Bob Myers Posted October 30, 2009 Share #1 Posted October 30, 2009 Trying to calculate the engine rpm of a bike at a given speed. Constants I know are, primary drive ratio(2.184), 5th gear ratio(.857od), final drive ratio(17/39=2.294), tire diameter(25.5=790 revs per mile). So, would it be as simple as 2.184x.857= 1.8716x2.294=4.293x 790(tire rev at 60 mph)=3391rpms? Link to comment Share on other sites More sharing options...
BigBoyinMS Posted October 30, 2009 Share #2 Posted October 30, 2009 Trying to calculate the engine rpm of a bike at a given speed. Constants I know are, primary drive ratio(2.184), 5th gear ratio(.857od), final drive ratio(17/39=2.294), tire diameter(25.5=790 revs per mile). So, would it be as simple as 2.184x.857= 1.8716x2.294=4.293x 790(tire rev at 60 mph)=3391rpms? All you need is... Tranny final gear ratio (.857) (or ratio of whatever gear you are in when you want to know the RPM.) Tire height (25.5") Final drive ratio (2.294) RPM = MPH (60) x Trans Gear Ratio (.857) x Final Drive Ratio (2.294) x 336.13 / Tire Diameter (25.5) RPM will be 1554.8646 in this example. Link to comment Share on other sites More sharing options...
Bob Myers Posted October 30, 2009 Author Share #3 Posted October 30, 2009 (edited) To get actual engine crankshaft rpm you need to figure in the reduction of the primary drive too. What is the 336 number? This is on a kawasaki and kawi claims the rpm should be about 2950 at this speed. Edited October 30, 2009 by Bob Myers Link to comment Share on other sites More sharing options...
dingy Posted October 30, 2009 Share #4 Posted October 30, 2009 Seems right You might lose some people at the 790 revs of tire per minute. Mile = (5280 ft x 12inches) = 63360 inches per mile / 25.5 (dia. of tire) x 3.1414 (Pi) =80.11 (circumference of tire) = 790.91 revs per mile or 63360/80.11=790.91 Link to comment Share on other sites More sharing options...
Bob Myers Posted October 30, 2009 Author Share #5 Posted October 30, 2009 My reasoning on the 790 is, 25.5 diameter gives us 790.9 revolutions in a mile, and at 60 mph the 790.9 would also be the rpm of the rear tire. Link to comment Share on other sites More sharing options...
dingy Posted October 30, 2009 Share #6 Posted October 30, 2009 Seems like 1554 rpm at 60 mph is low??? That's not much more than a fast idle. I have an 83 and I am around 3000 rpm at 50 mph All you need is... Tranny final gear ratio (.857) (or ratio of whatever gear you are in when you want to know the RPM.) Tire height (25.5") Final drive ratio (2.294) RPM = MPH (60) x Trans Gear Ratio (.857) x Final Drive Ratio (2.294) x 336.13 / Tire Diameter (25.5) RPM will be 1554.8646 in this example. Link to comment Share on other sites More sharing options...
Bob Myers Posted October 30, 2009 Author Share #7 Posted October 30, 2009 All you need is... Tranny final gear ratio (.857) (or ratio of whatever gear you are in when you want to know the RPM.) Tire height (25.5") Final drive ratio (2.294) RPM = MPH (60) x Trans Gear Ratio (.857) x Final Drive Ratio (2.294) x 336.13 / Tire Diameter (25.5) RPM will be 1554.8646 in this example. Using your formula and inserting the primary ratio you left out I get a final answer of 3395.8, which is pretty close to what I came up with. I spose this formula would work well enough with any speed to use it to prove or disprove a gear change. Link to comment Share on other sites More sharing options...
atlm Posted October 30, 2009 Share #8 Posted October 30, 2009 You're missing a variable, but the number you came up with is correct because you happened to choose 60 mph. For mathematical conversions, You need to look at units of measure. You're starting with speed (miles/hour) and trying to calculate RPM (engine revs/minute). Here is the formula: miles/hour * 1 hour/60 minutes * tire revs/mile * engine revs/tire revs = engine rev/minute miles per hour / 60 = miles per minute miles per minute * tire revs per mile = tire revs per minute tire revs per minute * engine revs per tire revs = engine revs per minute You calculated tire revs per minute correctly (790.91). You calculated engine revs per tire rev correctly (2.184 (primary) * .857 (5th gear) * 2.294 (final) = 4.294) The primary is engine-to-transmission (constant), 5th gear is internal transmission (changes with gear), and final is transmission to rear wheel (constant). So for your 60 miles per hour example 60 miles per hour = 1 mile per minute 1 mile per minute times 790.9 tire revs per mile = 790.9 tire revs per minute 790.9 tire revs per minute times 4.294 engine revs per tire rev = 3396 engine revs per minute For other speeds, your formula would be mph*790.91*4.294/60 Link to comment Share on other sites More sharing options...
Bob Myers Posted October 30, 2009 Author Share #9 Posted October 30, 2009 You're missing a variable, but the number you came up with is correct because you happened to choose 60 mph. 60 was chosen intentionally because I had not thought through far enough to arrive at what you did. I knew that wheel rpms at 60 would be same as 1 mile/ 1minute With your even more simple formula I can now be assured that between the 3 metods of calculating presented here that I can get my theoretical engine rpm, since they all agree! Link to comment Share on other sites More sharing options...
Squeeze Posted October 31, 2009 Share #10 Posted October 31, 2009 but you need to compute around 3 Percent Tire Slippage within your Calculation. Link to comment Share on other sites More sharing options...
Bob Myers Posted October 31, 2009 Author Share #11 Posted October 31, 2009 but you need to compute around 3 Percent Tire Slippage within your Calculation. OK, I'll bite. Why 3%? Would this vary with the amount of weight on the rear tire and pressure in the tire? Why choose 3% as a number? In the given parameters above that only equates to 102 rpms Link to comment Share on other sites More sharing options...
Squeeze Posted October 31, 2009 Share #12 Posted October 31, 2009 3 Percent is the commonly acknowledged Number on Tire Slippage at Speeds above 50 mph. Of Course, you're right, with Load, Tire Diameter, Rubber Compound and what not else Parameters come into play here, but if you use the 3 Percent, you won't be far off in your Calculations. It's near enough and better than not using any correcting Factor if you need a Result near Reality. Link to comment Share on other sites More sharing options...
Bob Myers Posted October 31, 2009 Author Share #13 Posted October 31, 2009 Ok, I'll remember that! I was presuming with my calc that something should be there but not knowing what I just called it a "theoretical rpms" . Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now