Landracing Forum
Introductions => Formulas => Topic started by: landracing on March 21, 2008, 09:16:32 AM
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Here is a gear calculator for motorcycles, Have never tried to input for a car.
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Note: To measure tire circumference roll bike and measure the length of one revolution of rear wheel. Then divide measurement by 3.1416 to calculate diameter.
I've gotten much better acuracy by measuring from the axle centerline to the bench with the bike loaded to get diameter as it takes the weight as well as the air pressure into effect. Then in a calc like this, multiply that number by 3.1416 to get the required circumfrence.
I've even gone as far as measuring my suspension travel in play, and then strap the bike down on the lift to that suspension compression and measure the tire there... the difference was minimal but was more accurate in the end result.
Todd
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Todd, thinking a spinning tire grows a little, and compressed radius might short ya a little, but make up for some of the slip at Bonneville... :roll:
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http://www.rbracing-rsr.com/gearspeed.html
I've used this one for Harleys and it's been right on.
Larry
Oh yeah. I couldn't open Jon's for some reason.
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Good point,
I don't agree that ALL tires grow though, the numbers show the oppositte sometimes(at least on our slow bike - lol). Deb's bike is in the "actual diameter" group while mine is in the "reduce diameter" group.
I just use numbers that I can justify a reason for and are repeatable and accurate with the tower numbers(no matter how I come up with them, though the axle down measurement seems the best overall and most accurate for me so far).
The suspension travel I'm refering to, is referenced at the traps. This is the closest that our calc that Deb and I made comes. We figured in a tire wear & tire growth area also(can be used to compensate for tire or even clutch slippage as well).
Todd
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Todd, thinking a spinning tire grows a little,
I don't agree that ALL tires grow though, the numbers show the oppositte sometimes(at least on our slow bike - lol).
Maybe I should retract that until my bike's actually able to spin the tire :-D
Todd
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Larry:
It probably won't load if you don't have MS Excel loaded on your computer.
Pete
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Thanks for the Gear calculator chart Jon. It downloaded ok for me but I have a quick question for anybody. The illustration showed an 18 tooth drive sprocket on all the examples. We are running a 22 tooth on the drive side and are playing with a 40 - 38 - & a 36 on the rear that we have sitting on the bench. My question would be. What would the actual formula be for calculating different sprocket combinations. I can see where this would help save time and engine ware from experimenting without the formula.
Respectfully
Uncle Jimbo
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In the upper left hand corner of the chart change 18 to 22 and everything happens!
Pete
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For those not familiar with Excel, what's nice is if you put the cursor in a cell watch the command line. If numerical data appears, you can change it and watch all new results appear all over the table automatically (if a formula shows, leave it alone!).
I color-code the font in my charts to indicate which is the "master cell", where input is variable - the "slave cells" will reproduce this without prompting, and are colored differently. Output values are also coded for easy recognition.
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Here's one for the common H-D big twin 4 & 5 speed boxes that just does RPM drops on shifting, with the correct tooth counts already input (10 ratio choices).
Shows % gained & lost on upshift, new RPM after shift, and lowest RPM reached on full upshift.
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I will re-format the sheet to show color coded user input fields. Usually I do that anyways but this one it was not done.
Jon
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Warning: on my downloadable .xls you can input new tooth counts in Columns B & C (even though they're not coded as variable), but only if the transmission is "conventional" where the high gear is locked to the output shaft (direct) in high gear. All-indirect transmissions will need some adjustment to get good output.
If you're trying to make a new one and have trouble, e-mail me I might be able to sort it out.
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Thanks Pete ! That is way cool, I would never have thought to try that. I am a bit of a cyber neanderthal, and not afraid to admit it. Now to get the printer warmed up and make a few different calculations.
Thanks again.
Uncle jimbo
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Added primary drive ratio, front and rear sprockets, tire OD, and speed in MPH in each gear to the download file (above).
If you already got one, just download again and it will replace the older file.
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Thanks Panic I don't know how I did it but the file that I got downloaded is written by Tom Nichols. I substituted the sprocket numbers, tire dia. and bingo everything is there, speed at projected rpm and even rpm's at a given speed. This saves us tons of guess work.
You guys are great. Thanks You made this cyber neanderthols day.
uncle Jimbo
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I know that there have been several calculators listed. Here's one more. It can be opened from the website, to memory. Or, can be downloaded for use offline. No installation needed.
http://www.terra-glide.net/pgms/WheelSpeed.exe
Jack
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Jack that is a cool calculator.....but it is missing 5th and 6th gears!
:-D
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Jack that is a cool calculator.....but it is missing 5th and 6th gears!
:-D
Jonny -
Here's a little beta program in the works: http://www.terra-glide.net/pgms/LSRutil.exe . See if this takes care of the missing gears.
Jack
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Usually I remember my engine rpm through the mile during my first run and I calculate how fast I should go assuming no tire slip. Then I look at my timing slip and see how fast I actually went. The actual speed divided by the calculated speed is my combined slip/tachometer inaccuracy/tire growth, etc factor. It is specific for my bike at that meet. I make further gearing calculations and adjustments using that slip factor. All works OK as long as the salt conditions stay the same as they were during my first run.
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Here's a few sheets for the 2 or 3 of us that don't use sprockets.
I don't remember where I got this, but if it's yours and you don't want this posted, lemme know and I'll take it down.
Fun to play mix n' match.
Tonerjockey
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Motorcycle Gear Calculator Part 2) This adds a little more to my post yesterday. A club member asked me about the gear calculator and the tire slip I would expect at 170 to 180 mph. He has not built the bike yet, but he is figuring out if he can afford to make it achieve the record he wants. He is a smart fellow to think before making the plunge. I never use that kind of logic and I pay the price in lots of hours and $.
My bike puts down a mighty 50 horses on a bad day and maybe 60 on a good'un. Naturally, I do not know about tire slip or 170 -180 mph. The fellow that asked me the question is reading this, if anyone sees errors in my thinking or has anything to add, type up.
As mentioned yesterday, I put a combined factor in the gearing equation to account for tire slip, tire growth, tachometer error, etc. It is based on the calculated speed versus the measured speed through the mile. My tachometer is not calibrated so I do not know how much of the difference I see is tire slip or tach error. My factors are good for comparing runs and setting my gearing, but not much else.
The tach error factor can be neglected for theoretical calculations. My bike runs relatively narrow steel belted radials. I do not see how they can expand. The belts would not allow it. I do see, in my imagination, how the flat spot at the bottom of the tire where it contacts the pavement can disappear it high speeds. Centrifugal forces in the spinning tire would make it more round. I always assume the tire is fully round in theoretical calculations. Tire slip is where I do not know much. I figure my gutless wonder is good for 5 percent slip, maximum, in theoretical calcs. This is about all I know.