Roof top tent mount question

[sned]

We are looking at buying a Tepui 3 person roof tent that we can move between our car and TD.

My current build has 3/4" plywood sides with extra an 2.5" thick word block behind the wall for bracing to mount rain gutter roof rack mounts via two 5/16x2.5" lag bolts..

My concern is the wood and lag bolts holding the weight of the rooftop tent and two or three people. Does anyone think there could be issues with the weight?

Another option I was thinking is installing trailer stake pockets and making a roof rack that will extend down the side of the TD and mount in the stake pockets and use as needed.

[QueticoBill]

I'd suspect the 3/4" ply sides would provide all the vertical support necessary, but would worry about any lateral movement (strong wind even) and if there was any risk of the walls "folding". May not be. Its just if walls are not vertical and load is pushing them to lean at all.

[Aguyfromohio]

I'd suspect the 3/4" ply sides would provide all the vertical support necessary, but would worry about any lateral movement (strong wind even) and if there was any risk of the walls "folding". May not be. Its just if walls are not vertical and load is pushing them to lean at all.

I think you have that exactly correct. Back in statics classes we called it "buckling".
Like any column, the side walls are tremendously strong in pure compression.
The trouble starts when a bit of sideways movement sets in.


It's hard to quantify, that what the class was teaching us, Euler's formula and all that.

If the coach is solidly braced at the front and back bulkheads, with a strong corner connection at top and bottom to make a stiff box, it might be OK.
When there's two adults in the tent and one hornet flies in, things could get bouncing around pretty badly.

[QueticoBill]

I worry about the gust of wind more than the hornet or some nocturnal activity.

[tony.latham]

My big concern would be the zero-dark-thirty trip down that ladder. I'm talking night terrors.

My concern is the wood and lag bolts holding the weight of the rooftop tent and two or three people.

Is there a way to run a regular bolt through that blocking? It'd be much stronger. I assume the block is glued and screwed?

Tony

[sned]

Is there a way to run a regular bolt through that blocking? It'd be much stronger. I assume the block is glued and screwed?

Tony

Correct the block is screwed an glued in. I haven't installed the roof so I could run a bolt through.

Wasn't thinking about side load, hence the reason I asked here. To be safe either build a metal frame that connects roof rack to trailer frame or just leave tent on top of the car.

[tony.latham]

I haven't installed the roof so I could run a bolt through.

Could you drill it and put a T-nut on the backside?

T

[working on it]

My big concern would be the zero-dark-thirty trip down that ladder. I'm talking night terrors.

My concern is the wood and lag bolts holding the weight of the rooftop tent and two or three people.

Is there a way to run a regular bolt through that blocking? It'd be much stronger. I assume the block is glued and screwed?

Tony

• I'm also looking to add extra bracing to my 3/4" walls, both for making a roof rack support, later on, and an inner brace/weight support for the interior (I need to make an overhead lift "trapeze", since I have trouble getting out of bed- can't roll out!) My roof is bolted/glued directly on top of the walls, and needed no spars to support it, as it is well-supported all-around, and is of small overall size. I want to retrofit a 4x4 spar/beam , glued to the roof with PL Premium, and using long lag bolts to affix it to the walls. I'll cut it to 46.5" long (wall-to-wall dimensions), carving out a 2" wide center space (so it will fit over my central light bar on the ceiling), and using eye-bolts to tie my lift rope to (soft, braided 3/8" marine rope). Of course, painted to match the decor! You might consider something similar, though it might be better to lower the beam's height (as low as you could), to make a stronger bracing structure.

• 

  BRACING PROPOSAL.png (23.83 KiB) Viewed 2380 times

• My modification? to come

• 

  trapeze-bed lift proposal.png (245.97 KiB) Viewed 2380 times

[djohnsonnv]

I'd suspect the 3/4" ply sides would provide all the vertical support necessary, but would worry about any lateral movement (strong wind even) and if there was any risk of the walls "folding". May not be. Its just if walls are not vertical and load is pushing them to lean at all.

I think you have that exactly correct. Back in statics classes we called it "buckling".
Like any column, the side walls are tremendously strong in pure compression.
The trouble starts when a bit of sideways movement sets in.


It's hard to quantify, that what the class was teaching us, Euler's formula and all that.

If the coach is solidly braced at the front and back bulkheads, with a strong corner connection at top and bottom to make a stiff box, it might be OK.
When there's two adults in the tent and one hornet flies in, things could get bouncing around pretty badly.

An erected RTT will add additional lateral wind loading. Lateral resistance isn't hard to overcome with a rigid skin and bulkheads. This is demonstrated by the cheap IKEA/Walmart bookcases. These bookcases will fall right over without backing yet becomes fairly sturdy when a thin sheet is applied to the back. IMO a solid skin will provide better resistance than blocking or bracing.

Also buckling wouldn't come into play. Buckling is a failure that occurs when the supporting member is overcome by the internal stresses. This occurs when very thin members have very large spans. I would elaborate further but also had a hard time staying awake during structural analysis.

[Aguyfromohio]

I worry about the gust of wind more than the hornet or some nocturnal activity.

That's a great point QB.
I was trying to find a lighthearted way to say that even in calm weather normal human activity can put some large sideways loads on the tent platform. In our normal daily life we just don't think about the sideways loads we put on our floors, because they are so tremendously strong the subject never comes up. For a roof top tent it's important. And wind loads are indeed a sure source of sideways loads on the system.

[Aguyfromohio]

An erected RTT will add additional lateral wind loading. Lateral resistance isn't hard to overcome with a rigid skin and bulkheads. This is demonstrated by the cheap IKEA/Walmart bookcases. These bookcases will fall right over without backing yet becomes fairly sturdy when a thin sheet is applied to the back. IMO a solid skin will provide better resistance than blocking or bracing.

