Harness bar strength?
I don't know if anyone here can help me figure this out, but I am confused by something.
I recently decided to build a harness bar for my Elise. I looked at the OEM and aftermarket ones, and for various reasons didn't like any of them. Most of the options are made from 1.5" DOM tubing in wall thicknesses from .120" to .135". My understanding is the OEM one is similar as well.
Not wanting to just copy this without knowing why, I looked up the requirements, and what I found is this:
For DOT safety, the requirement for seat belt attachment locations (from 49CFR 571.210 if you want to look it up) is that the lap and shoulder belts must simultaneously support 3000lbs for 10s.
I couldn't find an applicable racing requirement for attachments, but for belts it is similar, SFI 16.1 states that shoulder belts must support a load of 3300lbs for 10s, and have a breaking strength of 4500lbs.
Race cars are often designed with a peak deceleration rate used for loading, generally around 15-30g, so 3000lbs to support 1/2 of a 200lb driver seems pretty close to me.
Now, a harness bar in the Elise is about 3.5 feet long, and we can assume the loading from the shoulder belts is symmetrically applied about a foot from each end.
Running the calculations with a 1.5" steel tube, we would find that the tube isn't even CLOSE to being strong enough to support this kind of load. Max bending stresses for a simply supported tube with this loading are on the order of 200ksi (about 3-5 times the strength of steel, depending on grade).
So, my question is, what am I missing here? Are all the available options for harness bars just underbuilt, at least according to these standards? Am I an idiot that can't calculate simple bending, or making some invalid assumption here?
Let me know if anyone has any ideas
02 Triumph 955i
93 Mazda RX7 3 rotor
09 Mini Clubman
14 Mercedes GLK250
99 Mygale FC