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Tyre characteristics and modelling 267
Braking force F x (N)
Dry road
Good tread on wet road
Slip angle = 0
Camber angle = 0
Poor tread on wet road
Aquaplaning
0.0
Fig. 5.20
Slip ratio 1.0
The effect of road contamination on braking
values of braking force as expected reduce and that for a locked wheel with
poor tread a dangerous situation known as hydroplaning or aquaplaning
can arise where the tyre runs on a film of water and traction is effectively
lost. The curves of braking force can also be categorized by two coefficients
of friction associated with the peak braking force and that associated
with total sliding at a slip ratio of 1.0. On dry roads it is possible to obtain
a coefficient of friction for good tyres in excess of 1.0, on wet roads this
would typically reduce to about 0.5 or lower for tyres with poor tread while
a road contaminated with ice may only achieve a peak value of 0.1.
It can be seen when examining the curves in Figure 5.20 that the longitudinal
stiffness is relatively unaffected by surface contamination. This is particularly
dangerous for a road with ice or the poor tyre on a wet road. In these
conditions the peak braking force occurs rapidly at low slip ratio causing the
vehicle to skid before any possible corrective action from the average driver.
In addition to the above it is also known that an increase in vehicle speed
will reduce peak values of braking force and that other parameters such as
tyre inflation pressure will have an effect, a more detailed treatment of
which is given by Gillespie (1992).
5.4.5 Driving force
During driving the transmission will impart a driving torque T D to the rotating
wheel as shown in Figure 5.21. As the angular velocity of the wheel
increases a driving force is generated that tends to move the contact patch
forward relative to the wheel centre. This effect will introduce circumferential
compression in the tread just before entering the contact patch and tension
on leaving as opposed to braking.
As the wheel is driven the tread entering the contact patch will initially
bend forward under the action of shear stresses for a short distance. As the
tread approaches the rear of the tread the pressure begins to unload releasing