An Objective Analysis of the Effect of Tire Tread Depth on Crash Causation and Wet Road Vehicle Dynamics

This paper explores tire placement with given tread depths on vehicles from two distinct perspectives. The first area explored is an analysis of crash data recently reported by the National Highway Traffic Safety Administration (NHTSA). In this report, thousands of tire-related crashes were investigated where the tread depth and inflation pressure were logged for each tire and assessments were made as to whether tire condition was a factor in the crash. The analysis of the data shows that in regards to accident causation, it is not statistically significant which axle has the deepest tread. What is significant is that a tread depth at or below 4/32″ anywhere on the vehicle leads to an increased rate of crashes. To understand the physics implied by the NHTSA data, a study was performed on how the placement of tires of various tread depths affects the steering, handling, and braking performance of a modern sport utility vehicle. The test vehicle was instrumented with on board video equipment and a computer with transducers to measure driver inputs as well as vehicle responses during the testing. The vehicle was tested on a uniform wet test surface at a test track specifically designed for this purpose. Specific repeatable tests were performed to study the wet surface steady-state and transient handling performance as well as the straight line stopping distance during limit braking. These tests included an SAE J266 one hundred foot circle test, a closed loop single lane change, a slowly increasing steer test, and a limit ABS brake stop. These tests were each performed three times on the vehicle with tires of various makes and manufacturers and different levels of real world customer wear. A pair of “shaved” tires was also evaluated. The results show that placement of tires has a definite effect on the vehicle dynamic performance of the utility vehicle tested and that deeper tread depth on the rear suspension is not always the best configuration for overall vehicle performance.