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Plus size wheels and performance hit

Discussion in 'Wheels & Tires' started by crisslo, May 15, 2009.

    crisslo guest

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    All, anyone, everyone,
    Have you ever investigated plus size wheels affects with respect to the Rotational Moment of Enertia of the wheel and tire?
    A large "I" (moment of inertia) can cause your car to accelerate less, effect handling, suspension, general ride and fuel mileage.

    So how does this do this?
    If you assume for simplicity, plus not including the tire to start with, a wheel as a disk of uniform density, then the formula for "I" is I = mr*2. In English that says the moment of inertia equals the mass times the radius squared. But computing mass can be a problem. However, if you are looking only for comparative differences in wheel weights and their diameters, as compared to other wheel weights and their diameters, then you can do the following:

    % difference (from one wheel as compared to another) = weight (of a certain wheel) times its radius squared divided by the weight (of a different wheel) times its radius squared all times 100.

    Or in formula form % = (wr*2/w'r'*2)100
    where w = weight of a certain wheel of r radius. and w' = weight of a different wheel of r' radius

    Lets do a simple comparison. One 17 inch wheel weighs 19 pounds. Another 18 inch wheel weighs 20 pounds. Calculating the % difference gives the larger wheel to be about 18% larger in "I". 18% is a noticeable difference. You can really take a hit because of the radius being squared.
    If you install these larger wheels, you will notice right away your car with less performance than before. Sure the car wheels will look "snazzier" but with a performance hit. Your engine has to work harder to move the larger wheel as compared with the smaller wheel.

    This was a real "eye opener" for me after I did these simple calculations that I found in my old physics book.

    On the upside, if you can find wheels that are lighter in weight and the same diameter/width as your stock wheels, then you will gain in acceleration etc. Remember even if the next plus size is equal to the weight of your present wheel, you will still take a hit in performance. Reason is because the radius is squared in the formula not a linear progression.
    I am not saying you shouldn't go to larger diameter wheels that is your decision. Just keep in mind the Laws of Physics are constant for Newtonian Physics. Since we aren't approaching the speed of light here (186,000 miles/sec), this formula applies.
    Interesting huh??

    I have been considering the idea of changing my wheels to some other style and diameter. I notice a lot of car owners will buy larger diameter and wider wheels to make their car look "classier" without considering the downsides of different weights and diameters of the wheel. Even wider wheels of the same diameter will weigh more and give you a performance hit.
    Now when you add the weight of the tire and consider its overall diameter (keeping in mind to keep the same overall diameter of the tire/wheel combination for your car) things can get worse especially if you choose wider tires which weigh more yet.

    I was considering going to an 18 inch wheel (my stock 17 inch wheel for my 2001 525i sport model), but now I am not so sure I will.
    Newton strikes again -------- something like Murphy's Law.
    Comments anyone? I am open for correction/additions/etc.
    • Member


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    You are basically correct, The factor you purposely overlooked is probably as important as anything here. That is, a wheel's mass is not uniformly distributed, as your calculations assume. A greater portion of the mass is in the rim ('the outside part where the tire mounts', for all those morons who refer to the entire thing as a 'rim') than in the rest of the wheel. The result is that the effect you calculate is exaggerated because there is more mass at the outside edge with a plus one fitment. If it is also a heavier wheel, the odds are that an even greater portion of that mass is near the outside edge. Even if it were the same weight, the mass is distributed farther from the wheel center. Finally, if the wheel is wider, then the rim must be commensurately wider (and probably heavier) as well.

    Unfortunately, there is no way I can think of to measure the different distributions of mass unless you are using two- or three-piece wheels where you could independently weigh the centers and the rims. [Edit: Thinking more about it, if you were willing to sacrifice a one-piece wheel for the sake of getting the information, you could cut a known section out of it (maybe 1/4 or less) and determine the center of mass of that section. This would provide you with at least a 'half-and-half' analysis and further circumferential cuts could refine that. Bent wheels + machinists = theoretical fun!]

