The attached pic is the manual for a 15 buck K-Mart bicycle 'computer'. It's a BELL F20 that attaches to the bars. Its main funtion is a speedometer. There's a 'pick-up' attached to the forks and a tiny magnet attached to a spoke. The computer reads the revolutions of the wheel. As there are a lot of different bicycle wheel diameters the unit is capable of being programed to accomodate a wide range of diameters. So a code is put in the computer, by pressing buttons like it was a digital watch, to match the wheel.
It occured to me that a wheel with a much smaller diameter might work if two, rather than one, magnets were attached to the rim. With 9 possible bicycle wheel diameter codes it does appear to be fine for a couple of common mini bike tire sizes. The codes are the millimeters of circumference the various wheel diameters.
I measured several wheel diameters on mini bikes I own. Two tire sizes seem to be rather close to matching a bicycle tire code if two magnets are used to fool the computer.
A 410/350 X 6 tire is 12 inches in diameter. That's 37.7 inches in circumference. [Dia X 3.1416]. Multiplied by 25.4 [there are 25.4 mm in an inch] equals 957.6 millimeters. Two magnets means multiply by 2, so the final number is 1915 millimeters. The code for a 24" bicycle is 1916. That seems pretty close.
A 410/350 X 5 tire looks to have a diameter of about 10.5 inches. That's 33 inches circumference or 838 millimeters. Times 2 equals 1676 millimeters if two magnets are used. The closest code is for a 22 inch diameter bicycle wheel: 1759. Sounds like a lot of error? It's only about 6 percent. That is a whale of a lot closer that any speedometer on any of my motorycles except for one Harley.
O.K., I know that my stated mini bike tire diameters are not exact. Not only did I not make an effort to be exact when they were being measured but there are also a lot of other variables like tire wear and inflation pressure and the weight of the bike and rider. But if you want a speedo for your mini and wish to see if your tire diameter will, with two magnets, work with one of the available codes I am listing them below.
1596 1759 1888 1916 2045 2073 2124 2155 2237
You do your wheel measuring and the math.
It occured to me that a wheel with a much smaller diameter might work if two, rather than one, magnets were attached to the rim. With 9 possible bicycle wheel diameter codes it does appear to be fine for a couple of common mini bike tire sizes. The codes are the millimeters of circumference the various wheel diameters.
I measured several wheel diameters on mini bikes I own. Two tire sizes seem to be rather close to matching a bicycle tire code if two magnets are used to fool the computer.
A 410/350 X 6 tire is 12 inches in diameter. That's 37.7 inches in circumference. [Dia X 3.1416]. Multiplied by 25.4 [there are 25.4 mm in an inch] equals 957.6 millimeters. Two magnets means multiply by 2, so the final number is 1915 millimeters. The code for a 24" bicycle is 1916. That seems pretty close.
A 410/350 X 5 tire looks to have a diameter of about 10.5 inches. That's 33 inches circumference or 838 millimeters. Times 2 equals 1676 millimeters if two magnets are used. The closest code is for a 22 inch diameter bicycle wheel: 1759. Sounds like a lot of error? It's only about 6 percent. That is a whale of a lot closer that any speedometer on any of my motorycles except for one Harley.
O.K., I know that my stated mini bike tire diameters are not exact. Not only did I not make an effort to be exact when they were being measured but there are also a lot of other variables like tire wear and inflation pressure and the weight of the bike and rider. But if you want a speedo for your mini and wish to see if your tire diameter will, with two magnets, work with one of the available codes I am listing them below.
1596 1759 1888 1916 2045 2073 2124 2155 2237
You do your wheel measuring and the math.
