How much benefit are lighter wheels rotational mass wise
 

Pretty simple. I have gramm"LIGHT" wheels that weight about 22 lbs each.

I want to know how much benefit is it really to go to a 16 lbs or so wheel (so a benefit of about 12-13 lbs of rotational mass in the rear wheels).

Don't care about the benefit of overall weight reduction. I'm only asking about rotational mass savings allowing the engine to spin faster. Like a lighter Flywheel, LWCP, etc...


Experts on the subject... GO!

Edit: Good discussion on the subject can be found here

http://www.ft86club.com/forums/showthread.php?t=32858

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Why is the sky blue?
How is a rainbow made?
How does a posi-track in a plymoth work?....

It just does. :p

Probably depends on where the weight is shaved from too. Weight at the edge of the wheel takes more energy to spin than weight towards the hub, right? But then there's that whole conservation of inertia and shit, fuck if I know... I'm out.

I compare it to running in a pair of boots, then swapping boots for a running shoe. There is no comparison.

Everything is easier - starting, stopping, turning, reacting and feel are all better with lighter wheels.

Now, a set that's 1lb lighter per corner? You may notice it... But if you drop 5-6 lbs per corner? It's outstanding.

dont forget to take in account of tire weight too. a wider wheel of the same weight as the skinny stock wheel might be considered "light" for its size, but the added tire mass and weight will add it back... and at the outer edge of the rotation at that.

now if you compare a X-lb 17x7 wheel vs Y-lb 17x7 wheel, with Y-lb being lighter, then you can see and experience the direct difference weight makes from a lighter wheel alone.

Inertia = resistance to change in direction and/or velocity

Heavier wheels will be harder to get spinning, harder to turn for cornering, and harder to stop under braking. Vice versa for lighter wheels. Car will just feel more "nimble"

Huge

It pays to go light

I'm on 235 40 17 track day in r spec and its a dream on light rims

A typical 18x10 wheel and tire weighing 40 lb has an equivalent mass of about ~55 lb. A wheel and tire weighing 30 lb has an equivalent mass of about 41 lb. 14 lb x 4 wheels = almost 3/4 of a tank of fuel.


Here is some code:

function calculatewheel(){
var umass = document.wheel.mass.value;
var massunit = document.wheel.massunit.value;
// mass in kg
var mass = umass * massunit;
var diameter = document.wheel.diameter.value;
var diameterm = diameter * 0.0254;
// ratio = rim mass percentage of total mass
var ratio = document.wheel.ratio.value/100
var rimmass = mass * ratio;
var spokemass = mass * (1 - ratio)
var rimri = ri_pointmass(rimmass,diameterm/2);
var spokeri = ri_rod(spokemass,diameterm/2);
var totali = rimri + spokeri;
document.wheel.inertia.value = totali.toPrecision(5);
document.wheel.rimri.value = rimri.toPrecision(5);
document.wheel.spokeri.value = spokeri.toPrecision(5);
var emass = e_mass(mass,totali,1,document.wheel.rollingradius. value);
var emassratio = emass/mass;
document.wheel.emass.value = emass.toPrecision(5);
document.wheel.emassratio.value = emassratio.toPrecision(3);
}

function ri_pointmass(mass,r){
ri = mass*r*r;
return ri;
}

function ri_rod(mass,r){
ri = mass*r*r/3;
return ri;
}

function e_mass(m,i,gr,rt){
// computes equivalent mass given
// m - static mass
// i - rotational inertia
// gr - gear ratio. gr = 3 means the component in question rotates 3 X as fast as
// the drive wheels
// rt - rolling RADIUS of the drivewheels
var n = gr / rt;
emass = m + i * n * n;
return emass;
}

I know it works, that is not the question. My question was how much of a difference does it make. Maybe I didn't ask the question properly, I meant does it feel marginal like replacing LWCP, or does it feel way more than that, and thus worth around $1K.

It would be basically same for same change, only difference would be lighter weight. Same tires, same 18X9.5 wheel size.

Don't know if you know my tag, but either way this was brilliant.


I mean, how much? I want someone who can quantify a difference having run heavier wheels before. (You probably did, hence why I ask). Would it be worth $1K, considering the improvement.

I'm a mechanical engineer, you lost me at the word "code". JK, thanks, this is very helpful.

On a 60 second autocross course a few tests show the difference is in the noise....as an engineer this pisses me off so I still bought lightest wheels I could :)

You're an ME and can't answer your own question? Anyway, the above shows the formulas (code) needed to figure it out.

