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Why care about Aerodynamic Drag & Rolling Resistance?

Riding a bike fast is all about generating power to overcome the resistive forces acting on the rider and bike, or reducting those resistive forces. These include:

Force % Resistance on the Flat % Resistance on Climbs
Gravity 0 % Upto 90 %
Aerodynamic Drag Upto 90 % Upto 20 %
Rolling Resistance Upto 30 % Upto 20 %

For most people getting faster is all about improving fitness. That means increasing power or reducing weight.

But what if you want to go as fast as you possibly can, with no more training time? That is where reducing the resistive forces of Aerodynamic Drag and Rolling Resistance helps - a lot!

Aerodynamic Drag (CdA)

The air in our atmosphere is made up of many molecules of gas which vary in density depending on weather conditions and altitude. The faster we ride the more we start to notice the resistive force caused by these molecules. The question is how can a rider reduce this resistive force?

Frontal Area is one factor. The size of a rider and machine, when viewed from the front, is a huge determinant of the amount of resistive force. Reducing that area helps.

Slipperiness is another factor. The shape of a rider and machine is another determinant. In the same way that a fish can move more effortlessly than a brick, some riding positions or bike configurations result in lower resistive forces.

The combination of Frontal Area and Slipperiness is known by aerodynamicists as CdA. It is a number which can be measured, and usually reduced, leading to faster bike riders. One of the purposes of Fast Aero Lab is help you carry out and then analyse simple field tests, using a power meter, which will determine rider CdA. Then you can make changes to reduce it and find “free” speed.

Typical Power Loss to Aerodynamic Drag

Rolling Resistance (Crr)

As we ride along the tyres of our bike are constantly flexing where they make contact with the road. This flexing consumes power and represents a resistive force which increases with rider and machine weight, and with speed. But can it be reduced?

To some extent reductions in rider or machine weight will reduce rolling resistance, but there are more achievable gains in intelligent choice of tyre and inflation levels. Some tyres are faster than others but they all have an inflation level sweet spot depending on the road surface and the rider weight.

Rolling resistance is measured with a coefficient called Crr. Again it can be measured, and reduced, leading to faster bike riders. Another purpose of Fast Aero Lab is to help you carry out and then analyse simple field tests, using a power meter, which will determine rider Crr. Then you can make changes to reduce it and find “free” speed.

Typical Power Loss to Rolling Resistance


How To Do A Field Test

The Analyse page on this site provides a simple way to run the calculations on a power meter file (.tcx or .fit) that will reveal a rider’s CdA, Crr, or both. But first you need to record a suitable file, representing the right kind of field test, and these instructions are here to guide you through the process.

Overview

Field tests used to establish CdA &/or Crr must comprise multiple laps. They can be laps of a velodrome, or laps of a road circuit. In the velodrome case you need to know the length of the velodrome, and in the road case you need to know the length of a lap or tag each lap with an interval. Every lap must start and finish in the same place - this is an important detail which lets the test protocol work.

Road laps should be at least 250 metres in length and run on a circuit where the rider will not have to touch the brakes (this invalidates the data) or where there is traffic (this also invalidates the data as it influences airflow).

CdA or Crr only field tests can then be used to calculate a Cda or Crr for every valid and completed lap, allowing you to experiment with different positons or configurations across laps. A CdA and Crr test uses a slightly different protocol which requires multiple laps completed at different speeds to calculate a CdA an a Crr together. We recommend starting with a CdA only test as it’s the easiest to complete and focusses on the resistive force which promises the biggest gain for most bike riders.

CdA Only Test

Evaluating CdA is done at high speeds, with laps above 32kph (20mph) where this resistive force is most evident. We can calculate a CdA for every lap completed above this speed.

Crr Only Test

Evaluating Crr is done at low speeds, with laps below 16kph (10mph) where this resistive force is most evident. We can calculate a Crr for every lap completed below this speed. To focus on tyres and inflation, not the effect of road surface, tests should be run on the same track or road circuits.

CdA and Crr Test

This protocol requires the completion of 10 flat (e.g. track) laps, at increasing speeds, with a speed increment off at least 1.6kph (1mph) per lap. Laps which don't fit the speed requirement will be automatically discounted but will not stop you running the calculation, so long as there are at least 10 valid laps. There should be no changes in position or configuration. We will calculate one CdA and one Crr for the series of 10 or more laps.

Parameters

The recorded file provides most of what is needed to run the calculation, with 2 exceptions.

You must know Rider & Machine Weight (combined). We suggest weighing both together immediately before the test.

You must also know Air Density. You can get this from a weather report (see the link on the calculation page) or it can be measured just before the test with a hand-held weather meter (available on ebay).

Checklist

Calculation

Simply use our analysis page to get your numbers. The steps to follow should be self explanatory.

Analyse

Integrating Aerodynamics & Biomechanics

Reducing aerodynamic drag is a big priority in getting faster - but only so far. There comes a point where extreme riding positions will have a negative impact on a rider's ability to deliver power to the pedals. This can be a temporary loss, until the rider adapts to the new position, or indefinate. A rider's goal should be to maximise the ratio of sustainble power (watts) to aerodynamic drag Watts/CdA and this is where things can get complicated.

A bike fitter with the right training can help with this issue of Integrative Aerodynamics & Biomechanics and you can use this site to set up an online consultation with a world class expert. All we need is photo or video files of you riding, on the road or a static trainer, plus a few key fit measurements.

Consult