# Does anyone have this speedometer???

Discussion in 'Spare Parts, Tools & Product Developement' started by hot70cc, Jan 13, 2008.

1. ### hot70ccGuest

I got this off of ebay a while back and relized it only read in KMPH. I never used it bought it about 6 months ago. I wanted to use it for my other bike and i'm having a problem how to program the parameter number. Even though it only reads KMPH. I was wondering if anyone know the number to program in it for a 20" wheel. And if there is an alternate number to put in to read MPH instead of KMPH. I know this sounds confusing, but if anyone has this and knows the numbers that will be a big help. I tried to do it myself with the manual but the number i came up with isn't correct. Thanks

http://cgi.ebay.com/BIKE-CYCLING-SPEEDOMETER-BICYCLE-COMPUTER-ODOMETER-US_W0QQitemZ110213368670QQihZ001QQcategoryZ30108QQssPageNameZWDVWQQrdZ1QQcmdZViewItem#ebayphotohosting

2. ### bigdanMember

speed = d/t

speed is measured in kph

t = time
d = distance

so for 30 kph
t = 1 hour
d = 30 km

distance is calculated by number of wheel rotations WR * wheel circumference C

assuming the loaded wheel diameter is exactly 26" (which it's not but, for the sake of making the math easy we will assume it is) we can calculate the circumference of the wheel.
circumference = pi * the diameter = 3.14 * 26 = 81.64 inches

30km = 1,181,102.36 inches = 18.6411358 miles
wheel circumference = 81.64 inches
# of rotations = 1,181,102.36 / 81.64 = 14467.20

d = WR * c = 14467.20 * 81.64 = 30km

1 Kilometer = 0.621371192 miles
So the magic number MN is .621371193

If we multiply the wheel circumference by 0.621371193 we get
81.64 * 0.621371193 = 50.72874419652 as the adjusted wheel circumference

to convert inches to kilometers multiply by .0000254
put that back in the equation to check our work

Number of rotations * Wheel circumference * Magic number * Conversion factor = MPH

30Kph = 14467.2 * 81.64 * 0.621371193 * .0000254 = 18.6411334

so if you want the speed measured in Kph to display Mph so that the number is correct you will need to multiply either the wheel circumference or the loaded wheel diameter by .621371193

I just checked the date of the original post and it's a bit old. Oh well.