and 
, the corresponding road-tire friction
coefficient, is assumed.
In the Tire statement, a linear relationship between and , the corresponding road-tire friction
coefficient, is assumed.
Figure 1 Linear Friction Model
(9)
:
Static Friction Coefficient
:
Dynamic Friction Coefficient
The friction circle concept allows for different values of
longitudinal and lateral friction coefficients(
and 
) but limits the maximum
value for both coefficients to 
.
The relationship that defines the friction circle follows:
(10)
or 
and 
where, 
The Fiala tire model uses only 
and not 
and 
. The UATIRE model uses 
, 
, and 
.