Sports scientist Rob Kitching explains how to estimate your drag and make yourself more aerodynamic on the bike

How to calculate your dragIn cycling, aerodynamic drag – or air resistance – is a critical factor that affects the speed you can achieve for a given level of fitness. Reduce the drag created by your bike and body, and you’ll go faster with no extra effort.

Aerodynamic drag is a product of an object’s drag coefficient (Cd) or “slipperiness” and its size – critically, its frontal area (A). Hence the scientific measurement of aerodynamic drag is Cd x A, which is written as CdA and known as “drag area”.

The gold standard method of measuring drag is a wind tunnel session, but these can be very expensive (£1,000 for a couple of hours). However, you can estimate your drag without spending any cash at all.

To do this, you need to calculate your frontal area using a digital photo and some photo editing software such as Adobe Photoshop or the freeware Paint.NET.


HOW TO ESTIMATE YOUR DRAG

1. Find a photo

Get hold of a head-on photo of yourself riding your race-day bike on your tri-bars.

2. Create a cutout

Remove the background to create a ‘cutout’, as in the screengrab below. This is a fairly easy job – depending, of course, on your familiarity with the software. If you don’t know how to create a cutout, you should be able to find a step-by-step guide in the program’s Help files.

Screen shot 1

3. Find your frontal area in pixels

Select the rider and bike, and the software should give you the area in pixels squared (px²), normally at the bottom of the window. In newer versions of Photoshop, you’ll find it in the Histogram palette instead (make sure you select the layer, not the whole image, and hit the refresh button). In this case, the figure is 738,096px².

Screen shot

4. Measure your front wheel in the photo

Now you need to turn that photographic frontal area in pixels squared into a real-world frontal area in metres squared. One simple method is to use the height of your front wheel and tyre. First, select the front wheel in the photo and note down its height, which should either be displayed at the bottom of the screen or in the Info palette. In this case, it’s 876 pixels.

Screen shots

5. Translate pixels into metres

Now you need to relate that height in pixels to the height of your actual front wheel. If your bike is to hand, simply measure the height of your front wheel and tyre. If you don’t have your bike with you, a standard 700c wheel has a tyre bead diameter of 622mm, giving a total diameter of 668mm (0.668m) when fitted with a 23mm tyre (23mm + 622mm + 23mm). Use the following sum to calculate how many pixels there are per square metre:

(Height of wheel in photo, in pixels / Height of wheel in real life, in metres)²

eg (876 / 0.668)² = 1,719,710px per m²

6. Calculate your real-life frontal area

Once you’ve worked out how pixels in the photo relate to metres in real life, it’s time to calculate your frontal area in metres squared, using the following sum:

Area of bike and rider in photo, in pixels squared / Pixels per square metre

eg 738,096 / 1,719,710 = 0.429m²

7. Estimate your CdA

The final step is to calculate your approximate drag area. For this, you’ll need to use one of these rough drag coefficients as a starting point:

Riding with hands on handlebar tops: 1.15
Riding with hands on brake hoods: 1.00
Riding with hands on drops: 0.88
Riding with hands on tri-bars: 0.70

Here’s the sum:

Frontal area x Rough drag coefficient

eg 0.429 x 0.70 = 0.300m²

Congratulations – you’ve calculated your CdA!


WHAT NOW?

After calculating your drag area, you can make changes to your riding position, clothing, helmet, etc and see what difference it makes by going through the whole process again. It may seem like a hassle, but it could make a significant difference to your bike splits this season. It’s a small investment of time, compared to the time you spend training.