3-Point Y-Harness Lap Belt Mounting Location?

Stretch32 · 02-09-2023, 06:11 AM

I'm building a '71 911E LWT hotrod and I bought a set of reproduction Repa ST style Y-harness to use in the build. I welded in the parcel tray shoulder harness mounting plates (under the back window) like the ST's used but I'm having trouble deciding where / how to mount the lap belts in conjunction with my reproduction Recaro RS seats.

My issue is: the harness lap belts are designed to use the factory Repa seatbelt L-brackets / bolts that are normally attached to the seat bases / frames of the factory comfort and sport seats. I'm 6'4" so I have the seat slid all the way back and this places the lap belt L-brackets forward of my hips (at about 10 degrees forward of straight down). Looking at lap belt mounting info, the lap belts need to be at about a 45-60 degree angle aft of my hips so this forward location won't safely work.

I've been looking at various weld in / bolt in harness lap belt arrangements but I'm not sure if they will work for me. The obvious solution (I think) is to mount the lap belt harness mounts on the floor pan behind the seat at the proper 45-60 degree aft angle but this will make the lap belts wrap around the back of the seat to some extent instead of coming straight down parallel to the side of the sport seats. I can't find any documentation to confirm whether or not this is an acceptable / safe mounting arrangement or not.

The car is being built for street driving (not track) so I need to find an arrangement that will meet those safety requirements. Any insight or suggestions on this or an alternative method of mounting these lap belts would be appreciated.

Stretch

David Inc. · 02-09-2023, 07:48 AM

If it's being street driven only I would caution against using a harness. A harness fixes your back in place and in case of a rollover that means your head and neck have nowhere to go in case of roof collapse. A full roll cage will fix that problem but adds the problem of you smashing your head against the steel tubes in case of an accident, which means you'll want to wear a helmet all of the time.

Walt Fricke · 02-11-2023, 05:39 PM

If you plan ever to track it, you can't use a Y harness. PCA won't allow it, nor will the other organizations I know of that put on track days.

I wouldn't worry that much about street driving, though. Our Porsches have a quite sturdy roof structure which usually survives an ordinary roll over, as does the driver.

Getting the lap belt angles and alignment, however, could be critical. Dale Earnhart's left lap belt broke. NASCAR didn't attribute 100% of the cause of his injuries to that, but it is hard to avoid the idea that he would have survived had it not failed. It failed because the way he mounted his pull up adjusters led them to interfere with the seat and cause the pull on the belt to be at a substantial angle to the adjusters. Engineers call this "belt dumping," and the NASCAR video on the crash showed a laboratory demonstration of this. Easy to imagine that if all the force is applied to one edge of a belt, the thread there will break, and then the next, etc. I believe he mounted to the floor pan behind the seat. I did, but promptly moved mine after that experience.

Doesn't your car have stock lap belt mounts on the longitudinals under the door? Those are usually in a pretty good position. Same with the mounts on the center tunnel.

Anyway, believe the diagrams showing the routing as basically in a plane from mount to your tummy to the other mount.

Other than the potential to melt the fuel lines, welding mounts to the center tunnel with good backing is fairly simple. Not sure just how to do that to the longitudinals where the factory mount is, but with some cleverness you should be able to fabricate something which can be welded in. A plate under the floor, a metal piece bolted on top which extends to where you need the belt attachment to be without bumping into the seat adjustment parts. However, my experience is with G bodies cars mainly. My '68 track car called for all sorts of welding to get this right.

SWS911 · 02-12-2023, 05:14 AM

Walt, F body cars had the seat belts attached the seats themselves via an "L" bracket bolted on underneath. Not a very good or safe design. They, Porsche, figured out the error of their ways and rectified this problem with the G bodies.

Walt Fricke · 02-12-2023, 03:13 PM

Yes, the seat mounts until the Boxsters were not confidence inspiring. Putting all your last ditch safety eggs in one basket isn't an appealing way to do things.

Stretch32 · 02-12-2023, 06:16 PM

There are several options available of weld in plates with a 7/16-20 nut welded in the middle. Can I use these and weld them directly to the chassis or are these designed to be installed behind the sheet metal (inside the tunnel and heater channel)?

Also, there are several tunnel lap belt mounts (Brey Kraus makes one) that appear to weld on top of the transmission tunnel for the inside lap belts. Has anyone used these?

Stretch