Sinkage is an empirically significant factor in vehicle performance as it can result in an immobile vehicle or environmental damage.  Bernstein first proposed a pressure sinkage relationship in 1913, subsequent work by Janosi and Hanamoto and Hedegus concluded that longitudinal wheel slip also plays a role in sinkage.  Shinone, Nakashima, Takatsu, Kasetani and Matsukawa identified a linear relationship between slip and sinkage on a lightly loaded tire.  In this study the effects of vertical wheel load, wheel slip and tire inflation pressure on wheel sinkage on soft sand were investigated, in order to relate sinkage to a vehicular operating condition.

The tests in this study were conducted using the Cranfield University Single Wheel Tester (SWT) on soft, desert-like sand in the Cranfield Off-Road Dynamics facility soil bin.  The SWT uses a closed loop servo-controlled hydraulic ram to actively control vertical wheel load and similar active wheel speed control via a hydraulic motor.  The SWT apparatus is mounted on an independent prime mover tractor unit which controls forward speed.  True forward speed is continuously measured against a fixed reference point and used to calculate the required wheel speed in real time to give the desired slip profile.

A series of controlled load tests were conducted using a Goodyear G90 tire (7.50 R16C) on a dry desert sand material. Four discrete inflation pressures (10, 20, 30 and 40 psi) and four vertical loads (1, 2, 3, 4 and 5 kN) were chosen to represent the operating range of the tire.  Each test run consisted of a slow (30s) ramp of slip ratio from 85% (driven) to -15% (braked).

Although a near linear response was identified for slips greater than 10% as Shinone et al. also found, the overall relationship between slip and sinkage was found to be non-linear.