Seat belt reinforcement plate attachment

[chevynut]

I am working on installing seat belts and I have made some steel reinforcement plates for the mounting points. I was going to plug weld them onto the sheetmetal, but I got to thinking that maybe I should epoxy them instead. The strength of the seatbelt mount comes from the fact that it's behind the sheetmetal and would have to tear through it to fail. The plate interface with the sheetmetal would be in shear. The plates are 4.25X2.75" or about 11.7 square inches. Epoxies typically have a shear strength of 3500-5000 psi, so this should support 41K-58K pounds. Four 1/4" plug welds would only support about 10K pounds. So it seems like the epoxy is actually a lot stronger.

Is there any reason you would recommend NOT doing this? I have some high strength aircraft-grade epoxy that I'm thinking of using. The nice thing about it is that if I ever wanted to remove it, I think I could do it by heating it.

[markm]

It soumds like a good plan to me, but I dont wear seatbelts.

[Rick_L]

The plate's attachment to the sheetmetal needs only minimal strength. Its primary force in a crash, relative to the body, is compression, for which it needs zero strength. I suppose there might be some shear also, but the bolt takes care of most of that.

Plug welds spaced away from the bolt hole will not degrade the overall strength with a heat affected zone or a stress concentration.

I think you're overthinking this one.

[chevynut]

Rick, this is for my 3-point front seat belt. I have the upper reinforcement plate welded into the b-pillar and I need to add two more plates.....one for the reel, and the other for the end of the belt. They will both be on the inner quarter sheetmetal. The plate for the reel will be mostly in shear since the belt goes from the reel to the b-pillar anchor point. That's the one I was most concerned about. If it was the rear belts, the force would be mostly perpendicular to the sheetmetal so I don't think the attachment is very critical. I actually have different plans for the rear belts so I don't have to put the plates under the car.

I was wondering if there was any other reason besides the strength of the plate to not use the epoxy. Let's say the epoxy degraded and failed for some reason. Would the bolt alone be strong enough? In other words, let's assume I bolted the belt to the plate but didn't even attach the plate to the sheetmetal. The bolt would have to tear the sheetmetal in an accident as it pulls the plate toward the b-pillar anchor. Would that be strong enough? I'm thinking it wouldn't since the sheetmetal is so thin.

Markm, you've got to be kidding. I thought virtually all states required seat belts. I personally don't drive without one.

[Maddog]

It soumds like a good plan to me, but I dont wear seatbelts.

Do you smoke cigarettes also?

[Maddog]

Either will work, I do think you're over thinking this too. But consider since the welds do nothing harmful but you know they will never "degrade" and have unknown concerns with the epoxy, weld it.

[chevynut]

Nope, probably gonna epoxy it. My calculations show that the epoxy is stronger than the plug welds, and I'd rather not weld it if I don't have to. The 1/8" plate is actually going inside the car and the reel outside the inner quarter. According to my plans, the only place you'll see the belt is from the b-pillar attach point at the top and down to the floor. If I can make it work, it's going behind the headliner.

[Maddog]

Nope, probably gonna epoxy it. My calculations show that the epoxy is stronger than the plug welds, and I'd rather not weld it if I don't have to. The 1/8" plate is actually going inside the car and the reel outside the inner quarter. According to my plans, the only place you'll see the belt is from the b-pillar attach point at the top and down to the floor. If I can make it work, it's going behind the headliner.

So why did you ask?

Your real concern is the shear of the attachment bolt, if anything is going to fail that will be it.

[chevynut]

The reason I asked is to see if there's any other reason I shouldn't epoxy the plate that I was over-looking besides the strength. The bolt is grade 8 and 7/16" diameter so it should support around 13,500 pounds of shear load. If that's good enough for a seat belt mount then the epoxy should be good enough too.

[Maddog]

I haven't looked at every car, and mostly older ones at that but all OEM have multiple spot welds. I think your inability to reproduce the same quality of spot welds at the plate is the major concern with welding other wise I would weld it. Hopefully the epoxy holds for years to come and the plate doesn't become movable.