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Rotational mass question

  1. Jul 12, 2004 #1
    im not much of a mechanincal engineer but im hoping someone can explain to me. i appreciate all input... thanks..

    suppose we hand 2 identical cars both accerlerationg from 0-60mph but one car had wheels that weighs 20lbs and the other car had wheels that weighs 27lbs.

    wheel diameter= 19"
    wheel car a = 20lbs
    wheel car b = 27lbs

    1) who would get to 60mph the fastest?
    2) what effects would the weight of the wheels have?
    3) is there some quick reference you can say regarding each pound of the weight of the rim at rest vs. during rotation?

  2. jcsd
  3. Jul 13, 2004 #2
    First off..

    this is a really silly question. Ok, yes, the wieght of the wheels can make a diffrence but in your senero there is not enough of a weight diffrence to matter much. As for who gets to 60 first, who ever is the better racer, and has the better engine.
  4. Jul 13, 2004 #3


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    Little bit rude, don't you?.

    It is not a silly question at all. Chester Rockwell have something of reason for questioning it. If you want a quick reference, you will have to search in the technical manuals of each car. In Vehicles Theory, there is a factor:


    This factor (a,b constants, x=gearbox transmision ratio) measures the vehicle rotative inertia. Thus, the virtual mass of the car is not M, but gM (greater). This factor is provided, and has into account transmission inertia, wheels and shafts inertias.

    By the way, the car with less rotative inertia will accelerate more than the other that has greater one.
  5. Jul 13, 2004 #4


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    Here's just a single example but relevant to your question. On the Speed channel is a show called "Sports Car Revolution" where they have a project Acura RSX. When they went to larger diameter discs that were two pounds lighter and larger diameter wheels with the same overall weight, they lost 12 horsepower. They're using a dynojet which uses the acceleration of heavy drums to measure horsepower. Their source concluded that even though the overall mass of the brakes and wheel/tire combination was less, the distribution at a larger radius accounted for the 7% loss in power.

    Without knowing more information on how the weight is distributed in your 19" wheel and tire (compare the carcass of a Bridgestone/Pilot/Kumho etc tire and its surprising how different they feel) your calculations would be approximations at best. Why I thought you'd find the example interesting, its raw data you can interpret how you see fit. ;-)

  6. Jul 13, 2004 #5
    sorry for not posting enough information for your my questions, but i do appreciate your answers.. even yours mistic. reason why i ask is .. just wondering if purchasing a ligther wheels will yield any gains vs price. that's all. i have no mechanical engineering experiece.. so that why i seeked help from you experts here.

    Clausius2, your response seems that i can benifit from the lighter wheels.. but then Cliff_J responses argues that? okay.. im still confused.

    note.. these two cars are 100% identical.. except on car has light wheels.. tires are the same.


  7. Jul 14, 2004 #6


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    Chester - if you needed to pry open a crate, would you prefer a 6" crowbar or a 3' crowbar?

    Same for flywheels. Think back to some pictures of the really old-school stuff and you'll see some flywheels where its basically spokes supporting a large ring on the outside, all the mass is concentrated at as large a diameter as possible. Here's a link to a site I found via google to illustrate what I'm talking about:

    For a wheel, one could have a very heavy center section and very light rim, the other a super-light center section and heavier rim. As a flywheel they could work out to be either way. I hope the flywheel metaphor works for you, its pretty simple to think that if it stores energy it must have energy input into it to get it up to speed. And if you're using engine power to spin up the flywheel, that leaves less power to accelerate the car.

    Guessing, the heavy one loses everytime. The example I gave was to point out that weight distribution is more important than actual weight in some cases.

  8. Jul 26, 2004 #7
    Its a rather SIMPLE answer.. i dont know why you ppl are going around it Soooo much... the one with LIGHTER wheels will... (unless you apply traction differences which noone has mentioned so i wont bring it into mine).. it is lighter so it has less mass to accelerate.. however 28 pounds is hardly anything, so you wont see much or feel much of a difference... and btw guys.. he said the carsa were IDENTICAL except for the weight of the wheels.... but the cost difference will NOT outweigh any miniscule benefits.. expect u r losing 28 pounds with the lighter ones... so if you must.. pack lighter bags if u get the heavier ones.. lol
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