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Silly question for today.

Just picked up some strengthening plates from Rimmers ,to fit replacement inertia seat belts in my 6. These provide an additional fixing through the rear wheel arch.

Common sense tells me that ideally these should be welded in place but checking the car in their showroom I couldn't identify any welding. Not having access to welding equipment would a chemical weld such as JB Weld or similar suffice.

 

Brian

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Hi Brian, 

             the plate on the back should be good enough.

Welding it to thin mild steel may actually make the assembly weaker (at the weld in the mild steel sheet)

If you had access to some aerospace araldite adhesives then that would be OK but JB Weld is not for this application.

make sure the hole through the sheet steel is round and free of burrs and sharp edges.

Roger

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I don't understand why a good quality epoxy wouldn't work - as in full strength (24 hour cure time) Araldite.

A seat belt only has to do it's job once and the lap shear strength of Araldite is around 2,600 to 2,700 psi.  If your backing plate is approx. 2" x 2" that gives you over 10,000 lbs (4.5te) of shear strength which ought to be enough to restrain someone weighing perhaps 200lbs.  If you want to do the maths, V - U = at, S = Ut + 1/2(at^2) and F = Ma

Rgds Ian

PS the above is ignoring the strength of the sheet steel with the plate behind it.  The shear force required to pull the bolt sideways through the steel will be enormous.  Bear in mind that the tensile strength of seat belt webbing is only about 7,500 lbs.

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I use this stuff a lot in my business. 

It is used to glue modern car panels on. You could use this which is waterproof, can be welded through, paintable etc to glue your plates on the underside of the inner wing.

http://www.innotecworld.com/products/sealants-glues.php?productid=29

Iain

Edited by iain
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When bonding metal to metal joints the key issue is the preparation.

Even the very best aerospace adhesives have extremely fine quality preparation - this will not be achieved on any old car.

Aerospace Araldite is unbelievably strong BUT only after the prep!!!!!!! 

Stainless rivets are not really the way to go. Consider the joint !

the rivet is simply holding the plate insutu  - and replacing the rivet diameter of mild steel with the rivet diameter of stainless - a very small improvement (assuming that you actually do the riveting correctly).

Monel rivets are the better option but no added strength etc.

PLUS -  the rivets require holes in the mild steel sheet. These are stress raisers  and any benefit the rivet gave would be removed tenfold by the extra holes.

If there was a need to use the seat belt :o then the mild steel sheet would come under a sudden tensile load - holes are bad news. 

I would still recommend an unsupported plate to spread the load

 

Roger

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I had a look at the Motorsport UK (MSA) book. 

https://www.motorsportuk.org/assets/158-197sectionkrevised.pdf

 

and to be honest it doesn’t give that much info. It does mention fixing to chassis but I think this is a generic term for a modern car shell. 

Main thing is the bolts should be in shear stress

B70FFB56-8C2D-46E6-9E45-531365685AC2.png

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Hi Hamish,

                        most bolts have their tensile strength quoted. There are very few, if any,  specific shear bolts on cars.

Aerospace have them and have specific uses.

Generally on our cars tensile bolts are fitted and 60% of the tensile strength is quoted for shear strength.

However on thin(ish) gauge material once the seat belt comes into use any shear forces on the attaching structure turn into tensile loads as the structure starts to bend and deform.

Roger

 

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I might be being incredibly thick (which is not unusual) but surely the only reason to fix the plate is to aid fitting the bolt or stop the plate falling off if you take the bolt out.

You also need to stop water ingress between plate & body shell to prevent corrosion.

Why would rivets be any better than an adhesive as neither would seem to be adding any strength?

Dave

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25 minutes ago, D Murton said:

I might be being incredibly thick (which is not unusual) but surely the only reason to fix the plate is to aid fitting the bolt or stop the plate falling off if you take the bolt out.

You also need to stop water ingress between plate & body shell to prevent corrosion.

Why would rivets be any better than an adhesive as neither would seem to be adding any strength?

Dave

I don’t think you are being thick! I suggested rivets simply to hold the plate in place to make it easier to remove/ refit a bolt without the plate falling off. Most seat belt plates are either in a cage assembly made of sheet steel or spot welded. If the wheel arch popped out as well when you hit something then you’d be bloody unlucky! Four holes in the plate for location purposes filled with stainless rivets or even bolts are unlikely to cause an issue in my opinion. You could also fit an additional plate on the inside for added strength if you were concerned about it.

Kevin

Edited by boxofbits
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58 minutes ago, D Murton said:

Why would rivets be any better than an adhesive as neither would seem to be adding any strength?

 

They wouldn't, they'd just create stress points that would weaken the area.

Is anyone else disappointed that the MSA don't seem to know the difference between traction and tension?

Pete

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As we are saying that the bolts should be in shear, then if we assume a vertical bolt passing through a horizontal metal surface (car body ) with a plate underneath, clamped with a nut.  If the plate is not actually attached to the body then when a horizontal force acts on the bolt it will tend to be pulling on the edge of the body panel only, the plate will just follow the bolt. So, I would think it is pretty important to firmly attach the two.

Bob.

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28 minutes ago, Lebro said:

As we are saying that the bolts should be in shear, then if we assume a vertical bolt passing through a horizontal metal surface (car body ) with a plate underneath, clamped with a nut.  If the plate is not actually attached to the body then when a horizontal force acts on the bolt it will tend to be pulling on the edge of the body panel only, the plate will just follow the bolt. So, I would think it is pretty important to firmly attach the two.

Bob.

If you ignore friction between the two surfaces then your analysis is correct but the shear force required to tear the inner wing, especially when it’s constrained by the backing plate and the back of the eye bolt will be significantly more than the breaking load of the belt webbing. 

Rgds Ian

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