Also buckling wouldn't come into play. Buckling is a failure that occurs when the supporting member is overcome by the internal stresses. This occurs when very thin members have very large spans. I would elaborate further but also had a hard time staying awake during structural analysis.

Your points are all good.
A well-made box structure intended to hold a RTT will do just fine. For a TD that means
- the sidewalls need to be stiff in bending, like 3/4 plywood without seams, or strongly glued foam sandwich panels. Studs with weakly attached skins would not be as good.
- the front and aft bulkheads need to be attached strongly to the side walls, even if sidewalls are strong and stiff.

You are correct that the folding up of a box structure is not a pure, classic column buckling failure mode.
It would have been better if I'd called it a connection failure at the corners and not posted the standard column buckling figures.

I do remain concerned, though, that QB is correct and the box might fold up under a RTT.
The box can be plenty strong enough for normal use and still fold up with a heavy sideways load applied at the top by RTT.
We should probably also mention the risk of tip-over in a strong wind. That's a big sail high up on a narrow wheelbase.

All we can do is point out the risk unless we have the details of construction and slog through the calculations.
Perhaps someone here has really done this and tell us their actual experience, and either put our concerns to rest or confirm them.
Live field testing is hard to beat.

[QueticoBill]

I agree with the concept it's not the gravity load that's a problem. Two pieces of 3/4 ply will hold the weight and not buckle or just bend. But for them to not fold they need to be braced. Assume the bulkhead between sleeping and kitchen compartments takes care of that end, the question is does the roof/front wall do the job for that end. A sheet of plywood attached well to the ply sides is probably good enough.

I can't recall if 10% or 20% is the value I use for lateral load for some structures but calculate gravity load - 3 people, gear, and tent - maybe 500-700 pounds? - and see if 15% or so of that applied to top of wall - laterally- has any effect. Nothing like empiracle engineering.

[KCStudly]

I'm with Tony. The typical TD box with good construction, bulkheads and cabinetry is probably going to be more than rigid enough. It's those pesky lag bolts that worry me. Not the first time, probably not the fifth time, but into soft wood, probably won't last forever before wallowing out.

Not an apples to apples comparison, but case in point, the pumpkin chunkin rules were updated a few years back to disallow lag bolts. Too many wooden catapults were falling apart when their lag bolt joints wiggled apart.

Through bolts with washers or T-nuts won't pull out.

[working on it]

I'm with Tony. The typical TD box with good construction, bulkheads and cabinetry is probably going to be more than rigid enough. It's those pesky lag bolts that worry me. Not the first time, probably not the fifth time, but into soft wood, probably won't last forever before wallowing out.

Not an apples to apples comparison, but case in point, the pumpkin chunkin rules were updated a few years back to disallow lag bolts. Too many wooden catapults were falling apart when their lag bolt joints wiggled apart.

Through bolts with washers or T-nuts won't pull out.

• You're very aware of the different forces involved in the simple stresses a RTT would place on fasteners, compared to the violent actions of a punkin-chunker (trebuchet, for instance); the RTT lies flat on the trailer roof, with the most prevalent forces pushing directly downward on one wall, while possible lifting up on the other wall (mainly shearing forces), while the trebuchet will be torquing the supporting framework in a twisting manner, while storing/releasing energy. Unless the RTT is going to be storing/releasing tremendous energy, I don't see how deeply inserted lag bolts (say, 1/2" diameter, inserted at least 4" into the crossing 4x4 bracing/support beam), would be subjected to any such twist. It has already been determined that the usual monocoque box structure of a TD or TTT (especially of 3/4" plywood walls, like mine has) is quite resistant to torque or twist. The RTT platform on top could not impart as much force as the chunker, for sure. Here's a table illustrating the pull-out force needed to defeat the lag bolt in a non-twisting structure (the shear strength would not be a factor, but anyway, the shear strength of a 1/2" lag bolt is far in excess of the downward/upward stresses-there are none, really- imparted from the walls to beam interface).

• 

  Lag-screw pull-out strength chart.PNG (82.58 KiB) Viewed 2273 times

  1/2" lag x 4" insertion depth = 1748 lbs force needed to pull-out of Southern Pine
• The usual 4x4 I encounter around my home is Southern Pine, which I plan to glue and lag-bolt to use as a spar/bridge beam for my lifting "trapeze". I now for sure, from experience how strongly the PL Premium will hold, and the lag bolts will just be for insurance against adhesive failure (with my luck, I can just see a 4x4 crashing on my head).

[KCStudly]

You are correct, WOI hence the not apples to apples comment. However, whether it is a catapult or a RRT, they both require good solid structural and joint designs, but... even with the RRT we are not talking about a strictly static situation with just gravity or some wind force acting on the RTT. The whole thing is bouncing down the road with hundreds of thousands of changes in the direction that the mass of that tent (80 lbs maybe more ? x rate of acceleration, 3-4 g's on a good bounce?) is trying to move. Sure, you might argue that the trailer has springs and it doesn't really bounce that often, but let's be realistic, these forces are still there and every time, every little jostle its trying to overwhelm the friction in the joint and put stress on those fasteners. A few hundred lbs worth of people (perhaps heavy people... Americans are getting heavier on average, ya' know?) will be climbing in and out, and moving around. Will lag bolts hold? Maybe. Are thru bolts better? Yes. That was my point.

Not sure where on a TD there would be 4 inches of wood to screw a lag bolt into, except for end grain, and that would be an even poorer joint design. Just saying.

Not trying to start an argument, just trying to elaborating on my informed opinion. Peace.