    Plus one (or more) fitments may bring with them some performance advantages in turning because of the shorter tire sidewall, but when it comes to acceleration and braking, they are, as you said, a losing proposition. You will notice that many more people will choose the performance disadvantage, nonetheless, in a demonstration that fashion is more important than function - to them, at least. :cool:

    jonathonbarton guest

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    Conversely, you could just take it to the Dyno and see what the performance cost/benefit is in real-world terms.

    Just for reference - the wheel/tire combinations from that article, 17" and 39# and 46.5# respectively. If you were to go from the SSRs to the stockers (Can you say Snow Tires?) it's a 142% difference (624890/439569*100), and in the real world, that translates into 0.179 seconds longer from 25 MPH to 80 MPH...


    Really, it looks very much as though the performance hit is - for anything short of trying to wring those last thousandths of a second our of the car - negligible.

    crisslo guest

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    I would like to reply to both jonathonbarton and CRKrieger. Both inputs are interesting. CRKrieger's input is definitely on target. My formula would be a lot more complicated trying to come up with what he described more accurately. He is right, most of the weight is concentrated further out on the wheel. He is also correct that it is difficult to come up with a true number comparison due to uneven distribution of the total mass.

    jonathonbarton input is also of interest.
    Jonathan, could you please tell us where you got that information graph from (the wheel/tire combinations from that article, 17" and 39# and 46.5# respectively)? I love to see things graphically. Your data makes me scratch my head, but I will accept any correction.
    Thank you both for the quick response.
    • Member


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    The effect is not nearly (for most slightly different setups, not crazy heavy 22" type junk) as great as internet myths lead people to believe. You need to calculate the increase as a percentage of total rotational inertia not just the wheel to wheel comparison. Hence the % increase in rotational inertia is much smaller when analyzed properly (i.e. total engine, flywheel, transmission, driveshaft, differential, half-shafts, wheels, brakes, etc). Once you know the total rotational inertia, then you can analyze the percentage change (of course once you actually properly calculate the I for the two wheels).

    Of course since we're talking about rotational mass here, the effect is greatest when acceleration is the highest hence the effect is largest at slow speeds and falls as speeds increase (acceleration falls).

    Most people worry way too much about 3-4lbs of wheel weight imo.

    This can be easily estimated with Cartest2000 to get an idea of the order of magnitude of the changes, and then acceleration changes analyzed.
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    In day-to-day driving, there really is no performance hit. For drag racing, or track racing, there might be. In all honesty, ~.1 of a second is too little of a time for a human notice. As long as it's not dramatic, like 5 seconds, it shouldn't matter what size wheel/tire a person is running as long as it fits, doesn't rub, and doesn't look ridiculous.

    (And as long as it's not chrome and/or a spinner.)
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    Whats wrong with spinners? You don't like this look? How could you not? LOL!! :D

    jonathonbarton guest

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    Click the link in my post for the original article, and much more interesting information, such as HP/time and Torque/time graphs.
    The '97 M3 gained 1HP and 1 foot/pound of torque by going to the lighter wheels, but for daily driving and/or autocrossing (unless you're ekeing out those last thousanths), the effect would be negligible, as the car accelerates through the first two gears so quickly with either set of wheels/tires that the car gains only 0.03 seconds in each gear.
    Seems like a good investment if you're already a perfect driver. =)

    Going the other direction, the 'performance hit' from having a heavier wheel/tire package that you like looking at every day doesn't seem to be appreciable. Your car isn't suddenly going to start accelerating like a Karmann Ghia because you're rolling on 19's. =)
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    Well, there ya go. I am an autocrosser and I've both won and lost by smaller margins over much longer periods of time. ;)

