Harness connection solution

[drl911]

After a lot of research and thread reading, I spent this last weekend designing and fabricating my version of inside lap belt attachment points. This is strictly a DE / autocross / street 911 Carrera with no intent of racing. I just really didn't care for any of the thru the tunnel solutions or any of the aftermarket pieces that connect to the seat mounts so I made this......


The bracket is made from 3/16" x2"x2" angle with all mating pieces MIG welded. The bracket has 4"x8" backing plates on the underside on each side. The cross plate that carries the eye bolts retains the angle steel to maintain rigidity (can't see clearly in pic). Mounting bolts are 7/16"x20 grade 8 zinc plated from Ace hardware with locktite.


Conventional stock outside mounting point with eye bolt.


Shoulder straps to DAS rollbar. I still need to make some strap keepers.

The submarine belts (6 point) connect to a cross bar on the seat sliders like I read about on the latest cup cars. This way they stay in the optimal position regardless of seat position.

Thoughts?

Thanks,
Dan

[KTL]

Nice job on the orientation of the eye bolts. You've got them installed vertically like they should be. A lot of people (me included) have them installed horizontally, which reduces their load capacity due to the bending load applied. Vertical install creates a tensile load which is better.

[drl911]

Any comments or PCA DE tech inspection experiences regarding submarine belts connected to an underseat crossmember instead of to the floor pan?

[John Rogers]

On our 1987 Turbo I used the inside belt mounting holes as well as the outside as you did. Why not use the inside mounting holes? Second question is what is behind the middle eye bolts? If there is just a washer or lock washer then I would worry about the eye bolts ripping through.

You mentioned that this is a DE car but in the San Diego PCA region there have been lots of auto-x and DE cars wrecked, probably as many as wheel to wheel racers....

Check this to see what might happen: Car Crash Example

[drl911]

Only '87 and newer have the inside mounting holes, thus the reason for the project with '86 and older. You have to use backing plates when attaching the eye bolts thru the thin floor pan, but these should be fine attached to 3/16" steel plate.

Thanks for the comments,
Dan

[175K911]

I applaud your ingenuity.

Obviously the prefered method for any mount is to use the factory threaded bosses where possible, they're the strongest part of the car. I've got an '86 as well so know full well that the only threaded location is the two outboard spots.

On the inboard locations, like john rogers I'd want to know how the eye bolts are attached. Do they go through the new mount and through the tunnel? If not, I'd be concerned about flex on the bracket you made. You're transmitting the load quite a long distance from the eyebolts over and down to the floor. I've always just drilled two holes in the top of the tunnel, right under where you've got your eyebolts. I use large flat steel plates underneath the tunnel to reinforce. This is the same location that the factory used to attach the inboard seat belt receptacles for decades, until they switched over to the receptacle attached to the side of the seat. I've seen a 911 that had rolled and those inboard eyebolts mounted as I've suggested that didn't even budge.

My interpretation of the PCA Club Race rules is that anything that attaches to the seat frame or seat rail is not allowed. So the cross bar in your system wouldn't pass tech in my mind. Are you trying to avoid drilling 2 holes in the floor to mount the sub belts?

As a temporary solution for the shoulder belts on the roll bar, you can get a couple of 1.75" muffler clamps and put one outboard of each belt. Just make sure you point the threaded ends toward the rear of the car. Not elegant but it works.

[drl911]

Here's another pic of the bracket from the front. I intentionally retained about 1" vertical on the cross member for rigidity. The eye bolts do not extend into the tunnel. Therefore, I can easily monitor and re-torque as necessary the eye bolts. I can't conceive the cross member bending any more than the tunnel sheet metal w/ washers versions. I do have 2" diameter washers under the cross member for each eye bolt. My mechanical perspective is since the eye bolts are inside the perimeter of the 4 mounting bolts they all share the potential load to the point of overkill.

I didn't fix the submarine straps to floor pan eye bolts because I didn't want to mess with relocating the computer. And I read somewhere about the seat cross bar being a factory option now with the advantage of the angle and length of the strap not changing as the seat is moved forward/backward. I guess I need to get official approval or drill some holes....

