While the car is moving on the road or along what can be remotely called a road, the wheels suspended on the chassis elements are constantly moving, changing their position relative to the car body The engineers faced a problem related to the constancy of contact of the tire of the wheel with the surface on which it rolls. The simplest solution was the introduction of such a geometric parameter as wheel camber.
Note
Wheel camber has become a compensating parameter, which helps keeping the wheel in contact with the road.
Wheel camber is the angle between the vertical and the plane of rotation of the wheel.
Note
In case if there is a minus in the wheel camber designation, then the upper part of the wheel is directed towards the inside of the car body. In case if the camber value is positive, then the wheel is directed outward (see Figure 6.28).
Figure 6.28 Wheel camber.
Figure 6.29 Example of toe-in of front steering wheels.
Everything that is associated with uncharacteristic, uneven or excessive wear of the tire tread of the wheels, in one way or another, is the result of a change in such a geometric parameter as camber.
Wheel toe-in is the angle between the direction of movement and the plane of rotation of the wheel (Figure 6.29). It is measured in millimeters or degrees.
The wheel is suspended on the chassis elements on one side only. During movement on the surface (in case if this wheel is only steerable), forces and moments associated with rolling resistance act on it, which try to turn the wheel out. The wheels are toed-in in order to compensate for these forces.
Note
Ideally, the toe-in of the wheels should be changed as the loads on them increase, for example, as the speed increases. However, in practice, this is extremely difficult, if not impossible, to apply. Therefore, while lying in the geometric parameters of the chassis, the convergence is calculated relative to the cruising speed of the vehicle.
Note
Toe-in is mainly used for the wheels of passenger cars. But there is also a parameter, which is opposite to the toe-in – toe-out. It is used in sports cars where increased control sensitivity is required.
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.
While turning, the steered wheels of the car cover different distances. In case if both wheels turn at the same angle, the car will move out of the trajectory, and the tires of the wheels will wear out much faster.
In order to avoid this, the steering is designed in such a way as to ensure that the inner wheel turns at a greater angle relative to the outer wheel (for more details, see the "Steering" chapter).
Figure 6.31 Diagram of car turn.
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