Bell Spinfit computer wheel setting for 26x2.1 tire

Mark Wolfe said:

Hello,

I have a bell "spinfit" bicycle speedometer, and the closest setting the
manual lists is for a 26x2 tire, with the setting 2099. I believe the
setting is supposed to be diameter in millimeters X Pi. It has various
settings for 26x1.5, 26x1.x, etc.. I suppose the wider tire makes it stick
up a little more in the center? Should I go with something just a wee bit
larger than the 2099?

Thanks a lot!

Dear Mark,

Let's not bandy loose expressions like "just a wee bit
larger" around on rec.bicycles.tech.

Lay a ten-foot tape measure on a smooth stretch of
concrete. Line up a spoke, a mark on the tire, or whatever
strikes your fancy with the beginning of the tape. Roll it
next to the tape measure until the mark comes around again.

(Ignore the neighbor's snickering children, who don't
appreciate accuracy.)

Your mark should be sitting at a bit under 83 inches. Don't
bother to try to measure to more than an eighth of an inch
(0.125"😉.

Repeat this a time or two to convince yourself that the
measurement will stay about the same.

Multiply inches by 25.4 to convert to millimeters.

Worry about whether your weight on the tire will change
things. Realize that the difference is insignificant. Same
goes for whether it was 83 inches or 83 and 1/8th.

81.875" = 2079.6mm
82.000" = 2082.8
82.125" = 2086.0
82.250" = 2089.2
82.275" = 2092.3
82.375" = 2095.3
82.500" = 2095.5
82.625" = 2098.7
82.750" = 2101.9
82.875" = 2105.0
83.000" = 2108.2

Either enter the exact value, rounding as you please, or
pick the closest value allowed by your speedometer.

(Come the revolution, they'll let us enter values in tenths
of a millimeter!)

If you're off by an eighth of an inch (horrors!) and use
2092 instead of 2089, you'll be three miles off after about
2,100 miles--about 0.14% error.

Most speedometers read in half-mile per hour increments, so
you're going about 20.0 mph, plus or minus half a mile per
hour or 2.5% either way.

This means that your eighth of an inch margin of error
measuring the tire circumference is much less than a tenth
of the best accuracy that the speedometer can offer.

Obsessively yours,

Carl Fogel