29er MTB - Wheel Setting for Polar RS800CX

[eheather]

Please could someone assist me with the correct setting in (mm) for my Polar RS800CX for a 29er Wheel?

 

The sizing of my wheel is as follows: 29 x 2.0 (50-622).

 

I have visited a website www.bikecalc.com/wheel_size_math & used the corresponding size of 2273mm. I however measure a race of 40km still around 5km or so too short....

 

I also performed the exercise where I have measured my rim by making a mark on the ground at the valve & then another after one full revolution. The distance then measures a total of 2282mm, however this size also does not help me to get close to closing the gap of a 5km difference.

 

If someone has done this before on a Polar, kindly share with me or let me have your valuable suggestions?

 

Thanks

[splat]

Measuring the circumference of your tyre at correct pressure ist he most accurate.

Sit on the bike and get someone to make the marks for you.

 

And dont assume that the advertised distance of a mtb race is accurate...

[patches]

are you wheelie-ing/manualing a lot?

[AndreZA]

Measuring 1 roll of the tire, not rim, under load is the correct way. And are you sure the 40km race was actually 40km. I've never done any race where the distance was what they said.

[eheather]

Thanks Splat...

 

LOL Patches...

 

AndreZA: Agree what you say, I also had that thought in the back of my head however after this weekends 94.7 MTB Challenge the route was set to be 50km, then organizers mentioned 53km at the start however some guys ended up measuring 56km. Our Polar's only registered 46km.... That is what sparked my concern!!!

[siggie]

I use 2232 for my setting. Did the Engen MTB and got 64.9km on my polar and 65.4km on the Garmin

 

Regards

 

Siggie

[NINER_boy]

Stan's Flow rim, Scwhalbe Racing Ralphs.

 

Wheel measured under load, and circumference came to be 2240mm

 

Good luck with that

[eheather]

Thanks Siggie & NINER_boy....

 

I also think lesser of a measurement is the trick... I will try the settings you mentioned !!!

 

Much appreciated...

 

[Shebeen]

Thanks Siggie & NINER_boy....

 

I also think lesser of a measurement is the trick... I will try the settings you mentioned !!!

 

Much appreciated...

 

So you did a '40km' race and only measured 35km?

setting a smaller value just because someone else measured his tyre at that is

a)unscientific

b)the wrong direction

 

you've done it correctly - i would ask someone who did the same race to give you their reading first before you start a wild goose chase. only other guess is that your sensor isn't close enough to the magnet, and doesn't read every revolution at high speeds or something silly like that.

Edited November 14, 2011 by SteveCT

[eheather]

SteveCT,

 

Point taken, I am however quite meticulous about my magnet being close to the sensor as the Polar is rather sensitive, therefore I have ruled out this error.

 

This past weekend after the last two Trailseeker Events showed different results to the actual distance, I also went the route as to ask people about their own calculations on the 94.7 MTB Challenge. For instance, a friend of mine with the same wheel size, rim & Polar got the same 46km instead of some guys measuring 53 & 56mm respectively. The organizers mentioned 53km at the start.

 

I know what you say, however I am still stuck at which way to go...

 

I also agree my initial process was the most logical / scientific way, but why the big discrepanies? I will ahve to do a lot more investigation into this!

[NINER_boy]

Another suggestion:

 

Pedal next to friend of yours with a Garmin (or something with GPS linkage), and compare the speed.

km reading should be the same, and little variance on the .00 rwadings acceptable.

 

Good luck

[Vinnydabutcher]

I measured mine at 2300mm.

 

That was on Canis 2.2 - seems quite accurate - was out by 1km at van Gaalens.

 

Hope it helps.