Originally Posted by sleeper50' post='848073' date='Apr 15 2009, 12:23 PM

Good post, good questions. I am learning more as a result of your inquiry. thank you

Question for ya though....how did you sav 44 pounds? that equates to 11 pounds per tire. OEM wheels weigh in the range of 25-26 pounds an up(124's for example). dinan rims come in at 20 pounds, enkai racing rims come in at 21 pounds. so lets say I have 124's with general tires ---> rear wheel/tire combo weighs 26+25+51 pounds. If I replace rims with dinan I only save 5 pounds per wheel. Where does the other 5 pounds come from? Tires? If so, which tires weigh 20 pounds? Are there rims lighter than dinan?

this is not meant as a challenge (dinan claims their set up saves 47 pounds!), I just want to know more on this subject.

Originally Posted by ObD' post='848087' date='Apr 15 2009, 09:39 AM

His baseline was probably Bridgestone which is ~ 28-29 lbs/tire. I know my Dunlop numbflats are in the 29 ~ 32 lb range. Lightest tires are Michelin PS2's and General Exclaim UHP. My next tires are the PS2's whether I stick with the 124's or go with 19" forged.

Originally Posted by 545OH' post='802645' date='Feb 26 2009, 10:07 PM

I was wondering today how much rotating mass decrease equals x number of HP increase. Now as a lot of people think, this has nothing to do with the rotating mass being unsprung mass. (I am talking about light wheels/tires specifically although I know there is rotating mass in the engine/tranny as well.) A misconception is that reducing unsprung mass is much better than reducing sprung mass as far as improving acceleration and power-weight. It actually is only true that unsprung mass decrease increases and improves handling a TON but only a decrease in rotating mass such as wheels or tires can expotentially (not literally) increase acceleration out of proportion to the weight decrease. For example if you decrease the weight of your car by 50 pounds from taking the spare tire out, this will have a much less effect than if you decrease your wheel weight by 50 pounds.

I noticed this when I put new wheels and tires on that amounted to 44 pounds lighter for all 4 wheels w/ tires compared to stock. I noticed marked improvement in acceleration and speed as well as handling.

Now my question is, does anyone have a clue how to mathematically factor the increase in acceleration by knowing how much lighter the wheels/tires are? Also is it money better spent buying ultra light wheels/tires and saving up to 50-75 pounds of wheel rotating weight or spending that same money on horsepower improvements or upgrading a car to one with more HP. I am basically wondering how much lighter you need to make a car, specifically rotating weight to gain for example 100hp equivelents (just chose a number.)

I just think it is always cool to have the lightest car possible rather than a heavier one with more horsepower. I would love to continue shaving weight off my 545 and make it fast rather than buying an m5 for example. (just example.) It got me thinking with all the carbon fiber stuff available, you could pretty much replace every body panel and roof for about 6k and save a good 250 pounds or so. Then replace your wheels/tires and save 50 pounds or so. Just a thought.

Originally Posted by C's Bimmer' post='848104' date='Apr 15 2009, 09:53 AM

You sound like member redwhiteblue/sleepyca31...

Originally Posted by C's Bimmer' post='848104' date='Apr 15 2009, 11:53 AM

You sound like member redwhiteblue/sleepyca31...

Originally Posted by DRANGED' post='848113' date='Apr 15 2009, 01:01 PM

Pretty sure RWB/sleepyca31/quebecois claimed to have gone to school in Ohio. Very similar writing style and confrontational tone. He also coincidentally went from a 530 to a 545. Very curious that we now have a 545OH.

http://forums.e60.net/index.php?showtopic=48195&st=0

Originally Posted by C's Bimmer' post='848142' date='Apr 15 2009, 12:28 PM

quebecois...Hahahaha! I totally forgot about that name too. This is totally him.

How long before he gets banned as 545OH?

Originally Posted by DRANGED' post='848169' date='Apr 15 2009, 01:48 PM

http://forums.e60.net/index.php?showtopic=66899&st=146

Originally Posted by NobleForums' post='802814' date='Feb 27 2009, 02:32 AM

Conventional wisdom says that conservatively, 1lb worth of rotating mass lost is roughly equivalent to a 4lb sprung mass lost. This being said, I don't think there's any way to get an exact measurement. Even where the rotating mass is lost will make a difference. 1lb lost at the tire tread will make a bigger difference than 1 lb lost near wheel bolts.

In terms of converting that to horsepower, it depends on the power to weight ratio before the weight loss.

For instance, let's just assume we agree with the 4/1 ratio for rotating vs spring.

Let's further assume we're talking about a 4000lb vehicle with 500hp (ie. much like an E60 M5). We're starting off with a 8 lbs per hp ratio.

Let's take 50 lbs off of the rotating mass. Given our 4/1 assumption above, that is equivalent to losing 200 lbs. We're now at 3800lbs and 500hp, giving us 7.6 lbs per hp.

If we wanted to do this by increasing horsepower instead of losing weight:

4000 / 7.6 = 526hp

In other words, we could gain 26hp and we'd be at the "same place."

How much weight would we need to lose to be equal to a 100hp gain in this example?

4000 / 600 = 6.67 lbs per hp

6.67 * 500 = 3333.3 lbs

4000 - 3333.3 = 666.67 lbs sprung weight lost

666.67 / 4 = 166.67 lbs rotating weight lost

166.67 lbs is a LOT of rotating weight to lose.