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Unsprung Weight Reduction...

  1. Jan 18, 2017 #1
    Looking for effective weight lost or horsepower gained....

    Original wheels weighed 29 lbs
    front wheels 20" x 10"

    rear wheels 20" x 11"


    New wheels weigh 22 lbs
    front wheels 19" x 10"

    rear wheels 19" x 11"


    for the purposes of the exercise I will assume tire weights are the same since the side wall is taller but the diamater is smaller...


    thanks much in advance!

    John
     
  2. jcsd
  3. Jan 18, 2017 #2

    Ranger Mike

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    there are several posts on this mech eng forum discussing this subject..do a search
     
  4. Jan 21, 2017 #3
    You don't gain any power at all. What you do is improve the function of the suspension and reduce the force needed to accelerate the wheels. Note that power is time-dependent (it's a rate) so it varies at constant force with changing speed. Stated crudely and very simply, constant force and constant mass = constant acceleration = increasing power. Constant mass and constant power = decreasing acceleration as speed increases.
     
  5. Jan 21, 2017 #4

    jack action

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  6. Jan 22, 2017 #5
    There are too many unknows. For example, you state the reduction in weight of the wheels, but you don't know where that weight is coming off. It makes a big difference if the rims are lighter, a lesser difference if the hubs are lighter. Moment of inertia of the wheels is dependent on the mass distribution across their radius. This will affect how much torque it takes to accelerate the wheel's rotation. Restating, the power difference depends on the angular velocity of the wheel. For example, if you save, say, 10 ft-lbs torque for a given angular acceleration, the power difference is greater at 100 RPM than at 500 RPM. Power is time dependent.
     
  7. Jan 22, 2017 #6

    Randy Beikmann

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    Unless you're racing the car, the difference you'll see in straight-line performance from the effect on the car's effective mass will probably be too small to notice.

    What you'll likely notice more is the improved consistency in traction over uneven pavement, since the suspension will be able to control vertical wheel motion better (it results in less variation in sidewall deflection). It will be easier and safer to drive the car fast. But as Ranger Mike said, do a search on this site for discussions on unsprung mass.
     
  8. Feb 18, 2017 #7
    You would be surprised how big a difference it can make. Here's a Car and Driver test comparing aluminum and carbon fiber wheels on a Shelby GT350.
     
  9. Feb 20, 2017 #8

    Randy Beikmann

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    I wish they had done the comparison, between wheels, with the same type of tires. With the amount of horsepower that car has, traction is also important, not just wheel inertia and mass - even with the "running start" technique they used.

    The coast-down runs they did were more useful. In fact if done right, would have been all that was necessary. But the coast-downs shouldn't be run from 60 mph to 1 mph, as they did. It's not very repeatable. They should be more like 60 to 40, or 70 to 40.

    While reducing wheel weight is a good thing, I think the way they ran their test was unscientific, and may have exaggerated the effect - we can't know. It's too bad they did all that work and left so many loose ends.
     
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