Brad96

iTrader: (1)

I built this harness bar, to temporarily mount 5-point harnesses in my '88 Formula. It's made from 1.5 OD x .120" wall mild steel tubing. The upper "arms" are welded to 3/16" plates, which mount to the ceiling using the factory shoulder harness screws. The lower "legs" bolt to the rear seat belt receptacle mounting points on the transmission tunnel. I could have gone to the outer seat belt mounting points, but that would require removal of more interior plastic.

I know that a proper roll bar would provide shoulder harness mounting points, with the significant added benefit of rollover protection. But, this car is my daily driver, and my kids sometimes ride in back (not right now, of course). Plus, the interior is original and quite nice, so I don't want to hack it up. The idea here was to make a truly bolt-in piece. After the race is over, I can remove the bar and reinstall the factory seats and belts.

Removing the headliner to access those upper mounting points was a pain, but also gave me a tiny bit more helmet clearance, which is nice.

And don't worry, I'm not going to race the car with the stock seats. I have Corbeau A4's on order, which will work better with the 5-point harnesses.

Comments are welcome.

xpndbl3

iTrader: (5)

Doubt that would hold up well in a crash at all. But it's your decision to run it, I would have put in a 4 point main hoop and bars to the back and made the harness bar removable. But really once you're at that point of needing a 5 point harness, it's a 2 seater from then on out.

Brad96

iTrader: (1)

I appreciate your thoughts.

Admittedly, a 4-point roll bar would be safer. I had a good one welded into my '96, so I know what that looks like. I just couldn't bring myself to start cutting and welding the nice interior of my "nearly-classic" car.

The removable harness bar in a 4-pt hoop is an interesting idea, but you still have to worry about back seat passengers banging their heads on the hoop or rear bars in an accident. Like you said, once you add 5-pt harnesses, the car quickly becomes a 2-seater.

The better answer for me is to reassemble my OTHER car, which has a full cage. Here's a crazy thought: race the RACE car! Somehow I always get caught up modifying the DD instead.

gscz28

iTrader: (2)

I doubt that will pass any inspection at a drag strip

Roostmeyer

iTrader: (3)

As its sits you'd be safer with factory belts.

Run a tube back on each side to the rear shock mounts and weld it to a plate that sits flush on that sheetmetal. There should be enough room to get 4-6 1/4" bolts throug the rear shock mount sheetmetal on the top and front flat surfaces. Either that or if your car had rear shoulder belts you could tie it into the factory shoulder mount.

Even with that you should probably run another tube behind the main bar and truss it out to take the crash loads since that bar may bend in the middle during a wreck. Don't forget a seat back mount and collars to limit the harness's movement side to side. Right now your shoulders could slide all the way over the passenger side and vice versa.

BlackenedBird

iTrader: (5)

Not sure that roof point it is attached to is very structural. You coulda just made one like the 4th gen ones that are available.

My only issue with the 4th gen ones is I feel IMHO, that it should have a pair of legs coming off the hoop & attaching to the floor, like under the front seat mounts would keep it fully bolt-in.

Roostmeyer

iTrader: (3)

Probably not a bad idea to run gussets from the bar to the bolts, but the attachment themselves should be plenty strong considering thats where the factory belt attachment is.

The LG harness bar has been out of production for years, I'm not very familar with how they mount.

BlackenedBird

iTrader: (5)

No rear seat belts mount that high up. They mount just behind the rear seat back just over the rear wheel well.

Brad96

iTrader: (1)

For those who think that this harness bar setup is inferior to running the stock belts, could you please elaborate? I will grant that in a roll over, it's going to crumple. It's a harness bar, not intended to provide roll over protection. If the roof caves in, I have more serious things to worry about than whether my shoulder harness mounting points moved around or failed entirely.

Do you think that in a frontal collision, the bar will fail to sustain the tension that the shoulder harnesses apply? Or are you thinking of a nasty T-bone scenario where the bar actually gets hit and broken? Or, something else entirely?

Roostmeyer

iTrader: (3)

In a frontal crash every bar is in bending and something will fail as it sits. Structure in bending is inefficient for its weight. The loads from two bodies at 5g will literally be a ton. Would you want to hang a ton from the harness bar right now?

