i have been debating if i should get 17's on my 2000 gsr, vs. keep my stock
15's or go 16 inch rota's.
a lightweight 17 inch rim (example, velox hot die forged rim is 12.5lbs)
would be lighter than my stock rim, plus if you'll look at part of the chart
i got from toyo tires below http://www.toyo.com/tires/tire_specsheet.cfm?id=2

it shows that the 17 inch rim only has 1 revolution per mile less, which is
almost nothing. the 17 inch tire is 2lbs more, however, but then again i'll be
getting i lighter weight rim. i believe my 205/50/15 azenis are like 20+ lbs...
plus my stock rim..so a lightweight 17inch rim and a lighterweight tire should
not slow me down, right?

Tire Size,
Rim Width Range (inch),
Tire Weight(lbs.),
Tread Depth,
Overall Diameter (inch),
Overall Width (inch),
Inflated Dimensions,
Static Loaded Radius (inch),
Max Load (lbs.),
Max Pressure (PSI),
Revs Per Mile,


195/55R15 85V
5.5-6.0-7.0,
16.5,
10.3,
23.5,
7.9,
10.8,
1135,
44,
885


205/40ZR17 84WRD
7.0-7.5-8.0,
18.5,
10.3,
23.5,
8.2,
11.0,
1102,
44,
884


[Modified by EVOL, 7:06 AM 2/21/2003]


[Modified by EVOL, 7:09 AM 2/21/2003]


[Modified by EVOL, 7:12 AM 2/21/2003]

 

Even if you took a 15", 16", and 17" rim that all weighed the same and had the same diameter tire, the 15" would still be quicker. The reason is that it's how far out from the center of the wheel the weight lies that matters. The furthur the weight from the center, the more inertia it has holding it back. Try taking a dumbell in each hand and spinning with your arms out, then bring them into your body and try it. You'll notice you could spin faster when they are close to you. However, the problem with a taller tire (and a smaller rim) is it will be squishier in hard cornering. So, it's all a matter of preference. Peace.

 

isnt' my stock gsr rim like 16lbs and my falken azenis tire like 20+ lbs?
so what about a 12.5 pound seventeen inch rim mated w/ a 18.5 pound seventeen
inch tire?
that's like a 10 pound savings w/ my 17 combo over stock plus azenis.
where the mass is?
go here:http://www.miata.net/garage/tirecalc.html
stock size 195/55/15 vs 205/40/17 is virtually the same.

plus a lighter 17inch rim and tire vs. stock...

 

yep a 17" wheel places most of its weight farther from the spinning axis than a 15" combined with the fact that the tire weighs more also, that weight is farther away too. Inertial forces make the 17" harder to accelerate and decelerate.

 

my 15 inch azenis weighs 5lbs more than the 17 inch toyo tire.
the 17 inch rim i'm comparing my stock rim to is also lighter.

 

Ok... the thing with larger wheels of the same weight (even less weight) is that when you place a weight farther from the axis of rotation the polar moment of inertia increases. And when you do this it takes more force to accelerate an object of the same weight.

Think of it this way. If the wheels weight the same, but one is bigger, basically all you are doing is moving the weight a little farther away from the center, right?

Well, take a foot long stick and tape a 5lb weight to the end of it. Now, hold your hand straight out with the stick horizontal to the ground. Nw rotate the stick up using yoru wrist until it is vertical. Now do the same thing while strapping that 5lb weight to the end of a hockey stick. The hockey stick is going to be much harder to turn. Trust me!

That is the same thing that happens when you increase the tire diameter. You are thinking of it in terms of top speed and how fast the revolution of a tire compares to the diameter.

You should be thinking of it as bigger wheels take more force to accelerate, so you are waisting some out torque on just getting the tires turning instead of using it to accelerate our car.

Bigger rims will cost you a couple ft*lbs of torque... which will inturn hurt acceleration.

PS
I can crunch some numbers for you to show you an example if you need me to.


[Modified by PSU-TEG, 10:58 AM 2/21/2003]

 

[/QUOTE]my 15 inch azenis weighs 5lbs more than the 17 inch toyo tire.
the 17 inch rim i'm comparing my stock rim to is also lighter.[/QUOTE]

I don't know the equation for this off the top of my head, but it's related to physics. The lightweight 17" you speak of may have less inertia than the heavier 15", but if you're going to buy rims and you care a lot about performance, buy some good, light 15's. If you like performance, but think 17's look better, then a light 17 is your answer. If you still want to know the exact inertia % difference between the rims, you can probably find the answer (or equation) in a book, or a book. Good luck.

 

Damn this thread makes me want to keep my kosie's and buy another one to replace the one that cracked.....

