Why plano cases on roof?

yeah I’m gonna see how to make it stronger. Flat bar seems like the easiest way given how I have it on so far.

right now I drilled in the box oval holes underneath (there are 4).
Roam and Sherpa have a nicer attachment system but it costs a lot more.

even saw a guy run not two but THREE boxes one in center that looks good but don’t know how easy access the middle one is reaching over two on the sides where those ones ARE easy to access. And plan to screw struts into them.

used a moving blanket cut in half as padding but might switch to a yoga mat or something.
Because folding the moving blanket over top of gear pads it well except the box tries to pinch it at the corners which could interfere with lid gasket sealing when closed.

the Home Depot bolts going through the box right now I have thread into the Sherpa T slot nuts that slide into their crossbars. I’ll check the site to see if they have any listed info on what the strength of that is.

the more the better but I know if I add ratchet straps over top,
1. They’ll look ugly, and flap around
2. Will come loose over time
3. Interferes with box opening access

so far I don’t have the issue others had with the boxes gradually sliding rearward as my bolts simply go into the rails and that doesn’t really move.

but the concern is a failure of the plastic in the box due to washers not spreading the load and if anything ever makes them rip off it would not be good.

last time I plugged holes it was a pain but alright result. On an HVAC surrounding trim.
Used a combination of plastic welder, metal mesh heated into it, epoxy laid down, sanding, etc. which could result in even stronger than the surrounding material, so patching holes on these boxes to change anything would probably not be a big deal.

the guy with 3 boxes I can’t help but wonder if he fit maxtrax axe and shovel in the center to not have it out exposed taking space needing more hard to find mounting solutions and reducing clearance.
Truck barely fits in parking garages and if I get the maxtrax bolted onto the box lids I think that would be what puts it over the edge of too tall never again fit anywhere.

this one:



I can always test fit. At least if MaxTrax shovel and axe DOES fit into a third center box
1. It would be more anti theft by being harder to reach and less visible the contents
2. May still have room for other stuff

labels would be nice. But let folks know what you have.
Right now just relying on memory of how I laid it out. Left side camping right side tools recovery tow and whatnot; mechanical things

 

Using Allen head furniture bolts can smooth and lower the profile inside the case. It’s wider and thinner than a hex head or nut.

 

If your rails have t-slots, then one option would always be to get carriage or hex bolts (usually 1/4 or 5/16 diameter will be the sizes to look at) that fit the t-slots and just put a nylon nut/fender washer combo on inside the box. As long as you're not carrying fragile stuff in the box, the bolt tails hanging out won't be a big deal, and if you're really worried about it, you can always cut the bolts to length and put a rounded cap nut on above the ny-lock.

Unless you're planning to run the Baja 500 while hauling a full load of gold bullion in the box, four 1/4" or 5/16" bolts at the corners of the box with 1 1/4" fender washers (use grade 8 washers if you can find them, or double-stack two if you're really worried about strength) will be plenty strong to keep the box from coming off. If you're using the heavy-duty weather proof cases, the effective area that would need to shear through with a 5/4" diameter washer is around 0.75-0.90 sq in; if the box material (I can only find information on the kind of foam that's used to fill the box) has only 10% of the strength of aluminum, the pull-through load for one bolt location would be close to or over 1000 lbf. That's stronger than a standard HD 1/4" zinc bolt, and probably stronger than your roof rack. Having a full span of flat bar between two of the bolts would add some strength, but it's not adding strength where it would matter.

The only reason to add ratchet straps over the top of a roof-mounted box would be if you're using light weight rifle cases, but since those aren't generally weather-sealed or made from UV-resistant plastic needing straps to keep them closed would be the least of your concerns.

As far as padding the boxes, keep your tools in a roll/bag (GI surplus tool bags are great for this) and they'll be fine without padding. Towing/recovery gear shouldn't need any padding either. If you're worried about stuff shifting around (probably only an issue for tool rolls/bags, really), mounting a couple of eyebolts through the bottom of the box will create attach points for bungee cords or friction straps wherever you need them; I personally mounted a piece of e-track into the box so I can add/remove/relocate tie-down points as needed although I rarely use them since I usually just keep jumper cables, tire chains, and a few other odds and ends in the roof box. This kit would probably be ideal if you're looking to mount two boxes.

$47 DC Cargo Mall Indoor E Track Tie Down Rail Kit 3' (2 Pack) for Garages, Vans, Trailers, Motorcycle Tie Downs, ATV Mountings - Powder-Coat Black - Secure Cargo & Heavy Loads Up to 2000 lbs

www.amazon.com/DC-Cargo-Motorcycle-Mountings-Powder-Coat/dp/B08NTW5Z17

and combined with these you'll be completely set

www.amazon.com/DC-Cargo-Mall-Bungee-Tie-Down

It'd be tricky to fit maxtrax, which are usually 48" long into rifle cases made to fit guns that are rarely much over 42-43". Maxtrax have enough lockable mounting options to not need to be stored inside a case (if vehicle height isn't a big issue for you, it'd even be possible to mount MT carrying brackets onto the top of one of your boxes), and while they're somewhat valuable to a particular kind of buyer, they're not something that the average junkie knows how to fence quickly, same goes for roto-pax (although in CA it's getting close to where a gallon of gasoline could be worth trying to steal)

 

I never thought I'd be the one to say this to someone else in my life, but you're really overthinking it on this. Usually that's my move, I rarely see it from this perspective.