There also is nothing to keep the harnesses from moving side to side.

Brad96

iTrader: (1)

Roostmeyer, thanks for your comments. I completely agree, structure in bending is inefficient (strength/weight ratio). Bars in direct tension would be much stronger. I mounted it this way to make it a bolt-in arrangement. But, I don't want to risk my life unnecessarily for the sake of preserving a stock interior. In the grand scheme of things, it is just a car.

Could I hang a ton from the bar? I think so, although I haven't done any testing or structural analysis to demonstrate it. Here's my thinking, feel free to tell me if you see any errors:

1. I chose 1.5 OD x .120" wall mild steel tubing, because that's what the rules (BBORR, GT3) specify for roll bars (required in the faster classes) in cars weighing <1500 lbs. Those rules also recommend attaching the shoulder straps to the roll bar. I know, my car weighs a lot more than 1500 lbs, but I figure the car's weight is only a factor for roll over protection, not harness mounting. If the roll bar can handle frontal impact shoulder strap tension in a Miata, it should be able to handle the same ton or so of load in a Firebird.
2. If I had a proper 4-point roll bar, installed per the rules, those tubes would also be in bending in a frontal impact. The main hoop and horizontal bar would all be built in one vertical plane. The rear braces would be in almost pure tension, but the hoop and horizontal bar would be stressed very similarly to my setup.
3. In fact, my harness bar arrangement is not all in one plane, like the roll bar. The arms and legs run back at a slight angle, which helps them to work at least partly in tension.

So, if a 1.5x.120 roll bar is strong enough for shoulder harness mounting, I think that my harness bar should be, too. I'll admit that I am making some assumptions. And, this bar is heavier than a design that simply mounts to the rear shock towers.

I could argue that a typical roll bar's horizontal is only supported out on the two ends, while mine also has supports near the middle. But then, you could point out that the outer ends of my bar are being pulled upward, while the middle portion is being pulled down. That creates some additional bending in the vertical direction. So, the horizontal bending is reduced by having those supports near the middle, but the vertical bending is increased. I don't know if the net effect is better or worse.

IMHO, the weak link in my setup might be the upper rear mounting screws. I'm using the factory mounting points and screws, so they should be strong enough to support shoulder harness loads, if the loads are applied in the same way. To the extent that the harness bar structure behaves as a rigid body, all of the mounting screws are loaded mainly in shear (in a frontal collision), which is good. But, if the structure deforms significantly, those upper bars will have a cantilever effect on the mounting plates, which could put the rear screws in tension, which is bad. For that reason, it might be better not to gusset those upper mounting plates, so the plates can bend a little under load.

As far as side-to-side movement, I agree. The way I'm driving the car right now, the belts could slip off of my shoulders. My Corbeau seats are on the way. I assume that the shoulder belt holes in the seats should be adequate to locate the harnesses laterally, but I haven't researched this. The roll bar in my '96 had welded sheet metal pieces to capture the belts on the bar. I could certainly add something to this one to accomplish that purpose.

diablizzard

iTrader: (8)

so the belts don't slide off. why don't you weld a loop that the belts goes under. That way the there is a block and the belt can't some how jump over that block.

Roostmeyer

iTrader: (3)

With the upper and lower tubes being so far apart it’s really not safe. This pretty much cantilevers out both of the tubes. Neither tube is stabilized well enough to transfer any of this bending to the chassis either. In particular the lower bar is just going to pivot around the single bolt. The upper bar with a non-gusseted plate held in by two bolts is going to bend the plate as well.

If you can get a continuous run from a single vertical tube bolt at the top and bottom then you’d be about as safe as a harness bar on a four bolt roll bar. I'm not a fan of a harness bar on a four point either except on maybe a miata with a really short lower leg. If you have to keep this thing bolt in then maybe you could run the lower tube at a diagonal to meet up with the upper tube at the outside of the harness bar. It’s still not a good solution, but it would be better than what you have.

IMO the only way I’d feel safe with that is to run another tube back on each side to the rear shock mount plates. You could bolt through the carpet if you had to.

But I'm just a hack, you don't have to listen to me. I've never built a cage thats been certified by any sanctioning body.