Page 169 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 155

same time, the coefficient of adhesion component of friction under dry
conditions is seriously reduced to a value less than unity as the area of
contact is reduced by the grooves. Nevertheless, the overall effective
coefficient of friction under wet conditions is considerably increased; a
value of/=0.4 for locked wheel skidding on a wet road is typical. The main
role of the grooving on the surface of the tyre is to drain excess water from
the tyre footprint in order to increase the adhesion component of friction.
Thus, an adequate tread pattern offers a compromise between the higher
Figure 4.52
and lower friction coefficients that would be obtained with a smooth tyre
under dry and wet conditions respectively. The usual requirement of the
designer is to ensure that the grooving or channeling in the tread pattern is
capable of expelling the water from the tyre footprint during the time which
is available at high rolling speed. Figure 4.52 shows three basic tread
patterns which are used today, called zigzag, ribbed and block.
According to experimental findings the differences in performance of
each type are not very significant. Apart from the basic function of bulk
water removal, the tyre tread must also allow for a localized tread
movement or wiping, to help with the squeezing-out of a thin water film on
the road surface. This can be achieved by providing the tread with spies or
cuts leading into grooves. There are a number of important design features
which a modern tread should have:
(i) channels or grooves. The volume of grooving is almost constant for all
tread types. On average, the grooves are approximately 3 mm wide and
10mm deep;
(ii) spies or micro-cuts leading into the channels or feeder channels. Their
main function is to allow for tread micro-movement which is
characteristic of the rolling process. Usually, spies do not contribute to
the removal of water from the footprint directly. Figure 4.53 shows the
arrangement of channels and spies typical for the zigzag pattern;
(iii) transverse slots or feeder channels. Their size is less than the main
channels which they serve. The transverse slots are not continuous but
end abruptly within the tread. Their main role is to displace bulk water
from the tyre footprint. They also permit the macro-movement of the
Figure 4.53 tread during the wiping action.

4.14.5. The mechanism of rolling and sliding
Both rolling and sliding can be experienced by a pneumatic tyre. Pure
sliding is rather rare except in case of a locked wheel combined with
flooding due to heavy rainfall. Then, the same tread elements are subjected
to the frictional force and as a result of that, the wear of the tread is uneven
along the tyre circumference. Severe braking but without the wheel being
locked produces wear in a uniform manner along the tyre circumference
since the contact zone is continually being entered by different tread
elements. The extent of wear under such conditions is less, because the mean
velocity of slip of the tread relative to the road surface is much lower.
During the rolling of the tyre, four fundamental elements of the process