Write up for the 944 Strut Brace

When I designed the bar I did take the shear load into account.  I would like to address each of the components separately. The flexing and the resultant stress loads are a function of the ultimate side loading on the tires before they break loose and how that stress is placed into the shock tower.  I took into account the geometry of the front end and the bending moment of the tower and determined the expected tension on the bar, joints, ends and the hardware.

 

The center bar and end joints are not in a shear load.  As the shock towers try to flex in and out the joints and bar are in either tension or compression.  I have selected the material and thread design that will have an ultimate tensile strength that is over 20,000 psi or 20 ksi using the SI unit of measure. This is many times the maximum tensile load that the tires can exert onto the towers. 

 

The mounting plate was designed to also carry many times the calculated stress.  If you look at the design, the mount base and the vertical member are welded in such a way as to carry the shear load down the full length of the weld.  In other words, the shear loading is carried over the entire length of the weld an in the longitudinal axis of the vertical member.  The material that I selected for these parts has very good shear and tensile strength but also have great weld characteristics.  When calculating the shear strength of this part, even taking into account the slight reduction in strength in the weld heat effected zone (HAZ), the ends shear and tensile far exceed the strength needed to maintain the front end geometry of the car without failure.  According to the United States Steel Corporation (USS) Handbook Of Plate Products, the material we selected has an ultimate strength if 24,000 PSI or 24 KSI using the SI measure.  The smallest area of material under load is approximately ½ resulting in that area maintaining a minimum of 12,000 pounds of ultimate strength.  The material is also very ductile and therefore not susceptible to vibration cracking.

 

The last area of consideration was the selection of a 3/8 grade 8 bolt.  The bolt is the smallest part that is in shear loading.  In reviewing the ASTM table for grade 8 bolts, the 3/8 bolt has a body shear strength of 9,940 Pound Feet (lbf) of strength.  The threads are not in shear so their strength was not calculated.  With the placement and selection of the bolt, its shear strength far exceed the calculated stress.

 

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In final summation we should look at the car as a whole.  The car has a total weight of 3115 pounds.  The cornering loads are primarily maintained within the structure of the cars thin sheet metal frame.  This camber truss is designed to be a secondary stiffening devise to hold the loads that the frame cannot maintain.  The weakest part of the truss can maintain over 9,940 pound feet of loading.  In reality the weakest part of the cars front end is the sheet metal that is used to build the shock towers and they do not seem to have many problems. 

 

In the final analysis we could lift the weight of four 944s off the ground and suspend them without a fear of failure.  I hope this answers you question about the design methodology and strength.