Scrub radius
On steerable axles of vehicles, the horizontal distance between the wheel center plane and the point of intersection of the spreading axis (steering axis of rotation) through the roadway is referred to as the steering scrub radius or steering scrub radius . The scrub radius is influenced by the camber , the spread , the wheel carrier and the offset of the rim .
Meaning of the term
The scrub radius is 0 when the imaginary line of the spreading axis exactly meets the center of the wheel contact area. If the penetration point is further out, the scrub radius is negative. If the intersection is on the inside, the scrub radius is positive. In the case of brakes fixed to the wheel carrier, the scrub radius determines the strength and direction of the torque around the expansion axis during braking. This brake arrangement is standard on modern vehicles.
With internal brakes, the scrub radius loses its importance as a lever arm for the braking force. In this case, the disruptive force lever arm is decisive. This arrangement of the brake was z. B. realized with the Citroën DS , SM and GS . Here the steering axis was exactly vertical and led through the center of the wheel. The disturbance force lever arm as the decisive lever arm for braking and driving forces is thus zero.
The typical value range for steering wheel radii in cars is between −20 and +80 mm. Front- wheel drive vehicles tend to have negative steering wheel radii. So that as far as possible no drive influences are noticeable on the steering wheel, the disruptive force lever arm must be low. With the widespread MacPherson front axle , this can only be achieved by a larger spread , which results in a negative scrub radius. Small steering wheel radii are generally aimed for so that control interventions cannot be felt on the steering wheel during ABS braking.
Driving behavior
- When braking, vehicles generally pull to the side with the better grip with different frictional adhesion on the individual lane sides, so that the driver has to counter-steer .
- In the case of a vehicle with a negative scrub radius , when braking on roadways with different coefficients of friction, the braking force should turn the wheel towards the weakly braked side. This counteracts the destabilizing yaw moment due to the differences in braking force. The Fuldamobil introduced in 1951 and the first Audi 80 from 1972 were the first two German vehicles with a negative scrub radius. In order to avoid the adjective "negative", Audi referred to it as "track stabilizing" in its advertising. In order to be able to move the ball joint far outwards, the new Audi, like the Golf I presented in 1974 , had deeply shaped wheel arches and floating caliper brakes that require little space in the rim. The track-stabilizing effect of the negative scrub radius also made it possible to use a two-circuit brake with diagonal division, i.e. the brakes on one front wheel and the diagonally opposite rear wheel are combined in one brake circuit.
- A vehicle with a steering scrub radius of 0 does not transmit any torque caused by braking forces to the steering when the brake is fixed to the wheel carrier, but it requires more effort when steering the stationary vehicle.
Examples
All early automobiles had a positive scrub radius. The steering was therefore prone to wear and tear and sensitive to uneven ground. The steering axis was almost perpendicular to the ground and the spokes, and later also the brake drums, took up the space that the kingpin would have to occupy for a smaller or negative scrub radius. The scrub radius could be reduced by a wider spread and positive camber.
Probably the first vehicle with a negative scrub radius was the three-wheeled Fuldamobil developed by Norbert Stevenson in 1950. A patent was only issued to Fritz Ostwald in 1958, but was initially not pursued further. The Oldsmobile Toronado came up with this detail in 1965, and the new Audi 80 in 1972 . The Mercedes S-Class of the 116 series presented in the same year had, like the Citroën DS 18 years earlier, a double wishbone wheel suspension with a zero scrub radius at the front .
With the type E23, BMW introduced a front axle with MacPherson struts and a lower wishbone split into two bars. The two link rods are mounted next to each other on the suspension strut (wheel carrier). This results in a lower fulcrum located further out at the intersection of the extension of the longitudinal axes of the two links ( instantaneous pole ). The wheel carrier rotates around an axis that is determined by this intersection and the dome bearing. Because the lower pivot point is further out, the spread is larger and therefore the scrub radius smaller.
Others
In the development of series vehicles, all properties of the chassis are coordinated, including the scrub radius. Subsequent changes to the chassis can change the driving behavior of a vehicle. Retrofitting a vehicle with wider tires and the associated wheels with a smaller offset increases the scrub radius and it can be positive instead of negative. The assembly of spacers , such as spacers under the standard wheels can cause. However, since not only does the scrub radius change when the track is widened in this way, but also the impact radius and the wheel load lever arm, this has an effect not only when braking, but also when driving over bumps, for example.
Individual evidence
- ^ Konrad Reif, Karl-Heinz Dietschke: Kraftfahrtechnisches Taschenbuch . 27th edition. Vieweg & Sohn, 2011, ISBN 978-3-8348-1440-1 , p. 318 . : ( limited preview in Google Book search)
- ^ Metin Ersoy, Stefan Gies (ed.): Chassis manual: Basics - Driving dynamics - Driving behavior - Components ... 5th edition. Springer Vieweg, 2017, ISBN 978-3-658-15468-4 , pp. 37-41 . : ( limited preview in Google Book search)