Which Circumference do you use on the speedometer (Cateye Strada Wireless) when running a larger rear tire versus a smaller front tire (25 verus 23)? Does the larger tire determine speed, distance, etc. over the smaller tire?
Thanks, George
The tire on the wheel you are metering decides it.
These devices work by counting revolutions of the wheel and turning that into distance. So you need the circumference of the wheel you're using the device on.
answered Feb 1 '19 at 9:16
Assuming your bike computer is the sort that reads wheel revolutions using a magnet on a spoke, and a pickup sensor on the fork, then that's the only wheel that matters.
Test - Imagine a bike with different sized wheels - say a 16 inch wheel with 1.5" tyre, and a 700c. The 16" has a circumference of about 1185 mm, and a 700c wheel with a 28mm tyre has 2136mm circumference.
So the back wheel will roll around once, and the front would roll 1.8 times. If your magnet sensor was on the front wheel you would be overreading by 1.8x if you used the back wheel. Likewise you'd underread by 0.556x using the front wheel circumference on the back.
In reality those sensors are notoriously inaccurate. You can do a rollout test counting how far the bike goes in 3 ~ 5 wheel revolutions, but its still up to 5% out. So I tweaked mine till it gets about what strava returns, my 26" wheel measures 2030mm, but 2020 is what matches.
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This makes me wonder how accurate the sensors based on accelerometers, like the ones Garmin make for cranks and hubs, are. – Gabriel C. Feb 1 '19 at 19:37
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@GabrielC. I'd expect garmin to have invested in R&D to make better products than my $5 bike computer. Note those sensors are for power measurement, and less for calculating speed although the head unit will doubtless take them into consideration. – Criggie♦ Feb 1 '19 at 22:45
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No, I'm talking about these. Speed and cadence sensors, I have them along with the older magnet-based one. – Gabriel C. Feb 2 '19 at 0:07
