Is there any safe way to mount harness

[Sjoerd]

Hi guys,

I was hoping one of you might be able to help me out. Today I measured the position of my seat relative to the rear seatbelt mounting hole. It gave me an angle of 48 degree. The manufacturers and the FIA all recommend a mount with max. 45 degree, preferably <20 degree. This is not going to work then in my case.

Next up I calculated the possibility to mount it in the M8 bolt hole on which these black supports for the original rollers are bolted. When you have these the forces are distributed over three M8 bolts, which should be fine. However a single mount (as for my 3 point harness) uses just one bolt. This creates another problem. Can you check my calculations and perhaps give me a different idea on where to mount them?

Say I drive 100 km/h which is 27.78 m/s. With a mass of 75kg this gives me a kinetic energy of 0.5*75*27.78^2 = 28.9 kN. This leaves out the energy being absorbed by the crumpling zone though. It'd be rather hard to find that out though...

If we look at a standard 8.8 M8 x 1.25 bolt according to the relevant DIN spec (as used by Ford) the max. allowed shear force is 320 N/mm^2. The actual shear occuring is tau = 28.9*10^3 / 36.61 = 789.4 N/mm^2.

Not even close. A 12.9 bolt can not solve this problem as well with a max. shear of +/- 600 N/mm^2.

So in short, if we want to mount a harness and do it the safe way we seem to be pretty much f :twisted: ked if we don't have a roll cage. I have seen loads of pictures of people mounting it on the rear seat or the bolt hole, but it seems to me it simply is not safe enough. How do you guys cope?

Sjoerd

 

[Flying Scotsman]

With what has already been said, yes thousands of people will use the same method of utilising rear bolt holes from the rear seats for this purpose.
When manufactures recommend a MAX level, its not the real max, its a safety max, so you'll find the equipment actual max is not stated.
But totally understandable.
So in light I would say if you are considering this, why don't you either drill a new hole on the lip of where the boot floor drops away, you will find the angle to be much less than 45 degrees, or, weld in a roll cage bar between the rear suspension turrets and use as a harness bar. You will also find this to be less than a 45 degree angle.

Now if neither of these two are an option for you then I would say why are you trying to use an FIA spec safety restraint system when it is not required or in light of your calculations, potentially a flaw in installing it.

 

[Sjoerd]

With what has already been said, yes thousands of people will use the same method of utilising rear bolt holes from the rear seats for this purpose.
When manufactures recommend a MAX level, its not the real max, its a safety max, so you'll find the equipment actual max is not stated.
But totally understandable.
So in light I would say if you are considering this, why don't you either drill a new hole on the lip of where the boot floor drops away, you will find the angle to be much less than 45 degrees, or, weld in a roll cage bar between the rear suspension turrets and use as a harness bar. You will also find this to be less than a 45 degree angle.

Now if neither of these two are an option for you then I would say why are you trying to use an FIA spec safety restraint system when it is not required or in light of your calculations, potentially a flaw in installing it.

Offcourse there is always a safety factor in these kind of things. But that doesn't mean it is good to use that margin. Especially not when it comes to my personaly safety imo.

I prefer to do the least drilling in the body possible to preserve it. If I ever need to sell it.. Plus it'd be quite hard to weld a plate + nut on the other side there. Can we reach that place from the bottom of the car? A rollcage is not in my budget (yet).. To have installed a proper one you pay +/- €1500 here..

Why I use this FIA spec is simply my safety. The 3 or 4 point harnesses under the wrong angle have a tendency of submarining. This will put so much vertical forces on your back that you break it. It doesn't sound like a pleasant idea.

 

[Flying Scotsman]

Me of all people get the want for harnesses, they are more secure and safer for the body should an incident occur.

The area I mentioned, yes you can access it underneath, remove the spare wheel tray and you'll have plenty of access to get a hole drilled and supported.
With the roll cage bar, i'm merely meaning one bar, not a roll cage as such but something resembling a strut brace simply for the use of harnesses.
And how dare you sale 'sell it' lol

 

[Flying Scotsman]

I had mine mounted to the rear seat bolts but changed the seat belt plates with dual ones.



But I guess the angle would also depend on how tall or raised your seat was.

 

[Sjoerd]

I had mine mounted to the rear seat bolts but changed the seat belt plates with dual ones.

But I guess the angle would also depend on how tall or raised your seat was.

I guess it kind of depends on your seat as well, but I am using a relatively small racing bucket allready.. I have a set of Sparco Evo's in it on the standard mount brackets from Sparco + Ltec aluminium brackets and Schroth belts.

By the way, do you still have the original seats? Looks like it anyway. How does that work with the shoulder straps? No risk of them sliding off in a crash? Did you feed them through the headrest?

 

[Flying Scotsman]

The seats in the picture are FRP seats, but I now have a Sparco Rev in mine

 

[Sjoerd]

The seats in the picture are FRP seats, but I now have a Sparco Rev in mine

I'm thinking of buying a second bucket seat as mine is way past the FIA date and is getting worn. What do you think of the Rev? Is it a good alternative to the Evo? Worth saving the money or just keep on saving up?

 

[Flying Scotsman]

Well I didn't check sizes of the seats before buying lol, so ive had mine fitted and its too tight on the legs, so I was going to get an Evo but im having some unique ones made now. But the Evo is a good seat, id keep it.

