Upper seat brace

[jswisc]

I've read various debates on the pros/cons of the upper seat bracket. I was showing my car to an engineer friend of mine today. He questioned the purpose of this brace and when I explained it to him he was rather startled that we were required to install this. I have an adjustable upper bar so the seat can be adjusted for and aft and he suggested using a plastic bolt in place of the grade 5. I laughed but after a few minutes it made we wonder. I'm sure tech would object if they notice one, but it might be worth a try. Any thoughts?

[Matt93SE]

As much as I agree that they're not a good idea with a proper FIA seat, they're the rules on a non-FIA setup.
If tech catches you with something as blatantly uncool as plastic fasteners, they're likely to tear your car apart looking for anything else they can find, IF they let you fix that without just telling you to put it on the trailer.


I will tell you the back brace has saved my butt before. I went off once and went airborne over some curbing. the seat was through-bolted through the floor of the car, and the contact with the curb sheared all the bolts off the bottom of the seat. the only thing hold in me in place was the back brace and the seatbelt. Had the back brace not been there, I would have been sitting on top of the fuel cell in the rear hatch.

[Chip42]

jswisc - was your seat FIA? there's so little structure to many a non FIA seat, and SFI seats are built to be back-supported, so either way they should have one. (and you might consider upgrading)

if the seat is FIA then I think everyone knows my stance on the issue.

to Matt: good data, thank you for sharing (seriously). this is reason #18462 to avoid bolting the seat/mounts directly to the floor when possible. one of the things that has evoked the arguments for back braces on FIA seats is bottom (meaning of seat, not FIA mount type) mounting failures, particularly with the involvement of sliders, thus the current rule's wording.

The correct response is to change the rules to enforce mounting the seat to a higher standard - through added cage members, added material to the floor, factory mouting bosses when possible, hardware grades, etc.. while the back brace CAN work as you described, it CAN ALSO cause problems with seats not designed to accomodate it (rear impact in a fiberglass shell with holes drilled in it for the brace would be ugly). better to address the known failure than add under-thought solutions. BTW, I'm glad your situation worked out well - you seem a decent fellow!

[jswisc]

All your points are well taken. The point my friend was making was a good design would allow the adjustment bolt on the adjustable support to be the weak link in a rearward collison. Kind of like a shear pin. Plastic would be on one end of the strength continuum, grade 8 on the other. Maybe something in between?

[Matt93SE]

Yeah, I've learned a bit since we built that first one.. (which was a Lemons car, but the point is still very valid..)

My seat is still bolted through the floor, but I have large, thick plates welded to the sheet metal, and nuts welded on the bottom of them. the "frame rails" on this car are also much taller so there's less risk of bottoming out on the floor like that.

I'm also using 1/2" GR8 hardware instead of the 5/16" that first crapheap was done with. (a bolt size limitation due to using sliders to accommodate multiple drivers)

Umm what else....
Fiberglass seats.. use a LARGE plate on the back for them if you're going to run one. I forget the minimum size for a back brace, but there's nothing wrong with sticking a 3/16" plate of aluminum or so back there and making that plate much larger. it can fasten to the seat in multiple places to spread a side impact load (so it won't tear off the seat), and it will spread the load over your entire back in the event of a rear collision.

Would you rather be punched in the back, or punched in the back while you're supported with a huge plate of metal? :)


All your points are well taken. The point my friend was making was a good design would allow the adjustment bolt on the adjustable support to be the weak link in a rearward collison. Kind of like a shear pin. Plastic would be on one end of the strength continuum, grade 8 on the other. Maybe something in between?

I haven't looked at the rules recently, but does it say that it must be a completely rigid mount?
I'm envisioning a spring/shock type of device back there to help absorb a rear impact, but be restrained so it will not stretch during a frontal impact?

I would not want a shear-pin of any sort back there, unless there were multiples staged. if I'm in a wreck bad enough to break that, I don't want to go flying free as soon as pin breaks. If you get hit that hard from the back, there's likely going to be rolling/bouncing/additional collisions there and I still want to be in place. Something to help absorb the shock would be good though.

But then again, that's why FIA seats are hard shells with padding. they spread the load over your entire torso so you *shouldn't* need shock absorption and such.

[Chip42]

All your points are well taken. The point my friend was making was a good design would allow the adjustment bolt on the adjustable support to be the weak link in a rearward collison. Kind of like a shear pin. Plastic would be on one end of the strength continuum, grade 8 on the other. Maybe something in between?

IF you want he brace to fail (its on a seat that doesn't want it), then a plastic bolt would make sense. as a tech inspector, I'd ask you top forget this idea.

load spreading braces such as matt describes, GLUED to the seat, work. huge PIA and not necessary if you do the rest of the install right (I think 1/2" bolts for the mount is a bit mutch tho...)

whatever you do - palstic bolts or not, DON'T drill holes into a fiberglass shell.

FIA seats are designed to flex a bit, to absorb some energy like the damper matt describes. I'd have to check on the legality of that.

[Matt93SE]

whatever you do - palstic bolts or not, DON'T drill holes into a fiberglass shell.


good point. I didn't think about that. never messed with a glass seat- they scare me. I'd rather have metal that will flex and bend in an impact vs. shatter and slice. :)

as for hardware, DOT requires that seats and belts be mounted with a 7/16"-20 bolt. even in a Japanese car, those are SAE bolts. 1/2" is overkill, but they're cheap and readily available at any hardware store if you need to replace one. And I wouldn't be worried if I had to replace it with a Gr5 bolt since it's so big.

[jswisc]

I'm pretty happy with my seat mounts. I have an aluminum Kirkey. A square tube frame is attached to the floor at four points with 10 guage plates welded in place. Same plates on the bottum of the car. The two sides that run the length of the car are uni-lug rails with moveable plates that are threaded. The seat can be moved fore and aft by backing off the 4 bolts on the seat base and removing the bolt in the back brace. Allows for appr. 6" of travel. My son has long legs! Back brace is quite solid when tightened. I guess designing a system that would save you from a severe rear hit but not leave you vulnerable to secondary hits would be diffcult. Would be nice though.