Fitting seatbelt mountings

[Mags]

I'm going to be fitting 3 point inertia seatbelts to my Frogeye, I have OG drawings from Austin Healey on the mounting points and I was going to make up spreader plates myself but it looks like it's probably easier for me to order 6 FIA approved anchor plates with 7/16 UNF bolts already welded to these via a motorsports shop on Ebay.

Should I be welding the spreader plate on the reverse surface of the fixing, so underneath the car, the outside face of the rear wheel arch and the inner face (propshaft side) of the Propshaft tunnel so that it is impossible for the nut and spreader plate to pull through the panel or does it depend on my ability to perform a decent weld all around the spreader plate?
Obviously safety is the key concern here but in the case of the prop tunnel there is no way I can weld inside this, but I can get my hand in to hold the plate whilst fixing the eye bolt to it, it just means the spreader will not be welded to the body, what's the correct method?

Thanks
Mags

[tapkaJohnD]

Mags,
You cannot do better than consult the MSA Yearbook, Section K Safety: https://www.msauk.org/assets/153191commonregulatio...
In particular paras 2.1 and after "Seats, seatbelts and headrests"

Although it's common practice to seam weld back plates in place, it is not "required".

John

[Mags]

Thanks, very helpful. It shows the plate needs to be on the outer surface but just says "fixed to the cars chassis" so open to some interpretation I guess.

[finishing touch]

It all sounds a bit iffy to me. For starters welding a high tensile nut or bolt destroys its mechanical properties so that's out, FIA approval or not.

I also wouldn't regard a sheet metal inner wing or a transmission tunnel as being structural chassis members so once again, a bit iffy.


The IVA manual (for kits & home builds) states gusseted into a triangulated part of the structural chassis.


No point in hanging a five tonne chain on a half tonne hook I say.

Paul G

[Mags]

finishing touch said:

It all sounds a bit iffy to me. For starters welding a high tensile nut or bolt destroys its mechanical properties so that's out, FIA approval or not.

I also wouldn't regard a sheet metal inner wing or a transmission tunnel as being structural chassis members so once again, a bit iffy.


The IVA manual (for kits & home builds) states gusseted into a triangulated part of the structural chassis.


No point in hanging a five tonne chain on a half tonne hook I say.

Paul G

Hmmm, interesting. There must be lots of older vehicles where seat belt mountings are reinforcing plates onto sheet metal? I know the mounting points on my VW Camper are just that from the factory.
I'll download the IVA manual and have a look, I would rather do something safe than what may have been under-researched in the 60's or 70's.

[finishing touch]

If you or I were to repair a seat belt mount in a road going car for MOT purposes or indeed a chassis or floor repair within a certain distance
of a s/b mount, (I don't know the exact figure) it would need to be a continuous weld.

This contradicts what John says, no doubt in good faith, " Although it's common practice to seam weld back plates in place, it is not "required".


Seat belt mounts are usually designed to take a force many times greater than the weight of a person and are often near structural folds, (sill to floor)
or have more layers of metal, or thicker metal, or all three.

I would also bare in mind that the thickness of a car panel could well be already compromised by corrosion.


Paul G

[Mags]

Thanks Paul, I looked at the IVA manual and I read for Steel Monocoques the mounting should be in Structural box sections. I will have closer look at the structure before I go any further and see what I think makes sense.

[finishing touch]

I think you have cracked it with your last sentence.

Look and see what makes sense.

Cheers,
Paul G

[PaulKemp]

Large thick spreader plates on the outside of the monocoque (I think I used 3 or 4mm plate) drill through both and use the FIA plates outside of that utilising the small hole to hold the FIA plate to the load spreader
Most kit seven replicas use the same system welding the spreader to the chassis which is just 1.5mm 25mm square box.
Pulling through is better than side loading the plate but both pass IVA

As usual excellent advice from Paul G who's car you should see for the amazing metal work.

[227bhp]

I recently fitted some to an old trackday car, I used some cut out of another (scrapped) car! When you look at how they are designed it's a good idea as they turn the edges up so it's a bit like a tea tray and can't bend in on itself as easy when put under extreme load, a bit like these here:

http://www.ebay.com/itm/Safe-Tboy-Seat-Belt-Anchor...

[finishing touch]

PaulKemp said:

Large thick spreader plates on the outside of the monocoque (I think I used 3 or 4mm plate) drill through both and use the FIA plates outside of that utilising the small hole to hold the FIA plate to the load spreader
Most kit seven replicas use the same system welding the spreader to the chassis which is just 1.5mm 25mm square box.
Pulling through is better than side loading the plate but both pass IVA

As usual excellent advice from Paul G who's car you should see for the amazing metal work.

Is that bull st I smell Paul ?

On a more serious note, a photo of my interpretation of "gusseted into a triangulated part of the structural chassis."

This is the rear side of the transmission tunnel / chassis backbone and the 5mm plate has a 25dia boss x 20 long welded on the back, drilled 10.2,
and tapped M12 through plate and boss. This results in a 25 long thread as opposed to a 10mm long thread that a nut welded over a hole would give.

Ignore the hole to the left. Just a water drain hole in the floor.

Cheers
Paul G

[Mags]

Thanks again, so where do I get see some welding porn Paul?
I'm sure elsewhere on PH there is a reference to Bad Obsession Motorsports Mini build on YouTube, if you haven't seen it I highly recommend it. I don't share the You Tube comments on the humour but the fabrication work looks amazing to me.