 

[Sjoerd]

Well I didn't check sizes of the seats before buying lol, so ive had mine fitted and its too tight on the legs, so I was going to get an Evo but im having some unique ones made now. But the Evo is a good seat, id keep it.

I definately like it. It served me well for three years now. It's time for it to move to the passenger side though. I'd like a newer seat for myself. If you don't have a way to shed the Rev I'm definately interested I will need to find some mounting that is lower though. Seems the only practical solution to fixing my seatbelts..

 

[Flying Scotsman]

You're in Holland are you not? Bit far to ship it don't you think?

With the seatbelts, you could weld in harness eye loops.

 

[Sjoerd]

You're in Holland are you not? Bit far to ship it don't you think?

With the seatbelts, you could weld in harness eye loops.

Might be, but it might still be cheaper than buying it here. Unfortunately parts in NL are +/- 25% more expensive mostly. 9 out of 10 times the same parts from the UK are cheaper including postage.

Oh and by the way. I'm not in Holland. I'm in Netherlands. It's a huge difference! Holland is only like 25% of our little country :wink:

 

[Noobrider]

Heres where I mounted mine and used the original seatbelt bolts, kept the angle nice and shallow

 

[Sjoerd]

Heres where I mounted mine and used the original seatbelt bolts, kept the angle nice and shallow

I checked on my car and it seems to be standard M10 holes on mine. Is this the case for your car as well or does it have a 7/16UNF bolt? As I calculated above it doesn't seem to be able to hold the shear forces in a crash! Be careful man!

 

[Ziggy]

Do your energy calculations take into consideration the effect of the harness material stretching and the harness stitching coming apart in a controlled fashion ? The crumple zone deforming will also have a huge impact on the energy transferred to the occupants as well. What do the seat belt manufacturers suggest for suitable mounting hardware ?

 

[Sjoerd]

Well for example Schroth has clear instructions on their website to only mount it on the rear seat belt lower bolt. Which I cannot use because of the angle and such. Also I have always been thought in my studies to use fine thread bolts, minimum 7/16 UNF or above.

Schroth also says:

Die nachfolgenden Informationen basieren auf einer optimalen Installation für aufrechtes Sitzen während eines dynamischen Crash Tests. Sie stellen die Daten für einen rechtwinkligen Frontalaufprall mit einem 75 kg schweren Dummy und/oder einer Aufprallgeschwindigkeit von 50 Kilometer pro Stunde und einer Knautschzone von 400 mm dar. Die Maximalverzögerung ist 30 g gemäß FIA Standard 8854/98:
· Die Beckenlast, die leicht 14 kN überschreiten kann, wird das Gurtband auf jeder Seite dehnen und die weichen Körperteile in Bereich des Beckens komprimieren. Das Becken wird um etwa 80 bis 100 mm nach vorn rutschen.
· Jeder Schultergurt wird wahrscheinlich mit über 7 kN belastet. Er wird sich entsprechend dehnen, der Oberkörper wird sich in den Gurt rollen, die Versteller werden aus ihrer Lage heraus bis zu 200 mm nach oben in Richtung Hals verlagert und die Kopf-vorverlagerung wird bis zu 400 mm betragen.

So in a 30g, 50km/h crash with a crumpling zone of 400mm the force on each shoulder strap will be a minimum of 7kN. In total 14 kN on the single bolt that we have. Say we crash at 100km/h (not unthinkable when you have a serious mishap on track), it'd be atleast double that. My calculations above seem to be pretty much in the right direction then.

Now when I dive into it deeper:
v_i = vehicle impact speed
a = decelleration
s_fz = size of crumpling zone
v_i^2 = 2a * s_fz

Assuming the Puma's crumpling zone is 1,0 meter. According to the literature, the average for 90's car of comparable size.
Our impact speed is 27.8 m/s.

27.8^2 = 772.84 = 2*a*1 ---> a = 772.84/2 = 386.42 m/s^2 ---> 1g = 9,81 m/s^2 --> 386.42/9.81 = 39.4g.
This seems to kill you probably or atleast seriously injure you. Hmm..

Now I made a quick stress analysis with two pieces of flat steel and one standard M10 bolt at 14kN. It ain't Patran or anything but I guess it gives you a good understanding. Clearly the bolt is shearing.

Right now I'm looking for a CAD-model of a 7/16 UNF bolt, maybe one of you guys can help me find it? It ain't so common to get here, with 'normal' sizes, so I never installed the content library..

 

[Noobrider]

its a 7/16 UNF bolt, as I said its the original seat belt mounting bolt the black one with the torx head

 

[Sjoerd]

Hmm thats odd. Apparently Ford changed the belt mountings midlife as well!

 

[Ziggy]

Isn't that also assuming you are hitting something 100% square on with infinite resistance ? What happens if you hit armco at say 20 degrees to head on ? or sideways ? or even rear end first ?

Great theoretical study though

 

[Sjoerd]

Isn't that also assuming you are hitting something 100% square on with infinite resistance ? What happens if you hit armco at say 20 degrees to head on ? or sideways ? or even rear end first ?

Great theoretical study though

It is. I have a nice sheet of calculations under my bed for a so called offset-collision. Will definately detail that before I post it here in the next few days. A quick stress analysis of a 'standard' 7/16 UNF bolt showed about 15% higher forces before shearing. Still not enough though for a harness as the forces are different to that of a regular belt.