Citation

Veith AG. Transp. Res. Rec. 1977; 621: 113-125.

Copyright

(Copyright © 1977, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

unavailable

PMID

unavailable

Abstract

The traction forces generated by a rolling pneumatic tire are influenced by a number of factors. Some important ones are: tire construction, tread pattern, tread material, vehicle speed, pavement texture and the presence of interface lubricants such as water, mud, etc. This paper is concerned with the influence of tread pattern on tire wet traction as measured primarily by the locked-wheel traction coefficient, micron sub s. A first-order assessment of tire wet traction performance is obtained from phi, the "fractional groove volume". This is defined as the total footprint tread volume that is occupied by the tread groove void. The volume fraction is obtained from the footprint area by use of a reference groove depth of 10 mm (0.394 inch). In the 40-60 MPH range the coefficient micron sub s, exponentially approaches a limiting or maximum mivron sub s value as phi is increased for straight rib (groove) tires. This maximum value is a function of external factors, mainly pavement texture and water depth. At lower speeds the exponential approach to a limiting micron sub s is not found. Tires with "zig-zag" groove patterns (groove pitch and throw), show slightly inferior performance to straight rib patterns at the same phi values. This is attributed to an increased resistance to front-to-rear water flow through the grooves in the contact patch. This allows a larger fraction of the footprint area to be borne by a thin water film and thus promotes hydrodynamic lubrication. The influence of phi on 60 MPH wet cornering traction is similar to the skid behavior; an exponental approach of the cornering coefficient to a limiting value, as phi is increased. The influence of pavement macro-texture, characterized by its void volume, is very similar to the void volume of the tire tread pattern. Pavement texture numbers which are proportional to the void, were obtained from outflow measurements with a drainage meter. The 45 MPN locked-wheel skid coefficient of a standard tire increases in an exponential manner as pavement texture is increased and approaches a maximum value analogous to the behavior with varying tread groove void and a standard pavement. The relationship of these experimental findings to tire wet traction performance in terms of vehicle operational severity is discussed.