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wheels and negative drag numbers?(3 posts)

wheels and negative drag numbers?DougSloan
Jul 2, 2002 3:03 PM
Look at wheel aero drag data, at certain yaw angles disk wheels are reported to have negative drag numbers. What does that mean? Are you getting a "sailing" effect, meaning the wind/wheel is really pushing the bike? If so, this is a good thing, right?

I realize that this effect is very small (-.005 lb), and likely offset by greater drag on other parts and the rider in the crosswind; no one is going to be sailing down the road without pedaling in a 45 degree headwind. Nonetheless, negative drag is always better than positive drag, isn't it?

Any help understanding this, engineers?

Thanks.

Doug
You've pretty much got it.Kerry Irons
Jul 2, 2002 5:19 PM
The thing to remember though is that the price for this very small sail effect is that you have to fight that sideways force to maintain control of the bike. The net effect is that despite the negative drag of discs in some cross wind conditions, most people avoid discs in cross winds.
re: wheels and negative drag numbers?yfoiler
Jul 3, 2002 8:35 AM
Doug,

We have to keep in mind there are "two" winds happening.
The true wind which you experience when you are standing still. And the apparent wind, which is a vector of the true wind (speed and angle) and your bike speed. The faster you go the more the apparent wind moves to the front of your bike. Until you (in theory) go fast enough that you have created a dead on headwind. Regarding the lift effect of the disk wheels. Certain shapes do experience negative drag numbers by creating "lift" in a vectored direction relative to that shapes attack angle into this apparent wind. So put simply, the disk wheel "may" create lift, but it's in a direction that we don't want to go. Still, if the lift's direction is forward of 90 degrees to the direction the bike is traveling then this "lift" is a very small benefit. ...very small. The same is true of certain "aero" frames. I have a FAT Trek Y-foil bike that some say is "no way" aerodynamic, that is until it's put in a wind tunnel and measure the drag at broad attack angles. Then it shines compared to round tubing.

Marty