As I'm sliding in the trailing arm pivot bolt, I notice that it's only a grade 5.

Now, my tiny little non-engineering mind thinks, "Gee, there's a lot of important stuff hanging offa this here bolt!" :eek:

Why not grade 8 bolts here and in some other places that "seem" structurally important. Can't just be cost savings. Chevy would never do that on a Corvette would it? ;)

Okay engineers, give me the full treatment. :crazy:
:D

 

Rest easy at night, Hvn. You only have to consider the maintenance history of these cars to realize that failure of that Grade 5 bolt is non-existent as far as I know. It's dem dam high maintenance rear wheel bearings dats usually training IRS Corvette owners in "Sphincter Muscle Crunches". :D :D

The task of GM engineers was to figure out the least expensive component that could be used in an application safely, and corporate liability for owner safety was not taken lightly. Paying more for a Grade 8 fastener because it was "safer" would be overkill, and GM would never run up the manufacturing cost without good cause.

Because of the relatively close lateral clearances, the loads on the bolt are taken primarily in shear, i.e. the edges of the hole are trying to cut the bolt perpendicular to its length. There are no significant bending stresses to consider.

I don't know the maximum loads that this bolt could see, but it is, what, a 9/16" or 1/2" diameter bolt? That means the cross-sectional area is about 0.2 square inches. Assuming an ultimate shear strength of 50 ksi for a Grade 5 bolt (heck, I don't know, you engineers speak up :D ), that calculates out to about 10,000 pounds maximum load before failure.

Considering they probably used a safety factor of about 5, that means they were actually designing for loads around 2,000 pounds at the fastener. Actual side loads at the wheel would be less because of the moment lever of the trailing arm, but I doubt you could get enough friction between the road surface and the tire to come close to overstressing that bolt.

Fatigue stress was generally of greater concern, but in this application, the number of cycles was probably nowhere even close to those of, say, a connecting rod bolt.

 

Stronger is NOT always better.
While Gr.8 bolts have a greater tensile strength, they are also more brittle.

 

A grade eight bolt can be torqued to a higher value, so it will provide more clamping force that the same size grade 5, but grade 8s are more expensive due to differences in alloy and heat treat. A slightly larger grade five bolt - one with the same design tightening torque as the smaller grade 8 will provide equal clamping force, so that's typically what OEMs use. It's very rare to have a bolt break in service, but they can break due to material loss from corrosion or over-torqueing.

Duke