Aerodynamics for Two-Wheelers

The resultant aerodynamic forces in the Two-wheeler implementation is considered to act on the pressure center point, usually located above the center of gravity of the motorcycles.

Figure 1.

The aerodynamic forces are applied to the vehicle body are as follows:

Drag force represented as Fa,x: Fa,x = 0.5*Cx(τ_L)* ρ * A* v_r²

Lateral force represented as Fa,y: Fa,y = 0.5*Cy(τ_L)* ρ * A* v_r²

Lift force represented as Fa,z: Fa,z = 0.5*Cz,(τ_L)* ρ * A* v_r²

Where C_i express the aerodynamic coefficients, τ_L the relative airflow incidence angle, v_r the resulting velocity, ρ air density, and A frontal area.

Resulting Velocity and Dynamic Pressure

A diagonal airflow results in a flow velocity v_r with an incidence angle τ_L to the vehicle’s longitudinal axis. Both parameters v_r and τ_L can be calculated by vector addition of the road speed vector v and the wind velocity vector v_w.

The dynamic pressure can be calculated using the air density ρ, the resulting velocity v_r, using the following formula.

P(v_r) = 0.5 * ρ * v_r²

The air density is dependent on the ambient temperature and pressure of the environment and the ideal gas constant for air and it is often regarded as constant.

Aerodynamic Coefficients and Area

Reference by Vittore Cossalter ², the product of drag coefficients and area Cx*A can vary from 0.18 to 0.7m². The frontal area A reference value vary from 0.6 to 0.9m² for large displacement touring motorcycles, 0.4 to 0.6m² for sporting models, and 0.4 to 0.5m² for grand prix motorcycle. The product of the lift coefficient and area Cz*A can vary from 0.06 to 0.12 m².