lead to the area of
a little over zero, on a rough surface, a toothing cogging contact becoming narrower, giving the advantage of im-
effect can occur, which causes a further increase in the proved aquaplaning behaviour as the distribution of
coefficient of friction, then: ground pressure becomes more even (Fig. 10.1-9). Lower
tyre pressure and contours with larger radii make aqua-
m x;w 1:3 (10.1.6b) planing more likely; this also applies to wider tyres
(Fig. 10.1-19) particularly when tread depths are low.
When the road is wet, the coefficient of friction reduces,
but is still independent of the speed. This situation However, the greatest influence by far is the speed, es-
changes as the amount of water increases and also with pecially when the water level increases and tread depths
shallower profile depth. The water can no longer be are low. This is why reducing speed is the best way to
moved out of the profile grooves and the m value falls as lessen the risk of aquaplaning, and is a decision drivers
speed increases. can make for themselves.

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