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Do you know your recumbent's CdA?

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Do you know your recumbent's CdA?

Old 11-03-09, 12:08 PM
  #26  
GreenGrasshoppr
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The only thing I can measure is my maximum speed at terminal velocity, coasting down the biggest hills around town
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Old 11-03-09, 12:22 PM
  #27  
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Coastdowns have been a mainstay of recumbents for years. The problems with them are, you have to normalize weight and there's always a difference of opinion on the validity of the details - what speed to start, do you measure acceleration, top speed, total roll-out, etc. When adding fairings, the weight thing can be important. And I'm not sure how any of them would be turned into a magical CdA number. Measuring power at the hub would seem to be more reliable, but few non-racers are willing to buy a PowerTap hub or similar equipment. (I know a few are, but they don't talk about it.)

Even 'aerodynamic' TT uprights seem to get more benefit from drafting than a typical lowracer. That tells me something, even without knowing numbers. I've been in a 5-bike lowracer paceline and the benefit is barely noticeable - certainly not the kind of help it gives the uprights.
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Old 11-03-09, 01:35 PM
  #28  
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Originally Posted by BlazingPedals View Post
Coastdowns have been a mainstay of recumbents for years. The problems with them are, you have to normalize weight and there's always a difference of opinion on the validity of the details - what speed to start, do you measure acceleration, top speed, total roll-out, etc.
Yeah, I've seen lots of references to coast downs but the protocols used vary tremendously, as you've observed. That's one of the things I was interested in: if you do coast downs, what protocol do you use. A common protocol is to try to control entry speed and mass and then to do either terminal rollouts, terminal (i.e., maximum) speed, or speed at the end of a known distance. In my experiments, all of these have limited precision. BTW, for some coast down protocols you don't actually need to control entry speed or mass -- in fact, you can improve precision by altering them. If you want precision (and not everyone needs or wants it) then the key isn't controlling speed, it's getting a precise measurement of it.

Measuring power at the hub would seem to be more reliable, but few non-racers are willing to buy a PowerTap hub or similar equipment. (I know a few are, but they don't talk about it.)
A lot of recumbents have unusual drivetrains that make something like a Power Tap inappropriate. It's true a Power Tap makes things faster but one of the alternatives I've been playing with doesn't actually use the PT hub--it uses a wired PT head and the old harness just as a way to record speed. Something like a GPS that had a wheel pickup for speed (rather than relying on the satellite positioning for speed) would also work.

[Edit:] BTW, someone just pointed me to this, which appeared today.

Last edited by RChung; 11-03-09 at 02:34 PM.
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Old 11-04-09, 09:18 AM
  #29  
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RC, Thanks for the info and the article (I will try to follow the series).
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Old 11-06-09, 11:04 PM
  #30  
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You might be entertained by my efforts at measuring drag. Go to http://mysite.verizon.net/resob0n9/bicycle_performance/
If you wish you can email me at
foxconner@verizon.net
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Old 11-07-09, 02:59 PM
  #31  
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Originally Posted by foxconner View Post
You might be entertained by my efforts at measuring drag. Go to http://mysite.verizon.net/resob0n9/bicycle_performance/
Interesting. I'm surprised that your estimate was so sensitive to speed, especially at less than 8m/s. I've tested at speeds roughly 50% higher and didn't see that *except* when there was wind.
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Old 06-13-19, 06:43 PM
  #32  
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I was searching for something and came across this thread from nearly a decade ago. Reading through it again was nostalgic. Man, you guys were suspicious back then.
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Old 06-13-19, 07:59 PM
  #33  
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I wasn't suspicious so much as I was unable to add anything useful.
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Old 06-14-19, 12:00 PM
  #34  
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I retract my earlier suspicions. Robert is an ok guy.

I guess.
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Old 06-14-19, 12:40 PM
  #35  
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Originally Posted by Steamer View Post
I retract my earlier suspicions. Robert is an ok guy.

I guess.
Um, thanks, but even I don't think I'm OK.

I'm just saying I didn't have ulterior motives.
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Old 06-14-19, 01:33 PM
  #36  
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Originally Posted by RChung View Post
Um, thanks, but even I don't think I'm OK.

I'm just saying I didn't have ulterior motives.
As I think I might have mentioned to you once, I suspected your ulterior motive was to put down recumbents. Not may non-recumbent folk show up in the places where recumbent folk hang out, so your mere presence made you guilty until proven innocent. Sorta like Mormon missionaries showing up at a biker bar. They aren't there to down some suds. Not there is anything wrong with Mormon missionaries, or biker bars. Ok, I'll stop now.

Although I do stand by my one old statement:
Consider that if you are a college professor who ponders questions like this for a living, then it is likely you are much more suited to answering your own question than anyone here. ​​​​​​​
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Old 06-14-19, 01:59 PM
  #37  
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IIRC, weren't you doing something with leaning trikes a long time ago? I remember they looked pretty interesting, whoever was working with them.
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Old 06-14-19, 02:46 PM
  #38  
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Originally Posted by BlazingPedals View Post
IIRC, weren't you doing something with leaning trikes a long time ago? I remember they looked pretty interesting, whoever was working with them.
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Old 06-14-19, 03:54 PM
  #39  
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Originally Posted by Steamer View Post
Although I do stand by my one old statement:
Consider that if you are a college professor who ponders questions like this for a living, then it is likely you are much more suited to answering your own question than anyone here. ​​​​​​​
And I stand by my old statement:
You have a higher opinion of professors than I.
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Old 06-14-19, 04:10 PM
  #40  
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Originally Posted by BlazingPedals View Post
IIRC, weren't you doing something with leaning trikes a long time ago? I remember they looked pretty interesting, whoever was working with them.
Were you thinking of PurplePeople? He designed and built a tilting trike.
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Old 06-14-19, 07:54 PM
  #41  
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Originally Posted by RChung View Post
And I stand by my old statement:
You have a higher opinion of professors than I.
In fairness, you did pretty much answer your own question.
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Old 06-14-19, 08:06 PM
  #42  
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Still have not measured the CdA of my bikes.
But do have a general idea of what CdA means.
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