Calculating time savings
Using the drag chart, we can compare the drag force from two different wheels. However, we then want to be able to understand what difference this will make in the real world, i.e. how much time will a rider actually save. To do this, we need to understand two things:
- How a unit of drag translates to a unit of time saved
- What the “average” yaw angle should be
Fortunately, both of these factors are becoming increasingly well-understood.
CONVERTING DRAG SAVINGS TO TIME SAVINGS
The drag equation states that drag force is determined by 4 things:
- The density of air
- Frontal area of the object moving through the air
- Drag coefficient
For the purposes of cycle testing, we can assume the first two remain broadly constant. We can therefore calculate the drag coefficient for a particular speed. Reducing the drag force by a known amount (e.g. 100g) at that particular speed will then give a lower drag coefficient which we can use to calculate a higher speed that would be achieved by saving that known amount of drag. By knowing the increase in speed, we can see how much time would be saved over a defined distance (e.g. 40km).
That may sound relatively straightforward, but it is complicated by the fact that the increase in speed will also have a slight impact on drag force. However, for the range of speeds we would consider achievable on a bike (i.e. not considering 100kph+!), this has a very slight effect. To put this another way, a faster rider may see a higher reduction in drag than a slower rider, but the slower rider will have their benefit for longer.
This is how we are able to reach the “rule of thumb” that a 100g reduction in drag will result in a 40 second time saving over 40km.
AVERAGE YAW ANGLE
A number of studies, by both frame and wheel manufacturers, have been conducted to evaluate the “average” yaw angle seen by riders. Recent research and testing has shown that cyclists and triathletes are exposed to lower yaw angles than previously believed. However, this will be impacted by weather conditions – if you ride a very windy course you will see a far wider range of yaw angles than on a completely still day.We have taken the findings from these studies and applied them to our wind tunnel results. Rather than taking a simple average for each wheel, we have given a higher weighting to the more commonly-occurring yaw angles to more accurately reflect the real world. As a result, the time savings for each wheel design reflect a weighted average yaw angle of 6-7 degrees.