Reference

Surface Classification and Bearing Capacity

Classification system for surfaces encountered on documented routes. Covers substrate identification, bearing capacity estimation by type and moisture condition, traction characteristics, and tire pressure adjustment by surface class and vehicle mass. All values derived from direct measurement on documented routes.

Surface classification
Primary surface types
Class Description Bearing capacity (dry) Bearing capacity (wet)
Intact asphalt Continuous surface, minor cracking, vegetation in joints only >20,000 kPa >20,000 kPa
Degraded asphalt Broken surface, potholes, vegetation intrusion, base exposed in patches >10,000 kPa >8,000 kPa
Gravel (compacted) Angular gravel, compacted, stable under load 500–1,500 kPa 300–800 kPa
Gravel (loose) Uncompacted gravel, shifts under load, may rut 200–500 kPa 100–300 kPa
Sand (dry) Loose, dry, non-cohesive 50–150 kPa n/a
Sand (wet/compacted) Moist, compacted, surface crust 200–400 kPa 150–300 kPa
Soil (firm) Dry or low-moisture soil, vegetated, root-bound 200–600 kPa 80–200 kPa
Soil (soft) Saturated or high-moisture, no surface crust 50–150 kPa 20–80 kPa
Clay (dry) Hard surface crust, cracked, stable 300–800 kPa n/a
Clay (wet) Saturated, plastic, no crust n/a 10–50 kPa
Rock (bedrock) Exposed bedrock surface >50,000 kPa >50,000 kPa
Scree Loose angular rock fragments on slope 100–300 kPa 80–250 kPa
Basalt rubble Loose angular basalt, sharp edges 150–400 kPa 120–350 kPa
Desert pavement Interlocking cobble, wind-stabilized 400–1,000 kPa 300–800 kPa
Bearing capacity values are estimates for typical conditions on documented routes. Actual capacity varies with depth of substrate, underlying layer, moisture gradient, and vegetation root density. When bearing capacity is uncertain, test by placing a known load on the surface and measuring displacement before committing the vehicle.
Traction characteristics
Traction coefficient by surface type
Surface Dry Wet Notes
Intact asphalt 0.7–0.8 0.5–0.7 Moss or algae growth reduces wet traction to 0.3
Degraded asphalt 0.6–0.7 0.4–0.6 Exposed aggregate increases traction in some areas
Gravel (compacted) 0.5–0.7 0.4–0.6 Stable
Gravel (loose) 0.3–0.5 0.3–0.4 Lateral stability reduced on slopes
Sand (dry) 0.2–0.3 n/a Momentum-dependent; do not stop
Sand (wet) n/a 0.4–0.5 Compacted wet sand provides adequate traction
Clay (wet) n/a 0.1–0.2 Effectively frictionless on slope. Do not attempt grades >5%
Scree 0.2–0.4 0.2–0.3 Substrate moves under load. Low-range gearing essential
Grass/soil 0.4–0.6 0.2–0.4 Root mat improves traction over bare soil
Mud n/a 0.1–0.3 Traction depends on depth. Surface crust may give false confidence
Tire pressure adjustment

Tire pressure affects the contact patch area, which in turn affects bearing pressure and traction. Lower pressure increases the contact patch, reducing bearing pressure on the surface (beneficial on soft surfaces) but increasing rolling resistance and sidewall exposure (detrimental on sharp surfaces). The following table provides starting values; adjust based on observed performance.

Recommended tire pressure by surface (kPa)
Surface <2,000 kg vehicle 2,000–3,500 kg >3,500 kg
Asphalt (intact/degraded) 220–250 250–280 280–350
Gravel (compacted) 180–220 200–250 250–300
Gravel (loose) 150–180 180–220 220–280
Sand (dry) 100–120 120–150 150–180
Soil (firm) 150–180 180–220 220–280
Rock / basalt 220–280 250–320 300–380
Scree 150–180 180–220 220–280
Mud / wet clay 180–220 220–250 250–300
Do not reduce tire pressure below 100 kPa. Below this threshold, bead separation risk increases, particularly during lateral loading on slopes or during sharp turns. Re-inflate before transitioning to harder surfaces to avoid rim damage and excessive sidewall wear.
Revision

Surface classification and bearing capacity values derived from direct measurement across all documented routes. Traction coefficients measured under controlled conditions. Tire pressure recommendations are starting values; actual adjustment depends on tire construction, tread pattern, and load distribution.