Cycle Computer Accuracy ?

Discussion in 'UK and Europe' started by TJ, Jul 27, 2004.

  1. TJ

    TJ Guest

    Maybe I am missing something or is the computer really getting it wrong.

    I have a SigmaSport BC1200 comp which is seemingly recording my average
    speeds incorrectly.

    example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

    My own calcs make this 15.26 Mph

    If the comp is right can someone explain why ?

    Thanks

    TJ
     
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  2. On Tue, 27 Jul 2004 14:45:29 +0100, TJ <[email protected]> wrote:

    > Maybe I am missing something or is the computer really getting it wrong.
    >
    > I have a SigmaSport BC1200 comp which is seemingly recording my average
    > speeds incorrectly.
    >
    > example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    > My own calcs make this 15.26 Mph
    >
    > If the comp is right can someone explain why ?


    Might it be factoring out the periods spent stationary? Thus the time
    spent actually moving will be a little shorter.

    Colin
     
  3. MSeries

    MSeries New Member

    Joined:
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    I wouldn't expect these devices to be very accurate, TBH. There is no requirement for them to be calibrated. Perhaps internally the calculation uses more decimal places than the display shows you.
     
  4. TJ wrote:

    > Maybe I am missing something or is the computer really getting it wrong.
    >
    > I have a SigmaSport BC1200 comp which is seemingly recording my average
    > speeds incorrectly.
    >
    > example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    > My own calcs make this 15.26 Mph
    >
    > If the comp is right can someone explain why ?


    If your computer is like mine, it stops counting the trip time if you
    stop for more than 4 seconds (at junctions, traffic lights etc).

    So if you time your journey with your watch, and compare this to
    the trip time on your computer, the trip time will usually be
    shorter. Less time = higher average speed.


    --
    jc

    Remove the -not from email
     
  5. Gonzalez

    Gonzalez Guest

    On Tue, 27 Jul 2004 14:45:29 +0100, "TJ" <[email protected]> wrote:

    >Maybe I am missing something or is the computer really getting it wrong.
    >
    >I have a SigmaSport BC1200 comp which is seemingly recording my average
    >speeds incorrectly.
    >
    >example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    >My own calcs make this 15.26 Mph
    >
    >If the comp is right can someone explain why ?


    Assuming both the distance and the trip time come from the computer,
    at a guess I would say that the computer calculates distance and speed
    in km and kph and converts by using a look-up table into miles and
    mph. The look-up table is not sufficiently accurate to give a reading
    to two decimal places.

    1 kph = 0.621371192237 mph (approx 5/8)
    1 km = 0.621371192237 miles (approx 5/8)

    Converting 5.03 miles into kms, you cycled 8.05 km.

    You cycled 8.05 km in 1187 seconds. That's a speed of 24.41 kph.

    Converting 24.41 kph into mph, you cycled at... 15.26 mph.

    Hurrummph - your cycle computer's calculations are crap!!!
     
  6. TJ

    TJ Guest

    My computer does feature a dist timer as you mention which does stop within
    a second or two whenever the bike becomes stationary so I dont think that
    this this is an issue.

    I really don't see why it isn't calculating the Average Speed from it's own
    figures in the same way I am ?

    I'm using excel to calculate the A/Spd by simply dividing the Distance
    covered by the time taken to cover it.

    (Because a minute is only 60 secs and excel works in Decimal I multiply the
    seconds by 1.665).

    The entire result is then multiplied by 60 to give the MPH.

    e.g.

    This mornings Distance / Time was 5.00 miles in 15m 06s

    The 6 seconds are multiplied by 1.665 (.06 * 1.665) to give .10 this is
    done to give the base 10 representation of the number for the calculation

    so

    ( 5.00miles / 15.1 ) * 60 = 19.08 Mph

    I'm sure i'm right - Aren't I ?


    when I use other test data in my formula i.e. miles 10.5, Dist 10.3 10.5
    miles in 10 mins 30 seconds for example the result is 60mph as expected.

