Computer Settings (Mavic)

furiousd
furiousd Posts: 214
edited October 2012 in Workshop
Hi all,
Recently bought a Mavic Wintech speedo for my bike. I used to run an old Lidl one but snapped the cable ages ago so thought it was about time i got a new one and i like the way the sensor in on the skewer.

I measured the wheel and put the measurement setting. On my first ride it was over reading compared to Endomondo and my friend Mavic Wintech. I went to change the setting to that which is suggested in the manual for 700 x 23 but when i went to change it the number cam up as 83 instead of something like 2110. I had to reset the computer to be able to put a new wheel measurement in. Put the new number is as suggested by manual, which was 2130.

Went out for another ride with friend again and it was over reading again compared to his Mavic and both our Endomondo's. Do i need to reduce the circumference or increase it?

2110 was over reading
2130 was over reading
I cant remember which was over reading more.

Any help appreciated.

Thanks
D

Comments

  • furiousd
    furiousd Posts: 214
    Anyone?
  • Twostage
    Twostage Posts: 987
    According to the cateye documentation 700x23 should be 2096.
  • furiousd
    furiousd Posts: 214
    MIght try that then.
    I have just checked the mileage from Endo on map my ride for the same route and they were only 0.03 of a mile difference so that would suggest Endo is pretty accurate.
    Mavic documentation suggests the following:
    700 x 19 - 2100
    700 x 20 - 2110
    700 x 23 - 2130
    700 x 25 - 2140

    Think i will reduce the number and try it out again this afternoon. Just used excel to do some maths fore me an set at 2110 with 30,000 revolutions it comes in at 39.33 miles, 2130 came in at 39.71, so not a lot of difference but at least i know which was to adjust now.

    Cheers
    D
  • sungod
    sungod Posts: 16,869
    circumference will vary between different models of tyre, it won't be a huge difference, but you can just do a roll-out to get the correct setting

    inflate tyre to the pressure you typically ride at

    mark a spot on the tyre (a bit of tape is good)

    on a smooth floor, roll it out for two or three revolutions, make sure it's in a straight line, measure, then calculate circumference

    fwiw with a 700x23 gp4000s i measured 2107mm

    you say your mavic is over reading, how do you know that?

    be wary of consumer-grade gps as a reference, let alone a smartphone gps, accuracy is limited, epecially at lower speeds or if there are corners or altitude changes - 0.5mph error is fairly typical under good conditions, at 20mph that's only 2.5%, in other conditions it could be worse, a roll-out measurement will be much more accurate

    if your friend's mavic is set to a circumference that doesn't match the actual one then it'll be off too
    my bike - faster than god's and twice as shiny
  • furiousd
    furiousd Posts: 214
    Thanks Sungod.
    I believe it to be over reading compared to Endomondo on iphone and Mapmyride, manually putting in the route. These two are only .02 of a mile out. Just come back from 12.5 mile ride and the Mavic has an over read of .20 of a mile for the journal compared to Endo.
    This was using 2100 as the setting.
    D
  • sungod
    sungod Posts: 16,869
    gps distance depends on sample rate, essentially it takes each point and joins the dots, the curvier the route and lower the sample rate the greater the inaccuracy (typically it will read shorter), you can add a bit of sophistication by filtering to smooth out the edges, but the calculated course is still only an approximation of the real one

    gradient changes complicate things further, a simplistic distance calculation will simply assume travel over a 2d plane, because a proper 3d calculation requires accurate altitude which basic gps devices can struggle with, the 2d calculation will also under-report distance

    wheel-based distance depends on the actual distance the wheel rolls, if it's a curvy/hilly route, the wheel follows the curves/hills, so as long as the circumference is accurately set and the road smooth, it will give good accuracy

    for travel in a straight line, all gps devices should be extremely accurate for speed and distance, but when things get curvy in 3d a basic gps device has limitations

    set correctly, for a curvy course i'd always put my money on the mavic being more accurate for distance than the smartphone gps

    btw this article highlights the same issue, albeit for runners...

    http://www.nytimes.com/2011/12/20/healt ... d=all&_r=0
    my bike - faster than god's and twice as shiny