EDIT! Missed the J/K part... oops sry

The benefit is HUGE. There is some benefit in terms of acceleration and braking but that pales in comparison to the improvement in chassis dynamics. Unsprung weight is the enemy when in comes to suspension performance. Regardless the weight of your wheel/tire the road is going to "throw" it upwards when you hit a bump. Every time that happens the dampers and chassis have to stop it. Think of catching a 20 lb medicine ball vs catching a nerf ball. Which is harder to stop? How is your balance affected with the medicine ball? The same can be said for the car stopping a heavy vs light wheel.

well put

Hold on are you seriously a ME? If so this should be basic physics and dynamics I'm a Civil Engineer and could figure out the benefits.

I'm an Aerospace Engineer (at least, that's my degree) and so I shall settle this debate once and for all:

Microwave.

I guess you didn't read the whole thread. The question is not "why" or "how", I already knew the answer to both. The questions is quantity. How much of a benefit it really is. If you research, some people say it's negligible others say it's a big improvement so what I am looking for is people with experience on the subject. Someone who has actually changed wheels for same sized wheels but noticeably lighter and their personal opinion of performance improvement.

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rpf1? do it!
It makes everything more efficient. Brakes, Suspension, Acceleration, etc.
18x9.5 is much wheel. go 17x9 for racecar because racecar.
If you are near the limit with your current setup I would say its worth the money. Otherwise I would say spend money on consumables/track time/ stuff you don't have.
There is a reason why almost everyone runs 17's, even 17x8 or 17x8.5. And many run 225/45-17 instead of 245/40-17, car benefits more from acceleration(which it lacks) and nimbleness over outright grip. Tires are also lighter in smaller sizes.

maybe a tenth or two per lap.

I hate having the same wheels everyone has.... I am looking at TWS T66-F which are even lighter than RPF1s. But, they have two downsides. One more expensive than my current wheels, second, uglier than my current wheels. That's why I'm trying to gauge real actual quantifiable benefit of them. :cheers:

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rpf1's. DO EEEET. ONE OF US> ONE OF US.
seriously though, .1 up to maybe .2 per lap. +slightly faster with 17's. And tires are much cheaper, meaning you can get a better compound which is more important than the weight imo.
Your car is pretty built. I would suggest a lsd and maybe some aero instead.

I would never go back to 17's :D

The benefits I'm thinking about are, acceleration, specifically before the turbo spools up. The response from the car on initial WOT, and cutting down spool up time. Once I hit full boost, the wheel weight don't matter much. The transition phase when its spooling up is when I think these lighter wheels could make a big difference acceleration and response performance wise.

In the real world, it's not that big of a deal.


Grassroots Motorsports did a test on this in a past issue using a Miata (even lighter and more sensitive to wheel weight) and the changes didn't amount to anything more than statistical noise.


In other words, don't worry about it. It's just bench racing.

I hear ya on this.

Have a look at the PF01. They are much better looking than the RPF1 and according to Enkei site they are also stronger so should handle track abuse better. They are a little more expensive, which means you don't see many around.

Depending on the size you are looking at they are only marginally heavier than the RPF1.

as delivered by dear-teh SR

cuse Merica' that's why :burnrubber:

Oh, ok I missed that part when reading through I guess :bonk:

There was a good thread going for this already but I just found it today.. ooopps....


http://www.ft86club.com/forums/showthread.php?t=32858

If I remember correctly its also harder to slow down the weight at the end then at the center. Hmmm ...

Instead of the RPF1, do what I did and go with PF01. Different look, plus (supposedly) the mass is more centralized than the rpf1. the mass centralization part is important to have, but difficult for a lay person to actually measure. 'Visually' the PF01 'looks' more mass-centralized than the RPF1, but I've never driven them back to back for quantifiable measuring.

These in 17x8 with 225/45 are perfect for HPDE on most tracks.


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I answered earlier with the boot vs running shoe analogy... But yes, I have done a direct swap (exact same size wheel) and found the difference to be VERY noticeable and impressive. Got me addicted to shopping new wheels by weight first, THEN style, which would've seemed crazy to me before I had experience with lightweight wheels. Now, it's CRAZY to buy equal weight wheels, let alone heavier ones.

Honestly though, nobody can tell YOU if it's worth YOUR thousand dollars. But from a perspective of autocross and daily driving, there isn't much by means of "cheap" bolt on upgrades that will change as drastically as a set of light wheels.

In the end, you're going to have to take the plunge and decide for yourself. But yes, it's totally noticeable, and yes a lot of us think it's worth it.

Huge benefit, though I cannot quantify it. My change was dramatically noticeable because I lightened each wheel by 3 lbs. and lightened the driveshaft by 13 lbs., reducing rotational mass considerably. Significant performance gains accelerating and stopping which is evident to even the novice driver.
I would estimate these changes save me at least 3-5 seconds on a 1 minute autocross run.

