This statement unfortunately exactly illustrates my point. Strength is NOT the issue. STIFFNESS is the issue. They are two completely different structural properties. This is like saying that at drag car is "fast" when you mean "quick" (E.T. is not the same as MPH).

 

 

OMg, Chief, stop! LOL

 

OK…..this is becoming almost comical. Especially with Chief’s post of the little blue pill..

I can’t be certain of the grade of mild steel these brackets are made from as I just pulled the stock off my scrap rack. For discussion purposes we will assume it is A36. This is one of lowest grades available. Still the tensile strength of A 36 is 36 ksi or 36,000 pounds per square inch.

The cross section of my brackets at the location of my mounting hole is .25” x 2.75” = .688 sq. inches. Therefore the amount of internal stress required to begin to bend is 36,000 X .688 or 24,750 lbs.

I seem to remember the endurance limit of mild steel to be around 50% of tensile strength or for this example let’s assume 12,375 lbs. So as long as we stay under this amount of internal load we are safe from fatigue failure for an infinite number of cycles.

Now on any given day, we can assume the rear mounts of these brackets will be subject to approximately 100 lbs of downforce on a 3inch moment arm. I do try to avoid performing Duke of Hazard type of launches during my commutes and I do slow down for rail road tracks. I appreciate everyone’s concern here, but I think we’ve got this one covered. We are far below any area of concern of fatigue failure of these brackets.

tc

 

Sorry, that is completely incorrect. First, there are different properties for shear stress, tensile stress, and compressive stress. Second, NONE of this has anything to do with the STIFFNESS of the part, which is only a function of the SHAPE of the cross section (an I-beam is stiffer in bending than a bar), the area of that shape, and the modulus of elasticity of the material. Your calculation has only determined the tensile force necessary to tear the bracket apart. It has nothing to do with bending, shear, or tearout failures.

 

I assure you the stock mounting locations are not very strong. In HS I had a friend we called "Tiny", 7'4" tall and 350#. He could lean back and rip any GM seat (buckets or bench) out of the floor without breaking a sweat. The seats in these old cars are not a safety device, it merely serves as a cushion to support our humble a$$es while providing something to lean back on.

 

Still not a reason to make them LESS structurally adequate. Look, life is a series of risks. You evaluate the risks and choose them accordingly. The factory mounting points are MUCH stronger AND stiffer than any of these aftermarket add-ons. Your life, your call as to whether or not that matters to you. Just do NOT try to BS me with half-fast analyses that talk about the structural capabilities of some home-brew design.

I drive my 62 with the solid steering shaft pointed at my chest. I do this with my eyes open. I also installed three point belts. I realize they are not as capable as the ones in new cars, but they are better than no belts at all. I also took great care to locate and reinforce the attaching point at the top of the B-pillar.

 

Really, I think we are done here Joe. I appreciate your concern and comments but from this point on..I'm out of this discussion.

Good day.
tc

 

I did not consult MIL-HDBK-5, but the prior post just said "tensile strength". A36 is not a material typically used in aerospace structures and I don't have the allowables memorized. Yield or ultimate is irrelevant, taking area times tensile strength (yield or ultimate) has nothing to do with bending, only tension as seen in an Instron tester. It also has nothing to do with fatigue.

 

Not everyone took Statics, Joe.

 

Yup.

 

I think everyone can agree that most importantly it is the angle of the dangle

 

 

I guess this is one of the few times I'm glad I'm short!😁

 

Now what is the best method for retaining this information within my cranium for future and rapid access?

 

Now you're bending the discussion to thermodynamics? It most likely depends on who &/or how many are sitting in which seat.

If the angle of the dangle is inversely proportional to the mass of the *** the heat of the meat will be constant. I suspect there exists a relationship?

 

Depending on whether the shape qualifies as "compact" the maximum bending stress is 0.66Fy. Non-compact sections are typically 0.6Fy for bending. Shear stress if that is of interest is usually 0.4Fy.

