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Automotive Car subframe not falling off

  1. Aug 9, 2016 #1
    It boggles my mind how a car's subframe is only held on by a few bolts and some rubber bushings. How is it that the bolts don't just shear off? It would seem that the forces of driving, sometimes erratic, should easily overcome the strength of a bit of metal.

    People have been putting in aftermarket polyurethane bushings and even solid aluminum bushings into their subframes, and I can't help but wonder how long before those bolts snap. Or if those bolts are of sufficient strength, that they tear through the car's body where the bolts are bolted in to.

    Actually, solid bushings seem very appealing. Maybe reducing sprung/unsprung weight would reduce forces that would otherwise cause damage on an unmodified car.
     
  2. jcsd
  3. Aug 10, 2016 #2

    jack action

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    It is not the bolt shearing strength that holds 2 pieces together, it is the friction force caused by the compression generated by the bolt's tension.

    A low-strength steel bolt can withstand 55 000 psi of stress in tension before permanent deformation occurs (i.e. tension when correctly torqued). For a ½" bolt, that is 10 800 lb of force (##= 55 000 \times \frac{\pi}{4}0.5^2##). With a friction coefficient of 0.6, it means the friction force is 6500 lb between the 2 sandwiched plates. Even with a friction coefficient of 0.2, you still have 2100 lb of friction force.

    Even if friction is not present, the shear strength is usually 58% of the tension strength, so 32 000 psi for a low-strength steel bolt. In single shear, a ½" bolt can still support 6200 lb of force.

    It is usual to use high-strength bolts in those situations, which can withstand twice as much.
     
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