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Help design torque, power needed

  1. Jul 22, 2011 #1
    I am designing a transmission for my class but i have a couple of concepts i kinda confused, i have a 550lbs load, and i want to move 35mph=15.64m/s at a constant speed, assuming the drag coefficient to be Cd=.55 and the coefficient of rolling resistance to be Crr=.3 (tires on sand), i found out the two frictional forces going against a rigid body(my off road car) which are the sum of the Rolling resistance force and the force of aerodynamic drag which turned our to be F=815.21 N, i want to like to learn how to find the torque necessary to move it and the power required, do we keep power constant?, and does the power depend on torque or vice versa? Thanks in advance
    here are the eqn used

    http://upload.wikimedia.org/math/2/8/5/28560fb9ccae7b5f811de11f965d5478.png


    http://upload.wikimedia.org/math/9/a/b/9ab71e6e4f97b6a9d63cfa5aded81f1a.png
     
    Last edited: Jul 22, 2011
  2. jcsd
  3. Jul 22, 2011 #2
    Note that:

    Work = Force X Distance

    Power = Work / Time = Force X Distance / Time

    Velocity = Distance / Time

    Thus,

    Power = Force X Velocity

    You can compute your power from the 815 N force you figured and your 35 mph velocity.

    Then the power and torque are related as:

    Power = Torque X Angular velocity

    And you can find the angular velocity from linear velocity and gear ratios.
     
  4. Jul 22, 2011 #3
    thank you that cleared up alot, but how do you calculate the power and torque needed when the car is at rest to 35 mph?
     
    Last edited: Jul 22, 2011
  5. Jul 22, 2011 #4
    Compute Power = Force X Velocity from your numbers above.

    Then look at your engine rpm at 35 mph. Use the transmission gear ratio to find the transmission rpm. Using this power and rpm, figure the transmission torque.
     
  6. Aug 26, 2011 #5
    To calculate the rate of acceleration don't we need to know the average torque? That is from rest (to get the load moving) through the engines rpm range to 35 mph. Which would be calculated as a function F(x) torque (or maybe F(x) power)?

    Willy
     
  7. Aug 26, 2011 #6
    To calculate the rate of acceleration don't we need to know the average torque? That is from rest (to get the load moving) through the engines rpm range to 35 mph. Which would be calculated as a function F(x) torque (or maybe F(x) power)?

    Willy
     
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