smart51
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| posted on 27/5/10 at 04:15 PM |
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using the seat at the upper seat belt mount
Is it allowable in MSVA / IVA to use the seat as the upper seat belt mount? My new project idea doesn't have any chassis structure near enough
to mount seat belts on. I was thinking of mounting all the ends on the floor and looping the belt through holes in the seat back. Is this OK?
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BenB
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| posted on 27/5/10 at 04:20 PM |
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It's generally a bad idea because if you get thrown forward, the seat will be compressed downwards by the straps. If the seat crumples your
spine will be compressed with it by the harness straps.
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skinned knuckles
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| posted on 27/5/10 at 04:34 PM |
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the seat wont be strong to take the forces created when your body continues going forward when the car has come to a dead stop. the forces produced by
momentum are huge and therefore harnesses and seat belts must be mounted to something that wont budge under load ie. the chassis.
A man isn't complete until he's married, then he's finished
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turbodisplay
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| posted on 27/5/10 at 04:37 PM |
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I was under the impression that cobra seats were able to take the required load, think someone got a letter from them to prove to SVA man.
Darren
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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skinned knuckles
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| posted on 27/5/10 at 04:49 PM |
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quote:
Originally posted by turbodisplay
I was under the impression that cobra seats were able to take the required load, think someone got a letter from them to prove to SVA man.
Darren
surely that is a bit of a bold claim? just on leverage alone a 2000kg load at the top of the seat would translate to significantly more than that at
the base mounts? what is the seat mounted to? if bolted to the standard 25x25 RHS chassis tube it couldn't be strong enough could it?
I am in no way saying that cobra seats arent strong enough, more that your average 7 alike chassis would struggle to cope with those forces being
applied to chassis members that arent supperted by aditional triangulation. the mearest point of support for the chassis at the point seat mounts is
the tranny tunnell and there is no triangulation there. I wouldn't be comfortable attaching safety restraints to a seat bolted to a couple of
rubes that way.
A man isn't complete until he's married, then he's finished
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daviep
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| posted on 27/5/10 at 05:10 PM |
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quote:
Originally posted by skinned knuckles
the seat wont be strong to take the forces created when your body continues going forward when the car has come to a dead stop. the forces produced by
momentum are huge and therefore harnesses and seat belts must be mounted to something that wont budge under load ie. the chassis.
Bit of a sweeping statement?
Design the seat back and mounts for this application and there is no reason it cannot be done like this.
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smart51
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| posted on 27/5/10 at 05:11 PM |
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The way I see it, there would be two loads on the seat, one would put the back rest under compression, pushing it into the floor. The other would try
to push the seat forwards to straighten the belt over your shoulder.
Now if the seat is strong enough in compression, you would not be injured by the belts crushing you. If the seat were strong enough in bending then
all the force would be on the seat mounts. If the seats are mounted to the floor with strong enough bolts and the floor is strong enough to react the
load, then surely there's no problem.
What I want to know is this: Is there a rule against it or would the MSVA man object just because he can?
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MikeRJ
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| posted on 27/5/10 at 07:44 PM |
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It's a bad idea, which is why the old SVA specified a minimum mounting height for the upper seat belt attachment, and also why this would fail
any kind of competition scrutineering. I appreciate you aren't building a competition car, but the reasons for not allowing this are sound
IF the bottom of the holes in the seat are above your shoulders, AND the seat and mountings are strong enough, AND the mounting are a significant
distance from the back of the seat then you might get away with this, otherwise you will get a broken collar bone, or compressed spine, or the whole
lot will rip out of the floor. If you work out the forces that will be experienced by the rear mountings and the seat structure in this kind of
arrangement, the numbers get very big.
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BenB
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| posted on 27/5/10 at 09:46 PM |
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Well you could do it but that would have to be one seriously brick poo house seat  Might hurt the 0-60s a bit!!
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smart51
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| posted on 28/5/10 at 07:45 AM |
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quote:
Originally posted by BenB
that would have to be one seriously brick poo house seat
I've re-read the MSVA manual and it turns out that it is acceptable to mount the seatbelt to the seat (it doesn't specify where) so long
as the structure is strong enough. I only plan to pass the belt through hoops in the seat. the belt will be mounted to the chassis at about a 45°
angle to the seat.
A quick bit of maths shows a EuroNCAP typical crash exposes you to about 32g of deceleration. My 75kg weight converts to a figure equal to the
compressive strength of one 1" 16 SWG round steel tube. Just. The angle of the belt reduces the load on the seat and the space frame structure
of the seat would give me an order of magnitude of safety factor.
My project idea is based around what boils down to a ladder chassis, with the beams made from a space frame of small tubes. Rather than building a
tower to support the seat belt and roll over hoop, I thought I could build the seat in the same way as the chassis. The same amount of metal that
would build the tower would build the seat so no extra weight. I'm happy that I can make the seat strong enough and the advantage is that the
roll over hoop is always close to you, even if you slide the seat forwards. Is it still a crazy idea?
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skinned knuckles
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| posted on 28/5/10 at 08:55 AM |
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quote:
Originally posted by MikeRJ
It's a bad idea, which is why the old SVA specified a minimum mounting height for the upper seat belt attachment, and also why this would fail
any kind of competition scrutineering. I appreciate you aren't building a competition car, but the reasons for not allowing this are sound
IF the bottom of the holes in the seat are above your shoulders, AND the seat and mountings are strong enough, AND the mounting are a significant
distance from the back of the seat then you might get away with this, otherwise you will get a broken collar bone, or compressed spine, or the whole
lot will rip out of the floor. If you work out the forces that will be experienced by the rear mountings and the seat structure in this kind of
arrangement, the numbers get very big.
thats what i was tryingto say, but far more eloquently put
A man isn't complete until he's married, then he's finished
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