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Old 05-03-2010, 08:47 PM
meowy84 meowy84 is offline
Motorized Bicycle Elite Member
Join Date: Jul 2009
Location: Canada
Posts: 238
Default Re: digital tachometer

Wildemere don't forget that he also has to take into account his overall gear ratio to use his speedo for a tach. Our HT engines are not direct drive meaning the engine doesn't make 1 revolution for 1 tire revolution.

This is gonna take some math. There's a simple formula to calculate your rpm for a given road speed where you just plop in your numbers and ta-da but you still need to know your tire diameter and your overall drive or gear ratio. I don't have the formula in front of me but I'll walk you through the steps to calculate the rpm based on the road speed anyhow without the use of a formula. At least this way you'll know where all the values come from and how to logically apply the same steps to any vehicle.

Step 1. First, you need your tire size. If you're using 26" rims do not input 26 because you want the diameter of the tire and rim not just the rim since you're not running just on rims alone. This will vary depending on what tire you're using. Measure the diameter of the tire on rim. You might end up with say 29". Then use the pieXdiameter formula which will tell you the circumference of the tire or in other words what distance your tire is covering in one revolution. So for one revolution of a 29" tire your tire covers (3.14)X(29")=91.06".

Step 2. Figure out your 1st drive ratio. You have to count the teeth on your gears. Let's say you're using a 44 tooth rear sprocket at the rear wheel and the little drive sprocket at the engine has 11 teeth. That's a ratio of 4:1.

Step 3. Then you figure out the 2nd gear ratio inside where the clutch is. I don't have my engine in front of me so I can't give you the numbers but let's say the big gear where the clutch pucks are has 60 teeth and the small gear that drives it has 10 gears. That's a 6:1 ratio.

Step 4. Now you multiply these ratios to give you the overall drivetrain or reduction ratio. So 6x4=24. So your overall drive ratio in your entire drivetrain is 24:1. Which means that for 1 tire revolution your engine rotates at 24rpm.

Step 4. If you use miles/hour for your speed then you have to convert the distance your tire covers in 1 revolution from inches to miles. The conversion is 1"=0.00001578 miles. So if you recall for one revolution your 29" tire covers 91.06". So (91.06)X(0.00001578)=0.001436miles.

Step 4. Let's say you're riding at 25mph. Then your tire would have to rotate a total of (25)/(0.001436)=17398.24 times in that hour. Since we measured your engine speed in rpm (revs per minute) we have to convert the bicycle speed from from mph to miles per minute so the units are the same. So dividing (17398.24)/(60 minutes in an hour)=289.97 which is the number of revs your tire does going at 25mph. Now remember that 24:1 drivetrain ratio? Now all you do is multiply the 289.97 by that ratio. So (289.97)X(24)=6959 rpm AND YOU'RE DONE!!! So for the gear ratio given and teeth numbers given at 25mph your little China Girl would be turning 6959rpm.

ARE YOU GUYS STILL AWAKE? LOL The good thing is that you only need to calculate the drive ratios once and the distance your tire covers in one revolution once as well since those all stay the same and just use Step 4 and substitute different speed values to get your rpm at that speed. Once that is done you can write the values down somewhare and you'll know that if say you're going at 25mph you're turning 6959rpm. If you want to know what you're doing at 15mph or 20mph or 30mph use Step 4 to calculate your rpm and so on and so on.
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