What is meant by interactions?
When we speak of interactions in aerodynamics, we mean the effects that surrounding objects have on each other. In aerodynamics, a cyclist and his bicycle must always be looked at as one unit. It is possible to test individual objects for their wind resistance, but the value will change when the object is used as part of the whole system.
For example, a helmet can aerodynamically optimize or support an athlete’s head position. However, at the same time it is also the case that, due to these interactions, no general statements like “This helmet is aerodynamically always beneficial.” can be made.
The wind resistance value must always be determined individually for a specific athlete and his specific setup.