hey guys, i recently had my car dyno tuned and i was wondering if i should have run the car with a diffrent rim size. I have 19'' rims. If i would have run the car with maybe a lighter 18'' rim do you think that would have a big diffrence in my numbers, or a big enough diffrence to change to 18s permanently. thanks

 

It depends on the weight of the wheel, not its size. The unsprung weight affects power delivery insofar as it takes power to get each component turning. I would not worry too much about it, unless you have some 35lb cast pieces of crap and/or are considering Mg wheels. Since I doubt either is the case, don't sweat it.

 

19" wheels sized to fit an FD are more than likely pigs.

The difference between a light 16" and light 17" are not such a big deal. But if you have 19's then just about everything else will be faster.

Bottom line, if you want a light fast wheel then use stockers or a racing 17".

Dave

 

check this out, inertia calculation

lets say your 19s weigh 20lbs and the 18s weigh the same

I=weight*(radius squared)
early stock 16.8 12.5lbs
12.5lbs * (8 squared) = 800

late stock 16x8 15lbs
15lbs * (8 squared) = 960

19 x whatever 21lbs
21lbs * (9.5 squared) = 1895

18 x whatever 19 lbs
19lbs * (9 squared) = 1539


So basically yours 19s have twice the inertia of the stock 16's. This does not take into account tire weight, which I'm positive will add a few lbs to your 19s, not to mention you probably make the gearing different w/ the 19's because you cant fit the same diameter tire which will make it slower accelerating (not a deal w/ gobs of power).

Acceleration is probably the least big difference. I believe turning and braking are far more noticeable w/ sprung weight reduction and I'm sure is similar for unsprung weight.
Lighter unsprung weight is supposed to help incredibly w/ the shocks response (going up/down) and especially on bumpy/shitty roads it will handle them better.

Basically says that your all show and maybe a little bit of go :P
-Ben Martin

 

Unsprung weight doesn't affect power to the ground, the moment of inertia of the wheels, tires and driveline do. Sure they're related, but they're different. A 10" diameter wheel made of lead would have a lower moment of inertia than a 30" wheel of the same mass, because that mass is further out (that mass has more leverage).

It's not that simple a calculation. What you're calculating is based on the radius of gyration, which assumes that the mass is concentrated in a thin ring. A wheel has a lot of its mass in a ring, but it's not thin, and then there's the whole wheel face to deal with. A larger wheel has more mass out farther and will have a much higher moment of inertia. Wider wheels have all that extra mass right at the edge, and tires have most of their mass at the furthest distance.

If you're really concerned, then run the smallest and narrowest wheels and tires you can get away with, buy the lightest wheels you can afford, and look for the lighest tires that meet your performance criteria. That'll decrease your drivetrain drag and put more power to the pavement.

Less unsprung weight is better for handling, as the shocks and springs control the suspension movement, not the car movement, so if it has less mass to move around it'll react quicker and have an easier time controlling all that mass.

 

Listen to NOde and Black,

These guys know what's up.

You must calculate the change in the "moment" like Node shows.

YES IT WILL KILL POWER.

i.e. MY RX8, 17" NT03+M's vs. 19's OMG huge huge DYNO *** DIFFERENCE

Think of it like that.

Not to mention you will see a loss in fuel economy of about 12% for every +1 over stock. I tested on my RX8 and Miata.

Peace, Rishie