    Even if I skip the corrective adjustment I mention the result is still only
    19.92 yet the Sigma BC1200 is reporting the average speed this morning as
    20.12 Mph


    TJ



    "TJ" <[email protected]> wrote in message
    news:[email protected]
    > Maybe I am missing something or is the computer really getting it wrong.
    >
    > I have a SigmaSport BC1200 comp which is seemingly recording my average
    > speeds incorrectly.
    >
    > example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    > My own calcs make this 15.26 Mph
    >
    > If the comp is right can someone explain why ?
    >
    > Thanks
    >
    > TJ
    >
    >
     
  7. [email protected] wrote:
    > Assuming both the distance and the trip time come from the computer,
    > at a guess I would say that the computer calculates distance and speed
    > in km and kph and converts by using a look-up table into miles and
    > mph. The look-up table is not sufficiently accurate to give a reading
    > to two decimal places.
    > ...
    > Hurrummph - your cycle computer's calculations are crap!!!


    Allowing for the more sensible answers that have already been...
    Would it make sense to do anything to four significant figures?

    How finely are you able to calibrate the device in the first
    place? And any sensor that measures a single point on a 700mm
    wheel is bound to be anything up to about 0.25% of a mile wrong
    on any journey, however long or short, anyway. (A hundredth of
    a mile is only about seven rotations of the wheel.) How much
    further does the front wheel of a bike go than the back one?

    One of my grandfathers used insist on sitting in the front seat
    upstairs on a bus because he "went further" that way.
     
  8. On Tue, 27 Jul 2004 14:45:29 +0100, TJ wrote:
    > Maybe I am missing something or is the computer really getting it wrong.
    >
    > I have a SigmaSport BC1200 comp which is seemingly recording my average
    > speeds incorrectly.
    >
    > example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    > My own calcs make this 15.26 Mph
    >
    > If the comp is right can someone explain why ?


    The computer is using low resolution arithmetic to calculate the average
    speed, based presumably on the time and distance it has recorded. In
    principal, and certainly if you do it on your PC in excel, or write a
    program to do it, you will get an answer that is as accurate as the
    time and distance measurments up to several significant figures.

    However, the cycle computer uses not very many significant figures in
    its calculations, and so it looses accuracy as it calculates. As an
    example, suppose it uses only 2 decimal digits throughout the calculation,
    and simply throws away less significant digits, the answer you calculate
    will be 15.00. If it uses 3 you get 15.20, and to 4 you get 15.25 (if
    you use the same algorithm I did). Your trip computer will use either
    binary or binary coded decimal arithmetic, and so its calculation
    accuracy will not be the same as I have above.

    My own computer is always between 0.00 and 0.20 mph below the actual
    (excel calculated) average speed.

    --
    Trevor Barton
     
  9. Lee Garnett

    Lee Garnett Guest

    Surely

    average speed = distance/((seconds+(minutes*60)+(hours*60*60))/3600)

    by using number of seconds * 1.665 you are introducing an error, it may only
    be 1% error but that greater than the error in the original question which I
    believe was 0.85%.

    -----------
    LeeG
    -----------
    "TJ" <[email protected]> wrote in message
    news:10gct8s8e4ct0dc[email protected]
    My computer does feature a dist timer as you mention which does stop within
    a second or two whenever the bike becomes stationary so I dont think that
    this this is an issue.

    I really don't see why it isn't calculating the Average Speed from it's own
    figures in the same way I am ?

    I'm using excel to calculate the A/Spd by simply dividing the Distance
    covered by the time taken to cover it.

    (Because a minute is only 60 secs and excel works in Decimal I multiply the
    seconds by 1.665).

    The entire result is then multiplied by 60 to give the MPH.

    e.g.

    This mornings Distance / Time was 5.00 miles in 15m 06s

    The 6 seconds are multiplied by 1.665 (.06 * 1.665) to give .10 this is
    done to give the base 10 representation of the number for the calculation

    so

    ( 5.00miles / 15.1 ) * 60 = 19.08 Mph

    I'm sure i'm right - Aren't I ?


    when I use other test data in my formula i.e. miles 10.5, Dist 10.3 10.5
    miles in 10 mins 30 seconds for example the result is 60mph as expected.

    Even if I skip the corrective adjustment I mention the result is still only
    19.92 yet the Sigma BC1200 is reporting the average speed this morning as
    20.12 Mph


    TJ



    "TJ" <[email protected]> wrote in message
    news:[email protected]
    > Maybe I am missing something or is the computer really getting it wrong.
    >
    > I have a SigmaSport BC1200 comp which is seemingly recording my average
    > speeds incorrectly.
    >
    > example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph
    >
    > My own calcs make this 15.26 Mph
    >
    > If the comp is right can someone explain why ?
    >
    > Thanks
    >
    > TJ
    >
    >





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