Many cyclists ask themselves what the importance is of aerodynamics during a climb (assuming they have time to think about it when struggling to come to the top…). We all have the tendency to ride close behind our competitor (or mate) during a climb. Is this only a mental gain to do so, or plays aerodynamics a role as well?
The answer is not straightforward as this depends on many aspects as grade of the climb, power delivered, posture of the cyclists during the climb amongst, other factors.
But first of all: when do we call something a climb and what is the category of a certain climb? For the Grand Tours (Tour de France, Vuelta, Giro) there is surprisingly enough no clear definition. Searching the internet provides many discussions on this (e.g. https://velovation.co.uk/climb-categorisation/ ). Many trivial and non-trivial aspects come into this classification. Obviously, steepness and length of the climb are the base, but other aspects as moment during a stage, timing within the tour, and even sponsoring arrangements, come into the picture.
The base however on grading is the length and the percentage of a climb. Below the basic principle, which is used by the Grand Tours as base and by Strava as well.
Classification of climbs from 4th category to HC (Hors Catégorie) as used by the Grand Tours and Strava.
A cyclist has to overcome four resistances: drivetrain, rolling, gravitational and air. The first two are under normal conditions very small. Gravity and aerodynamic resistance take 80% to 90% of the power delivered. But under which climb is aerodynamics still important to consider and can drafting help you to reach the top?
The figure below shows for a pro-elite rider (65 kg; climb position [CdA=0.3]; FTP of 400 Watt) how the power is used under climbs ranging from 0% up to 10%.
- 50% aero resistance: slope of 2%
- 20% aero resistance: slope of 5%
- 10% aero resistance: slope of 7.5%
So for pro-elite riders at slopes somewhere between 5% and 7.5% drafting, so cycling close behind their competitors, and a more aero posture on the bike, provides clear gains. In pro-elite races one sees indeed often that during climbs above about 7% breakaways happening: the preceding rider loses the benefits of drafting.
Distribution of power usage of a pro-elite rider delivering 400 Watt under climbs ranging from 0 up to 10%.
What about the regular cyclists who can deliver 200 Watts? The figure below indicates that compared to a pro-elite rider delivering 400 Watts, gravity resistance plays already at less steep climbs a dominant role.
- 50% aero resistance: slope of 1%
- 20% aero resistance: slope of 3%
- 10% aero resistance: slope of 4.5%
Assuming that as long as aero takes more than 20% of total power delivered, one should consider aero as a serious factor. In this example for slopes up to about 3% aerodynamic riding provides clear gains. In other words, a more aero posture and/or drafting behind your mates should be practiced.
Distribution of power usage of a regular cyclist delivering 200 Watt under climbs ranging from 0 up to 10%.
