How much benefit are lighter wheels rotational mass wise

[Sportsguy83]

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

Sent from my SM-N900A using Tapatalk

[tahdizzle]

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

It just does. :p

[Foobar]

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.

[CatDaddysBBQ]

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.

[8R6]

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.

[chas3wba0]

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"

[kiichiro]

Huge

It pays to go light

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

[Hoosier Daddy]

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;
}

[Sportsguy83]

Originally Posted by tahdizzle

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

It just does. :p

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.

Originally Posted by BRGT86

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.

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

Originally Posted by chas3wba0

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"

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

Originally Posted by kiichiro

Huge

It pays to go light

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

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.

[Sportsguy83]

Originally Posted by Hoosier Daddy

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

[vroom4]

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

[Hoosier Daddy]

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

[kask2_6.0]

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.

[2much]

Originally Posted by kask2_6.0

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