6.3.4 Longitudinal angle of inclination of the axis of rotation of the wheel
Wheel castor angle (Caster) is the angle between the vertical and the projection of the axis of rotation of the wheel on the longitudinal plane of the machine (Figure 6.30). This angle is necessary for better vehicle stabilization while driving. This function is, so to speak, extremely dynamic. In this case, stabilization should be understood as the ability of the steered wheels to resist deviation from the neutral (corresponding to rectilinear motion) position and automatically return to it after the cessation of the action of external forces that caused the deviation. If we imagine a suspension, which constructively excludes the availability of a caster, then all actions to stabilize the car would fall at the discretion of the driver.
Note
The manifestation of the action of the caster in practice could be observed by everyone who rides a bicycle. After all, the fork of the steering wheel of the bicycle is also installed at an angle to the vertical. It is this angle that allows us to ride and turn the bicycle without hands (although we should not forget about the gyroscope effect).
Interesting
The effect of the gyroscope was observed, without exception, by everyone who at least once in their life played with an ordinary whirligig. Whirligig does not fall while rotating. In case if you apply force to the edge of the disk and release it, it will instantly return to an upright position.
Figure 6.30 Example of longitudinal inclination of the wheel turning axis.
Kingpin inclination (KPI) is the angle between the vertical and the projection of the wheel axis of rotation on the transverse plane of the vehicle. As it was already mentioned above, the caster angle of the steering axis stabilizes the car at speed. But what if the speed of movement is relatively low? Either way, the steering wheel shall be stabilized. For this, the parameter "Transverse angle of inclination of the axis of rotation of the wheel (Kingpin inclination)" was introduced into the suspension kinematics. In this case, the wheel is lowered while turning. Consequently, when the car is standing on the surface, when the wheels are turned, the body rises, and, as a result, additional forces arise that tend to return the wheels to their original position.
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