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den9
07-01-2007, 12:29 AM
how much will my car be effected by 18x8.5 28 pound wheels? right now i have 15x5? wheels.

yellow2000S/R
07-01-2007, 01:26 AM
how much will my car be effected by 18x8.5 28 pound wheels? right now i have 15x5? wheels.

Those are some heavy rims... Id say it would feel like there are 2 extra people in the car at all times as far as acceleration and braking goes...

den9
07-01-2007, 01:07 PM
these are only 18 pounds each, do you think they look good on my car
obviously it will be smaller and differnt since its just photoshopped, it looks goofy

http://img.photobucket.com/albums/v628/LBCsublime9/carrrr_w_18_axis_rimmm_copy.jpg

Wiisass
07-02-2007, 01:31 AM
It depends what the weight of your current wheel and tire package is versus the weight of the new wheel and tire. And even then it's not a direct relationship because you have to take into account the difference of rotational inertia which will be different due to locations of the masses away from the center of rotation.

Any cheaper wheels you find in bigger sizes, like 18x8.5 are probably going to be heavy. If you want lighter wheels, you're going to have to spend more money. It's just due to the different manufacturing processes that companies use to make them. And cheaper processes usually mean a heavier wheel.

htheduck
07-02-2007, 03:32 AM
It depends what the weight of your current wheel and tire package is versus the weight of the new wheel and tire. And even then it's not a direct relationship because you have to take into account the difference of rotational inertia which will be different due to locations of the masses away from the center of rotation.
wiisass's first paragraph pretty much sums it up.
two bodies of mass are increased, the wheel and the tire. as a result-so has the rotational inertia required to spin these bodies.

BALLPARK math is approximately 7:1 ratio for sprung/unsprung weight. to make the math easy let's just assume your 15x5 wheel is 14 lbs. you are going up 14lbs x 7 x 4 wheels = 392lbs of sprung weight. that's like two 196lb adults in the car-at all times. AND this doesn't include the increase in TIRE mass. :eek:

since the mass of the tire is now INCREASED AND ROTATES at 1.5 inches in radius-greater than before. i would use about 8:1 for similar math here. an 18x8.5 tire has goto be over 27-28 lbs. while a 15" tire is perhaps less than 20? (i'm not sure here).

good luck with your new gas mileage.

den9
07-02-2007, 09:28 AM
Yeah I'm def gonna get them 18 pound wheels

Wiisass
07-02-2007, 01:30 PM
BALLPARK math is approximately 7:1 ratio for sprung/unsprung weight. to make the math easy let's just assume your 15x5 wheel is 14 lbs. you are going up 14lbs x 7 x 4 wheels = 392lbs of sprung weight. that's like two 196lb adults in the car-at all times. AND this doesn't include the increase in TIRE mass. :eek:

since the mass of the tire is now INCREASED AND ROTATES at 1.5 inches in radius-greater than before. i would use about 8:1 for similar math here. an 18x8.5 tire has goto be over 27-28 lbs. while a 15" tire is perhaps less than 20? (i'm not sure here).

good luck with your new gas mileage.

I think that's missing a lot of things. The sprung to unsprung ratio is purely a weight ratio of how much mass is sprung versus how much is unsprung. It doesn't mean that x lbm unsprung is effectively y lbm sprung weight.

And for rotational inertia, you can't just consider the wheel. Well you could, but it would be pointless. If you get the right sized tires for the larger wheels, the overall diameter of the package would not change. But you will have different locations of the mass and it will effect it. It's too hard to estimate, it would be much easier to physically test the wheel and tire to find out what the rotational inertia of it actually is. This can actually be simlply done by swinging the wheel/tire from a string and applying the parallel axis theorem. And you will end up with a good estimate of the actual rotational inertia of the package.

den9
07-02-2007, 04:32 PM
headache! all i wanna know right now, are them wheels ugly on my car

htheduck
07-02-2007, 05:27 PM
I think that's missing a lot of things. The sprung to unsprung ratio is purely a weight ratio of how much mass is sprung versus how much is unsprung. It doesn't mean that x lbm unsprung is effectively y lbm sprung weight.

And for rotational inertia, you can't just consider the wheel. Well you could, but it would be pointless. If you get the right sized tires for the larger wheels, the overall diameter of the package would not change. But you will have different locations of the mass and it will effect it. It's too hard to estimate, it would be much easier to physically test the wheel and tire to find out what the rotational inertia of it actually is. This can actually be simlply done by swinging the wheel/tire from a string and applying the parallel axis theorem. And you will end up with a good estimate of the actual rotational inertia of the package.
i know it's missing a lot of things. (it's been 13 years since undergrad & engineering classes :o) that's why i wrote ballpark-for a simple example.

increase the mass of unsprung components...what is the result-lots of energy to move those components for every imperfection the road exhibits. though this is an indirect answer to his question, i kind of went on a tangent here.

i should have worded it more precisely in terms of increase of rotational inertia and/or energy. Expect that for every 1lb increased in mass of the rotating object it is (ballpark) equivalent to more energy required to MOVE x 7lb of sprung weight.

and yes you would also have to consider the increase mass of the wheel at a radius as well. if you can find the correct set of formulas please post.