    The other thing is, at least on my E28, I can easily feel the difference in which of my wheels & tires I have mounted at any given time. The 14"s are soft and smooth, as you would expect of a 60 series on a stock bottlecap wheel. The 15"s are close, feeling slightly rougher, probably due to the smaller sidewall. The 16"s feel surprisingly ponderous compared to even the 15s. I am fairly sure I could tell the difference in a blind driving test. Anybody want to change a buncha' tires to win a bet? :D
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    • Technical Service Advisor


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    That article only takes acceleration into account between stock and leightweight same size wheels. There is no comparison between stock and plus size, and as stated before, the extra weight on a plus size wheel is at a greater distance from the center than the stock size. It also completely ignores the penalties paid due to unsprung mass, as these also have a huge impact on transient suspension response. Going to 19s on an E36 (for example) is bad, bad, bad. And thinking it doesn't cause issues for day to day use is also misguided. I have driven E28s with 17s that handled horribly because they were not set up properly. 95% of wheel swappers do not make necessary suspension mods/adjustments to adapt for the larger size, change in contact patch distribution, extra unsprung weight, etc. and the penalty in day to day driving can be decreased traction, which can cause a crash under normal conditions. One E28 owner brought the car in because it kept activating the ABS. It did so because the car had been lowered and saddled with 17s. The result was sucha huge reduction in traction from camber and toe issues that are only accentuated buy tires with stiff, short sidewalls, that you could activate the ABS under hard (not panic, but normal hard) braking. This means that they were obviously losing traction much sooner than a properly sized set of tires and wheels at stock ride height. If the car had proper lightweight 17" wheels, was lowered with a good set of springs and dampers, and had an adjustible subframe setup properly, it would probably have been fine, but the ride would still have been harsh.

    And, as CR has said, races are lost by thousandths and that article shows that it can make the difference of well over a tenth of a second each time the car accelerates from 25-80, not to mention that the car will have a similar increase in braking time each time the brakes are used. That is why racers use light wheels.
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    I think I'm going to be sick.

    So same diameter but lighter has small perf hit, but bigger diameter and heavier would probably have a more significant hit?

    Is there a site to find out the weight of different stock wheels? I really want a set of CSL wheels but don't want to take a perf hit with the increased size from 18 to 19, so I've also been looking at light 18's or even 17's for track use. Issue with 17's is not all will clear the brakes of the M Coupe.
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    The best place to find wheel weight data is www.wheelweights.net There seems to be the most data there. You will see that the same size wheel can vary greatly. Also keep in mind that the tire weights can also vary a couple pounds between brands and also between batches of the same tires. With mountain bikes some people pick through batches of tires to pick out the lightest ones in the batch.

    95 525i
    91 525i
    87 325is (in repair)
    85 K100RS
    00 Z3 (wife's car)

    Wagon Boy guest

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    The performance differences seem small. If you are racing you know what to do. For the others it is about road feel and looks.

    My '92 525it rides on 225/60/15 Dunlop Summer tires, and 215/65/15 Nokians in the winter. These feel great to me but even with the Dinan engine and trans chips, the car is slow. My 1995 520i rides on 205/65/15s with snows for the winter and all season Nokian i3s for the summer. I have no power to spare and would go with 195s but I want the traction and load rating. I have a set of 16" rims but the selection is limited for 205/50/16 summer only tires. To me, this is a reason to go with 17" rims: to get good tires. The E34 never came with anything over 17" rims and the suspension was designed for 15" to 17" rims. From there it is a trade off from comfort to handling. For my daily driver I want soft but I changed the strut inserts on the 520 to Bilstein Touring. My wife does not like the ride. She calls it "jerky". I like the taught feeling and I don't think it's harsh at all but the difference is dramatic to me. When I change her wagon struts they will be Sachs.

    There is another consideration. Living in Pothole City, Chicago, it helps to have a high profile tire to maintain comfort and decrease rim damage. I know a guy with 40's who lost 3 tires last winter alone. But everyone has their ideas. My co-worker had 20" on his Cadillac and he loved them but with all the flat tires he went back to stock.

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