Board logo

using the seat at the upper seat belt mount
smart51 - 27/5/10 at 04:15 PM

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?

BenB - 27/5/10 at 04:20 PM

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.

skinned knuckles - 27/5/10 at 04:34 PM

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.

turbodisplay - 27/5/10 at 04:37 PM

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

skinned knuckles - 27/5/10 at 04:49 PM

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.

daviep - 27/5/10 at 05:10 PM

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.

smart51 - 27/5/10 at 05:11 PM

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?

MikeRJ - 27/5/10 at 07:44 PM

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.

BenB - 27/5/10 at 09:46 PM

Well you could do it but that would have to be one seriously brick poo house seat Might hurt the 0-60s a bit!!

smart51 - 28/5/10 at 07:45 AM

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?

skinned knuckles - 28/5/10 at 08:55 AM

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