Cadence sensor vs GPS

Gabbo
Gabbo Posts: 864
edited October 2012 in Road general
I'm currently using both, of course. Problem is, without my cadence sensor it takes me longer to clock a mile and the gps records me cycling slower than when I'm using my cadence sensor. Either GPS' are inaccurate or I've incorrectly installed my cadence sensor. But the whole point of this post is to get answers as to which device is lying!

Anyone experienced the same issue?

Comments

  • DHA987S
    DHA987S Posts: 284
    Is the wheel size correct in the cadence sensor (assume its a Garmin, you set it in options). GPS needs to be able to "see" and if you ride under tree cover/tunnels/buildings it will lose sight of the satellites and this causes the route to jump. The combination of the two sources is the most accurate you will get from a small bike based GPS.
  • antfly
    antfly Posts: 3,276
    Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.
    Smarter than the average bear.
  • sungod
    sungod Posts: 16,551
    a consumer-grade gps, especially one cut-down to fit a bike or smartphone, is poor at determining distance travelled or speed unless you only travel in a straight line on flat ground (no doppler, poor altitude accuracy, low sample rates, and the effects of trees/buildings on signal)

    on a twisty route, a basic cycle computer calculating speed/distance (calibrated to match wheel circumference) will be much more accurate than a bike gps
    my bike - faster than god's and twice as shiny
  • antfly
    antfly Posts: 3,276
    I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.
    Smarter than the average bear.
  • turnerjohn
    turnerjohn Posts: 1,069
    Gabbo wrote:
    I'm currently using both, of course. Problem is, without my cadence sensor it takes me longer to clock a mile and the gps records me cycling slower than when I'm using my cadence sensor. Either GPS' are inaccurate or I've incorrectly installed my cadence sensor. But the whole point of this post is to get answers as to which device is lying!

    Anyone experienced the same issue?

    I'm the other way round...when I had a wireless computer I ride fast up hill then my Garmin....I did wonder if Garmin speed takes into account % incline as you travel further actual then viewed directly from above aka a sat ?!
  • sungod
    sungod Posts: 16,551
    antfly wrote:
    I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.

    it's heavily dependent on route/conditions, and some gps units are better than others, i've got a garmin too, but i only use it for navigation, for training i use the cycle computer

    there's a running-oriented article here, there was quite a variation between different gps units...

    http://www.nytimes.com/2011/12/20/healt ... =all&_r=1&
    my bike - faster than god's and twice as shiny
  • antfly
    antfly Posts: 3,276
    Running is different from cycling.
    Smarter than the average bear.
  • rolf_f
    rolf_f Posts: 16,015
    antfly wrote:
    I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.

    Plus one - I find the differences to be so small as to be negligible. Unless you record distance to a tens of feet accuracy.
    Faster than a tent.......
  • Gabbo
    Gabbo Posts: 864
    antfly wrote:
    Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

    I use Garmin Edge 500

    Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

    Thanks
  • p1tse
    p1tse Posts: 694
    Also cost to consider
    Wanted: Cube Streamer/Agree GTC Compact / Pro/ Race : 53cm
  • sungod
    sungod Posts: 16,551
    antfly wrote:
    Running is different from cycling.

    to post such an irrelevant comment indicates that either you are trolling or that you do not understand the various sources of error

    neither makes you look good
    my bike - faster than god's and twice as shiny
  • MattC59
    MattC59 Posts: 5,408
    Gabbo wrote:
    antfly wrote:
    Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

    I use Garmin Edge 500

    Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

    Thanks

    You need to measure your wheel / tyre circumference. Put a chalk mark on your tyre, roll it along the ground starting with the chalk mark level with a fixed mark on the ground. When the wheel has moved one rotation and the chalk mark is back at the ground, mark that point on the ground. Measure between the marks. You can then enter this into the Garmin in the 'bike settings' menu.

    I found that the difference between the auto value and measured value was several cm. worth measuring and inputting yourself IMO.
    Science adjusts it’s beliefs based on what’s observed.
    Faith is the denial of observation so that Belief can be preserved
  • antfly
    antfly Posts: 3,276
    MattC59 wrote:
    Gabbo wrote:
    antfly wrote:
    Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

    I use Garmin Edge 500

    Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

    Thanks

    You need to measure your wheel / tyre circumference. Put a chalk mark on your tyre, roll it along the ground starting with the chalk mark level with a fixed mark on the ground. When the wheel has moved one rotation and the chalk mark is back at the ground, mark that point on the ground. Measure between the marks. You can then enter this into the Garmin in the 'bike settings' menu.

    I found that the difference between the auto value and measured value was several cm. worth measuring and inputting yourself IMO.

    There's no point in doing the roll method if you are not sitting on the bike while you roll it, easier and better just to put in the quoted figures for the tyre size in use.
    Smarter than the average bear.