911vet

I just bought a Schroth Profi 6pt Harness. It came with snap on brackets SEWN ON to 3" straps. So I can't anchor it to my roll bar unless I cut the the brackets off (i.e. can't wrap with them on there).

I have a Cabriolet with bolt-in DAS roll bar. Used on street and track.

Which is "better"?:
1) run the harness through the guides on the roll bar and anchor to the rear?
2) get new straps and anchor to the roll bar.

I don't yet have a HANS. I'll have to buy 2" straps when I get one. When I do, should I get open-end straps and anchor to my roll bar? Or continue to anchor to the rear?

kurt M

I am not an ex-spurt but the standard rule that has little argument is the shorter the belt the better. The shorter the belts the less take up before they go to work in an impact and the less you come out of the seat and rag doll around. Is there enough webbing for you to cut the ends off and still properly install them on the bar using a 3 bar clip? If so that is what I would do if it were my car. You can cut the webbing neatly by using a flat tip on a soldering gun or by heating an old butter knife on the stove "gooten hot".

As to the 2 inch straps. I think Hans has published that the standard 3 inch webbing works just as well.

chrisp

I would contact DAS to be sure but from the look of that bar (I had a look alike built for my car), I would strap to the rear hip seat belt bolts. When I think about the four mounting points for the bar and how it might move/rotate if pulled directly forward by shoulder belts I could see a lot of stress being applied to the two little bolts that secure the bar in the wheel well region.

Bull

If the mounting points of a roll bar can't take the stress of shoulder belts in an impact, then I wouldn't have it in the car in the car.

Mount the belts to a properly anchored roll bar. Can you exchange them?

kurt M

Looking at the bar in question I can see your concern. I would think that the bolts in question are far stronger in single shear than the 3000 pounds rating often quoted for shoulder belts. Does not hurt to ask and you can't uncut webbing.

Another note. If you do mount to the bar you might find the guide loops are not where you want the belts. Cutting them off is one way to cover this. Most bar, belt, seat combos don't need belt guides. if you need to cut them off and want guides you can purchase split ring collars that bolt on and act as ajustable guides.

chrisp

A harness bolt is a much larger diameter than those little bolts the DAS bar uses in the rear. Plus, there are only two bolts back there and potentially two sets of harnesses strapped to the bar which compounds the problem 4x in my mind.

I don't know the single shear rating on those bolts.

RedlineMan

Pffff....

Don't listen to Kurt's false modesty... well DO listen to him, cuz he knows what he's talking about!

I don't know what size fastener is used at the rear on the DAS? I can't THINK of a decent size bolt hole on the rear frame sill of a Carrera? is there not only a small bolt hole there somewhere? Seat back pivot? They certainly are not big enough as far as I can remember. I'd say if it is not at least seat belt bolt size (12mm, or 7/16" SAE), I'd not trust it to withstand harness forces... or any others. If I remember right, a 7/16-18 Gr.8 bolt has a 30k PSI shear rating. Those are good numbers.

If I were you, I'd seriously consider having an adjunct mount point welded in. You can get one in flush under the carpet that will not show, but be large enough to handle belt loads.

With the belts, if they are long enough I'd just cut them loose and wrap them. If not, I'd look to get an exchange from your supplier.

kurt M

Chrisp does bring up a good question. I think it would be a good idea to call the bar maker before cutting you belts. I would wonder about a cab bar that has rear mounts that are not even up to harness mounting point stress duty.

BTW how many rear mount bolts and what size are they ?

chrisp

I pulled this from another site.

I don't know the size of the bolt but it pales in comparison to the seat belt mounting bolt shown in the picture which is the same thread as a harness eye bolt. One small bolt per foot.

kurt M

I was looking too. This is one of those kinds of questions that I have to have an answer to. It loooks like most of the DAS cab bolt in bars use the belt bolt. The one pictured looks to be the later aircooled 911 cab and does not. You might be able to add some through body bolts and bring it up to snuff and have the desired shorter straps. The ones that do use the rear seatbelt bolts look like they lever the bolts a lot as well. It is common to see the belts woven on the bars and was PCA legal until the cage only rule came into play.

RedlineMan

Hmmm...

Looks like 8mm at best. Not good enough. I might be inclined to get in there behined the carpet and weld a receiver for a 7/16-18 bolt. This would make me a lot happier and make feel comfortable about mounting harnesses directly to the belt bar.

911vet

Thanks for the replies. You all bring up a good point that I hadn't considered.... I need to at the very least get a better grade bolt for the rear roll bar mounts. I think the idea of welding in a receiver for a larger bolt is a great idea. I don't weld but I can find someone who does

I'll anchor my harness to the rear seat belt attachments rather than the roll bar.

deep_uv

All DAS bolt in roll bars mount in this same fashion. Is there data showing failure of these rear mounting bolts? This bar has been used in club racing for years prior to the cage rule and there's a bunch of them out there with harnesses directly mounted to them. Don't forget that the main hoop is bolted to the front heavy duty seat belt anchors.

Given that neither setup is optimal, I would still say properly mounting the belts to the roll bar, even given the rear mount bolt size, is safer than mounting the belts to the rear seat belt anchors and draping them over the bar. My opinion, I'm not an expert.

All that said, I agree that securing the rear mount somehow would be a good idea and that nothing is better than a properly welded in cage. But, in a dual use car is the DAS bar configuration not a reasonable configuration?

RedlineMan

<< All DAS bolt in roll bars mount in this same fashion. Is there data showing failure of these rear mounting bolts? This bar has been used in club racing for years prior to the cage rule and there's a bunch of them out there with harnesses directly mounted to them. Don't forget that the main hoop is bolted to the front heavy duty seat belt anchors. >>

Steve, don't take this personally. This is an example of flawed thinking in my opinion. You can find many things that "everyone does" that are patently absurd, even now. Everyone needs to be a little more critical than just following the herd on such things. I'm no engineer either, but I have developed the facility for critical thought. It's nothing but a learned response to any such situation. When I see something, I immediately run it through my filters to see if it makes sense. I don't care what anyone else thinks. That "everyone else does it" snags in my filter like a brick!

Since at the very least, we are looking to this bar to manage belt loads, common sense would tell you that the mount bolt for any bar should be at least as big as a seatbelt bolt. We know they have specs for that, and that it is an important safety item that cannot be allowed to fail. I don't know what the rear bolts are, but they are not 11mm (Porsche belt bolt size) as far as I can remember. I'd guess they are 8mm. The difference in tensile yield strength of an 8mm bolt is roughly HALF that of an 11mm bolt. This is the type of thinking one needs to do.

Beyond that, I've never liked the main hoop mounting of the DAS bars. That offset plate they use to get over to the lower lap belt mount does nothing for me. It represents an inherent weakness in the design to me, relative to something that attaches the end of the tube DIRECTLY to the mount point. Whether it would be easy to get such a mount is another matter, and precisely why they DIDN'T do it. They're mount makes it easy, not optimal.

chrisp

Help me understand your thinking on this one. Are you trying to avoid stretch or are you saying that draping the belts will fail?

Is it definitely better than nothing assuming [IMHO] you don't mount the belts to the bar.



When I had my look-a-like bar I investigated adding a spreader bar between the rear feet to keep the legs from collapsing inward and reducing the chance of popping the little bolt heads off. I also looked at a totally kluge idea of securing the bar (ratchet strap, cabling, etc) to the firewall or inner rear seatbelt mounts to keep it from rotating forward in a frontal impact.