Slip (vehicle Dynamics) - Icon Vehicle Dynamics

In (automotive) vehicle dynamics, slip is the relative motion between a tire and the road surface it is moving on. This slip can be generated either by the tire's rotational speed being greater or less than the free-rolling speed (usually described as percent slip), or by the tire's plane of rotation being at an angle to its direction of motion (referred to as slip angle).

In rail vehicle dynamics, this overall slip of the wheel relative to the rail is called creepage. It is distinguished from the local sliding velocity of surface particles of wheel and rail, which is called micro-slip.

Longitudinal slip

The longitudinal slip is generally given as a percentage of the difference between the surface speed of the wheel compared to the speed between axis and road surface, as:

${\displaystyle slip={\frac {\omega r-v}{v}}}$

where ${\displaystyle \omega }$ is the lateral component of the rotational speed of the wheel, ${\displaystyle r}$ is wheel radius at the point of contact and ${\displaystyle v}$ is vehicle speed. A positive slip indicates that the wheels are spinning; negative slip indicates that they are skidding. Locked brakes, ${\displaystyle \omega r=0}$, means that ${\displaystyle slip}$ is -100% and sliding without rotating. ${\displaystyle v=0}$ and ${\displaystyle \omega r}$ ≠ ${\displaystyle 0}$, means that ${\displaystyle slip=}$∞.

Lateral slip

The lateral slip of a tire is the sideways motion of a tire which occurs when the sideway forces of a tire are greater than its friction resistance. This can occur, for instance, in cornering.

The slip angle can be defined as:

${\displaystyle \alpha =\arctan \left({\frac {v_{y}}{|v_{x}|}}\right)}$

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