General Discussion Gear Reviews & Questions » Wheel circumference help!!! Rss Feed  
Moderators: the bear, Ron Reply
2012-09-19 5:26 PM

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Regular
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2525
Cairns - Australia
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Subject: Wheel circumference help!!!

my bike tyres are 650 x 23c - I have a Sigma Bike monitor which I have used for a long time and my distance riden is pretty accurate.  I have just set up my Timex GPS watch to be able to monitor my distance etc as well.  This is my problem.

Today I rode by bike and my Sigma monitor said I did 36 km, my Timex watch said I did 69km ( I wish), I have set my wheel circumference on my Timex to 1950 (I googled this figure) - could someone please explain where I'm going wrong here?  Many thanks Liz



2012-09-19 7:57 PM
in reply to: #4419337

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Veteran
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Hudson Valley
Subject: RE: Wheel circumference help!!!
I'm guessing the actual calibration number you entered on the GPS unit was 1050 because 1950/ 1050 is very close to 69km/36km within a few percentage points.
2012-09-19 11:32 PM
in reply to: #4419515

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Subject: RE: Wheel circumference help!!!
Hi Six000MileYear , I just checked that I hadn't typed 1050 in my wheel cirmcumference and no I had put in 1950 - any other suggestions? Thanks Liz
2012-09-20 6:54 AM
in reply to: #4419337

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Pro
5236
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Greater Pittsburgh
Subject: RE: Wheel circumference help!!!
The circumference of a 650x23 wheel should be 1938mm

The calculations looks like this:
Rim, 571mm (ISO nominal)
Tire, 23mm

Total wheel diameter, 617 (571+23+23) times Pi, equlas 1938.36mm

There's something obviously wrong with your Timex... have you tried calling (or email) Timex customer service? A GPS unit should not be off much at all (you will have an error margin as if you go under tree cover, underpasses, etc. basically anywhere where you lose the GPS signal, the unit will calculate the distance based upon last known location, first new location and the average speed you traveled before signal was lost; obviously this will be wrong, but usually only by a small margin)
2012-09-23 5:36 AM
in reply to: #4419870

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Cairns - Australia
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Subject: RE: Wheel circumference help!!!

Thank you I will put in that wheel circumference and will check my gps, although it works ok while running.

I went for a ride today with 1950 and it said I had done 111 km but I had done half of this distance, so confusing!!!

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General Discussion Gear Reviews & Questions » Wheel circumference help!!! Rss Feed