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  1. #26
    Senior Member Nate552's Avatar
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    Quote Originally Posted by RChung View Post
    or (better) "spin" your legs slowly but without putting any power into the pedals so that you'll get a truer reading of your aero drag when you're pedaling.
    I wouldn't bother with that, we've tested leg rpm in the tunnel and haven't found it to be a factor ( I believe Jim Martin also spent some time trying to "see" leg movement and couldn't find it). I'd focus more on holding an identical position for each run. You'd be amazed at how much people change their positions and don't even realize it. I measure peoples head positions at the start of each tunnel run, and 99% of people can't nail their head position, run to run. (Exceptions being Racer Ex, who is like a dang robot) If you lower your head by a few cm's, the run is void.

    When I've done VE testing in the past, I have actually taken straws and cut them to length, then mounted them so that I can place my chin in the exact same position. Of course, I was looking for small changes.

  2. #27
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    Quote Originally Posted by Nate552 View Post
    I wouldn't bother with that, we've tested leg rpm in the tunnel and haven't found it to be a factor ( I believe Jim Martin also spent some time trying to "see" leg movement and couldn't find it). I'd focus more on holding an identical position for each run. You'd be amazed at how much people change their positions and don't even realize it. I measure peoples head positions at the start of each tunnel run, and 99% of people can't nail their head position, run to run. (Exceptions being Racer Ex, who is like a dang robot) If you lower your head by a few cm's, the run is void.

    When I've done VE testing in the past, I have actually taken straws and cut them to length, then mounted them so that I can place my chin in the exact same position. Of course, I was looking for small changes.
    Yeah, I wasn't suggesting turning your legs over because of a "windmilling" effect -- I was suggesting it for exactly the reason you were pointing out: I have difficulty remembering to get my legs in the exact same orientation from coast down to coast down. When I do "regular" VE runs under power, of course, my legs are turning so this is moot: it's just for coast downs that I forget where I put my feet. It's so unnatural.

    The straw thing is cool, as long as you don't get distracted and use big pointy spikes by mistake. Chung on a stick.

  3. #28
    Senior Member Nate552's Avatar
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    Quote Originally Posted by RChung View Post
    Yeah, I wasn't suggesting turning your legs over because of a "windmilling" effect -- I was suggesting it for exactly the reason you were pointing out: I have difficulty remembering to get my legs in the exact same orientation from coast down to coast down. When I do "regular" VE runs under power, of course, my legs are turning so this is moot: it's just for coast downs that I forget where I put my feet. It's so unnatural.
    That's a good point, I've changed my mind, soft pedaling is good.

    Quote Originally Posted by RChung View Post
    The straw thing is cool, as long as you don't get distracted and use big pointy spikes by mistake. Chung on a stick.
    Speaking of, any news on a commercially produced one?

  4. #29
    Because I thought I could ks1g's Avatar
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    I have a question for RChung about doing this test protocol in real life. I have a long downhill-flat section-downhill around the corner, I will have to check next commute how fast I need to be going to coast through the flat section. It looks like the protocl is get to a desired entry speed, coast through the test section while collecting the time-series speed data. Repeat with identical geometry/body position but at a much different entry speed. Use USGS maps, barometric altimeters, etc. to get altitude/slope data. I assume you use other means than hitting a "lap" button to note when you've entered the test section (since we're assuming no power meter that would indicate coasting).

    I wish I knew about this when my kids were a few years younger - they both did a bike tire rolling resistance experiment for school science fair (I was the "test pilot"). Data collection was limited to stop watches, and my ability (or lack of) to hit a consistent entry speed, lap buttons (gave up on that) and note max indicated speed for each run. I sat up as upright as I could to try to get consistent CdA*rho. No collection of data or adjustments for air density and I forget what we did about wind.

  5. #30
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    Quote Originally Posted by ks1g View Post
    I have a question for RChung about doing this test protocol in real life. I have a long downhill-flat section-downhill around the corner, I will have to check next commute how fast I need to be going to coast through the flat section. It looks like the protocl is get to a desired entry speed, coast through the test section while collecting the time-series speed data. Repeat with identical geometry/body position but at a much different entry speed. Use USGS maps, barometric altimeters, etc. to get altitude/slope data. I assume you use other means than hitting a "lap" button to note when you've entered the test section (since we're assuming no power meter that would indicate coasting).
    That's about it. In this example I did only two runs because I was only trying to show that it could be done, and done with pretty minimal equipment. I'd do at least one more in a real attempt, then compare the runs against each other. I don't hit a lap button -- I like to get my position set and start to coast before I enter the test section so I don't have to move my hands or head or anything, even if only to hit a lap button. Instead, I transform speed into distance and from the speed and change in speed and mass I build up a preliminary elevation profile for the course. It's pretty clear where the slope begins, or where it transitions to the flat, and then steepens again; so once that's identified I can count in distance backward and forward to match the test sections. May sound hard but it's not, really, and it frees me up from remembering to mark the sections. Basically, I try to make the data collection as simple and brainless as possible even if it means I have to do an extra calculation when I get home. Many other protocols require that you hit a certain speed at a certain point or hold your speed or equalize mass or something else. I'm usually too stupid to remember all those things when I'm on the bike so it makes sense to account for it later.

