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      07-04-2008, 08:20 PM   #1
Socale92
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so glad i didnt get 20's

sensation of power/acceleration loss from 18 to 19 inch rims is dramatic! caveat emptor

Last edited by Socale92; 07-06-2008 at 07:06 AM.
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      07-04-2008, 09:25 PM   #2
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depends. the oem 19's weigh less (tire plus wheel) than the oem 18. so the 19 should accell faster. plus they are forged and therfore stronger. 20's are just for bling not performance.
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      07-04-2008, 10:15 PM   #3
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Quote:
Originally Posted by sayemthree View Post
depends. the oem 19's weigh less (tire plus wheel) than the oem 18. so the 19 should accell faster. plus they are forged and therfore stronger. 20's are just for bling not performance.
word im sure your right, just reporting my initial observations on teh switch in size...
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      07-05-2008, 01:32 AM   #4
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Quote:
Originally Posted by Socale92 View Post
word im sure your right, just reporting my initial observations on teh switch in size...
so you went from 18's to 19's? the tire diameter shoudl be the same. unless the BBS LM;s are heavier than the OEM 19's which could be the case. how bout some pics?
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      07-05-2008, 01:35 AM   #5
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Originally Posted by sayemthree View Post
depends. the oem 19's weigh less (tire plus wheel) than the oem 18. so the 19 should accell faster. plus they are forged and therfore stronger. 20's are just for bling not performance.
wrong. overall weight is only one factor. rotational intertia is the key principle. 19"s have greater rotational inertia because of the diameter of the wheel is larger. i'm not a physics geek, but it is incorrect to say that 19" will accelerate faster just given the fact that they are lighter than 18"s.
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      07-05-2008, 09:48 AM   #6
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wrong. overall weight is only one factor. rotational intertia is the key principle. 19"s have greater rotational inertia because of the diameter of the wheel is larger. i'm not a physics geek, but it is incorrect to say that 19" will accelerate faster just given the fact that they are lighter than 18"s.
This doesn't make sense.....since the overall tire wheel package weight on the 19" is lighter and the overall height of the 19" is the same then the 19" will accelerate faster since its lighter??

http://www.miata.net/garage/tirecalc.html

Both the 18" and 19" tire-wheel package have an overall diameter of 26.3"... the overall diameter & radius are both exactly the same with only a slight difference in the circumference of .2" So

"IF" the 18" were significantly lighter or had a smaller overall diameter...say 25.0" then in theory they would accelerate faster...
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      07-05-2008, 01:16 PM   #7
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Someone should post actual weight numbers from the wheels, fronts and rears, 18s and 19s, and finally with and without tires. I seriously doubt the claim that 19s are lighter than 18s. As others have stated both the weight of the wheel and it moment of inertia are factors that affect the performance of a car with the wheels. The wheels have be be linearly accelerated like any part of the car that does not rotate (say a seat) but in addition they have to be spun up (angularly accelerated) which takes additional power.

Although I saw an old test report for the E46 M3 that showed some performance advantages (at least two of the following acceleration, braking, track times were improved) for the 18s vs 19s, I seriously doubt you can feel the difference, they are quite small. It is like saying you can feel a suitcase in your trunk in a car that weighs 3700 lb with 400+ hp. Not possible.
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      07-05-2008, 03:13 PM   #8
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Quote:
Originally Posted by swamp2 View Post
Someone should post actual weight numbers from the wheels, fronts and rears, 18s and 19s, and finally with and without tires. I seriously doubt the claim that 19s are lighter than 18s. As others have stated both the weight of the wheel and it moment of inertia are factors that affect the performance of a car with the wheels. The wheels have be be linearly accelerated like any part of the car that does not rotate (say a seat) but in addition they have to be spun up (angularly accelerated) which takes additional power.

Although I saw an old test report for the E46 M3 that showed some performance advantages (at least two of the following acceleration, braking, track times were improved) for the 18s vs 19s, I seriously doubt you can feel the difference, they are quite small. It is like saying you can feel a suitcase in your trunk in a car that weighs 3700 lb with 400+ hp. Not possible.
as stated - the 18 and 19's have virtually the same diameter so rotational inertia is not an issue. the 18 and 19 PS2 tires also are almost virtually the same weight so that would not affect inertia - if the 19 tire was one pound lighter than yes a slight disadvantage would occur as keeping the wieight closer to the axle is an advantage.

the wieghts of the two e92 OEM tires and wheels have all been posted here before. its clear the 19 tire/wheel wieghs less than the 18 tire wheel combo.
see link below.

