545OH

The basic weight is simple, however figuring how much rotating weight plays in the equation is what I am talking about. It is really easy to thnk of it in extremes. Imagine wheels that weighed 5 pounds each v. 50 pounds each. Imagine manually trying to spin the wheel. A heck of a lot more energy is spent moving the 50 pound one compared to the 5 pound one, especially when getting it moving or "accelerating" it. However take 50 pounds off a car by dumping your passenger off and you do not feel any difference. I think 1:4 may be a little low. I would be more inclined based on intuition that removing 1 pound of rotating weight that is far from the center (ie tire) would be more on the lines of 10 pounds of normal weight. Like the one poster said, getting light wheels is great but you have to see where the weight savings is. If it is right near the hub than the difference is minimal since weight here takes virtually no more energy to turn compared to weight farther from the center that takes MUCH more energy to turn based on simple leverage.

Anyway interesting. I Just think my point is people underestimate what spending 1500 bucks on a light set of wheels and tires and saving 40-50 pounds of rotating weight can do to their performance, handling, acceleration and not to mention good looks. The general exclaim and PS2's are the lightest I know of weighing 23 pounds for a 245-35-19 tire compared to many that weigh 28-30 pounds, especially stock runflats. Just by changing tires you save nearly 28 pounds. Add to that a set of wheels that weigh 25 pounds each compared to stock 30-33 pounds and you have 50 pounds of savings and probably equivelent to 25-30 hp.]


If you take a 545 that weighs 3800 and has 325 hp 12.5 pounds per 1hp roughly. So if you use a semi-conservative 1 pound rotating to 5 pounds regular weight, then that is 50 pounds rotating savings times 5 which is 250 which equates to around 25 hp.

That is not a bad gain IMO

EuroCarFan

I remember coming across a post on M5board where one of the members (Darren_Dallas) used a 1:9 ratio comparing rotating mass vs sprung mass. I vaguely remember him using them as factual numbers and not as an assumption.

Then again, I also came across an article on the web that says there are no differences in power benefit between reducing the two types of weights; that reducing 1lb of rotating mass is the same as reducing 1lb of sprung mass. Sure there's the benefit if improved braking and handling but that article said there was no more realized power "gain" from reducing rotating mass over reducing sprung mass.

Personally, it seems completely logical that a lighter weight wheel/tire set up should allow your car to accelerate faster like 545OH mentioned above. I've been on a campaign to reduce weight on my car, both sprung and unsprung mass. I sold my beloved R10 wheels to get the current 360 Forged Mesh Eight wheels which saved about 20lbs total from my R10 set up. I believe the R10s weigh about 31lbs each while the Mesh Eights are around 26-27lbs. I've only had the wheels since early Oct last year but considering switching them again to these new B9 Technic wheels from WheelExperts that are only 19-20lbs for 20" wheels.

Changing my brakes to the current Brembo BBKs probably also shaved about 20lbs or more off my car. Then there's the complete swap of the stock full exhaust including headers to the complete RDsport system which I'm sure saved close to 100lbs give or take. And yes, I did buy a lightweight battery and that should save another 40lbs.

Unfortunately, the ///M5 logo floor mats stay.

525iEnjoy

You'll have more to gain if you decreased rotational mass that is on the engine like valves, pistons, pullies, crank or wheels, tires etc..Stripping the car of unsprung weight will help but you'll get more bang for your buck the other way

525iEnjoy

The battery was good as far as unsprung weight, but you just got rid of some traction by getting a lighter battery. And the weight distribution isnt near 50/50

545OH

Careful here, and this is what I mentioned in my post, people get "unsprung" weight and rotational weight confused, thinking they are the same thing.
Sprung weight-all weight that the suspension holds
Unsprung weight-INCLUDES the tires and wheels
ROtational weight besides engine is anything rotating like wheels and tires.

So decreasing wheel/tire weight is decreasing both unsprung and rotational weight but they are not the same thing and it is the decrease in rotational weight that adds acceleration. Decreasing unsprung weight by getting lighter rotors, calipers etc will increase handling and ride but not acceleration (anymore than simply the weight of the pieces as it came off any other part of the car.

So unsprung weight has no advantage or ratio higher than sprung as far as accelration goes. Rotational mass is what the ratio is for and is much more effective in improving acceleration.

flyingpuck

GAH! Sixcard, you beat me to it!! Of course you were up earlier than me too!

miguex

Very interesting to learn that. thanks!

Also I found some good info to diet the car:

About more lightweight batteries: http://forum.e46fanatics.com/showthread.ph...ght=voltphreaks and http://www.voltphreaks.com/index.html but it is a very expensive battery and it is only 5 pounds! (OEM is about 48 pounds) About $1500 the unit!

EuroCarFan

Thanks for clarifying and I completely understand what you're saying. Man, I should just go splurge and buy a set of Dymax wheels.

EuroCarFan

Yeah we had a nice thread discussing lightweight batteries on here too: http://forums.e60.net/index.php?showtopic=...t=0&start=0

BTW, Voltphreaks was giving $500 discount for their batteries on M5board but even then, I can't justify spending that much on a battery.

sixcard

I'm guessing you posted at 0945 PAC time. I had a full day's work in by that time!