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Homework Help: Racecar Statics and Mechanics of Solids problem

  1. Nov 1, 2011 #1
    1. The problem statement, all variables and given/known data
    So I am designing the suspension for a small formula car. It's a pushrod suspension much like what is shown below:
    [PLAIN]http://f1-dictionary.110mb.com/Images/pullrod_poshrod_push.gif [Broken]
    I am trying to find the required tubing diameter for the loads the car will experience.

    2. Relevant equations
    Ʃf(x)=0
    Ʃf(y)=0

    σ=F/A

    3. The attempt at a solution
    The free body diagram
    ocx0cl.jpg

    The car is assumed to weigh 700lbs, have 60-40 front to rear weight distribution and be in a 2G turn, so the maximum force of the tire is 525lb. θ is assued to be 45°

    By the method of joints:

    At pinned joint A
    Ʃf(x)=0=FLCA-cos(θ)Fp
    Ʃf(y)=0=Ft-Fpsin(θ)

    At pinned joint B
    Ʃf(x)=0=-Fs-Fpcos(θ)
    Ʃf(y)=0=-FRy-Fpsin(θ)

    So Fp=Ft/sin(θ)

    And the maximum force in the pushrod is around 743lb. One of the tubing thicknesses under consideration is 5/8 .035 4130 chromoly which has a yield strength of 70PSI.

    Cross sectional area is
    ∏/4(.625in-.59in)=.0275 in2

    Calculating the stress at maximum load is calculated at around 27000 PSI :eek:

    I know my statics is rusty but this is wayyyyy off. This tubing thickness is within the range of what other people are using, maybe a bit on the small side. Can anyone point out where I may have gone wrong?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 1, 2011 #2

    SteamKing

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    Staff Emeritus
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    Homework Helper

    The cross sectional area of the tube is incorrect: A = (pi/4)*(do^2 - di^2)
    The min. yield strength for this material is 70000 psi (70 ksi), not 70 psi.
     
  4. Nov 4, 2011 #3
    Thanks! makes much more sense now.
     
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