Among the major sources of drag on a moving vehicle are upper wheel drag forces and frictional drag. The sensitivity of these forces increases with the speed of the vehicle. In order to minimize the overall drag on the vehicle, the upper wheel surface must be shielded from the external headwind. Depending on the configuration of the wheel, the uppermost point of the wheel assembly is typically exposed to the highest relative wind speeds. It is important to understand that the upper wheel drag forces can be significant. They are leveraged against the axle, which is a disadvantage to the propulsive counterforce at the axle.
The forward vertical profile of the wheel is usually symmetric in shape. At the same time, the vehicle propulsive force pivots around the same point of ground contact. The force applied at the axle magnifies the impact of lower wheel drag forces. The result is a significant increase in the force required to overcome the drag forces at the axle. The force required to overcome these forces is a function of the cube of the effective wind speed.
The total drag force on a vehicle with exposed wheels is a function of the total velocity of the vehicle, the relative wind speeds, and the effective wind speed at the axle. The sensitivity of upper wheel drag forces and frictional drag to vehicle motion retardation is greater than that of the lower wheel surfaces. When the lower wheel surfaces are shielded, the total drag torque on the vehicle is reduced.
The effect of the relative wind speeds on the overall drag force on the vehicle can be quantified by calculating the average moment at different points on the wheel rim. This is usually done using a pedestal mount and dynamometer. The force exerted on the pedestal mount is then compared with the force exerted on the vehicle. The difference is then used to calculate the net drag force on the model. This is a more accurate method for calculating propulsive power than using direct force measurement techniques. However, direct force measurement techniques can be inaccurate, and there is a need for an alternative technique.
The sensitivity of upper wheel drag forces and form drag to the vehicle’s motion retardation is also an important consideration. The forward vertical profile of the wheel is generally symmetric in shape, but it may be shaped differently if the rim is streamlined. As a result, the forward surface of the wheel has a tendency to be positioned closer to the top of the wheel. This is an effective means for reducing the upper wheel drag forces. However, if the upper surface of the wheel is shielded from an external headwind, the total drag torque on the vehicle may be increased. The upper wheel surfaces may be more sensitive to external headwinds than the lower wheel surfaces. In this case, the upper wheel drag forces increase more rapidly than the lower wheel drag forces.