Wiisass
07-02-2007, 05:43 PM
The way I look at it, is the increae in rotational inertia will require more energy from the drivetrain to start and stop. While it's moving it won't be as noticeable. And the spring-damper will control any of the vibrations caused by the unsprung mass. So increasing the unsprung mass will lower your wheel hop frequency and effectively soften the compression damping of the system. Depending on how much the change in unsprung weight, it will slightly change natural frequencies and damping ratios.

But all of this is pretty much worthless conversation if we don't know the actual weight of the stock package versus the weight of the new package. I really doubt those wheels in 18x8.5 are 18lbs, maybe in a different size they might weight that. Maybe I just don't recognize them, but they look like the standard knockoff, cast, heavy wheels. But maybe they're a lot nicer than I'm thinking they are and they'll be that light, but I really doubt it.

http://www.analyticcycling.com/WheelsConcept_Disc.html

Take a look at that, it's based on bike stuff, but about halfway down they talk about how to measure the rotational inertia of the wheel and tire. It's pretty interesting and if I ever have tires on my wheels and time to do this before I take them out and use all the rubber, I really want to try it. You might be surprised on some of the results.

Thinking about this a little more, I still don't think that ratio works out for this situation. It seems to be one of those every 100 pounds equals a 1/10s in the quarter mile or something like that. Just a rule of thumb that doesn't equate very well to real life. I mean it might, maybe I'm not seeing something or I'm making it too complicated because that's how it really is. But I know it's just a ballpark and I guess it could work in this situation, I'm just a nerd who wants to see some ft-lbm^2 values.

htheduck
07-02-2007, 06:31 PM
If you want the values take a look at this article I just found and the spreadsheet link-(I am still digesting the spreadsheet, but perhaps you will be faster).

It would appear that the ratio of equivalent chassis weight is about 10:1

Article:
http://www.mazda6tech.com/index.php?option=content&task=view&id=16&Itemid=32

Spreadsheet (at bottom):
http://www.mazda6tech.com/files/rotational.xls

Quick post on a forum:
http://www.rx8club.com/printthread.php?t=3681

Again, ballpark - a quick guide for quick calcualtions. Also, the original creator of the post never stated they were 18 lbs wheels.

den9
07-03-2007, 07:11 PM
http://www.axiswheels.com/axis_wheels_weights_and_lips.html

htheduck
07-03-2007, 08:50 PM
http://www.axiswheels.com/axis_wheels_weights_and_lips.html
yes, and?

*are we supposed to guess which wheel you photoshopped?

Wiisass
07-03-2007, 09:25 PM
Alright, I looked over the Mazda stuff and it seems that it's assuming constant linear acceleration derived from a 0-60mph time. And then also making assumptions on how to calculate the rotational inertia of the wheel and tire. But it makes 3 assumptions of different ways to approximate the rotational inertia of the wheel and tire. But then when they go to the end, it looks like the units don't even work out.

Oh well, I'm about to run out, we can talk more about this later.

den9
07-04-2007, 04:13 PM
yes, and?

*are we supposed to guess which wheel you photoshopped?
i thought i posted it in this thread, they are reverbs

malloyboy1
08-01-2007, 10:45 PM
the wheels will look good on the car go for them, or look for a similar style wheel but a lighter version

iampakman
08-06-2007, 03:14 AM
headache! all i wanna know right now, are them wheels ugly on my car

who cares? do u like em? why worry about what everyone else thinks? the main reason i got my wheels is they weigh 10.6 lbs each, without tire. and, they look good, to me

CleanNeon98
08-06-2007, 12:17 PM
This is why I wish I had 15s, not 17s

yellow2000S/R
08-06-2007, 02:28 PM
If you want the values take a look at this article I just found and the spreadsheet link-(I am still digesting the spreadsheet, but perhaps you will be faster).

It would appear that the ratio of equivalent chassis weight is about 10:1

Article:
http://www.mazda6tech.com/index.php?option=content&task=view&id=16&Itemid=32

Spreadsheet (at bottom):
http://www.mazda6tech.com/files/rotational.xls

Quick post on a forum:
http://www.rx8club.com/printthread.php?t=3681

Again, ballpark - a quick guide for quick calcualtions. Also, the original creator of the post never stated they were 18 lbs wheels.


Thats what some friends were saying that they were told in the High Performance class. They said it is just under 10:1.