Scrub radius

As a scrub radius or scrub radius is referred to steerable axles of vehicles the distance of an imaginary extended line of the steering axis to the center of the wheel tread.

The scrub radius is determined by the fall, the spread and the rim offset.

Term Meaning

Exceeds the imaginary line of the steering axis exactly the center of the tire contact area, so it is called a steering roll radius 0 Does it further out on, it is called a negative scrub radius, it shall take further inward on, is a positive scrub radius of the question.

Driving behavior

• A vehicle with positive steering roll radius has a good straight, but is susceptible to ruts and tends when braking with different frictional grip on the individual track pages to the side with the better adhesion to pull on a level road, so the driver has to steer.

• A vehicle with a negative scrub radius has a self-stabilizing directional stability when braking on surfaces with different frictional grip the wheel is steered by the braking force to the weaker braked page. Result, the vehicle is stabilized without driver intervention and prevents skidding. Further obstacles on the road do not lead to a strong " pass-through " in the steering. The Audi 80 (from 1972) was one of the first German vehicles have a negative scrub radius. In order to " negative" to avoid the adjective was designated in advertising this as a " track-stabilizing ". To make this possible, had the car like the later (1974 ) presented the VW Golf I specially shaped rims and floating frame brakes, which require little space in the rim. The track- stabilizing effect of negative scrub radius also allowed only the use of a diagonal two-circuit brake.

• A vehicle with a scrub radius 0 transmits roadway or vehicle- related forces ( flat tire ) not to the steering, but requires a higher effort when steering angle of the stationary vehicle. The scrub radius 0 is practically difficult to achieve, since any change in the tire diameter by change in air pressure, tire wear, load or deceleration ( = change in load on the front axle ) because of the spread increases or decreases the scrub radius. This applies not only to when the steering axis is exactly perpendicular and located in the center of the wheel, as in the case of the Citroën DS. As with the Alfa Romeo Alfasud, VW K 70, Audi 100 (up to 1974), Audi F103, NSU Ro 80, Citroën SM and GS models were, inter alia, in the DS disc brakes not ( as now almost ubiquitous ) in the wheels, but located on both sides of the differential (that is, the gear unit). Thus, the unsprung mass was reduced to one and the other by the space gain in the wheels possible (for DS, GS and SM ) of the steering roll radius zero. In these vehicles, transfer the drive shafts, the braking force and are more stressed.

History

Earlier cars all had a positive scrub radius, since the steering axis nearly perpendicular to the ground and stood for reasons of space could not be accommodated within the rim. The steering was therefore very susceptible to wear.

Probably the first vehicle with negative scrub radius was the three-wheeled Fuldamobil 1950 developed by Norbert Stevenson. A patent was issued in 1958 to Fritz Ostwald, but then did not pursue. 1965 Oldsmobile Toronado of waiting on with this detail, in 1972 the new Audi 80 The work presented in the same year Mercedes S-Class of the 116 series had a double-wishbone front suspension with zero scrub radius.

Others

In the development of production vehicles all chassis components are designed to work together to achieve the desired steering roll radius. By subsequent changes to vehicles in this area, the driving behavior of a vehicle can change fundamentally. Even retrofitting a vehicle with wider tires and the appropriate wheel rims can be sufficient to convert the negative scrub radius into a positive.

• Vehicle steering