Slip angle

Vehicle slip angle. View from above perpendicular to the roadway. M: instantaneous pole of the plane vehicle movement; R: radius

The slip angle is the angle β between the direction of movement of the vehicle in the center of gravity and the vehicle's longitudinal axis . The direction of movement is variable in the longitudinal direction of the vehicle. For other locations in the vehicle than the center of gravity, this must be specified, e.g. B. slip angle (rear axle). In the driving test, the slip angle was previously measured with a Peiseler bike . Optical processes are used for this today. Since measurements can rarely be made in the center of gravity, either the measurement location must be specified or a conversion into the center of gravity must be made.

Slow cornering

When cornering slowly, the lateral forces of the wheels and thus the slip angles are low. The rays perpendicular to the wheels approximately intersect at the instantaneous pole. In the case of a vehicle being steered on the front axle, the direction of movement on the rear axle is in the direction of the longitudinal axis. In the center of gravity, especially with tight radii ( turning circle ) in cars, there is a float angle of the order of 15 to 20 degrees, which only depends on the distance between the center of gravity and the rear axle and the curve radius. Since the latter depends approximately on the mean wheel lock of the front axle, a relationship between the steering angle of the front axle and the float angle can be derived with the single-track model : ${\ displaystyle \ delta _ {v}}$

{\ displaystyle \ beta = {\ frac {-l_ {h}} {l}} \ cdot \ delta _ {v}}

Assuming a constant total steering ratio is , with the steering wheel angle . The distance of the center of gravity from the rear axle is and the wheelbase. The ratio is about 0.6 for front wheel drive, 0.5 for standard drive and 0.4 for rear wheel drive vehicles. The minus sign shows that the float angle is negative in the left turn shown above. ${\ displaystyle i_ {s}}$ ${\ displaystyle \ delta _ {v} = \ delta _ {H} / i_ {s}}$ ${\ displaystyle \ delta _ {H}}$ ${\ displaystyle l_ {h}}$ ${\ displaystyle l}$ ${\ displaystyle l_ {h} / l}$

Fast cornering

Maserati 250 F with a large slip angle in the limit area . The vehicle is stabilized by counter-steering.

At higher speeds, lateral forces are required to maintain course when cornering. These are caused by tilting the entire vehicle. The front axle has a narrower radius than the rear axle. The float angle or the float angle gradient is used here as a measure for assessing driving stability.

On a dry road, the driving behavior of a car is still stable at a slip angle of less than 5 degrees, and in racing cars up to 10 degrees. With larger, controlled sideslip angles, one speaks in motorsport of the driving technique drifting , with uncontrolled sideslip angles in road traffic of skidding . In general, a driving state is stable when the original driving state is restored without the driver having to do anything after a fault . This is always the case when the moment of the lateral force on the rear axle with respect to the center of gravity is greater than the moment of the front axle. All of today's vehicles are therefore untersteuernd designed.

Measures to influence the side slip angle

Large sideslip angles, i.e. H. large slip angles of the rear axle can still, for. B. when changing direction quickly, or on slippery roads. With dynamic driving control systems , a yaw moment is initiated by braking the front wheel on the outside of the bend, which stabilizes the driving condition again. Another possibility to reduce the slip angle is offered by all-wheel steering in which the rear axle turns in the same direction as the front axle. It causes the longitudinal axis of the vehicle to rotate out of the direction of the curve and counteracts the tendency of the vehicle to turn the longitudinal axis of the vehicle into the direction of the curve. The steering angle of the rear axle can, for. B. stand in a fixed ratio to the front axle angle, which depends on the driving speed. Small slip angles of the rear axle and thus small sideslip angles with high lateral acceleration can be achieved conventionally by shifting the rolling moment to the front axle or by using wide tires on the rear axle. Sports vehicles make extensive use of this measure. The understeering behavior is then achieved through mixed tires .

literature

Bosch: Automotive paperback . 28th edition, May 2014. ISBN 978-3-658-03800-7 .
Bert Breuer, Karl Heinz H. Bill: Bremsenhandbuch , publishing Fried.Vieweg and son, Wiesbaden, 3rd edition 2006 ISBN 978-3834800640 .
Rolf Gscheidle: Expertise in automotive technology. 30th edition, Verlag Europa-Lehrmittel, Haan-Gruiten 2013, ISBN 978-3-8085-2240-0 .
Manfred Mitschke, Henning Wallentowitz: Dynamics of Motor Vehicles , 5th Edition, Springer Vieweg 1971 ... 2014, ISBN 978-3-658-05067-2

Individual evidence

^ Bosch, p. 765.
↑ Rolf Gscheidle, p. 479.
↑ Bert Breuer, Karlheinz H. Bill, p. 344.

[1] Bosch, p. 765.

[2] Rolf Gscheidle, p. 479.

[3] Bert Breuer, Karlheinz H. Bill, p. 344.