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Bigger wheels accelarate faster?

  1. Sep 13, 2007 #1
    HI i live in South-Africa and a few of us guys have been fighting on a car club about something......
    I say that if you get 2 of the same cars (Ford Fiesta ST150) which has a standard of 17" wheels. Now you keep the same wheel on the one car and on the other you change the wheel to a larger wheel (Same weight) the the standstil accelaration would be faster on the larger wheel.They say that a larger wheel makes the top end faster and the standing start slower?

    So the question is: Does a bigger wheel make my car accelarate faster from standstill but be more slower on top end?


    Hope to hear from you soon.

    Jacques
     
  2. jcsd
  3. Sep 13, 2007 #2

    rcgldr

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    Homework Helper

    You've stated it both ways in your posts.

    A larger wheel will result in slower acceleration and higher top speed if the engine has enough power. If not, at least the top speeds in each gear but the last will be higher.

    A smaller wheel will result in faster acceleration if tire spin doesn't become an issue, and top speed may be reduced if the car was properly geared for top speed before the wheel change.
     
  4. Sep 13, 2007 #3

    Danger

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    And the weight of the wheels is largely irrelevant, unless there's a huge difference such as tungsten vs. magnesium rims. What matters is that changing the wheel size (and this means tire diameter, not the rims) alters the gear ratio. Without using the actual numbers, going from 28" to 30" tires would be equivalent to swapping your 4:56 gearset for 4:11 unit. Again, those are made-up numbers. By following the power train from engine to ground, you can figure out the actual ones.
     
  5. Sep 13, 2007 #4

    WHat wil be the defference on a car that puts out 118KW at the flywheel? if you go from a Standard mag+Tyre 205/40/17" to (new mag) 215/35/18" ??

    The reason why i thought it would be faster on lower speed is cause like i bicycle when you give an amount of force on a bigger wheel it moves easier and the distance is a bit more that it covers on the road?
     
  6. Sep 13, 2007 #5
    Bigger wheel is equivalent to a higher gear; go faster ultimately, but take more time accelerating. Think of the radius as a lever arm.
     
  7. Sep 13, 2007 #6

    russ_watters

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    If the transmission and wheel size are matched to provide the same torque to the ground (to eliminate the complication Danger pointed out), the wheel with the lower moment of inertia (typically the smaller one) will provide better acceleration. Moment of inertia is resistance to rotational acceleration.

    If the wheels and gear ratios aren't matched, you also have to consider the moment of inertia of the flywheel. Also, this is all moot if the limiting factor is the friction of the tires.
     
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