Originally Posted by
Tourist in MSN
Hmmmm, first you take one number and key it into a calculator, ... ...
The difference is about 2.6 percent. Is that enough to worry about or not, your call. If one wheel is always right the other wheel would be 2.6 percent off.
I personally would want better accuracy than that for distance.
But, for speed that is close enough for me. A lot of my computers that I have used over the years rounded off to the nearest half MPH, I suspect for you they would round off to nearest half km/hr. At 19 km/hr, if it is off by 2.6 percent it would read almost exactly a half km/hr off, either 19.5 or 18.5. I would not notice that kind of error for speed.
When touring, I use both a computer (one that is over a decade old) and a GPS. I mostly use my computer for cadence or speed measurements. I almost always have my GPS screen on the map with no other data. But I toggle to my GPS data screen for distance measurements, both distance traveled that day and distance to destination. And sometimes estimated time to destination. My GPS does not have a pressure sensor in it, so I usually disregard any elevation related data in my GPS as inaccurate, but I have it configured to show elevation.
Sometimes my wireless VDO computers have added extra distance for no reason that I can figure out. So, I am less likely to look at their distance data, that is part of the reason I am more likely to look at my GPS for distance data. My older Sports Instruments (defunct company) wired computers were always spot on for distance but they lack cadence data.
When in a country that uses km instead of miles, I re-configure my GPS and computer to use the local units, I want my electronics to match the units on the road signs.
Or maybe plan B. If you configured your computer to the mid-point between the two wheel sizes, then both wheels would have a 1.3 percent error, one reads high the other reads low. At a 1.3 percent error, that becomes almost insignificant, I could live with that much error for both distance and speed.
Funny, that's the plan I've decided on. I posted this same question in the road cycling sub forum and someone suggested it. That idea gives a variance of 1.3 which seems acceptable. As I said there, the way I use a computer for touring around here is pretty basic and I only need to know distances for navigation within km's not meters.
FWIW, this is the maths I came up with
One revolution is 2100mm = 210cm's.
1km = 100000cm's.
100000/210 = 476 revolutions.
27.5mm = 2.7cm's deviation per revolution.
476 revolutions x 2.75cm's = 1285cm's
1285cm's = 12.85m
So... if my thinking is correct, I would have a discrepancy of 12.85 metres for every 1 km travelled.
or.. 12.85 meters x 100km's = 1.285 km's per 100 km's traveled.
If that is correct, would it also stand to reason that if I were travelling at 25km's per hour I would have a speed discrepancy of .32 km's per hour?
1.285 meters per 100km's / 4 = .32.