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It took me a few days, but I managed to crunch the numbers. My approach was thus: calculate the difference in rotational kinetic energy between the stock driveshaft and the aftermarket at 60 mph, and assume this energy is supplied by the engine in 6 seconds. First trick: calculate the driveshaft rpm at 60mph. Easy. I used my 235/45-17 tires, computed the circumference and used the axle ratio of 3.69 (NA). Driveshaft rpm work out to 216. Second trick: calculate the polar moment of inertia of mass for a cylinder, Jm. Used the equation from the 24th edition of Machinery's Handbook: Jm=1/2M(Ro^2+Ri^2). Used 2.95 as the OD of the driveshaft (NA) and assumed a 1/4 wall thickness. Used 21lbs and 11lbs for the weight (don't forget to divide the weight by 32.2 to convert to mass!) Jm=.0083 and .0044 lbs*ft*sec^2 respectively for the weights. Third trick: calculate the rotational energy Ekr. Machinery's Handbook to the rescue: Ekr=1/2*Jm*w^2. w is the rpm of the driveshaft in radians/second. The energies of the stock and aftermarket driveshaft are 2.13 and 1.12 ft*lbs, respectively. The savings, or the difference, is 1.01 ft*lbs. Fourth trick: convert energy to power. What you've first got to understand is that power is energy per time (ie ft*lbs/sec), so we've got to assume a value for time to complete this converstion. Since the driveshaft energy is based on it rotating at a road speed of 60 mph, I assumed a time of 6 seconds. So, divide 1.01 by 6 and you get .1683 fr*lbs/s of power. Now, we've got to convert that into HP, so divide again by 550 and MY FINAL ANSWER IS .0003 HORSEPOWER! Doesn't seem right does it? Well, stop to think that one horsepower is equal to raising 550 lbs one foot in one second. Now think that we're saving only 10 lbs from a small, slowly rotating shaft! If you're looking for HP gains, try switching to lighter wheels and tires. Those are FOUR, BIG, HEAVY, QUICKLY rotating flywheels that gobble up lots of energy. I've got an Excel spreadsheet that I'd be happy to share with anyone wishing to see the dirty work. If you've found this article to be entertaining, educational, or otherwise, would you PLEASE vote it to the tech section? Thanks. Brian
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20:49:14 05/24/00
