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Help: Aerodynamics vs Rotational Mass

Discussion in '2nd Gen. Tacomas (2005-2015)' started by powerban, Feb 1, 2010.

  1. Feb 1, 2010 at 7:29 PM
    #1
    powerban

    powerban [OP] Well-Known Member

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    Hey TW. Im looking to buy a set of wheels for my 09.
    I will be using the stock tires that come on the sport model. Will the pros from the lighter wheels be 'weighed' down by the extra drag from the negative offset? I drive about 90% highway and 75 miles each day.

    Aerodynamics - The wheels in question will have a -10 offset. The stock sport wheels have a +30 offset so it will increase the wind drag at highway speeds (65mph).

    Rotational/Unsprung mass - 10 lbs savings on each wheel. Engine works less to spin the wheels. Since its rotational mass, the truck will feel lighter and be quicker.

    Have any of you switched to lighter wheels to increage your mpg?

    Thanks
     
  2. Feb 1, 2010 at 7:43 PM
    #2
    topgun155

    topgun155 Well-Known Member

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    IMO the aerodynamics will be minimal unless the whole tire is outside the body. The front of the truck pushes a large air wall that most likely will make most of the air currents go around the wheel well.
    As for the rim weight it will help, how much just depends.
     
  3. Feb 1, 2010 at 7:56 PM
    #3
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    FlimFlubberJAM
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    Reducing rotational mass is one of the best performance oriented things you can do to a vehicle. On a truck, it would be more affective than worrying about aerodynamics.
     
  4. Feb 1, 2010 at 8:30 PM
    #4
    powerban

    powerban [OP] Well-Known Member

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    Thanks Chris. Would you happen to know the ratio from reduction of rotational/unsprung mass to sprung mass? Ive seen something like for every pound of unsprung mass saved, it feels like 8 lbs of sprung mass. Ive seen as high as 10 and as low as 5.

    By the way, is installing an 8" wide wheel on the stock sport tires ok? Stock wheels are 17x7.5
     
  5. Feb 1, 2010 at 8:52 PM
    #5
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    You should be fine with the tires. I cant remember the ratio of unsprung mass vs. rotational mass. IIRC, it was closer to 10 to 1
     
  6. Feb 1, 2010 at 10:57 PM
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    Juggernaut

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    Reducing mass will not help hwy millage as you are just cruising. Having less mass will help with city millage where acceleration is involved.
     
  7. Feb 1, 2010 at 11:05 PM
    #7
    powerban

    powerban [OP] Well-Known Member

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    Thats what i was thinking. I know the effects of reducing unsprung mass since im come from riding motorcycles and slight racing background. You accelerate quicker and brake quicker. Cant say much for cruising since you never cruise during a race.
    Im just gonna assume that the motor has to work less to spin 40 lbs of metal hence lighten the load and save gas.
     
  8. Feb 2, 2010 at 3:10 PM
    #8
    powerban

    powerban [OP] Well-Known Member

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    Anyone else have any input?
     
  9. Feb 2, 2010 at 3:35 PM
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    ElectronMan

    ElectronMan Well-Known Member

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    You are spending too much time thinking about this. 1) If you get lighter wheels, a majority of your gains, if any, will be seen in city traffic 2) You aren't driving fast enough to see a drastic change in mpg with cd changes.
     
  10. Feb 2, 2010 at 4:15 PM
    #10
    thinkingman

    thinkingman Well-Known Member

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    You are getting some good info here.
    Remember, it's not just the reduction of mass, but the radius of the mass that is also a factor.
    Remember this equation:

    Aerodynamics+truck=su(x)
     
  11. Feb 2, 2010 at 4:20 PM
    #11
    skistoy

    skistoy Make mine a Double!

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    What?
     
  12. Feb 2, 2010 at 4:40 PM
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    Juggernaut

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    I believe what thinking man was trying to say is the farther out the mass is, the greater the effect it will have on acceleration & mpg. In other words, a 1 lb increase in rim weight is not as significant as a 1 lb increase in tire weight as far as rotational inertia is concerned.
     
  13. Feb 3, 2010 at 1:55 PM
    #13
    thinkingman

    thinkingman Well-Known Member

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    Foglights are for fog, not oncoming traffic!
    Yes, thanks.
    Think of the radius of your tire/wheel combination and the application of weight farther out on the radius, the greater the effect of that mass.
    Radius is like a lever.
     
  14. Feb 3, 2010 at 3:15 PM
    #14
    powerban

    powerban [OP] Well-Known Member

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    Yeah, ive always understood that the further out the mass, the more effect it has on rotational inertia. So to conclude, having the lighter wheels on the same tires, will benefit anything having to do with acceleration and deceleration, and not so much with steady speeds. The wind drag from the -10 offset does not counter the pros of the lighter wheels that much. Plus new wheels will make a better looking ride, depending on your preference.

    Thanks guys
     
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