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  1. #1
    A tiny member bikeguyinvenice's Avatar
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    Smartphone app vs bicycle computer vs dedicated gps unit

    Curious as to which is more accurate for recording distance and speed and time. I have been using the cyclemeter app for iphone, which I have noticed sometimes has hiccups in the recorded speed, elevation and time. On todays ride the cyclecomputer recorded an average speed of 14.1 mph over 29.36 miles and my app recorded and average speed of 17.8 mph over 29.29 miles.
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    Smartphone apps will be least accurate because the GPS in a phone is not very responsive and the apps generally don't record a lot of points (to save on battery life). A dedicated GPS unit will be better, but not as good as a properly calibrated bicycle computer. A 3mph speed difference between units is unusually high, unless you have some kind of systemic issue like the different units are measuring different things. Is one of them set to pause at stops, while the other is not?

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    Quote Originally Posted by bikeguyinvenice View Post
    Curious as to which is more accurate for recording distance and speed and time. I have been using the cyclemeter app for iphone, which I have noticed sometimes has hiccups in the recorded speed, elevation and time. On todays ride the cyclecomputer recorded an average speed of 14.1 mph over 29.36 miles and my app recorded and average speed of 17.8 mph over 29.29 miles.
    What cyclecomputer?

    The cycle computer and the phone probably report the total time (including stops) and the moving time. You should be able to determine which time was used to compute the average (given that the distances are the same).

    NOTE: different computers might use different low speeds above (or stop times below) which the computer thinks you are moving.

    By default (i.e., without a wheel sensor), the Garmins use GPS for speed, which can be inaccurate over short periods of time and distance. The 500 and up units allow you to use a wheel-rotation sensor that avoids the GPS inaccuracy.

    GPS (basically) just determines location/position (and assigns a time to the location). Speed is determined from the position data by computing the distance between two points and dividing by the difference in time. Since GPS positioning has some error (+-30 feet or so), the speed between determined from points close together will be less accurate (more variable). GPS speed works better when traveling at higher speeds because the points being used are farther apart.

    Speed from wheel rotation is very fast and works accurately at distances much less than the normal GPS position error.
    Last edited by njkayaker; 06-19-13 at 11:44 AM.

  4. #4
    A tiny member bikeguyinvenice's Avatar
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    The cyclecomputer in question is a blackburn delphi 3, it uses magnets and sensors for both speed and cadence.
    My cycling blog http://kcmjr.wordpress.com/
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    Quote Originally Posted by bikeguyinvenice View Post
    The cyclecomputer in question is a blackburn delphi 3, it uses magnets and sensors for both speed and cadence.
    Not a lot of detail about how it computes the average speed.

    http://www.blackburndesign.com/black...ers/delphi.pdf

  6. #6
    A tiny member bikeguyinvenice's Avatar
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    Quote Originally Posted by njkayaker View Post
    Not a lot of detail about how it computes the average speed.

    http://www.blackburndesign.com/black...ers/delphi.pdf
    Well it must only calculate average speed of when the bike is moving, because at the half way mark, at the end of the trail I take about a 5 to 10 minute break and when I get back on the bike the average speed is the same as when I stopped.
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    I've gone 100% over to using my iPhone as a cycle computer using Cyclemeter from Abvio and bluetooth speed/cadence sensor from Wahoo Fitness. It replaces a Garmin 705 and a 305 before that. I don't notice any accuracy issues at all (at least as accurate as the GPS units). That makes it just one less thing to carry.

    J.

  8. #8
    Daily Rider Robert C's Avatar
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    I have a GPS bicycle speedometer. It is an i-gotU 800. The trouble it has is that the software is not bicycle specific and the desktop client also does not remove some total stops from the average time. As such, the average speed is messed up. If I really were concerned with accurate average speeds I would certainly get a unit with a wheel sensor.

    With that caveat, I like being able to come home and plug my gps into the computer and have a record; or, and more realistically reflecting what I do, the ability to plug the gps into the computer once every couple of weeks and get a record. The GPS I have works well with its included software to automatically log the rides on the day they occurred and create an activities calender.

    A phone app would not work well for me due to battery life. I tried one on my phone for a part of a summer and was never happy with it because of that issue.

    So, I would recommend a bicycle specific GPS with a wheel sensor; but, it is not the cheapest option.
    Last edited by Robert C; 06-19-13 at 04:45 PM.
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  9. #9
    A tiny member bikeguyinvenice's Avatar
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    Quote Originally Posted by Robert C View Post
    With that caveat, I like being able to come home and plug my gps into the computer and have a record; or, and more realistically reflecting what I do, the ability to plug the gps into the computer once every couple of weeks and get a record. The GPS I have works well with its included software to automatically log the rides on the day they occurred and create an activities calender.
    Well that is mostly what I use the gps app in my iPhone for, I start it at the beginning of the ride and end it when I am done then I email a gpx file to myself and upload it to ridewithgps, but as I said I have noticed on several occasions the app has recorded erroneous data, mostly max speed, average speed and elevation.
    My cycling blog http://kcmjr.wordpress.com/
    "Normal" is just a setting on a washing machine.

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