How to Calibrate your Speedo Correctly

Discussion in 'Motorized Bicycle General Discussion' started by Derks420, Jul 31, 2010.

  1. Derks420

    Derks420 New Member

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    Hey guys, I see alot of people making speed claims that are CRAZY! I am positive that may have not set up their speedo right. Here is the best way to find the size of your tire.
    1. Take a piece of rope and tape the end on the tire.
    2. Place tape every 8 to 10 inches until you have wrapped the ENTIRE TIRE (circumferance) once.
    3. Measure the length of the string in inches.
    4. Multiply your length by 25.4 ( 25.4mm=1 inch)
    5. When you program your speedo, put the number that you got in as the wheel size in mm.


    Example.
    My tire is 77 inches in circumferance. So I multiply 77x54.4 which is 1955.8mm. When I programed my speedo, I put it in as 1956. Now acurate within 1/2 mph.

    Good Luck
     
  2. weekend-fun

    weekend-fun New Member

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    cool thanks man
     
  3. Kevlarr

    Kevlarr New Member

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    Easier way.

    Put a drop of any kind of liquid on your tire (3&1 oil, grease, maple syrup, etc), get on your bike and roll it forward with your weight on it. Measure the distance between the dots on the ground, convert to mm and enter that number in as the wheel value.
     
  4. Derks420

    Derks420 New Member

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    Then you would need a clean surface to roll on, and should prob do 2 or 3 revolutions. then measure and use the average cuz oils smears and wouldnt be as accurate as the string...
    But Either way will do
     
  5. scotto-

    scotto- Custom 4-Stroke Bike Builder

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    Yes, or mark your tire with chalk and chalk the pavement at the same point and with the help of a friend have them steady you and roll forward until one revoluton and mark the pavement then measure the distance between the two marks. The rolling circumferance in inches or millimeter and input for the calibration. Same, same and same. All the methods work.
     
  6. Patr1ck

    Patr1ck New Member

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    Would having the weight of the rider on the bike when doing these methods be more accurate, since the overall diameter of the tire would be less than unloaded?

    Pat
     
  7. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    This is the best and most accurate way. Oil will spread, not to mention what it will do to your tires, string can be stretched.
     
  8. AslansMonkey

    AslansMonkey Member

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    Good suggestions. Personally I just lay a tape measure on the ground and line up the stem on the front tire so that it is perpendicular to the ground and at the beginning of the tape measure. Then I roll out one revolution and get the reading right off the tape measure.

    But the idea is to enter the actual tire circumference in millimeters into your cycle computer. Not all tires are created equal so using a general "guide" isn't always accurate.

    I suspect some of these speed claims are made with mechanical speedo's which aren't even close to accurate.
     
  9. severedcorps

    severedcorps New Member

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    on my speedo it only asks me for the wheel size, in my case 24 inches..

    my speeds seem accurate..
     
  10. AslansMonkey

    AslansMonkey Member

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    It's possible that it's close. The faster you go the more you'd notice any discrepancies. Because not all 24" tires (or any size for that matter) are created equal, however, the stock tire size entry in a cycle computer is a shot in the dark at best. The most accurate way to set a cycle computer is using the roll out method mentioned above.
     
  11. Derks420

    Derks420 New Member

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    Agreed. Any of the above methods will work
     
  12. severedcorps

    severedcorps New Member

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    This is good info.. I'll recalibrate when I get my new wheels, but until then I'm leaving it where it is.

    1916 is default for 24 inch tires on my instructions
     

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