545OH

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.

mhs525

I don't have the patience or the background to understand all the math of the physics involved although I have seen some pretty great right ups but I follow your theory. Less weight = more HP/Lb and less rotational mass increases HP available to propel the car plus makes the suspension, brake etc work better as they have less weight to manage. I run 18" BBS LM's that are forged and very light, these wheels combined with non flat tires took a lot of unsprung weight off the car and reduced the rotational mass. I could feel the difference the first drive after swapping the OEM with run flats to the BBS LM's with regular tires.

Less weight = a faster better handling car.

Lotuses can be as fast as Ferraris around a track but only have four cylinder Toyota engines. Less weight.....

The M3 Forum has some pretty smart guys that really get into this stuff, you might want to try some searches on that forum...

PraiseTheLowered

anytime you switch out panels for carbon fiber or switch cast for forged wheels the difference is very noticable

I love having forged wheels, pricey but worth it.

miguex

Maybe remove the auxiliary tire and change the batteries (to a small/light battery). Less weight.

JetBlack5OC

European Car Mag has a project Z4M. They installed a set of Forgeline wheels weighing 4 pounds/each less than the the stock wheels. They dynoed the car and gained 13hp/10tq.

NobleForums

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.

Trinity Autosport

HOLY CRAP!

That's a lot of numbers...LOL!

You the MAN

sixcard

Don't forget the floor mats.

selbycy

And if you eat a few burgers all that hard work lost

Mr.Bavarian

I thought it was quite simple ? For example,
4000LB M5 at 500HP.

Say we magically remove 2000LB from the car. Wouldn't that mean he HP is almost doubled relative to the original weight?