    I wish I knew about this when my kids were a few years younger - they both did a bike tire rolling resistance experiment for school science fair (I was the "test pilot"). Data collection was limited to stop watches, and my ability (or lack of) to hit a consistent entry speed, lap buttons (gave up on that) and note max indicated speed for each run. I sat up as upright as I could to try to get consistent CdA*rho. No collection of data or adjustments for air density and I forget what we did about wind.
    If you're serious about this kind of thing you really need to do it under calm conditions (or else have a way to measure wind on the bike, and that's not easy to do).

  6. #31
    Senior Member aaronmcd's Avatar
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    Quote Originally Posted by stedalus View Post
    I get Crr = 0.0064 and CdA = 0.378 m^2. Basically you do as above and write out the energy budget. It looks like

    alpha * Crr + beta * CdA = gamma

    where

    alpha = -mg*integral(v)
    beta = -0.5*rho*integral(v^3)
    gamma = 0.5 m(vf^2-vi^2) - mgh

    Doing this for each run you gives you a 2x2 set of linear equations that can be solved for CdA and Crr.

    I'd say the two runs are needed to disentangle CdA, Crr, and the unknown elevation profile. If you could just ride your bike on flat ground, or use the altimeter data, then only one run is needed since you could do a nonlinear fit to the expected v(t).

    In principle, now that you have CdA and Crr, you can calculate the force of gravity as the balance of all the other forces (aero, rolling, net). The peak would tell you when you hit max slope. But I don't think you have the time resolution to calculate dv/dt cleanly enough for that to work well. Then again I didn't try it.

    Do I get a cookie?
    Oold thread but I found it searching online. Problem is alpha is equal for both, since distance is the same for both. So Crr doesn't effect the energy equation (like friction down an incline fast or slow - same coefficient of friction, same distance, same energy loss).

    I get CdA = 0.387, and Crr =0.0062 assuming 5 m elevation loss.
    Max grade between 3.9 and 4.4%, depending on which data set.Elevation Profile.PNG
    That was fun.

  7. #32
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    Very nice. With those data I got slightly better fit with Crr=.0061 and CdA=.389 (but I probably make very slightly different assumptions about the course). In any event, for an actual test I would have done more trials, made sure to test on a wind-free course, and tried to get a better handle on the exact elevation change between start and finish. A month or two after I posted the challenge I saw a city surveying team out on that street and I stopped to talk with them. They said that with the equipment that had they could tell the absolute elevation at given sites to well within a centimeter, and I could call up the city engineering department for that info. Perhaps your city or utility district has an engineering department you can call for similar information.

  8. #33
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    I'd like to implement this calculation in an interactive web page and in a smartphone app, but I miss something.
    Once I recorded some values during coast down, and once I found the polynomial approximating it, how do I get Cd and Crr without using Excel, Aerolab or anything else other than pure math?

    What I can say is that the speed during coast down can be expressed as:
    v1 = K*v0^2 + v0 - H
    Where:
    K=-(Ar)/(2m) * Cx * T
    H = -g*Crr

    A= frontal area
    r = air density
    m = mass
    T = time between samples
    g = 9,81 m/s^2

    I was also able to find the formula needed to determine the coefficient of equation ax^2+bx+c which fits the samples.

    But I can't go further :-(
    How do I join the fittin equation and the data to get Cd and Crr??

  9. #34
    Killing Rabbits
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    I suck at math but if you have two equations and two unknowns you just use the substitution method.

  10. #35
    Man of constant sorrow Dudelsack's Avatar
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    Quote Originally Posted by RChung View Post


    Let's not make this too complicated. Cyclists in general, but racers in particular, don't have especially long attention spans if you know what I mean and I'm sure you do so it's good to keep things simple.


    You lost me with the title

    About nubile vixens: pics or it didn't happen.
    Possunt quia posse videntur. St. Dudel: Epic is stupid that you get away with.

  11. #36
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    Quote Originally Posted by jumpjack View Post
    I'd like to implement this calculation in an interactive web page and in a smartphone app, but I miss something.
    Once I recorded some values during coast down, and once I found the polynomial approximating it, how do I get Cd and Crr without using Excel, Aerolab or anything else other than pure math?

    What I can say is that the speed during coast down can be expressed as:
    v1 = K*v0^2 + v0 - H
    Where:
    K=-(Ar)/(2m) * Cx * T
    H = -g*Crr

    A= frontal area
    r = air density
    m = mass
    T = time between samples
    g = 9,81 m/s^2

    I was also able to find the formula needed to determine the coefficient of equation ax^2+bx+c which fits the samples.

