shiners780

The installation instructions for a particular belt say that the anchor point "must be able to withstand a minimum of 2,000 lbs of force". Obviously we all don't have testing procedures or equipment to determine the amount of force, so how do you know if what you have fabricated is sufficient? How do I determine whether or not the mount I fabbed will withstand a minimum of 2,000 lbs of force?

What I currently do is look at it while scratching my head and say "yup, that should do". Not exactly scientific....

gbaker

Not a bad summary here: http://www.engineersedge.com/fastener_thread_menu.shtml

Short answer: Determine the effective cross-sectional area of the bolt (not the major dia.) and the strength of the material. Then:

Strength = 0.5 * area * strength of the material.

(^ a rough approximation, assuming single shear.)

shiners780

That measures the approximate strength of the fastener itself though, right? Can the same formula be applied to the material the bolt is fastened to? I am not sure how I would go about determining the strength of any particular mounting point (sans the fastener itself).

How many people here actually determine the effective strength of their seat and belt mounting points? Anyone been surprised at the strength or weakness of any particular point?

gbaker

Right.

No. It's very case-specific.

Back in the Dark Ages (before computers), engineers would reference "Roark's Formulas for Stress and Strain" for generalized equations. I don't have this available, but the electronic version is rather ubiquitous. Try throwing the question out here. It's a pretty sharp crowd.

For specific mounts you would need to run a finite element analysis, or break a sample.

Check back if you can't find a Roark's. I can probably dig it out if the shovel is large enough.

shiners780

So what you're saying is that most people do what I've been doing...look at the mount while scratching head and say "yup, that should do".

schwank

If you've ever seen the cage discussions here Jim.... it's pretty obvious that most safety stuff comes down to that measurement methodology. You can make assumptions about any material... but any equipment is only as good as the installation.

gbaker

Yeah, pretty much.

adrial

You'd be surprised how small a fastener can take 2,000lbs of force ... even in single shear.

Shear is 4/3 * Force / Area ... limit around 150ksi for a typical fastener ... the fastener would probably fail in bending... but if you have plenty of safety margin for shear ... bending is probably OK.

No such thing as pure shear...most everything will fail in bending...unless it's torsion.

RedlineMan

Shiner;

About the best a mortal can reasonably do is study a stock mount and try to create nothing less. Reality; it is not that hard to outdo a factory mount. If you have a good understanding of how the forces are entered into the supporting panel, and how you should go about attaching your own fabricated piece so as not to diminish the strength of that panel, you can pretty easily come up with something effective.

Case in point. The stock mount in a 944 side sill is nothing more than the external 20-22ga. steel panel with a slightly thicker plate (maybe 18ga.) containing the threaded receiver, and spot welded on the back of the sill panel. A .125" plate spot welded in would obviously outdo that quite easily. however, continuous welding this piece in may weaken the host panel, and so it should likely be stitch welded in place, possibly including a couple of plug welds for good measure.

I cannot remember for sure, but it seems I remember a strength of 3500lbs as the accepted standard for passenger cars.