Drafting Power Savings Calculator

How it is calculated

Only the aerodynamic term is sheltered; rolling resistance is not. With v in m/s, aero is ½·ρ·CdA·v³ (ρ fixed at 1.225 kg/m³) and rolling is Crr·m·g·v. Solo power is (aero + rolling) ÷ 0.97; drafting power is (aero × k + rolling) ÷ 0.97, where k is the share of drag still felt. Second wheel uses k = 0.73 (27 % cut), back of the bunch 0.66, and mid-bunch 0.73 − 0.03 × (group − 2) with the group effect capped at 9 riders beyond the first two, so the deepest shelter is a 54 % cut. Group size must be a whole number from 2 to 200. Speed must be above 0 and up to 100 km/h, and rider plus bike weight up to 400 kg. CdA defaults to 0.32, Crr to 0.005, and the group to 6 when left blank. The speed gain is the faster speed you could hold in the draft for the same watts as solo.

Common mistakes

• Expecting the percentage of drag removed to equal the percentage of power saved. Rolling resistance gets no shelter, so a 45 % drag cut here is only a 38.86% power saving.
• Choosing mid-bunch and entering a huge group: the extra benefit stops growing after 9 riders beyond the first two, so a group of 20 gives the same 54 % cut as a group of 11.
• Typing a fractional or one-rider group. The group must be a whole number of at least 2, otherwise the tool shows no result.
• Typing speed in mph or m/s. The field is km/h and anything above 100 returns no result.
• Using the saving for a wide group or a crosswind. The model assumes riders in a straight line, with the wind from the front, so it cannot describe echelons or side wind.