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Wheel Circumference vs Strava
I've been using a little Pro wireless computer for about five years. Last year I got a good Lezyne track pump and found that my tyres now at 110psi were bigger and therefore had a great circumference. I dutifully laid out a tape measure and tried to get an accurate measure by wheeling the bike along the hall a number of times. So my computer was set to 2135mm. This summer I started to use Strava, mainly to record my route and have a bash at segments and see how others are doing. Strava gave me my climbing figures which I though was smashing (like a jaffa cake.) But I was still using the computer for my distances.
Now last week I did the first metric for the road.cc metric Challenge. I followed my carefully planned route and got back to my front door hungry (jelly babies and gels exhausted,) tired, painful cold feet, and two punctures down; to find my computer said 100.8km and strava only 98.1km. So I went out again to go around the block and come back with figures on the computer 103.5 and Strava 100.7 Here: https://www.strava.com/activities/248656576
Satisfied I had actual done a metric I had a look online on calculating wheel circumferences as I wanted to know why there was this discrepancy. Various websites gave a calculation from 2096 for a 70x23 tyre up to 2125. So perhaps I was overestimating my distances. However I have noticed sometimes Strava zigzags all over the route and doesn't always accurately record distances.
So how do you decide your tyre size? Just go with the lowest website value just to be sure?
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.
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7 comments
Thanks for great replies (when I though I wouldn't get any due to more popular troll threads.) I think I will go with the conservative 2096 measurement. It probably doesn't matter that I was overestimating in the past, it just makes it 2% harder to beat my previous distances, and given I am planning on going from 7000km to 8000 this year anyway then thats no problem.
I did a 160k audax the other week. Did slightly more than the 160 to make sure I did 100 miles. Downloaded in to Strava from my Garmin- 99.9 miles . Gutted.
I'd treat gps mileages with a little scepticism - I often record a ride both on a phone and a garmin, and every time the garmin ride comes up longer, by around half a mile. The trace on the maps for both devices is always the same. Climbing figures are always way out!
Park your GPS device on a table on the lawn recording and see how far it goes in an hour. My GPS phone did over a mile. At 10mph average speed that's a 10% inaccuracy. At 20mph it's 5%.
2135 sounds high. Remember to load the wheel as you roll it to mimic rider weight, or sit on it if you can, then should be pretty accurate. Tyre pressure will also make a difference, if you don't check every ride. Mine work out at 2095 to 2100 and agree with Strava to within 0.5%. As above, always add a little on, just in case.
GPS is not Accurate to the centimetre. I think at best its 3 metres is GPS. or 3 ft. Depends on variables.
Always do a bit extra for Strava, even if the Garmin says 100 give it a bit more. GPS signals bouncing off buildings, obstructed by trees or just poor reception means it's always variable.