There is absolutely no way lighter wheels and driveshaft save you 3 seconds on a 1 minute course. No way...

I was inclined to say the same thing.
Lightening the wheels will make a difference in unsprun weight and polar momentum (quicker response) which would make a good driver faster.

The reduced weight from the 13 lbs reduction in the driveshaft will help with overall car weight, but will do very little to the rotational inertia of the rotating mass.

but is it worth a grand?
for me, and you, I bet, yeah

I'll give it a shot, for the record, I studied to become a civil and structural engineering draftsperson, but got bored and switched to IT a couple of years in.

I'll use the numbers you quoted and ignore tyres (let's keep it simple). I'll also be using proper units (sorry US, Burma and Liberia).

Gramms:

22lb / 2.2 = 10000g

No diameter specified, so lets assume 17 inch (432mm)

Using this calculator: http://www.botlanta.org/converters/d.../flywheel.html

Assume average speed of 100km/h (wheel RPM of ~800) and even distribution of mass over the radius (disc): 818 Joules (Watt seconds)

Convert to horsepower: 818 / 746 = 1.09 hp seconds per wheel

16 lb wheels:

16lb / 2.2 = 7272g

Still 17 inch (432mm)

Same assumptions: 595 Joules (Watt seconds)

Convert to horsepower: 595 / 746 = 0.79 hp seconds per wheel

So, over 3 seconds of acceleration (say, accelerating out of a corner), the power required to get the wheels up to speed is:

Gramms: (1.09 x 4) / 3 = 1.45 hp
16lb: (0.79 x 4) / 3 = 1.05 hp

But, what if the lighter wheel also has it's mass concentrated towards the centre instead of evenly distributed (smaller moment of inertia)?

We just multiply the disc result by a ratio of the amount of mass that has been moved from the outer edge to the inner edge, lets say the mass is somewhat better distributed and give it a ratio of 80% (0.8).

595 Joules now becomes 595 x 0.8 = 476 Joules (0.64 hp seconds)

So now we have:

Gramms: (1.09 x 4) / 3 = 1.45 hp
16lb: (0.79 x 4) / 3 = 1.05 hp
16lb with better MOI: (0.64 x 4) / 3 = 0.85hp

Braking will obviously be better too, the wheel would slow down faster, so you could brake later.

Most of these numbers are very hand-wavy, without a dyno, they probably cannot be accurately quantified and even then, they may be lost in error margins.

Some dyno tests on a Camaro (heavier wheels and more power, so more measurable): http://www.bimmerpost.com/forums/sho...d.php?t=639783

Oh and as I said before, I'm only a partial engineering draftsperson so if I screwed something up or made a mistake somewhere, please point it out. I'm just calculating using info I have gleaned through my research over the last few weeks.

I went from OEM 16x6.5 inch wheels to Enkei PF01 17x8. By my reckoning I increased mass on each corner by around 1.5-2 kg, but it feels more responsive and quicker, might just be a placebo or the better tyres, but the PF01s seem to have their mass concentrated towards the centre, which would give them a lower moment of inertia.

Ugly? Blasphemy! 15.7 lbs of golden 18x9.5 goodness! I cried once after purchasing but have been nothing but smiles since.

Good luck on your search though.

http://i1087.photobucket.com/albums/...ps7d9dba39.jpg

I think the advantage is more in the lowering of the unsprung weight. The less weight your suspension has to control the better it can do what it was meant to do. The car should go faster, turn faster, and stop faster. Not sure in seconds what it amount to on a race track. Mikey for CSG should weigh in!

Welcome to car forums!! Where everyone can be an expert by regurgitating popular opinion/theory but no one can really answer your specific question. :bellyroll:

Personally, I've stopped caring about a few pounds of wheel weight and gone the budget friendly route since some of the fastest track guys I've ever seen run **gasp**

20 lb Rotas.

Lol everybody wants to write a novel about a bunch of information that wasn't asked for.

Question:

Is it worth the $1,000 price in a performance aspect?

Sure.

I mean, $1,000 can just about get you headers and tune which I would imagine would have a more noticeable gain. $1,000 isn't much though for modifying cars; so if you're already considering changing the wheels on your car, I would go for it. Just choose something that would give you what you're looking for style and weight wise.

I have 18" TE37s and am actually going to sell them simply because I'm going back to 17s. Not so much for weight concerns, but for the fact that 18s don't work with the additional steering angle my setup utilizes.

I suppose it just comes down to what you actually want from the car.

One may be legal where you are, while the other one isn't...