Bikedude, I don’t even want to get into the whole discussion on the accuracy of wind-tunnels, moving air over an object instead of objects through air, etc. That is a lengthy, lengthy debate. Wind tunnels are not 100% accurate, but they are still very useful. Let’s just leave it at that.
- Ernie covered the exact definition of a stall, so thanks for that. Here is a neat pic that shows the phenomenon he is talking about well. When the slope is horizontal, that is critical angle of attack, which varies for each airfoil.
http://www.centennialofflight.gov/essay/Dictionary/angle_of_attack/DI5G1.gif
- Angle of attack is a valid term. Because we have no lifting surfaces oriented horizontally, yaw angle and angle of attack ARE the same in our case, as I said in my first post, for aero tubes on a bicycle. It is important to consider both. Because you definitely can get lift. How can you say that you can’t get lift at bicycle speeds? 50+ mph on a downhill is plenty fast to get lots of lift. And even 20+ mph is enough to generate lift. Just think of a glider. The amount of lift is dependent on airfoil shape, but you can definitely get lift at low speeds. In a crosswind, the greater surface area of the aero tube that is exposed to the wind will generate force pushing left-to-right or vice-versa. This is lift, since the airfoil is oriented in the vertical plane. This is the reason you can discuss angle of attack, even though it would be out of place when talking about an airplane or car with primary forces in the Z direction (Up/Down). Since we are essentially operating in a 2-D world, we could rotate the bicycle 90deg and talk about angle of attack ionly and totally neglect yaw. End result is the same. I am not saying that a lot of lift is going to be generated, certainly not enough to make an aero frame “unstable” or anything like that, but you might feel it, and it will definitely be there. Yes, the purpose of using airfoil tubes is to minimize the drag, but that doesn’t mean there is no lift. In every case except Odeg yaw there will be lift. And the smooth departure is in large part dependent on how the wind strikes the bike. That is why Gerard and Cervelo are trying to delay stall as long as possible as yaw angle increases. Stall is a “unclean” departure, to put it simply, and it is dependent on angle of attack, among other things.
Gerard, thanks for the details.
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IHS - finally, a real reason. Thanks. I won’t ask for the FEA proof, it makes sense.
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Again, thank you. Makes sense.
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It looks like we are in agreement. I am not saying Aluminum frames didn’t preform better. I was just saying Aluminum is not inherently better than steel or Ti, which is what I interpreted from your post. I think it was just a result of your post being short, rather than anything else. If the best designers are using aluminum, those bikes will probably perform the best. If the most money was being poured into steel bikes, they would probably be best. ALuminum has enough other attractive features that it has been optimized to do a lot of things well, whereas the money may not have been spent to do the same for steel. Either way, I think we can put that to rest. Glad you were able to give a fuller response.
4… I really didn’t mean to open up a can of worms with the fatigue thing. It really seems to be a non-factor for bicycle frames, which is all I was curious about, and I ended up having to get into a big materials science debate. I never meant to imply that Aluminum would be no good because of a decreasing fatigue limit. So hopefully we can put that to rest since you cleared up that an Aluminum frame is not going to fail any time in the distant future from cyclical loading. But that got out of hand. And I agree, I am sure you would notice and your chainstay wouldn’t just bust one day. But that whole discussion got out of hand when it should have been simple. Probably my fault…
Let’s keep this ball rolling… Sorry I couldn’t post the pic instead of putting a link, but I use FireFox web browser and it doesn’t support the “advanced editor” for the BB.