Racecar Statics and Mechanics of Solids problem

  • Thread starter hks118
  • Start date
  • #1
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Homework Statement


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.

Homework Equations


Ʃf(x)=0
Ʃf(y)=0

σ=F/A

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?
 
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Answers and Replies

  • #2
SteamKing
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Science Advisor
Homework Helper
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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.
 
  • #3
19
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Thanks! makes much more sense now.
 

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