as far as e46 wheels? the e46 rear with a DUnlop SSR is 52 pounds.
the e92 rear 19 inch wheel with PS2 mounted is 51.4 pounds. my front e92 19 wheel was 47 pounds.

http://www.m3post.com/forums/showthr...ghlight=weight
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      07-05-2008, 03:22 PM   #9
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Despite weighing almost the same, the 19" rims will have slightly more rotational inertia than the 18" rims as the majority of the rim mass is distributed further away from the rotation axis (by 0.5" in this case). But that effect would be so small that one cannot feel it.
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      07-06-2008, 02:41 AM   #10
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i went from OEM 18's to OEM 19's and it feels even better. In terms of performance, pretty much identical but i just love the feel of the 19's much more
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      07-06-2008, 06:37 AM   #11
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Quote:
Originally Posted by Socale92 View Post
power/acceleration loss from 18 to 19 inch rims is dramatic! caveat emptor
Utter rubbish.

Wheels. Power. Hows that then Einstein? What, the wheels have upset the engine and it only produces 350bhp now? Duh.
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      07-06-2008, 07:04 AM   #12
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Originally Posted by Leg View Post
Utter rubbish.

Wheels. Power. Hows that then Einstein? What, the wheels have upset the engine and it only produces 350bhp now? Duh.
excuse me for sharing an opinion....bloody english...
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      07-06-2008, 07:26 AM   #13
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Socale92,

I understood what you meant mate, it's possible in theory to have a difference in acceleration between 18" and 19", the more side wall may allow for more flex and thus slightly more traction as the wheel starts to turn but the improve might need to be measured not in 0.?s but in 0.00?s of a second.
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      07-06-2008, 08:04 AM   #14
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Quote:
Originally Posted by Leg View Post
Utter rubbish.

Wheels. Power. Hows that then Einstein? What, the wheels have upset the engine and it only produces 350bhp now? Duh.
Have you ever driven the same car with an 18" to 19" wheel change? Maybee then you'd get your head on straight instead of making baseless comments like this. There is a reasonable performance loss with a move from 18" to 19" not because the engine doesn't produce enough power but because the drivetrain power has to motivate a much larger mass at each of the 4 wheels (the center of the mass is displaced outward by alot when moving any size up in wheels). Its basic physics. Its one of the reasons you see F1 cars with tiny (forged lightweight) wheels and big fat tires.

This is an effect you can easily feel and notice. Ever wonder why all the BMW test mules always use small wheels too? Because they are performance tuning it. They only move to large wheels at the end of test cycle because they want to run the car with the wheels that will go on sale.

Anyone with an e-pinion that this effect is small and negligible should simple find a friend with a BMW and do a wheel swap from 19" to 18" one day and see for yourself. The move from 19" to 17" is like a slap in the face, but thats too small of a wheel for an E90/E92

And no matter what your opinion is, theres no need to jump down the man's throat, grow up!
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      07-06-2008, 08:13 AM   #15
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Quote:
Originally Posted by sayemthree View Post
as stated - the 18 and 19's have virtually the same diameter so rotational inertia is not an issue. the 18 and 19 PS2 tires also are almost virtually the same weight so that would not affect inertia - if the 19 tire was one pound lighter than yes a slight disadvantage would occur as keeping the wieight closer to the axle is an advantage.

the wieghts of the two e92 OEM tires and wheels have all been posted here before. its clear the 19 tire/wheel wieghs less than the 18 tire wheel combo.
see link below.

as far as e46 wheels? the e46 rear with a DUnlop SSR is 52 pounds.
the e92 rear 19 inch wheel with PS2 mounted is 51.4 pounds. my front e92 19 wheel was 47 pounds.

http://www.m3post.com/forums/showthr...ghlight=weight

As lucid started to say, its not about the total weight (its only a gross reflection of the distributed weight) but its about where the weight is located in the mass. Obviously the wheel/tire combo will have a similar total diameter, but if one w/t combo has most of the weight centered towards the middle close to the hub its very easy to create the centripital force needed to move it round and round. If the mass is farther away its harder. And so thats the reason that a 19" wheel, even when lighter will be worse then a heaver 18" wheel. Now this won't work on the extreme situations (40lb cast 18" wheel vs. 16lb mag 19" wheel). But in most reasonable real world wheel decisions, this rule is almost absolute.
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      07-06-2008, 09:34 AM   #16
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John,

I can't argue with what you are saying because I don't have the physics degree to know whether you are right or wrong. The mass being moved out is correct though I don't know if the affect is felt to the extreme you are suggesting.

