Topic: Acewell digital speedometer wheel circumference calculation

[evanphi]

For those with acewell digital speedos, this is how you calculate the front wheel circumference when using the ANALOG-DIGITAL cable. If you are using the magnet method, just input the actual circumference in mm.

Something to keep in mind: SOHC Hondas all have the same speedo drive ratio. 2240:60. This means that the speedo drive turns 2240RPM when the bike is going 60MPH on stock wheels. Converted to metric, that is 2240:96.56


ALL OF THESE CALCULATIONS ARE DONE IN METRIC. USE MM and KM. KEEP AN EYE OUT FOR WHERE THE CONVERSIONS ARE MADE.


First, find the circumference of your TIRE as it is on the wheel. measure from the centre of the front axle to the furthest tire edge that touches the ground. Multiply by 2 and then by Pi (3.14159265359). For me, my circumference was 2075mm, or 0.002075km.

Another way to calculate circumference is use the following tire size calculator and then convert inches to mm: http://www.csgnetwork.com/tiresizescalc.html. Stock CB750 front wheel is 100/90-19 tire size (3.25" on a 19" rim). Circumference is 2081.66mm.

Now you need to convert this to Revolutions per Kilometer. One revolution is equal to the circumference, and this is also the distance travelled, thus 0.002075km/1Rev. Invert this to get revolutions per KM! Use the "x^-1"  or "1/x" key on your calculator. You should get a large, positive number. WRITE THIS NUMBER DOWN.

Now you need to find the Rev/km for the speedometer drive. Converting involves changing from RPM to RPH, and cancelling out the minutes and hours. This will give you Rev/Km. See image below. The result should be 1391.88Rev/km.

NOW...

Divide the Rev/KM of the WHEEL by the Rev/KM of the SPEEDO DRIVE. This will give you the magic ratio of how many revs the drive makes per wheel rev. WRITE THIS NUMBER DOWN.

Multiply your tire circumference by this number. The result will be the number you will input into the digital speedometer as your circumference.

In my case, my distance from axle to tire edge is 330.20mm (AKA 13"). This gave my circumference as 2075mm. My final, resulting, calculated circumference that I put in my speedo is

DRUMROLL PLEASE...


719mm

[raymond10078]

This method will net you some inaccuracy.  The "rolling circumference" will always be less that a measured circumference.  This means that your speedo will indicate a slower speed than you are actually travelling.

I have personally found that the best method (meaning most accurate) is to mark the tire, mark the starting point, move the bike forward at least ten tire revolutions, and mark that point.  Measure and divide by the number of revolutions.

[evanphi]

In my testing against my GPS, I was within 1-2KM/H. For the average road user, this is acceptable.

Really you can use whatever method you want to obtain the circumference, and this is reduces possibility for error in your method, such as turning the wheel, etc.