Quote:
Originally Posted by Bob@BPS
Not 7 pounds, but 7 times the weight because it is a rotating mass. That is the common factor used by most people. I'm not an engineer, so before I go spewing hearsay, I thought I'd do a little research. What I found was factors quoted as high as 10 times or as low as 2. What appears to be the truth is the farther the mass is from the axis of rotation, the more affect it will have. I interpret that to mean the actual factor will depend where the weight is saved. Tires should theoretically make a bigger difference than the wheels. What that would also mean is those REALLY expensive Dynamag wheels with CF spokes would make a bigger difference in unsprung weight than rotational mass, since they are saving most of the weight close to the axis of rotation.
Here are a few links I found during my Google search on "rotational mass":
http://news.carjunky.com/automotive/...nal_Mass.shtml
http://www.ux1.eiu.edu/~cfadd/1150/09Rot/RotDyn.html
Here is a good page that explains the difference between sprung and unsprung weight.
http://victorylibrary.com/mopar/sprung-c.htm
I also found a really good post at BimmerForums from 2003 here. The member StevenGee calculated a 0-60 difference of 2/10's of a second comparing a 36# wheel/tire combo to a 49# set on an E36 M3.
The bottom line is a lighter wheel/tire combo is definitely a performance mod. Another spec that is thrown around a lot is every 100 pounds of weight saved is equivalent to 10HP. Whatever the correct calc is, no one wil argue that weight is an enemy of performance (both speed and handling).
Bob |
All good points, lightening the rotational mass will improve not only acceleration, but handling and braking as well.
Also factor in tire weights in the final decision - especially since they are the furthest away from centerline. I've seen many users get the lightest wheel on the market and run an cheap/inferior or heavy tire because they haven't done the research or ran out of their budget.