How to interpret a drag chart
When riding a bike, the wind you feel can be split into two main components:
- Wind resistance from your forward motion. As you ride forwards, you (and the bike) are moving through the air in front of you, creating drag. Rather than riding forwards, this is simulated in a wind tunnel.
- Impact of the wind that is blowing that day (i.e. the weather). Clearly this can act upon a rider from any direction, depending on the conditions.
The two components combine to give the effective wind that a rider will encounter. To model it in the wind tunnel, we need to understand both how strong the wind is, and from what direction it is felt – the yaw angle. Given that most riders will be travelling significantly faster than the wind is blowing, the wind resistance from your forward motion makes up the larger share of the effective wind. It also concentrates the yaw angles seen when riding into a small arc in front of the rider. As a result, wheels are tested between 0 and 20 degrees of yaw angle, reflecting the yaw angles seen in the real world.
GRAMS OF DRAG
In order to quantify wind resistance resulting from a wheel, we measure the drag force it exerts when exposed to wind. The wind tunnel measures the force on the wind axis (i.e. based on the direction the effective wind is travelling), which is then converted to the body axis (i.e. based on the direction the rider is travelling).The drag chart shows the drag force exerted against the direction of travel for the rider. This is the component of the drag force that slows you down whilst riding. The higher the drag force, the more energy is needed to overcome it and move you forward. When reading a drag chart, this means that the lower the line, the more aerodynamic the wheel is. You should bear in mind though, when comparing wheels, a wheel may show a lower drag at one yaw angle, but higher at another.