Yaw know what I mean?

The new cyclist checks out a new pair of aero wheels. Let’s say it’s the Dura-ace R9170 C60 hoops for disc brakes. They feature the D2 rim shape, which Shimano says reduces air drag better than other aero-shape rims at yaw angles from zero to 15 degrees. “Zero to 15 degrees?” the rider might say. “That doesn’t sound like very much. What if I was heading north and a wind was coming right out of the northeast? That air would hit me 45 degrees. Those wheels don’t seem able to handle that.”

If you know your physics, you’ll catch the newbie’s mistake: equating yaw with wind direction. “Yaw is the vector sum of your forward speed and the wind angle,” says Andrew Buckrell, ceo of Stac Performance. He knows about wind and its effects on a rider and gear. His company has developed a method of scanning a rider and his riding positions. This information can then be put in Stac’s virtual wind tunnel to see how the rider can get more aerodynamic.

“A 45-degree yaw is generally unrealistic,” Buckrell says. “In fact, the faster you are, the less yaw you experience. Take an extreme example: if you were travelling in an aircraft at 800 km/h with a 10 km/h sidewind, the yaw angle will be roughly zero degrees. For a cyclist, it would take a huge crosswind to get a yaw above 20 degrees.” Since speed affects the yaw, then a slower rider, Buckrell says, should look for wheels that work at a broad yaw range. If you are a faster rider, then wheels that perform well in a narrow range of yaw would suit you better.