Joe P. has valid points about stiffness and strength. You may put a chair at the end of a spaghetti noodle that is sufficiently strong to support you, but if it deflects 12 in when pushed in any direction, one will soon have a severe case of motion sickness. It may be strong enough to support you but it's too flexible. Generally, serviceability for humans requires enough stiffness that minimizes perceived movement and sufficient strength to comfortably support the load. Fatigue loading is a whole other ballgame that significantly reduces your allowed material stresses.

If you pay attention on bridge overpasses while you are stopped at a red light in the turning lane when an 18 wheeler drives by in the lane next to yours perceptive people will feel the bridge "bounce." Too much bounce will make people evacuate their place on the bridge.

 

My last comment on this discussion: I have a friend we all refer to as “House” as in he is as big as a house. At his biggest, he was 6’1” and well over 450 pounds. While he never broke the seat mounting bolts or floors (3rd gen Firebird), he did on a fairly regular basis destroy the seat backs. It wasn’t uncommon to find the reclining seat would no longer stay upright.

House had bariatric surgery about 10 years ago. He now weighs about 195. He still has his Firebird, it’s about 2 tenths quicker now than it was during his heavy years. Once again proving the rule of thumb that every 100 pounds removed from a car is worth about a tenth on the timeslip.

 

Or have a bigger car

 

I think it’s funny you guys believe its the seat that holds you in place during a collision 🤣🤣🤣

the seat belts do that, which are connected to the floor.

 

Winner!

 

I think it's funny that you guys lack the reading comprehension skills to understand that fatigue loading on a cantilevered bracket due to your fat @$$ bouncing up and down has nothing to do with crash survival.

 

Yup, nothing…, zilch, when the brackets in the seats themselves are thin sheet metal and are the weak link

 

Thinking about the cantilever argument, would it be helpful to wedge a right-sized block between the rear of these adapter plates and the floorboard to compensate for flex/bounce?

Something like this?

 

Bearing against the thin sheet metal floorpan doesn't solve the problem. Back to my original point, the factory crossmembers under the floorpan provide this reinforcement. Those are the hard points for seat mounts and seatbelt anchors.

I have to be honest, I've wasted waaaay too much time hear. People can do whatever the hell they want. Clearly I don't have a clue about structures design.

 

Thanks, Joe; appreciate the data.

 

you do waste a lot of time here

 

I'm still on factory braces. 😋

 

But you needed to take Statics first to take Mechanics of Materials......

 

Quite right.

Yes, for a head on collision. It's worth noting that the extra space created might get you a broken collarbone from the different geometry of the belts. More significantly, the seats are less strongly mounted in case of any other impact.

 

I don’t know if I should tell you or the other engineer this,,, but,,, I,,,, drive my old Oldsmobiles on the highway with drum brakes,, ALL AROUND 😳😳

 

Aren't you forgetting about the ratio of cushion size to quality of pushin'?

 

I hope the OP has garnered some pertinent information pertaining to the original question; the OP will need to decide on cushion size/quality.

 

Hi just wonder what are 6`6" in centimeters ? I have sometimes think about to trade the 98 for a Cutlass 66-67 or 70-71 to get more
space in the garage or doing something to garage. I am 182 cm tall myself.

 

6'6" = 78" = 198cm = TALL

 

Thanks maybe 66-67 is larger inside than 70-71. I used to drive a 66 Cutlass long time ago and this was no problem
with leg room.

 

I drive a 73 Econoline with 4 wheel drum. Drum brakes are fine. Neither myself nor the other engineer have problems with any stock system; it's the modifications dreamed up by some hack fabber that thinks he can out-design professional designers just because he can weld that is dangerous. Scarebird, since we're on brakes, is a good example.

 

Oh boy, we really lived life on the edge back then, we even road bikes without helmets and sometimes with no hands, look Mom!

 

Chemical Engineer. Why this was required is beyond me. As is why I kept the $17.xx textbook.

 

if we consulted engineers everytime we did a mod on our hot rods, the whole hot rod world would fail to exist.

im not a hack…although I do know some engineers that are hacks