Let's take a fast rider who can ride an hour record of 40km/h. If we give him an aero advantage (wheel, frame, helmet) that gives him a 2km/h speed advantage. When we give the same advantage to a slower rider, he gains 1.392km/h. So the slow rider gains less speed from the aero improvement. This is as expected, based on the drag equation where drag force goes up as speed goes up.
However, usually we race a fixed distance, not a fixed time. So let's look at the SAME graph, but let's compute how long it takes to ride a 20k TT distance.
Simple math; to get the finishing time, simply divide the distance by the speed. For example, if our fast rider goes 40km in one hour, it takes 30min to ride 20km.
Likewise, if you ride 42km/h, it takes 20km / 42 km/h times 60min equals 28min 34sec.
Do the same for the two speeds of the slower rider and the result?
- Baseline equipment: 20km / 30 km/h = 2/3rds hr, or 40 minutes.
- Aero advantage: 20km / 31.392 km/h times 60min = 38min 13sec.
The faster rider saves 1:26, while the slower rider saves 1:47. The slower rider saves more time!
If you agreed with the graph before, you agree with this. We didn't change anything! The reason is that the rider saves less per second, but is on the course for more seconds so saves more time overall.
We're talking about flat ground here. For a discussion about climbing and aerodynamics, see the article Weight vs. Aero.