View Single Post
Old 05-06-2010, 11:57 AM
meowy84 meowy84 is offline
Motorized Bicycle Elite Member
Join Date: Jul 2009
Location: Canada
Posts: 238
Default Re: digital tachometer

Originally Posted by wildemere View Post
Far out man, you have typed a novel, and all of your steps and numbers are wrong, I have attached some spreadsheets (not my own) to show you the correct maths,

For a HT

Initial engine ratio is 4.1:1 20t drive, 82t driven

Secondary is usually 4.4:1 10t drive 44t driven

It equals around an 18:1 reduction ratio. I like an 16:1 ratio on my single speed bikes.

Normal 26" bike wheels are usually between 26 oto 27" high, the bare rims are around 23"

Measure it all up and a speedo is a tacho.

A stock 26" HT setup turns around 5800rpm at 25mph. Not 6959 or any other random number.

LOL I did type a novel didn't I? Didn't think it would take that long to go through and explain all the steps.

HOWEVER MY MATH AND STEPS ARE 100% ACCURATE!!. I dare you to point out a step where I made a mistake. 6959prm IS NOT a random number. All my conversions are correct.

Like I mentioned in my 'novel' I didn't have my bike or engine in front of me so I guesstimated the numbers of teeth in the gear train and also the diameter of the tire/rim. Those are the only numbers that are not correct because they were estimates and they will vary on your bike based on what tires you're runnning and what sprockets you're running. 6959 is the rounded rpm you would obtain based on the figures I provided. Put the gear ratio and rim/tire #'s I provided in your spreadsheet and see what comes up. Anyone can plop numbers into a magical formula. To make things faster absolutely use the formula but I thought some might actually have enough curiosity to want to know the logic and steps in calculating the rpm so they could understand the process. I apologize for my rogue attitudes and for making anyone's head hurt from all the math.
McGyver could fix that with bubble gum.
Reply With Quote