 

the laws of physics state that the amount of rotational inertia is equal to one half of the product of the mass of the wheel times the radius of the wheel squared (I=.5mr^2). so.........although increasing the weight of the wheel does increase the amount of inertia, increasing the size of the wheel increases the inertia EXPONENTIALLY.

your stack wheels have at least 562 lbs./in. of inertia.
the 17 in. velox has 452 lbs./in. of inertia
the 16 in. rota has only 336 lbs./in. of inertia

the 17s have 35% more inertia, so it sounds like the 16s are your best bet. alright i'm done; my brain hurts.


[Modified by topgunpilot20, 3:28 PM 2/21/2003]

 

rev/mile is how much they turn over a distance. doesnt really have anything to do with time or acceleration. think about it; where in rev/mile does time even come in. the only thing that you can predict to be effectively changed is the accuracy of your odometer. two identical rims with the same tires (which equals the same overall diameter) that weigh different weights still have the same circumference. revs/mile are gonna be identical because over one mile, both are turning on the same exact amount of surface. doesn't matter if one weighs 1 lb and another weighs 100 lbs, they will still have the same rev/mile. so using that isn't a legit comparison for speed or acceleration.

 

Buy, what you want. Plain and simple. Are you going to be drag racing the car? Are you going to be road racing the car? Are you going to be autocrossing the car? If no then who cares. Put what you want on your car.

 

lol you just proved one of my points...both are turning the same amount of surface.
so size should not be an issue, right?
and then i get a lighter weight 17 inch rim, and a lighter 17 inch tire than my stock
or my azenis...so if my rim and tire combo is lighter than stock..turning on the same exact amount of surface, wouldn't that promote acceleration?

 

the 17s would let you accelerate faster than your stock, though the 16s would accelerate faster than the 17s. but it really comes to how much it really matters. either set would be an improvement, and it sounds like you're kind of leaning towards the 17s anyway so go ahead and get them if you like the look of them. they're still quicker than your stock.

 

it just seems like everyone has this deep belief that just because they're 17's
they'll slow you down.

 

i'm actually leaning towards rota slipstreams in gunmetallic...
but black or bronze volk se-37's would look sooo nice on my 2000 red gsr.

 

Are you getting larger brakes?

If not, get 15's (there big enough for 11" disks) and get a GOOD tire.

I.E. Azenias have a very hard side wall and performance is awesome.

Oh, one more thing. I just want to know if someone wants to crunch the numbers.

what would a 14" 11.75lb rim have for inertia? (Another reason HX's are no joke!)

 

actually my last post doesnt prove your point. it only proves that ur use of comparing revs per mile is pointless is determining which wheel will be faster. for most people up or down-sizing their rim size i'm gonna assume they are gonna try to keep their overall diameter the same. so rev/mile and surface etc dont mean a thing in this case. if you want to compare the difference in acceleration speed between the two wheels you have to consider weight and momentum. and to do it correctly you also have to determine where the weight is distributed depending on the construction of the rim.

my first post was simply a reply to your post concerning the comparison of revs per mile of each setup. i was trying to show you why you shouldnt use that solely as evidence of which wheel/tire combo will be faster/slower in terms of acceleration. good luck in your choice

-alan

 

14 in. 11.75 lb. rims have 288 lbs.in. of inertia

 

How about the 13" vx wheel? I think 9.5 pounds...

 

13" 9.5 pound wheels have 201 lbs.in. of inertia.

 

It's called rotational inertia, people, had you been paying attention in physics.

Any spinning component has it, and the larger the diameter you have, the slower the rotational acceleration will be. Big diameter wheels, tires, brakes will slow down the car during acceleration, and make stopping distance longer. This is why people put on lightened flywheels, clutches, and pullies. Not for the outright weight savings, but for the increased acceleration. This knowledge has been around for literally hundreds of years...

 

weight and weight distibution...lighter rim and lighter tire.

 

Assuming 100% of the mass is 14" away from the axis of rotation. Which isn't true.


There is NO feasable way to measure this without disassembling rims and determining the exact mass distribution of EACH rim.

*edit* changed weight to mass so that my physics professor doesn't rise from his grave to kill me.


[Modified by qtiger, 1:08 AM 2/23/2003]

 

this is exactly what i mean. just b/c something weighs X lbs doesnt mean you can just put all that mass on the outside and calculate it. you have to look at how the weight is distributed, which depends on the construction and design of the rim.

you can't just do what you're doing and say one rim and tire is lighter than another so it automatically gets better acceleration. assumptions like that just aren't true. unless you plan to really do all the math, get whatever best suits your cars use or whatever suits you best.

 

well, you can get good approximations as long as the metal density and distribution are near to uniform....there were some articles in i think scc that dealt with this issue some time ago.

also there is a misconception that a sower profile tire will necessarily mean better handling - which they proved to be untrue.

i don't really see how there is much more discussion - because of the inertia principles mentioned already, a smaller diameter wheel even if slightly HEAVIER will outaccelerate the larger wheel

btw, one of the all time champs is the racing hart cp 035 15 in/ wheel @ 7.5 lbs...