Page 168 - Tribology in Machine Design

154 Tribology in machine design

Q" = —2G cKy(y + c). The total cornering force is thus

Self-aligning torque can be found by taking moments about O

The infinite traction at the rear of the contact zone produces slip, so that the
deformed shape k(x) has no discontinuity in gradient and conforms to the
condition g(x)=fp(x) within the slip region.

4.14.3. Functions of the tyre in vehicle application

The most widely known application of pneumatic tyres is the vehicle
application. In this application the pneumatic tyre fulfils six basic
functions:
(i) allows for the motion of a vehicle with a minimum frictional force;
(ii) distributes vehicle weight over a substantial area of contact between
the tyre and the road surface;
(iii) secures the vehicle against shock loading;
(iv) participates in the transmission of torque from the engine to the road
surface;
(v) allows, due to adhesion, for the generation of braking torque, driving
and steering of the vehicle;
(iv) provides stability of the vehicle.
When in a rolling mode the resistance to motion comes from two sources:
- internal friction resulting from the continuous flexing of tread and walls;
- external friction due to micro-movement within the contact area between
the wheel and the road.
The tyre, from a design point of view represents a complex problem, the
solution of which requires a compromise. For example, increased ride
comfort means greater shock absorption, but also increased power
consumption in transmitting engine torque.

4.14.4. Design features of the tyre surface
Another example of a compromise involving the tyre is the incorporation of
a tread pattern into the running band. Under ideal conditions (no rain or
dust deposits on a road surface) the coefficient of sliding friction of about 5
would be attained with a perfectly smooth tread since the adhesion
contribution to friction is maximized by a large available contact area. The
existence of a thin film of water would, in this case, easily suppress the
adhesion and produce very dangerous driving conditions characterized by
a coefficient of friction as low as 0.1 or less. Therefore the tread pattern is
provided on the surface of the tyre to eliminate such a drastic reduction in
the coefficient of friction. However, there is a price to pay, because at the