Dan

[John Rogers]

When finished, make sure to either lockwire or cotter pin those belt hooks or you will surely fail inspection.

[Walt Fricke]

Kevin

Given the enormous strength of the forged eyes, I'd not think that the bending moment when mounted horizontally would be an issue. The FIA specifies withstanding a 25G load for belt anchors, and from (perhaps faulty) memory I think there is or was a US Federal spec of 3,000 lbs for anchors. Do you think such eyes would fail (not just bend) if these loads were applied perpendicular to the plane of the eye?

This is an inventive idea for drivers whose driving position allows installation of the anchors this far back while still maintaining the optimum lap belt geometry. I'd be inclined to a belt and suspenders here, with the threaded eyebolt extending into the tunnel, and a 3/16 inch custom backing plate inside the tunnel with holes to match. In this location you ought to be able thread the nuts on on the inside, and tighten them. A little Loctite blue would deal with any worries about nuts backing off, as these anchors don't get repetitive loads of any significance. In such a design I'd want to be sure that the upper nuts or whatnot were drawn firmly down onto the sheet metal, the beter to resist any bending.

That ought to insure that the cross piece could not twist much.

Could not an engineer equipped with the modulus of elasticity of the hardware store cold rolled steel (and it is not weak stuff, just not as strong as the higher priced stuff) used in this actual design, and the dimensions and the loads ,calculate, without even resorting to FEA, how much movement there would be in these anchors if both were simultaneously loaded in the forward direction (driver and passenger loads)?

Compared to the stretch in the belts, my instincts tell me this should be insignificant, but that's not engineering.

[drl911]

FWIW the black sharpie mark in the pic above is the centerline position of the outside factory mounting bolts. So my bracket puts the inside eye bolts about 2.5" behind. My intention was to put the inside eye bolts inline with the outside eye bolts, but the bracket interferred with moving the seats all the way back.

This weekend I will review using longer threaded eye bolts and sandwiching the tunnel material between two nuts as Walt's suggested improvement.

And the PCA website for DE minimum standards does not state anything about anchoring to the floorpan (particularily submarine belts), but only must follow manufacturer's instructions for installation.

Dan

[Walt Fricke]

Right - for DEs the minimum standards, as one might expect, do not require more than what the car came equipped with (at least for cars built from the point where three point belts were installed).

Regions, though, can impose stricter standards. Usually this would only be done on modified cars, on the theory that the car now had more performance potential than what it was designed for. But you have to check to know for sure.

[KTL]

I agree the harness bar needs some means to keep the harnesses from moving laterally. In absence of retaining the lateral position of the harness, you're counting on the holes in the seat's head area to support the harness. Seat holes are only for access, not support of any harnesses.

Ed's suggestion about the muffler clamps is a very good one. If you want something prettier, you can buy some steel or aluminum bar clamps like these shaft collars

McMaster-Carr

Walt,

You're right that the eyebolt isn't going to fail. I was just giving credit for pulling the eye in the best position to carry the load. Also, it seems the clip-in harnesses work best when you've got vertical eyes. Horizontal orientation sometimes causes the clip-in hardware to get hung up in my experience with my own harness installations.

John also makes a good point about the cotter pins in the spring clip. Our race tech inspection guy always checks for those. So a DE tech inspection could possibly check too. May seem like overkill or nitpicky, but I think they're a very good idea for any application. I have had spring clips that were became partly "open" due to the belt loose and allowing the clip door to be aligned with the eyelet. Then when I pulled the belt tight, the clip door was trying to open via my pulling on the belt while in contact with the eyelet, which bent the clip door. With a cotter pin in there, the clip door is not able to move out of the closed position.

If you ever make a version 2.0, i'd suggest adding another piece of flat stock to the right angle "bend" between the floor plate and the eyelet plate. Only reason I say that is because a tech guy could say, I don't know how good your welds are. If you add another piece that provides some additional support, the guy is less inclined to question it because you've got multiple welds that have to fail for the bracket to displace?