    But I can't go further :-(
    How do I join the fittin equation and the data to get Cd and Crr??
    The simplest way is to convert from a speed-and-power equation to a work-and-distance equation that solves for a CdA for a given Crr, and vice versa. If you have two runs taken at different initial speeds, you'll have two equations for two unknowns. If you have more than two runs, you can solve using a regression-like procedure. Let me know if you need help.

  12. #37
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    Quote Originally Posted by RChung View Post
    The simplest way is to convert from a speed-and-power equation to a work-and-distance equation that solves for a CdA for a given Crr, and vice versa. If you have two runs taken at different initial speeds, you'll have two equations for two unknowns. If you have more than two runs, you can solve using a regression-like procedure. Let me know if you need help.
    I can have all runs I want... Currently I have 6 runs available in the example spreadsheet.
    Can you please provide some examples about methods you're talking about?

  13. #38
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    Quote Originally Posted by jumpjack View Post
    I can have all runs I want... Currently I have 6 runs available in the example spreadsheet.
    Can you please provide some examples about methods you're talking about?
    Sorry to take so long to respond. Real world life interruptions.

    You've read the the .pdf, right? Slide 70 discusses integrating the power equation up to a work equation for any segment of a test. For a coast down, the segment power is zero and you'll know your mass, the height of the drop, rho, and the second-by-second speed. That will let you create six equations from your six runs, with only two unknowns (Crr and CdA). So solve for the Crr and CdA then --> importantly <-- reconstruct the VE profiles for each of the six runs and overlay them. If they overlay, you're golden. If not, you'll typically be able to identify what caused the misfit.

    A couple of years ago a guy wrote up an Android phone app to do this with his phone's GPS. GPS speed, especially for phones, turns out not to be clean enough for this method. (He subsequently tried it with a surveying dGPS instrument on top of a car and got very, very consistent results, so he knows it was the phone's GPS and not the method or his data collection technique).

  14. #39
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    why aren't phone gps suitable? I was trying to write such an app because I was not able to find it, have you got any link?

  15. #40
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by jumpjack View Post
    why aren't phone gps suitable? I was trying to write such an app because I was not able to find it, have you got any link?
    Not accurate enough and slower sampling rate. If you've ever compared a phone gps speed to a computer with a wheel magnet you'll notice a slight lag after higher accelerations. It appears to me that the gps apps do a real-time Gaussian smoothing (a kind of average over successive readings). The bike computers do that also but with a higher sample rate, every 7 feet or so. For his kind of calculation, the better instantaneous data you have the better the model is.

  16. #41
    ride lots be safe Creakyknees's Avatar
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    Quote Originally Posted by wphamilton View Post
    the better instantaneous data you have the better the model is.
    that's why I use 36 magnets for my coast-down tests.
    "have fun and be kind"
    - an internet post

  17. #42
    ride lots be safe Creakyknees's Avatar
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    somebody suggested that I buy a touring wheel to get more spokes and magnets, but come on that's just silly.
    "have fun and be kind"
    - an internet post

  18. #43
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    Quote Originally Posted by jumpjack View Post
    why aren't phone gps suitable? I was trying to write such an app because I was not able to find it, have you got any link?
    GPS's determine speed by the change in location over time. They have the time piece right but they can be off by a couple of meters in location. That's not much but if you're trying to measure speed every second and you're off by a meter or two, that's the difference between thinking that your speed is 8 m/s followed by 8 m/s and maybe 10 m/s followed by 6 m/s. In addition, GPS location can be off if there are trees or buildings that attentuate a signal, so if there's a missing point the speed over the missing point gets interpolated.

    I don't have a link to his app, but here's a link to one of his papers. If you write him he can probably explain what he did with the app, and why it didn't work very well.

  19. #44
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by Creakyknees View Post
    somebody suggested that I buy a touring wheel to get more spokes and magnets, but come on that's just silly.
    I'd think you'd be better off upgrading the magnetic switch first if you haven't already done so.

  20. #45
    Wheelsuck Fat Boy's Avatar
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    Quote Originally Posted by Creakyknees View Post
    that's why I use 36 magnets for my coast-down tests.
    For the stuff I do, we run up to a 72-tooth trigger wheel (and there are some who use more) when using traction control (36 when not). Our GPS data comes in at 20 Hz. If you're sampling is too coarse, you're just kidding yourself. You'll never get it really right.

    I've worked with some engine builders that sample things like cylinder pressure in 1 degree of crank rotation. Those things pull _a lot_ of data. It used to be a big deal, but it's amazing how no one even cares about file size anymore.
    Austin doesn't have hippies. They have slightly rebellious Methodists. - Racer Ex

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