An Experimental Study on the Tractive Performance of a Single Tire according to Normal Load in Sandy Soil

Purpose The purpose of this study was to experimentally investigate the effects of normal load variation on the tractive performance of a single tire under sandy soil conditions using an indoor soil-bin test system. Methods Straight-line traveling tests were conducted at a constant theoretical traveling speed of 0.5 m/s while varying the applied normal load at five levels (0, 10, 20, 30, and 40 kg). Dry sandy soil with a particle size of 0.18–0.20 mm and negligible water content was used to ensure experimental repeatability. For each load condition, the tire traveled 3 m, and each test was repeated five times. Soil thrust was estimated from the driving motor torque, slip ratio was calculated from the difference between theoretical and actual traveling distances, and slip efficiency was derived from the slip ratio. In addition, a gross traction coefficient was introduced as a normalized index by dividing soil thrust by the vertical dynamic load. Results The average soil thrust increased from 27.07 N at 0 kg to 47.19 N at 40 kg, corresponding to a 1.74-fold increase, indicating that normal load strongly influences soil thrust generation under sandy soil conditions. In contrast, slip ratio and slip efficiency exhibited non-monotonic behavior, with the minimum slip ratio (46.90%) and maximum slip efficiency (53.10%) occurring at the 20 kg condition. The gross traction coefficient ranged from approximately 0.04 to 0.05 and also showed non-monotonic variation with increasing normal load. Conclusions The results demonstrate that increasing normal load enhances the absolute magnitude of soil thrust but does not necessarily improve tractive performance due to increased slip under high-load conditions. An optimal normal load condition exists in sandy soil that cannot be identified based on soil thrust magnitude alone. The soil-bin-based experimental results provide reliable benchmark data for understanding soil-tire interaction and for validating numerical soil-tire interaction models.