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KEEP M3POST ALIVE BY DOING YOUR TIRERACK SHOPPING FROM THIS BANNER LINK! 
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07082009, 06:11 AM  #2  
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 The circumference of your tire/wheel combo is larger than before (this actually will reduce force applied to the ground, but at the same time proportionally increase vehicle speed per wheel revolution, so it doesn't actually affect power to the ground  in other words, in gear acceleration will be reduced)  Your tire/wheel combo is heavier  The contact area of your tire/wheel combo is increased  this will be negligible, but greater contact area = greater rolling resistance. Of course the plus is that you get more grip. Note there are many other contributing factors including tire compound, pressure etc.. 

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07082009, 06:39 AM  #3 
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^ has the nice answer.
But generally speaking yes it will make your car slower.
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07082009, 06:57 AM  #4  
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Thanks for the detailed info mixja 

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07082009, 12:34 PM  #5 
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07092009, 12:23 AM  #6 
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The bigger the heavier, the heavier the slow pick up≥
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07092009, 12:35 AM  #7 
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Actually, if all four wheels (19" & 20") are the same brand/model, have the same offset, the same diameter, and the same overall rolling tire diameter...
The 20" wheel/tire combo will probably be a 'tick' faster in the acceleration department. Again...the measurable aspects listed above must be exactly the same, or this doesn't work...
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07092009, 10:17 AM  #9 
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07092009, 11:15 AM  #10  
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Acceleration w/ 20s
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BTW, you didn't reply to my response to that post.
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07092009, 11:19 AM  #11  
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07092009, 11:53 AM  #12  
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07092009, 01:51 PM  #13 
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lol...you guys are killing me here...
The reason I said the 20" wheel/tire combo may actually be a 'tick' faster (and I literally meant a couple hundredths of a second), is because you cannot simply assume that a larger diameter wheel will automatically have a detrimental impact on your performance. That's not necessarily true. Now I laid out a scenario that would prove this, and no one seems to understand that my opinion is based on a set of tightly controlled parameters. What I was saying in my earlier post, was that if you were to have two identical wheel designs, with the same offset and widths...the only measurable difference would be the outer diameter (19" vs 20") of the two wheels. (putting weight aside for the moment) It's highly probable that your acceleration would be ever so slightly improved by the 20" wheel/tire combo rather than the 19" wheel tire combo. Now some of you are probably perplexed to hear me say that, but this is not wild speculation on my part. The following numbers will tell the story.... If you have two hypothetical wheels with the following specifications: 19 x 9 +20 offset , wheel weight = 21.0 lbs 20 x 9 +20 offset , wheel weight = 22.0 lbs. These are both realistic weights for quality forged wheel products sold by several manufacturers. Tire specificationsboth tires are Michelin PS2's to level the playing feild) 255/35/19, section width: 10.2", rolling diameter: 26.0", weight: 25.0 lbs. 255/30/20, section width: 10.2", rolling diameter: 26.1", weight: 24.0 lbs. Note: These 2 tires are as close as I could get for a 19" vs. 20" tire having almost identical section widths and rolling diameters. Quick math shows: 19" wheel tire combo weight = 46.0 lbs. total 20" wheel tire combo weight = 46.0 lbs. total Big deal they come out to the same weight right? Well on the surface it would seem so, but if you look a little deeper you will see the reason why the 20" wheel/tire combo has the slight advantage. Calculating a given wheel/tire combinations weight impact on performance is very anecdotal to say the least. There is no scientific formula (that I know) which will give you a 100% correct no doubt about it result. The best that anyone can do, is to minimize the variables when comparing to samples, and use time tested experience (and a little math) to prove or disprove your theories. Now the reason the I believe the 20" tire wheel combo will edge out the 19" tire wheel combo is based on a combination of simple physics theories. If you analyse the outer most portion of each wheel/combo listed above, you would find the the total weight at the most distant point away from the center point of the cylinder (middle of the wheel bore) belongs to the 19" wheel tire combo. The 19" tire has taller sidewall vs. the shorter sidewall on the 20" tire. Why is this significant? Because the outer 3" of each wheel/tire combination will impact acceleration performance more than anything else. And since both combinations have nearly identical exterior rolling diameters, the sum total wheel/tire weight in that specific zone, will be higher on the 19" sample. Steelbelted radial tires will generally weight more per square millimeter than a forged 6061T6 Aluminum wheel. So that means the 19" wheel/tire combo has a total weight disadvantage on the outer 3" portion of the two samples listed above. That fact, combined with the unsprung weight + rotational force + moment of inertia formulas that will multiply (times 4) the effect the farther you get away from the center point of the wheel = a very slight performance disadvantage for the 19" wheel/tire combo. Believe it or not, the theory is solid. That's why the I said the 20" wheels performance might not be such a forgone conclusion. This is also why I don't like to go into long drawn out explanations about this stuff...
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07092009, 02:03 PM  #14 
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That depends on what radial section of the wheel and tire you are looking at/comparing, meaning this is a 3D consideration not 2D. The 20" wheel has its barrel further out from the rotation axis by 0.5" (and the barrel has significant mass). You are saying the taller sidewall of the 35 aspect tire will have a larger impact than that on rotational inertia. Without seeing the actual numbers, I wouldn't reach any conclusions, but I doubt that is the case.
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07092009, 02:54 PM  #15  
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By contrast, the weight of any automotive wheel (OE or aftermarket) is spread out over a much larger surface area...thus reducing it's negative impact on performance on the outer edges of the cylinder. Factoring in the density and weight of the materials discussed here, and it's not even close. Yes , the barrel is slightly farther out...but it's the much lighter weight material. A forged 6061T6 aluminum wheel barrel is much thinner than you may think. Only a few mm in overall thickness is not uncommon. Therefore, it does not impact the performance (MOI) as much as the tire weight does. Remember the overall rolling diameter is essentially the same, so the outer cylindrical weight is skewed in favor of the 20" combination. There is no denying this fact. I reduced the the variables to an absolute minimum, so I could draw an applestoapples conclusion based on this theory. Look at my data more closely...
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07092009, 04:17 PM  #16  
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Assume: * Thickness of barrel is negligible, and thus I = m x r^2 * The approximate 1lb difference in tire weigths is entirely due to the higher sidewall (tirerack numbers look rounded to me) * The extra tire mass is concentrated in a 19.5" dia cylinder, also with negligible thickness * Density of forged 6061T6 aluminum is = density of 6061T6 aluminum (which is not true, but I don't know what the variation is) * Thickness of barrel wall is 3 mm Calculate and compare I for the 20" barrel and the combined I for the 19" barrel + the extra tire sidewall mass (according to the assumptions above), you get this: So, they come up pretty much dead even. Let me know if you have some exact figures for any of this such as barrel thickness, or better yet, weight, or the exact tire weight difference. Cheers. EDIT: This calculation does not include the extra weight associated with the slightly longer (and perhaps thicker) spokes of the 20" wheel.
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07092009, 05:03 PM  #17  
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Just so you know, the 3mm barrel thickness is too high. Try 2.5 mm 2.7 mm for barrel thickness on a 6061T6 forged wheel. Even that calculation won't tell you the whole story... This is why I don't like to state something so complex on internet message boards. The negative or positive impact on performance is not a simple mathematical calculation. There is great disagreement, as to which formula should be used to accurately calculate the gains or lossess derived by manipulating the location of weight on a hollow or solid cylinder. The location of the weight on a rotating unsprung mass, is not a black and white calculation. The effect is not linear in regards to aftermarket wheels. So therefore your math is a bit off. It's not completely wrong, but the equation leaves a lot to be desired. A straightforward mathematical calculation will never tell the whole story. This is why I didn't answer Greg. You want a tidy little equation that can give you a definitive answer. There is no definitive answer. Math is a part of the answer, and the other part comes from real world performance numbers you collect over a long time period. On the track, on the dyno, time trials, braking tests, suspension performance, steering feel. Things of that nature. The 'jist' of my post was to prove (to a certain degree) that you cannot immediately assume that a larger diameter wheel combination (that retains the same rolling diameter) will automatically have a negative impact on performance. (i.e. a bigger wheel will hurt your acceleration performance) Although the math is fuzzy, you basically proved my point. I've known this for many, many, years...and I didn't need a mathematical equation to figure that out. BTW: Calculating the affect of the placement of the weight on a rotating unsprung mass does not exist. That equation assumes a linear affect on performance. It doesn't work that way with aftermarket wheels and tires. (where the actual weight is NOT consistently spread out evenly)
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07092009, 05:10 PM  #18 
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I'll chime in. We debated this in the past. I did some very similar calculations as those above. See here. My conclusion is that in general a larger wheel and tire combination that maintains roughly the same overall diameter will be accompanied by a larger moment of inertia.

