yep, you did

lol

I figured out my 18" combo and I got considerably less than you did for the 16" rim and thought something was not right so I went back to yours and did my own calcs and then discovered you used the diameter.

 

these calcs aren't right either.....looks like you also used diameter and not radius..

for my rear 18x10.5 iForged rims and 295/30/18 PS2 tire....

rim:
M=23.5lbs
R=0.75'

I=6.61 lb ft


tire:
M=26lbs
R¹=0.75'
R²=1.04'

I=21.4 lb ft

for a total combo inertia of ~28 lb ft.....

 

I also think you would want to subtract the two radii of the tire and not add them........?? The tire is hollow and therefore you want to know the inertia of just tire and not part of the 'donut hole'. The way I see it, by adding them you are adding the inertia of an Xlb item with radius X to another with the same mass but different dia. I think you want to subtract the inertia of the tire 'hole' out of the equation.

 

Ok thanks. Time to rework the brain.

 

It's just a matter of guessing where the centroid is, which is the cumulative centroid of both the wheel and the tire. I would say it somewhere close to the edge of the wheel radius, but that's just a guess.

 

MOI is important. Slung or rotation mass is much more than static mass. Go read one of the mags where they pull the heavy 18-22" wheels off some ricer, and replace them with the stock 15" wheels. The car goes a second quicker in the quarter...

Now, with our cars, its isn't as drastic, but it does make a difference. Think about this. Go from a Factory 17-18" GM Truck/Suburban/Tahoe wheel to a factory 20" wheel and you loose at least 1MPG. More mass farther out eats up more energy.

 

Ok a Freind and I have been working on this one a bit more. Here is what we have caculated.

What effect does the lower weight of a race tire have on a car's
performance? This is a follow-up of some data that Tom and I were
kicking
around. The setup:

CCW + Michelin PS = 51 pounds
CCW + Hoosier slick = 39 pounds

The lighter combination will have less translational energy at speed,
and
less rotational energy as well. This impacts acceleration, braking,
and
cornering. How much?

The total wheel/tire energy is 1/2 mV^2 + 1/2 Iw^2 at speed.

I took some measurements of my own CCWs to roughly calculate the moment
of inertia I:

I (race setup) = 0.9168 lb*ft*s^2
I (street) = 1.1989 lb*ft*s^2

At 140mph (back straight at VIR), the energy stored in all 4 wheels/tires
is:
Street setup:
stored wheel energy = 133556 + 86140 foot-pounds
(translational+rotational
energy)
Track setup:
stored wheel energy = 102131 + 65871 foot-pounds (translational +
rotational
energy)

What does this all mean? Some fun facts:

The total energy stored in the wheels/tires is about 10% of the total
kinetic energy of the vehicle at 140mph (not counting
rotating/reciprocating drivetrain parts). This percentage drops as speed drops.

The energy difference between a street tire and a race tire gives the
race-tire equipped vehicle about an equivalent 5 rear-wheel horsepower
advantage on VIR's back straight. On slower straights, this advantage
decreases with speed.

The race setup is overall 48 pounds lighter than the street setup.
However, to the brakes on VIR's back straight, the race setup "feels" about 70 pounds lighter due to having to dissipate less rotational energy. This 70
pound number drops as speed drops. The effect will not be as great on a
100mph straight.

+ the race setup has less gyroscopic effet in corners. I didn't
calculate
that one Light tires do make a difference.


On longer high speed straights on road race ciruits there is a big difference. not to metion the effect of braking from high speeds,

 

So what did LostCause make after the wheel switch and mods?

 

Yeah any updates?

 

You've been working on this for a long time.

I gave up as soon as I realized I used diameter instead of radius.