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Old 09-03-2009, 08:36 PM
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Russell Russell is offline
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Join Date: Apr 2009
Location: MA
Posts: 1,144
Default Re: Why I don't trust mechanical speedometers.

Originally Posted by Finfan View Post
OK, I am basing this solely on the cheap Taiwanese speedometer that I got from Spooky Tooth. If you check the picture below you will see that 200 rpm equals 21 mph so I'll start from there:

200 rev/min = (21 mi/hr) / (60 min/hr)

converting hours to minutes gives:

200 rev/min = 0.35 mi/min


200 rev = 0.35 mi

Doing some unit conversions:

200 rev = (0.35 mi) * (5280 ft/mi)
= (1848 ft) * (12 in/ft)
= 22,176 in

Now divide both sides by 200 :

1 rev = 110.88 in

One revolution is equal to the circumference of the wheel and the circumference of a circle is equal to pi times the diameter so the diameter is equal to the circumference divided by pi:

diameter = (110.88 in) / (3.14159)
= 35.29 inches

So unless you have a mighty large front wheel these speedometers are useless! Likely they are a kids toy designed to make them think they are going a lot faster than they actually are.
I took mine apart and shortened the hair spring up until the speedo was accurate between 20 and 30 mph.
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