I do know the Leg's car came with 18" and has since changed up to 19" aftermarket rims, so I reckon he sould be able to notce the difference that you are talking about.

I choose 18" myself but purely from a handling and ride perceptive, the 18" should have a more forgiving ride and a nicer balance at extremes of grip than the 19" and all other things like steering feel, etc.
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      07-06-2008, 08:58 PM   #17
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You don't need a physics degree to see this in action. Next time you are in the grocery store get a big box of kitty litter and put it in the front of an empty shopping cart. Next, turn the cart around a corner and see how difficult it is. Now, move the kitty litter to the back of the cart closest to you and take the same turn. You will notice that it is much easier to turn the cart when the mass is closest to you. This is the same idea with the wheel. The mass is the same but it's distance from the center of rotation affects the force required to change it's direction. Resistance to change in direction is the layman's definition of inertia. It may not seem like an inch on a wheel would make much of a difference but then again, we are talking about hundreds of rpm, not a shopping cart hanging a left in the parking lot. Realistically, there wouldn't be much seat of the pants change in straight line acceleration with all things equal, grip, rolling diameter etc. There would, however, be a difference in seat of the pants acceleration if the rolling diameter changed which would cause a change in effective gear ratio. Hope that helps!
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      07-06-2008, 09:21 PM   #18
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Quote:
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depends. the oem 19's weigh less (tire plus wheel) than the oem 18. so the 19 should accell faster. plus they are forged and therfore stronger. 20's are just for bling not performance.
but the GTR has 20" wheels...
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      07-06-2008, 10:54 PM   #19
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but the GTR has 20" wheels...
and 480+hp.....
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      07-07-2008, 03:52 AM   #20
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Quote:
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You don't need a physics degree to see this in action. Next time you are in the grocery store get a big box of kitty litter and put it in the front of an empty shopping cart. Next, turn the cart around a corner and see how difficult it is. Now, move the kitty litter to the back of the cart closest to you and take the same turn. You will notice that it is much easier to turn the cart when the mass is closest to you. This is the same idea with the wheel. The mass is the same but it's distance from the center of rotation affects the force required to change it's direction. Resistance to change in direction is the layman's definition of inertia. It may not seem like an inch on a wheel would make much of a difference but then again, we are talking about hundreds of rpm, not a shopping cart hanging a left in the parking lot. Realistically, there wouldn't be much seat of the pants change in straight line acceleration with all things equal, grip, rolling diameter etc. There would, however, be a difference in seat of the pants acceleration if the rolling diameter changed which would cause a change in effective gear ratio. Hope that helps!
Theres nothing wrong with your physics, but you like others above are making the assumption that the weight in the 19's is distributed in the exact same manner as the 18's, and therefore, having a slightly wider radius that means they must be slower.

But lets be honest. None of us know how the weight in these wheels is actually distributed, since they are different designs. For all we know the 18's have really dense and heavy barrells while the 19's have most of their weight at the hub.

So we are left with the basic equation of a slightly lighter wheel (19's) with a slightly larger diameter vs a slightly heavier wheel with a smaller diameter and bundles of usual internet speculation. I would be willing to bet that the difference between the two is negligible and people claiming they can really feel the difference are at best suffering from a placebo effect and at worst talking trash.

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      07-07-2008, 05:58 AM   #21
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Mick,

I can assure you that you should feel the ride improvement for a start and the way the car behaves in both steering and way the car finally lets go of grip should also be different. How much this is notice is up for debate and depends on the quality of the driver but the facts are that there will be a difference no matter how small.

The thing about acceleration differences I can't see how but I am remaining open minded of this subject.
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      07-07-2008, 06:28 AM   #22
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Quote:
Originally Posted by Socale92 View Post
sensation of power/acceleration loss from 18 to 19 inch rims is dramatic! caveat emptor
Dramatic might be over stating it just a little don't you think.

I would agree if you were talking about handling characteristics. A small/lighter wheel will readily move in response to road bumps and will have more grip when tracking over an imperfect road.

But as far as rotating mass goes, 18s to 19s would not be dramatic. You could be seeing max 3-4 hp gain, hardly dramatic.

The TV show Sports Car Revolution (SCR) took an Acura RSX and changed the tires/wheels from 15" to 17" and did a dyno comparison. Both sets of tires and wheels weighted about the same, so no change in unsprung weight. The end result was that with the 17" wheels the dyno measured 7 less horsepower at the drive wheels when compared to the 15" package. So with a two inch smaller wheel, they reported a max 7 hp.

You also have to take into account wheel design. All wheels have their weight distributed in different ways, due to different wheel designs. This will affect the whole rotational mass equation.
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