recursive

It's not perpetual motion because it requires a yaw angle > x, for some positive x that is probably a function of bike and wind velocity. That counts as energy input. Theoretically.

CastIron

My understanding, far from educated on the matter it may be, is that the wheel produces a net loss of drag when attached to a bicycle as compared to some other wheel. It doesn't introduce energy per se, rather creates an aerodynamic effect where the whole is greater than the sum of the parts. It isn't a magic flying saucer. Would be a cool graphics package, though.

If I'm wrong, someone please explain it to me in English.

ElJamoquio

No, it's actually when compared to a theoretical no-wheel-at-all. In other words, it would be propelling the bike, like a sail.

I seriously doubt that it actually produces negative drag often, if at all. In numerous other threads I've detailed my arguments against Zipp's assumptions.

That said, I'm certain that if you tested Zipp's wheel under the same conditions that they tested it, it would produce a very slight forward thrust.

CastIron

I was shooting for some kind of fairing effect. Another fine thought process that clearly should've been devoted to booze.

Triguy

This is the same thing that a lenticular disc like a Mavic, Hed or Campy disc does. As stated earlier, it isn't anything new for a disc to do. But as always Zipp finds something and runs with it.

Lenticular negative drag is often dependant on which way the yaw is going.

Voodoo76

I think this is where the problem lies, Lift is not "negative drag". If at some yaw angle Lift > Drag that is not negative drag.

ravenmore

this thread is getting boring. Put something sexy in it. Dare ya.

recursive

Sirrobinofcoxly

Yay!

Waldo

Negative drag occurs on the range of 11-18 degrees with the minimum value of -80 grams occurring at 15 degrees. For those playing along at home, that was with our tire; with the standard Vittoria Corsa CX we use as a baseline, the drag approached zero but did not quite go negative.

That's true, it's not anything new to achieve negative drag when yawing from zero degrees out to thirty. However, the protocol we developed years ago (that has since been copied by the majority of the companies that do visit the tunnel) is to start at thirty degrees and decrease the yaw angle. There were a few reasons for this change. If you do run from 0-30 and then back to zero, you typically see significant hysteresis issues, where the 0-30 plots can look abnormally good as the wheel can be yawed slowly enough to delay airflow separation (this effect is most pronounced with lenticular discs). The difference we saw was that our protocol yields results within approximately 5%, while it is not uncommon to see ±20% with the older protocol, so the potential for error in deriving data from a single run is higher. Also, as real-world airflow is generally turbulent, and not the perfectly straightened flow we see in the tunnel, the change in protocols yielded not only more repeatable data, but more accurate real-world data as well, plus it removes any temptation for manufacturers or whoever to selectively 'choose' what data to publish. We have tried to take this one step further by publishing only data averages of a minimum of 3 runs, and having the wind tunnel staff handle all the data to eliminate the claims that we are manipulating or selectively publishing some data over others, but doing both of these things is very expensive and would not be possible for everybody's testing.

This discussion also neglects the increase in drag associated with lenticular discs when installed in a frame, but we'll save that for another day (if someone is really bored, I think I've posted a more in-depth explanation of that somewhere on here).

I was going to put a picture of the offending disc in here but it's going to be hard to top recursive's post.

garysol1 

^^^I need a cup of coffee before I read Waldo's post^^^

CCFISH81

Bernoulli is turning in his grave.

Voodoo76

Waldo, im a ChE so a bit out of field here. But I understand "drag" to be a force component parallel to relative wind direction. I understand the idea of Pressure Recovery, shape creating high pressure areas pushing an object, but anything ive seen on the subject has this only approaching the drag produced by high pressure areas at the front of an object. Based on the math and intutively Im having a hard time with a negative number for drag. Arent you simply describing Lift? Or are you saying the Drag Coeff < 0 ?

Fat Boy

FWIW, I understand everything that Waldo is saying. I've been there, done that. I do not believe that the disc produces thrust, regardless of the testing protocol.

For the sake of argument, let's say that the drag is very, very small. In anything close to the actual environment it's used it, it will have significant measurable drag. Of this, I have no doubt.

To further some of what Waldo is saying. Real world airflow is _always_ turbulent, not sometimes. If we're dealing with a laminar flow situation, then you're basically not moving. Turbulent flow is based on a Reynolds number, not flow straighteners. The airflow is often also unsteady and chaotic, flow straighteners in a tunnel will change that. These conditions are different from simply turbulent. Put on top of that the discs are only used for the rear which has the inconvenience of a rider and a bike in front of it not to mention spinning in a direction that works against you.

Let's deal with reality here. A wheel will not push you through the air.