Top Speed and Math Just For You

tigmaster · 06-10-2007, 11:06 AM

Ok, so I have noticed that many people are not only unclear on what the speed of their bike is and they have now way to find it except to guess. I sure have seen my fair share of guesses so very extravagant and others a little low. So I figured that it was time to throw some of my knowledge your way and teach you how to calculate the top speed of your bike, or any speed for that matter.

First of all you are going to need to gather a few essential pieces of data. This will be a breeze if you have a shop manual. In other words if you don’t have a shop manual you should buy one because they will always come in handy. If you don’t have one though these numbers can be found by Googling your bike name plus the name of the ratio we are looking for. You will need the primary reduction ratio. This is the ratio between the crankshaft and clutch basket gears. If you don’t know what a ratio is, Google that too. Next ratio to be found is your 5th or 6th speed ratio depending on how many gears your bike has. This is the ratio between the clutch hub and the countershaft, or the shaft that your sprocket is on. Lastly on ratios are the sprockets on your bike(drive ratio). Divide the rear sprocket tooth count by that of the front sprocket count. Say you ran 13 front and 47 rear. 47/13 = 3.6. Ok next you need to multiply the primary ratio by the 5th or 6th speed ratio by the drive ratio. You should get a number somewhere between 8 and 15(this is your total ratio). What this number means is that for every one rotation of the rear wheel the engine has to turn that number of times. So say your total ratio is 9. This means that for every 9 rotations of the engine the tire will turn once. Next thing you need to grab is something called the aspect ratio of your rear tire. It’s that crazy number that looks like 120/90-19 stamped on the side of your tire. The 19 in this example means that your rim is 19 inches in diameter. Now the 120 in this case refers to the width of your tire in millimeters, I don’t know why they chose to use inches and millimeters but they did. Next is the 90, this number is the percentage height of your tire from the 120mm. So in this case the height of the tire is 90% of 120mm or 108mm. We want the diameter so we multiply this number by 2 giving us 216. Now whatever number you got needs to be converted to inches* and added to the diameter of your rim. So in this case we would add it to 19 inches. Ok now multiply that number by pi(3.141592654) and there goes the circumference of your tire in inches. We however want to calculate Miles per Hour though and your tire currently is in inches. This means we need to convert the tire circumference to miles(there are plenty of easier ways to do this, I am just trying to make clear the steps). Take the number you received after multiply it by pi and divide it by 12 to convert to feet and then divide by 5280 to convert it to miles. This number should be a real small decimal like 0.0015678765 or something. Last thing you need to locate is how high your engine revs to. Most MX series 4 strokes rev to somewhere between 12000 and 13500. You can Google this too. Now take the rev limiter of your bike and divide it by your total ratio. Then multiply that number by the circumference of your tire in miles. This number gives you miles per minute. All you need to do is convert to hours by multiplying by 60, giving you miles per hour.

There you go, now you know how to do it. I know some of that may seem complex, but it’s just because of how hard it is to explain to someone how to do something without talking directly to them. I use this calculation everyday and know you will be able to in no time at all. Go ahead, impress your buddies. Remember try to be accurate and use lots of the decimal and a bike hitting its rev limiter is sometimes nearly impossible.

*If you wanted to calculate the speed of your bike in Kilometers per Hour then do not convert this number to inches, but instead convert the diameter of the rim to millimeters. Remember to multiply the 120/90 number by 2 because we want diameter, then add it to the rim diameter in millimeters and multiply by pi(3.141592654). Divide this number by 1000 to receive meters and again by 1000 to receive kilometers. You could just divide it once by 1000000. Now you have the circumference of your tire in kilometers.

06-10-2007, 11:06 AM	#1 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,659 iTrader: (1)	Top Speed and Math Just For You Ok, so I have noticed that many people are not only unclear on what the speed of their bike is and they have now way to find it except to guess. I sure have seen my fair share of guesses so very extravagant and others a little low. So I figured that it was time to throw some of my knowledge your way and teach you how to calculate the top speed of your bike, or any speed for that matter. First of all you are going to need to gather a few essential pieces of data. This will be a breeze if you have a shop manual. In other words if you don’t have a shop manual you should buy one because they will always come in handy. If you don’t have one though these numbers can be found by Googling your bike name plus the name of the ratio we are looking for. You will need the primary reduction ratio. This is the ratio between the crankshaft and clutch basket gears. If you don’t know what a ratio is, Google that too. Next ratio to be found is your 5th or 6th speed ratio depending on how many gears your bike has. This is the ratio between the clutch hub and the countershaft, or the shaft that your sprocket is on. Lastly on ratios are the sprockets on your bike(drive ratio). Divide the rear sprocket tooth count by that of the front sprocket count. Say you ran 13 front and 47 rear. 47/13 = 3.6. Ok next you need to multiply the primary ratio by the 5th or 6th speed ratio by the drive ratio. You should get a number somewhere between 8 and 15(this is your total ratio). What this number means is that for every one rotation of the rear wheel the engine has to turn that number of times. So say your total ratio is 9. This means that for every 9 rotations of the engine the tire will turn once. Next thing you need to grab is something called the aspect ratio of your rear tire. It’s that crazy number that looks like 120/90-19 stamped on the side of your tire. The 19 in this example means that your rim is 19 inches in diameter. Now the 120 in this case refers to the width of your tire in millimeters, I don’t know why they chose to use inches and millimeters but they did. Next is the 90, this number is the percentage height of your tire from the 120mm. So in this case the height of the tire is 90% of 120mm or 108mm. We want the diameter so we multiply this number by 2 giving us 216. Now whatever number you got needs to be converted to inches* and added to the diameter of your rim. So in this case we would add it to 19 inches. Ok now multiply that number by pi(3.141592654) and there goes the circumference of your tire in inches. We however want to calculate Miles per Hour though and your tire currently is in inches. This means we need to convert the tire circumference to miles(there are plenty of easier ways to do this, I am just trying to make clear the steps). Take the number you received after multiply it by pi and divide it by 12 to convert to feet and then divide by 5280 to convert it to miles. This number should be a real small decimal like 0.0015678765 or something. Last thing you need to locate is how high your engine revs to. Most MX series 4 strokes rev to somewhere between 12000 and 13500. You can Google this too. Now take the rev limiter of your bike and divide it by your total ratio. Then multiply that number by the circumference of your tire in miles. This number gives you miles per minute. All you need to do is convert to hours by multiplying by 60, giving you miles per hour. There you go, now you know how to do it. I know some of that may seem complex, but it’s just because of how hard it is to explain to someone how to do something without talking directly to them. I use this calculation everyday and know you will be able to in no time at all. Go ahead, impress your buddies. Remember try to be accurate and use lots of the decimal and a bike hitting its rev limiter is sometimes nearly impossible. If you wanted to calculate the speed of your bike in Kilometers per Hour then do not convert this number to inches, but instead convert the diameter of the rim to millimeters. Remember to multiply the 120/90 number by 2 because we want diameter, then add it to the rim diameter in millimeters and multiply by pi(3.141592654). Divide this number by 1000 to receive meters and again by 1000 to receive kilometers. You could just divide it once by 1000000. Now you have the circumference of your tire in kilometers. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here*

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