mkent93 Posted April 23, 2006 Report Share Posted April 23, 2006 I was reading this over on the corvetteforums. I found it interesting since I've suspected this theory ever since I put the heavy chrome c5 wheels on my Z. http://forums.corvetteforum.com/showthread.php?t=1328054&highlight=c6+wheel+weight In summary, 17" stock wheel weighs about 16.5 pounds less than 18" chrome wheel. 17" stock wheel dynoed 12rwhp more and 9rwtq more than heavy 18" chrome. Quote Link to comment Share on other sites More sharing options...
SPLN SUX Posted April 23, 2006 Report Share Posted April 23, 2006 :jerkit: glad weve all finally discovered the effects of ratational mass. Quote Link to comment Share on other sites More sharing options...
Nitrousbird Posted April 23, 2006 Report Share Posted April 23, 2006 :jerkit: glad weve all finally discovered the effects of ratational mass. Have you seen dyno results on a before/after wheel swap? Exact numbers, not a freaking guess. Quote Link to comment Share on other sites More sharing options...
SPLN SUX Posted April 23, 2006 Report Share Posted April 23, 2006 Yes... 2002 Civic Si on stock wheels 138.2bhp....same car on Rota Slipstreams, 146.1bhp. Even a lil itty bitty civic sees it. I dont remember the torque figure off the top of my head sorry, but i think the gain was like 1.4 or something equaly insignificant. Granted, these wheels were the same size... a smaller wheel would have yeilded even greater change. If i was to change from a 16x7.5 to a 15x7... thats also lighter... theres no question its going to effect the power the engine makes. Not only are you removing rotational mass from the system, like a pully, or flywheel, but your alos changing the over all gear ration from the crank to the ground, unless you use a taller tire to compensate for the change in diameter of the wheel... then that aspect is uneffected. It also matters where the weight is distributed. A disc has less effective rotational mass, or rotational inertia, than a ring. Meaning: If you had a wooden disc and a wooden ring that had identical mass, and you placed them on edge at the top of a ramp... if you released them from the same level of the ramp and at the same time, the disc would out accelerate the ring severely... which is why i think the Outlaw/2 wheels are so popular with the old muscle cars that are "track prepped". Its a way to reduce rotational inertia without giving up wheel size and often width. Julius Summner Miller FTW EDIT: Numbers mixed up.... Before - 136.2, After - 138.1... my bad, its been a while lol. Quote Link to comment Share on other sites More sharing options...
Drunkendubber Posted April 24, 2006 Report Share Posted April 24, 2006 "glad weve all finally discovered the effects of ratational mass." Physics?? cr doesn't believe in those, doesn't fit into the bench racing equations Quote Link to comment Share on other sites More sharing options...
desperado Posted April 24, 2006 Report Share Posted April 24, 2006 I fail to see how this actually is a surprize. A chassis dyno is only an indicator or wheel horsepower, and works on the principal of inertia, if you add rotating mass to the driveline that is being tested it's gonna show different numbers. It would be no different than adding weight to the drum on the dyno and not telling the computer to compensate (sp?) for it. Look at all the different mathematical equations for figuring horsepower with weight of the car and 1/4 mile time. This is really not that far off from that, change the weight then the numbers of course change. Quote Link to comment Share on other sites More sharing options...
Orion Posted April 24, 2006 Report Share Posted April 24, 2006 im not even sure what "ratational" mass is. Quote Link to comment Share on other sites More sharing options...
Guest Removed Posted April 24, 2006 Report Share Posted April 24, 2006 We track tested this with my integra, my old fan blade wheel's wieghed 70 pounds at each corner(with tire's), Best time 14.5 @96mph, eagle 5 star wheels and taller tire's (to make up the traction loss, etc) 14.2@ 99 mph, and thats just changing the front wheels!! Quote Link to comment Share on other sites More sharing options...
mkent93 Posted April 24, 2006 Author Report Share Posted April 24, 2006 My car is about 1mph slower and 2 tenths slower while racing with my 18's. It cuts nice 1.91 60's though but it's still slower in the end. To the smart asses who stated the obvious about rotational mass--I'm willing to bet I've had as much or more physics education as you to know this was the case. However, many people in the world don't really think about these things when buying wheels to increase the appearance of their car (or make the car look hideous in extreme cases). SPLN SUX, I like the way you explained the effects--I could never be a professor/teacher because I can't explain science and math comprehensively as you just did. Quote Link to comment Share on other sites More sharing options...
SlowZ06 Posted April 24, 2006 Report Share Posted April 24, 2006 im not even sure what "ratational" mass is. Beat me to it. As I was reading this post, I wondered if I was going to get to point it out. Quote Link to comment Share on other sites More sharing options...
Buck531 Posted April 24, 2006 Report Share Posted April 24, 2006 Beat me to it. As I was reading this post, I wondered if I was going to get to point it out. you know.. you have rotational mass (points to belly). Quote Link to comment Share on other sites More sharing options...
SPLN SUX Posted April 25, 2006 Report Share Posted April 25, 2006 My car is about 1mph slower and 2 tenths slower while racing with my 18's. It cuts nice 1.91 60's though but it's still slower in the end. To the smart asses who stated the obvious about rotational mass--I'm willing to bet I've had as much or more physics education as you to know this was the case. However, many people in the world don't really think about these things when buying wheels to increase the appearance of their car (or make the car look hideous in extreme cases). SPLN SUX, I like the way you explained the effects--I could never be a professor/teacher because I can't explain science and math comprehensively as you just did. I was always very fond of physics. I took both law and theory in highschool as well as AP Comprehensive... then took two semesters in college... most of which had more to do with static discharge and behavior of stimulated matter (it was for electroncis engineeing). I was never any good at spelling though The basic theory here goes back to the equation for Force: Force = Mass x Acceleration In this case, your force is going to be engine output. The mass is going to be the total rotational mass of the drive train, and the acceleration is going to be the inverse of the total static load factor. If your engine makes 200bhp/200lb-ft on a brake dyno, then that doesnt change... so its a constant. Thus, if you know that Mass and Acceleration are inversely related in that equation, meaning if one increases, the other must decrease in order to keep the equation true, if you decrease the mass or the entire drive system, the acceleration will in fact yield a higher figure. Example: 200 = 100lbs x Acceleration -- static load factor of .5 200 = 50lbs x Acceleration -- static load factor of .25 So what does this mean? Well what it means is, the percentage of effective mass you remove from the initial system, is the percentage decrease in the static load factor. Meaning, the percentage of decrease in the static load factor, is the percentage of increase in acceleration when multiplied by the load factor itself. So, if you can manage to remove 50%the effective mass of the drive line, you can increase your accleration by as much as 25%. If you remove 25%, as much as 12.5%. Of course, this is all relative to the inital factor... and every car is different. I really dont know any other way to explain it... theres too much involved for me to know how to explain it any better. Ive always been a bit of a smart ass too... ask my mom Quote Link to comment Share on other sites More sharing options...
Trouble Maker Posted April 25, 2006 Report Share Posted April 25, 2006 Yes... .... I dont remember the torque figure off the top of my head sorry, but i think the gain was like 1.4 or something equaly insignificant. .... EDIT: Numbers mixed up.... Before - 136.2, After - 138.1... my bad, its been a while lol. Uhhhh.... I'm willing to bet I've had as much or more physics education as you to know this was the case. I'll take that bet. Quote Link to comment Share on other sites More sharing options...
Cougar1647545494 Posted April 26, 2006 Report Share Posted April 26, 2006 Light weight wheels ftw Quote Link to comment Share on other sites More sharing options...
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