I put mine on a bathroom scale a while a go and got something around 42.5lbs for the rear rim and tire. Don't know how accurate the bathroom scale is though.

 

a general rule of thumb is that for every pound loss of rotaitonal mass u save, that's the equivalant of 10 pounds less weight for the car. rotational mass on a rim is sorta like...take 2 wooden dowels. tape them together in a "T" shape. Now, at the top of that "T" tape 2 weights to it, equal distance from the center..tape them close. Now, try to spin it between your hands..it's pretty easy. Take those weights, and move them out to the outer edge, and repeat. It's harder to get that momentum built up ,and get them spinning. it's also harder to stop it. this same idea is used on wheels..it takes longer to stop and accelerate wheels..the further the weight is out from the center axis, the harder it is to accelerate.

-A. Swift

 

alan you aren't thinking about the rotational mass. you aren't just getting rid of 50 lbs. that the car has to accelerate in a straight line, you are getting rid of 50 lbs. that the car has to spin at thousands of rpms.

tie a 5 lb. rock to a string and spin it around. then tie a 10 lb. rock to a string and try to spin it around. then extend the rope by 3 feet and try to spin it again. this should give you an idea of how rotational mass can affect acceleration. bigger heavier wheels WILL slow your acceleration. it's a proven fact. i assume you've never put any aftermarket wheels on your car?

(btw, you showed your ignorance by asking what "unsprung weight" was. that is a very widely known phrase in terms of handling)

 

alan, that was more of a total effect larger wheels might have on a vehicle. Whileu and I might not notice this at all, it is a difference..minute as it is. rotational mass and rotational inertia is what everyone is concerned about..I think I explained it better above.

-A. Swift

 

[QUOTE=TsunamiDC2R,Dec 8 2004, 12:20 PM] But its not just about straight line acceleration.........

the root of the big wheel/little wheel is not weight.... its rotational inertia (as someone mentioned earlier)

the physics equation has 4 constituents

k, m, r, i
where k is some constant......> m is the mass of the wheel....> r is the radius and i is the moment of inertia

the relationship is thus:
i = km(r^2)

what this means is that even though you have a larger wheel that may weigh the same as a smaller wheel the radius is the more important factor...

a 20" wheel that weighs 10 lbs has TWICE the moment of inertia as a 10" wheel that weighs 20 lbs...

moment of inertia is not only related to resistance to rotational acceleration..... but to changes in rotational direction...> in other words the wheel with the smaller moment of inertia will offer better steering response as well as better acceleration.... a few examples i can offer here are thus>

> ever take the front wheel off a bicycle, hold it by either side of the axle, and spin the wheel as fast you can..... and try to change the direction... the wheel will naturally resist this

>try the same test with a smaller wheel .....and the resistance is vastly reduced...


the second and more relevant example> polar moment of inertia in a car> the steering response from the moment of inertia of the wheel acts IDENTICALLY to the rotational responsiveness of a mid-engine car..... its because the weight is more centrally located. The same concept can apply to the chassis of the S2000, given the bulk of the weight is placed behind the front axle, giving it a 50/50 weight distribution.


yes, the tire may be one of the heavier portions of the unsprung mass of the car.... but even moving the sidewall inwards by making it taller... you are putting more "air" at the outside of the equation... effectively reducing the "r" portion for most of the individual portions of mass, of the rotating wheel/tire combo.......


nevermind that having a taller sidewall allows more forgiving handling... reducing the breakaway speed and making the car more controllable & predictable at the limit....

 

If the unsprung weight is that important....I should choose a light weight 16" over the 17" then. The CE28 16" is like 11 or 12 lbs. Save 5 lbs each wheel. Nice!

 

thanks guys I have now bagged a set of 18" CE28N to reduce my unsprung weight!

 

17" would've been better, and 16" SW-388s would've been best. but congrats.

 

There is a reason that Porsche's high end wheels have hollow out spokes. Unsprung weight is very important.

I think 17's are a good compromise. The 16's look a bit small to me, but it sucks to lose performance to get a better appearance.

 

Thanks guys
I want the best of both worlds and 18" wheels.
So the CE28N's are a good compromise.
I guess I could have gone TE37's, Gram Lights or Mugens but I prefer these and they are loads lighter than the EVO5's I have on at the moment!!
Plus they have the same tyre fitment