Computer Settings (Mavic)

furiousd

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

furiousd

Anyone?

Twostage

According to the cateye documentation 700x23 should be 2096.

furiousd

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

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

furiousd

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

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