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Effect of wheel diameter in a car

  1. Jul 17, 2014 #1
    Hello guys

    I want to know how the wheel diameter of car affects the car's performance parameters(physical significance) such as
    1)speed
    2)torque
    3)handling.

    I would also appreciate if some equations are used to substantiate the physical understanding of these.
     
  2. jcsd
  3. Jul 17, 2014 #2

    jack action

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    Just to make sure, are you referring to the wheel diameter or tire diameter ?
     
  4. Jul 17, 2014 #3

    Ranger Mike

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    Steering wheel or wheel for the tires?
     
  5. Jul 18, 2014 #4
    Thanks for replying . i actually mean the rim diameter
     
  6. Jul 18, 2014 #5
    wheel for the tyres.
     
  7. Jul 18, 2014 #6

    rcgldr

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    Assuming the wheel material is denser than the rubber in a tire, then a larger wheel increases the combined weight of wheel and tire. Assuming a fixed outer diameter of a tire, the side walls can be either too tall or too short for optimal handling, depending on how stiff the sidewalls are. As an example, in the case of a car like a Porsche 911, a wheel diameter around 18 inches is usually optimum for racing tires, while a Formula 1 car uses a smaller wheel, and relatively taller and stiffer sidewalls.
     
  8. Jul 18, 2014 #7

    jack action

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    The following assumes the tire diameter stays constant.

    Inertia:

    By modifying the wheel diameter, you will modify the total inertia of the wheel/tire assembly, thus affecting the acceleration of the vehicle.

    It may increase or decrease, depending on the shape and design of the wheel. Here is a calculator to get a crude evaluation for rotational inertia of wheel/tire.

    Tire stiffness:

    With a small wheel diameter, the vertical stiffness of the tire usually increases. This means that the tire radius will vary more under vertical load (such as weight transfer) (deflection = load / stiffness). This affects the force and speed measured at the contact patch (force = torque / radius; speed = rpm * radius).

    The preceding is also true for braking, which will affect front/rear brake bias.

    Having a higher tire vertical stiffness also means that the tire can loose contact more easily with the road - if it is relatively too rough - hence affecting handling.

    Similarly, the longitudinal stiffness will most likely decrease with smaller wheel diameter. This means that a very large acceleration/braking force can be absorbed, giving a smoother force transfer to the ground, like on drag tire for example:

    0810chp_02_z+mickey_thompson_drag_tires_tips+hard_vs_soft.jpg

    Tire rigidity:

    Having a smaller wheel diameter means a less rigid tire. This means that it is easier to remove the tire from the wheel under a lateral force, i.e. under lateral acceleration; A beadlock is then necessary.

    A more rigid tire usually also means lower rolling resistance.

    ----------

    Also, as the wheel diameter increases (no matter the tire diameter), the force between the rim and the tire increases, which can lead to slipping; Again, a beadlock may be necessary.

    Finally, a larger wheel usually means more space for a larger brake system which can absorb more heat.
     
  9. Jul 18, 2014 #8

    Ranger Mike

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    we had a great discussion on mech engineering post

    Alloy wheels and fuel economy
    by Dr Lots-o'watts
    use search above to check it out..
     
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