Installing rivnuts in the trunk

[BigAl205]

Since the car has a trunk, it's less of a concern...but if it was in the cabin with me, bolts with washers would be the only way to go. Back in my younger years. I had a Tacoma with two 12's behind the seat, and the box was screwed down to the sheet metal with regular screws. While driving down the road, a deer jumped out in front of me. I slammed on the brakes, the rear screws sheared off, and the box tipped forward and slammed into the back of my seat. If I had a rollover accident, I might not be here today. It was a good lesson in physics.

[Captain Buff]

Yeah, i dunno...
chilly-bin in NZ, because it is bin that is chilly.

Where I was born we keep things simple. KISS. Only thing is that whilst it is spelt (aka 'spelled' if you don't think the word looks right and check Google) 'chilly bin' it is pronounced 'cheaully bun...cuz'.

Yeah, i dunno...
I worked with a couple of really seasoned materials engineers, and they used peel tests for epoxies and adhesive tapes... and I think there is a Mil-Spec for the tests.

That metric tonne for an M8 sounds right. It is a lot, but if the bolts were along the front, and there is a moment arm to the centre of mass, and the snout of the car plowes into a wall, then it too could lever against the bolts and break them in tensile before they would ever fail in sheer. Hopefully it would just buckle the trunk and stay attached.

Mil-Spec for peel tests...sounds like something interesting for a bored Engineer to research! Normal people can leave that one alone
'Failure' is a difficult word to use when related to any structural member. Even my roughly quoted shear strength of an M8 bolt is based on calculations that assume the bolt is completely homogenous, the materials that the bolt is in are both perfectly aligned with each other with no gap in-between them and can move with no friction against each other...and the list of assumptions gets more ridiculous from there.
'If the bolts (or Velcro, or any other joining mechanism) are at the front' is another assumption.
I think most people with general common sense would take a square board and put an 'attachment' somewhere near each corner (or if a glue or Velcro then along two edges) when fitting it to something else. Just sort of makes sense...?

Using the riv nut fixing as an example (trying as best as possible to stay relevant to the OP's query) - assuming (again!) that the riv nut is fixed well and the bolt has good thread contact with both the riv nut and the MDF - failure would be a combination of the MDF crushing, the bolt bending, and the threads pulling out of the MDF board. Assuming (again) that the board has a load that isn't astronomical (as in 300 kg/660 pounds) and/or that load has a centre of gravity close to the board (not 3m/10ft from the board); it would take a massive amount of force to bring those bolts to failure.

Velcro/glue Holmz has said a bit and I couldn't comment on as I didn't study them, other than to say with common sense that the riv nut and a bolt is likely the strongest fixing method of the group (if that is your concern).

In my particular instance the fitted carpet top is fairly well locked in place. I can't remember exactly where I put the Velcro - but if at the front and a 'levering' type issue arose - then the sub box would lift about 30mm (1 inch-sh) before it hit the boot/trunk roof. So that could not create an issue.

In general if people have vehicles with large tools or caravans or those homes on wheels (I think they are called or a brand is Winnebago's?) then common sense surely plays a part. Transporting (ie.) a pile of bags of cement in a van or ute and having say a dozen bags at the very back of the vehicle whilst travelling at freeway speeds or even just driving around corners is 100% dumb in the ass. Maybe a lame example but the point is there.

[Holmz]

.... Maybe a lame example but the point is there.

If the example was a strawman to attack, then it would be lame.
But we see people do questionable things a lot - or at least those are the videoed examples.

So my post is best viewed as more more of a public service announcement (PSA).

[Captain Buff]

Since the car has a trunk, it's less of a concern...but if it was in the cabin with me, bolts with washers would be the only way to go. Back in my younger years. I had a Tacoma with two 12's behind the seat, and the box was screwed down to the sheet metal with regular screws. While driving down the road, a deer jumped out in front of me. I slammed on the brakes, the rear screws sheared off, and the box tipped forward and slammed into the back of my seat. If I had a rollover accident, I might not be here today. It was a good lesson in physics.

You can learn all the science and mathematics applicable to many subjects - and it is definitely beneficial - but nothing teaches quite like your own experience (and to a lesser extent someone else’s). Crazy story man.

[Holmz]

Since the car has a trunk, it's less of a concern...but if it was in the cabin with me, bolts with washers would be the only way to go. Back in my younger years. I had a Tacoma with two 12's behind the seat, and the box was screwed down to the sheet metal with regular screws. While driving down the road, a deer jumped out in front of me. I slammed on the brakes, the rear screws sheared off, and the box tipped forward and slammed into the back of my seat. If I had a rollover accident, I might not be here today. It was a good lesson in physics.

Those Nort American deer look a lot like the local Australian skippy in the face... and they taste similar too.I hope you stopped and at least cut out the backstraps?

i do see a bit of a deer in your avatar.

[ckirocz28]

If you do use rivnuts (threaded inserts), definitely use steel or stainless steel, the aluminum ones suck in any application under any load.

[Theslaking]

Pl400 has a load strength somewhere in the neighborhood of 450lbs per sq in of adhesive. I can't remember the exact number but that's real close. So a 10 inch bead double up down the length of a 2x4 is going to hold more than any regular bolt. It's not even a fair fight.

Rivnuts get exponentially stronger with washers.

[ckirocz28]

Rivnuts get exponentially stronger with washers.

Where would these washers be used? Between rivnut and sheet metal, or on the screw that goes into the rivnut?

[Theslaking]

On the backside, however depending on the location you may not be able to use them.

[ckirocz28]

On the backside, however depending on the location you may not be able to use them.

Gotcha. Always good to learn some more common sense.