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07092009, 05:19 PM  #19 
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The use of such approximations are certainly valuable. Yes they can send you in the wrong direction but generally they will predict correctly. Let's think about the dip in the barrel (just for example). You can try to calculate the exact effect on moment of intertia compared to simply assuming a totally cylindrical barrier. But you can also use some common sense and it will tell you that small detail does not matter. Similar arguments apply to other small area of wheel and tire details. Get a problem conceptually correct and even a simple model will be predictive. If you really want to get precise you can make a detailed 3D CAD model. Such a model, along with appropriate densities WILL give you a near perfect result for the moment of inertia. Once you have the mass and inertia, the effects on performance are very well goverened by very basic physics. Alternatively you could also measure the wheel/tire combinations moment of inertia.

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07092009, 05:36 PM  #20  
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You must always try to limit the amount of variables involved. (or as many as possible) That gets you closer to the truth. This is exactly why I used exact weights and sizes. I repeatedly stated that this was not something one could prove beyond a reasonable doubt. The weight distribution of automotive alloy wheels makes it a 'tweener' in the world of physics. There is no way to definitively guess exactly how the weight shifting will affect real world performance. (to an absolute certainty) All the available formulas just 'guesstimate' an approximate value. A close value...but not it's not deadonballs accurate. If there is such a formula...please post it up.
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Last edited by Lemans_Blue_M; 07092009 at 06:18 PM. 

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07092009, 05:52 PM  #21  
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You make a bunch of "approximations" in your previous posts. How are the more detailed approximations I made irrelevant all of a sudden? Without getting all worked up, I think one can say that, once you factor in the weight of the longer spokes, it will pretty much be a wash, or the differences will be small/not noticable. I never said otherwise. If you really wanted to see how the different wheels behave in a more general sense, one can take months and years, and develope a much more complex and accurate performance model that would be predictive of those scenarios as well. No point in arguing against any of that as that is the basis for real world product development. I don't get your criticism of physics based mathematical modelling at all. Any such well conceptualized approximation, no matter how basic, is better than someone's opinion. The basic math for angular acceleration I did took me 10 minutes, so its implications are limited (do not extend to complex performance scenarios, etc).
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07092009, 05:58 PM  #22  
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