badwolf
Serious Thumper
Offline
Ridin' my Lil' Bagger - 153k miles so far
Posts: 802
Palm Beach County, FL
Gender:
|
Alright, we have no stock tach on our bikes. Folks have posted charts to tell us our speed vs. rpm in all gears. But if you, like I have changed final drive ratios, and or tire sizes, you just have to make a guess at your rpm at speed. NO MORE! I have done the math to get the ''magic numbers'' for each of the 5 gears.
1st - 44535 2nd - 30200 3rd - 21790 4th - 18349 5th - 16820
They don't make any sense on their own, but here is how to use them. First, measure the circumference of your back tire. Blow it up to normal pressure, and have a friend layout a tape measure 7 feet. While sitting on your bike, start with the valve stem straight down at 0 on the tape. Roll forward till the valve stem is all the way around to bottom again, have your friend read the number, this is the circumference of your back tire, write it down. Second, write down the final drive ratio on your bike. If you are running stock pulleys, they are 23 and 68 for a ratio of 2.96 to 1. Third, divide the tire's circumference in inches to the second decimal point by the gear ratio. Write this number down. It is the key number for your bike as setup. Example= My bike's tire is 73.50'' and my final drive ratio is 2.60 (Kaw. pulley conversion) giving me a key number of 28.27. Now divide each gear's magic number by your bike's key number. This will give you the engine rpm at 10 mph in each gear. Example for my bike is - 5th gear- 16820 / 28.27 = 595 rpm at 10 mph x 6 = 3570 rpm at 60 mph Do each gear, Example for me- 1st = 1575 2nd = 1068 3rd = 770 4th = 650 5th = 595 You can now multiply these numbers to see your rpm at different speeds for each gear, or divide your selected rpm by these to see your speed at that rpm. Example= Say I want to shift at 4,000 rpm, 1st - 4,000 / 1575 = 2.53968253968, or basically 25 mph 2nd - 4,000 / 1068 = 3.745, or 37 mph 3rd - 4,000 / 770 = 5.194 or 52 mph 4th - 4,000 / 650 = 6.153 or 62 mph
Now you can ''try'' different final drive ratios or size tires by making a new key number. My bike's stock tire was just 73'' and with the stock pulleys at 2.96 to 1 the key number was 73 / 2.96 = 24.66. So at 60 mph my old rpm was 16820 / 24.66 = 682 x 6 = 4092 My current key number is 28.27 - 16820 / 28.27 = 595 x 6 = 3570 So the Kaw pulley conversion and tire change has dropped me 522 rpm at 60 mph cruise. (Hey, I'm old and live in south Florida = no hills or twisties. I am on the Superslab a LOT and always have to throw away rear tires with the sides looking like new.) Now you can make-up charts and graphs CORRECT FOR YOUR OWN BIKE till your heart is content. This is in relation to REAL SPEED, not the stock speedo. Get a GPS app for your phone and you can see how much your stock speedo is off, but that is for another post.
Disclaimer...I have worked the math several times for this, and it will give you a good idea of the numbers for your bike. If someone wants to go to 29 decimal points and re-figure it they may find it could be off by a fraction. BIG DEAL!! We aren't plotting the path of the Space Shuttle here. I am giving you a way to figure how changes in your final drive and tire size will affect your rpm on YOUR BIKE.
Best wishes, Badwolf
|