Ground pressure

Ground pressure is the pressure exerted on the ground by the tires or tracks of a motorized vehicle, and is one measure of its potential mobility, especially over soft ground. It also applies to the feet of a walking person or machine. Pressure is measured in the SI unit of pascals (Pa). Average ground pressure can be calculated using the standard formula for average pressure: P = F/A. In an idealised case, i.e. a static, uniform net force normal to level ground, this is simply the object's weight divided by contact area. The ground pressure of motorized vehicles is often compared with the ground pressure of a human foot, which can be 60 – 80 kPa while walking or as much as 13 MPa for a person in spike heels.

Increasing the size of the contact area on the ground (the footprint) in relation to the weight decreases the unit ground pressure. Ground pressure of 14 kPa (2 psi) or less is recommended for fragile ecosystems like marshes. Decreasing the ground pressure increases the flotation, allowing easier passage of the body over soft terrain. This is exemplified by use of equipment such as snowshoes.

Examples

All examples are approximate, and will vary based on conditions

+ObjectGround pressure (kPa)(psi)Hovercraft0.70.1Human on snowshoes3.50.5Rubber-tracked ATV5.1650.75Wheeled ATV13.82Diedrich D-50 – T2 drilling rig26.23.8Human male (130 kg, standing on one foot)558Average human, flat shoes172.5M1 Abrams tank103151993 Toyota 4Runner / Hilux Surf17025Adult horse (550 kg, 1250 lb)17025Bagger 288 excavation machine17025Passenger car20530Adult elephant24035Mountain bicycle24540Road racing bicycle62090Commercial semi truck/Lorry793115Stiletto heel3,250471 Note: The pressures for average human and horse are for standing still position. A walking human will exert more than double his standing pressure. A galloping horse will exert up to 3.5 MPa (500 psi). The ground pressure for a pneumatic tire is roughly equal to its inflation pressure.

See also Contact patch Tire load sensitivity

Related reading Theory of Ground Vehicles

References