1. Why most of a cyclist's energy goes into pushing air aside
When riding a bicycle, most of the pedaling force is used to push air aside. A study by Kyle in 1979 states that 80–90% of a bicycle’s energy consumption comes from air resistance. This is much higher than the 4–8% for a person running. [1]
This 80–90% figure is a rough guide for when riding fast. When riding slowly or going uphill, the percentage is lower. Because air resistance makes up such a large share, letting the person in front take the wind has a big effect. [1]
2. How drafting behind another rider saves energy
Following closely behind the person in front is called drafting. The person behind is partially protected from the wind, so they use less energy. Kyle reported that at competitive speeds, the power output decreases by more than 30%. [1]
This has also been measured in the body. McCole and others reported that when riding outdoors at 40 km/h (about 25 mph) and staying at the back of a line of 8 people, oxygen consumption dropped by 39 ± 6%. The drop was larger than when following lines of 1, 2, or 4 people. [4]
However, studies of small groups in single-file lines suggest that the air resistance for the rider behind drops to 70–50% of what it is when riding alone. The next study notes that these numbers have been applied to large groups as well. [2]
3. Why the middle of a large pack feels less wind
In a large group, there are people not just in front, but also on the sides. A research team led by Blocken used computer calculations to model a group of 121 people in two different arrangements. They used about 3 billion grid cells for the simulation. They also checked the calculations with four wind tunnel tests using 121 models. [2]
The results showed that everyone in the group experiences less air resistance than when riding alone. In the middle-rear area, resistance dropped to 5–10% of the single-rider level. [2]
Air resistance is proportional to the square of the speed. So, if resistance is 5–10%, the wind speed is about 0.22–0.32 times the actual speed (about 1/4.5 to 1/3.2). For example, in a group riding at 54 km/h (about 34 mph), the wind feels like riding at 12–17 km/h (about 7–11 mph). [2][5]
It is important to note that 5–10% is the minimum value for the "middle-rear" position. Not everyone in the group experiences this; the wind felt varies significantly depending on the position. [2]
4. How team riders take turns leading to share the wind
The rider at the front experiences the greatest air resistance. Therefore, in team time trials, riders take turns leading. The rider who led moves to the side, returns to the back, and recovers while riding behind others. [3]
A study using wind tunnel tests and computer simulations of single-file lines in team time trials (up to 9 people) found that when 7–9 people ride very close together, the average resistance per person becomes about half of what it is when riding alone. [3]
It is said that riders behind give a small benefit to the rider in front. If the gap is within 1 m (about 3 feet), the resistance for the leader may decrease by about 2–3%. However, this number comes from a Japanese explanation and has not been verified in the paper. It is not a difference significant enough to make the leader’s job much easier. [6]
5. How to read the numbers from these calculations
These calculations are for groups in specific arrangements. In actual races, wind direction and the disorder of the lines differ from the calculations. The figure of 5–10% does not necessarily apply directly. [2]
What can be said for sure is that "the person in front takes the wind, and the wind is weaker behind." [2]
6. How to feel the effect of wind yourself in a safe place
In a safe place like a wide park or schoolyard with no people or cars, try holding your palm facing forward. Compare the strength of the wind when walking versus jogging. As speed increases, the wind felt on your hand should become stronger. This is practice for feeling how air resistance increases with the square of speed. [2]
Do not try riding a bicycle close to the person in front, as there is a risk of collision. [2]
If you want to research this, the abstract of the paper by Blocken and others is a good starting point. It is written in English. [2]