You have to consider that your custom mount invites scrutiny because it's not something that was built into the car by the factory, nor is it the tried and true mount-thru-the-floor-with-monster-washers. When tech people see home made stuff, it sometimes raises an eyebrow if it doesn't have a manufacturer's name brand sticker on it? That said I think it's nicely made.

I give you credit for asking for feedback on your install. Lots of people come up with some hokey ideas to connect harnesses. I've seen a few myself on friend's installs. I don't consider myself an expert by any means. It's just that i've seen people be silly because they don't want to drill holes or spend money on an "expensive" store-bought solution. Nice job making your own.

I agree that 3/16" angle stock is PLENTY beefy for this application. Even the factory seat belt locations in the chassis are only threaded bosses placed in somewhat thin sheetmetal. Next time you're under the car, take a look at the rear seat wells and you can see the bosses back there aren't heavily reinforced at all. Granted these are back seats where the Porsche engineers figured little people (weigh less than adults) are likely to be. But the point is, tech inspection people find those points acceptable to connect the harnesses to when you route the harness over the bar instead of looping it around the bar. Some tech people will suggest the harnesses be connected at the rear seat points in lieu of around the bar, because some bars are not well designed to anchor the belts, but only guide them instead.

[Walt Fricke]

Since this is a 911 list, most of you probably remember that for many years Porsche had two brackets which were screwed through the package shelf upholstery into the sheet metal. These were either for attaching luggage to the shelf, or for a snap strap to hold the rear seatbacks upright. They are a small potmetal casting, held with two ordinary sheet metal screws.

While teching for a DE I found a car with the shoulder harness straps attached to these.

[175K911]

Originally Posted by Walt Fricke

Since this is a 911 list, most of you probably remember that for many years Porsche had two brackets which were screwed through the package shelf upholstery into the sheet metal. These were either for attaching luggage to the shelf, or for a snap strap to hold the rear seatbacks upright. They are a small potmetal casting, held with two ordinary sheet metal screws.

While teching for a DE I found a car with the shoulder harness straps attached to these.

OMG, and I thought the guy I failed because he'd attached his sub belt to the same little stud and nut that holds the DME Relay under the seat was bad.

[drl911]

Thanks for an informative discussion guys.

And I am definitely going to add a mfg sticker to the bracket prior to tech inspection... lol!

[CRNT918]

I wish I would have saved it, but I have read somewhere that unless the rollbar or rollcage is actually welded in, the shoulder straps should be attached to the rear seatbelt mounting points.

[Walt Fricke]

CRNT918

I've never seen such a recommendation.

Perhaps you are thinking of the Brey-Krause advice that a harness bar not be used for shoulder belt attachment. That is, unless it is the nifty truss design which they also sell and is designed for those loads. The BK pitch is that standard harness bars are not adequate to do more than serve as guide bars for the shoulder harness loads - which is why they also sell a piece which they call a guide bar.

A properly installed bolt-in cage or roll bar (proper ones, not cosmetic ones like the Jeep folks sometimes favor) is plenty strong enough to serve as a harness attachment. I think welding in is a better mousetrap, but we are talking questions of degree, not yes or no.

For years, nobody paid much attention to the length of shoulder harnesses from shoulder to attachment. Lately, most have awoken to the fact that the longer the belt, the more stretch in it during a severe impact. While some stretch is good because it greatly reduces peak G loads on the body, too much is bad because it allows body parts to impact parts of the car. And the peak loads from that secondary impact are nasty indeed.

So now it is common to see recommendations that the shoulder harnesses be mounted as closely to the shoulders as can be. Which means to a horizontal cross bar in the main roll hoop, is positioned - as it should be - at or just below shoulder height.

Running the belts over a guide bar and back to the package shelf, or down to the floor pan, or back to the rear seat belt attachment points, is not optimum, as all leave quite a length of belt to stretch. I don't think anyone would fail tech with this, but it is somethng folks should be aware of.