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Homework Help: Calculating engine torque/horse power from datalogs

  1. Sep 14, 2005 #1
    The idea is using datalogs to calculate horsepower/torque. The given information is rpm, mass, time....

    Here is what I have so far:
    DragA=.5 * densityAir*Area of the front of the car*velocity^2



    Axlerpm= Engine Rpm/(Gear*final drive)

    Diameter(inches)= Rim+ 2(tirecross section * Aspect Ratio/100)/25.4

    v(ft/min)=(diameter/12) * pi * Axlerpm

    Now 1hp=33,000ftlb/min so could I simply say HP=v*mass ? That does not seem correct at all, but the units work.

    If I do F=MA, I get units of lb*ft/min^2 which is definitely not correct. Basically I'm confused and any help would be appreciated it.

    Thanks in advance for any help.
  2. jcsd
  3. Sep 16, 2005 #2


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    Staff Emeritus
    Science Advisor

    RE: "HP=v*mass ?"

    Energy is the product of Force applied over a distance.

    Power = rate of energy (produced/consumed) = product of Force and speed.

    Actually energy and power would be found by integrals, but I am assuming in this case that the force and speed are constant to simply the problem.

    Torque is a product of force and moment arm.

    The drag puts a force on the car. The tires must provide the force in balance, and in the opposite direction (assuming constant speed and not accounting for friction).

    The driving axle must provide the torque, and the engine provides a torque to the driving axle (hence the gear ratio).
  4. May 12, 2011 #3
    "Diameter(inches)= Rim+ 2(tirecross section * Aspect Ratio/100)/25.4"

    Could some explain this equation please? How does the tire cross section come into play and what is aspect ratio in this case? And 25.4?

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