Originally Posted by
ItsJustMe
Unless they have some unusual sensor on the wheel, it only gets one pulse per revolution. One revolution on a 700c wheel is about 2100mm or about 7 feet. 3 digits means that last digit is measuring a distance of 5280/1000 = 5.3 feet.
You're measuring 5 foot distances with a 7 foot stick. It's going to be jittery at best. on subsequent revolutions it'll go 7,14,21,28,35, 42,49,56 feet, which will translate to:
0.001, 0.002, 0.003,0.005,0.006, 0.007,0.009, etc
You have a cumulative error that's a significant percentage of the indicated value, so either that last digit is unneeded (which is probably the case, in which case why show it) or it's inaccurate (usually off by a large part of its indicated value) in which case why show it)
My bike computer requires the total mm of the circumference the wheel + tyre. As ISO chart is included is the user is lazy but one can also use a piece of string, which I did and I think it came out to 2096mm. When I change tyres, I recalibrate. I also compared the tyre that I got about 4000km out of an measured a 2mm difference in circumference (2 / 2096 = 0.95%), which I'm OK with.
http://202.215.251.86/data/resources...e_chart_v2.pdf
So I don't really understand this whole measuring a 7-foot length with a 5-foot stick, as the computer will also keep track of fractional units as well.