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Ensuring the seat isn't included in the seat belt test
Slimy38 - 1/5/23 at 08:29 AM

I'm just flicking through the IVA manual with regards to my seats. Thankfully my head rests are 800mm off the base, so with the test block and the required 700mm I'm good (this was going to be a major headache if not!!). But I can't quite understand how the seat belts are tested in terms of the upper bracket.

This is the specific rule I'm thinking about;

The "effective anchorage" is any point where the load from a belt would be applied; consideration should be made to the suitability of the seat to withstand the loads likely to be imposed.

While my seats are strong enough to hold a person, they're not strong enough to carry a seat belt.

I can see that the upper mount has to be at least 140mm from the longitudinal centre, and at least 450mm off the R reference point. Both of those are satisfied pretty easily, even if the chassis back isn't high enough I have the cross brace of the roll cage to work with. But as the seat belt webbing comes from the fixing point and over the shoulder of the occupant, are there any measurements I need to consider to prove that it gets to the occupants shoulder before it touches the seat? I'm thinking about this rule;

For information, I'm using a seat design similar to the MX5 'tombstone' seats, and the seat belt will come round the outside of the seat (there's none of those 'through seat' holes).

jps - 1/5/23 at 10:14 AM

My understanding is this effectively means your seatbelts must run freely through any seats cutouts, presumably when they are being worn (it’s the latter but which I guess matters for your question).

I.e. If you’re wearing harnesses and it goes over your shoulders, then goes through a cut out in the seat which changes the angle of the belt then the seat must be as structurally strong as your seatbelt anchorage points need to be. Makes sense really, in a crash your body weight will try and pull the seatbelt out of it’s anchorage, so if there’s something which is supporting the belt along the length of the belt, the force will go through that point too.

I can’t understand why running the belts round the outside of the seat solves the problem for IVA, but it seems to be what a lot of people do….

adithorp - 1/5/23 at 12:50 PM

When mine was tested they checked that the bottom edge of the cut out in the seat was lower than the upper mounting.

pewe - 1/5/23 at 01:03 PM

I overcame the problem by making braced towers from 25mm SST thick wall and welded-in threaded inserts to lift the rear anchorage points, then welded these to the cross rail running behind the seats. I should be able to post a pic if that helps.
Cheers, Pewe

Slimy38 - 1/5/23 at 01:25 PM

quote:
Originally posted by jps

I can’t understand why running the belts round the outside of the seat solves the problem for IVA, but it seems to be what a lot of people do….


That would be the thing for me, I'm not planning on cutouts on the seat and the belt would just be on the outside. In that case, does that mean I'm avoiding the issue altogether?

Slimy38 - 1/5/23 at 01:27 PM

quote:
Originally posted by adithorp
When mine was tested they checked that the bottom edge of the cut out in the seat was lower than the upper mounting.


Perfect, that is something measurable then. I think that's the case already, the 'shoulder' of the tombstone seat sits below the upper mounting.

jps - 1/5/23 at 03:26 PM

quote:
Originally posted by Slimy38
quote:
Originally posted by jps

I can’t understand why running the belts round the outside of the seat solves the problem for IVA, but it seems to be what a lot of people do….


That would be the thing for me, I'm not planning on cutouts on the seat and the belt would just be on the outside. In that case, does that mean I'm avoiding the issue altogether?


I’ve certainly seen plenty of pictures over the years where people have said that you can just run the belts outside the seat for IVA. The Haynes Roadster book shows this arrangement I think.

snapper - 2/5/23 at 06:32 AM

I had my belts tested going through the seats however it passed because there was no tension put on the seat by the belts what the driver was in, effectively a straight run from mounts to shoulders.
Running the belts around the seat is a good call as the seat will not have any affect on the belts, it’s a simple way of taking any doubt over the belt run.
I note that you have understood the reference block measurement from seat base to upper belt mount, something that more that a few people have come a cropper with. I had to have a bar welded across the roll cage to take my rear mounted belts up to the correct height. The belts were mounted far back in the boot area and only loop over the new height bar

Slimy38 - 2/5/23 at 07:17 AM

quote:
Originally posted by snapper
I had my belts tested going through the seats however it passed because there was no tension put on the seat by the belts what the driver was in, effectively a straight run from mounts to shoulders.
Running the belts around the seat is a good call as the seat will not have any affect on the belts, it’s a simple way of taking any doubt over the belt run.
I note that you have understood the reference block measurement from seat base to upper belt mount, something that more that a few people have come a cropper with. I had to have a bar welded across the roll cage to take my rear mounted belts up to the correct height. The belts were mounted far back in the boot area and only loop over the new height bar


Yes it did take a while to understand the reference block, but it makes sense why they do it as it removes a lot of ambiguity on the test. No messing about debating where the measurement should start from, just a block laid on the seat. I first used the reference point to ensure the head rest was suitable, as long as I don't use too much seat foam I should be fine.

Slimy38 - 2/5/23 at 07:21 AM

Here's a test fit of the belt;




The shoulder mounting is too low here, it was just clamped to the rear brace to see how things would work. It looks like it's going to have to be mounted to the roll bar cross brace as per the book. So it'll be about six inches higher (ish)?

adithorp - 2/5/23 at 12:17 PM

Didn't realise you're using 3point/diagonal belts. Go around the seat and you'll be fine I think.