 

I hope so.

never thought it about it before. Just drove. No issues.

later was advised it should be beefed up claiming “my method” (such as shown below pic) is too weak

and would result in the box flying off
by ripping through the plastic

Which so far after dailying and a trail it has not.
I’ve got no idea how much force is generated fighting the box from wind resistance driving
Or the weight of contents inside off road
without a wind fairing to block it and apply downforce. Unless I make one.

last “experiment” I moved a desk on a car roof. Even tied down it tried to fly up at slow speed. Stuck my hand out the sunroof pulling down the short drive home. Thing wanted to take off.



Sherpa brackets clamp down Roam from the top in a T shape but that’s a different box.



probably gonna at least put longer bolts and flat bar to brace it including factors like
-plastic over time due to aging eventually dries, weakens, fatigues, stretches, cracks



just wanted to know what everyone else did.

already have t slot nuts. Bolts. Washers.
Just don’t know if it’s strong enough.

sitting parked installed isn’t the same as forces on the truck driving.
Current bolts into t slot nuts both stainless 1/4-20. 4 connections per box. Def not the beefiest hardware.




should I overthink it? Yes. Because if the box flies off and through someone’s windshield or they run it over in a sedan that’s not a good scenario. Crack the box have a bunch of large strong metal objects scatter on the road.

Bit more consequence than having a burned out brake bulb. Or cab mount rub that affects driver and no one else.

maybe it’s fine as is.


I’ll double check the math but Apache 9800 front cross section 0.68 sq ft x 12.5 = 8.5lbs of wind force which sounds too low.

 

If that's screwed into a slot nut set in a t-track crossbar, the only thing I'd change would be to get a bigger diameter washer on there. The strength of a "pull-through" failure in a sitiation like that will increase at least directly with the OD of the washer, assuming the washer itself has decent strength. A stainless fender washer should be fine for that kind of application (two of them stacked will definitely spread the load out completely), or a grade 8 would work, but it can be hard to find fender washers in that grade.

The huge variable in all of this is what the actual shear strength of the box material is. That's something that shouldn't be hard to get some idea of if the material that the case is made from were known to me (every search I run says it's closed-cell PE foam, but I think that's the internal shapable foam and not the outer casing material.

If that's a 1/4" diameter bolt, then your current washer probably has a 5/8" OD. The strength there would be at least doubled by using a 1.25 OD washer, even better with a 1.5 inch OD, both of those should be easy to find at HD or Lowes (in my area Lowes tends to have a little better selection), and if not then most Ace Hardware locations should have them in stock.

The other thing to consider is how few real-world occurrences will actually create significant tension load in that joint. Even in a crash situation, a collision big enough to create enough "g force" on the box to pull hard on any corner of the box is one that you're not likely to survive unless the amount of mass in the boxes is so large that it'd be in danger of causing a failure of either your roof rack, or the roof (which is designed to hold up in a full rollover) itself if you hit a large bump at moderate speed.

 

having the bolts along the centerline changes things a bit as far as how much load could develop at any bolt location, and makes bending in the bottom of the box a more likely mode of failure than just pull-through. If you're looking to reinforce the box with strips of flat bar like you've got sketched there, you'll want to bolt the ends of the crossbars to the bottom of the box, and use steel for spanning that kind of distance since it's going to be in compression if the box pulls up (and not loaded if the box is pushed down since it'll be pushing against the rack), and aluminum will buckle at 1/3 the load that steel will.

Locating the bolts as close as possible (say, 2" away if you're using a large washer) to the edges of the box will eliminate the need for that kind of reinforcement. Even better, if they can be pushed that close to the corners, you'll be as strong as possible. Bolting the box down along its center-line will create bending loads through the plastic which would create a lot of tension load in the bottom face of the plastic, and that stress combined with the effect of the holes being located there could lead to the plastic splitting open which could then allow the bolts (especially with smaller washers) to slip through those rupture points. If the box is bolted down at the corners, the mechanism to create that bending through the bottom plastic doesn't exist, and even with the existing holes just plugged with molded epoxy putty, there's no danger of the bottom splitting at those locations unless you put a crushing amount of weight inside the box.

Tension strength of 1/4" diameter grade 2 (60ksi steel) bolts is over 2000 lb. That 220 lbf "pull-out" load capability is based entirely on what material that hardware is attached to; that strength level seems to be for a 1/4-20 bolt/rod connected to threads tapped into a 1/8" thickness of 6000-series Aluminum (or a 1/8" thick aluminum nut), where the Aluminum part is limiting the strength of the joint. As long as you're not using thin aluminum nuts in the t-track (there's no good reason to because steel t-track nuts are standard parts which aren't hard to obtain, and the t-track might actually be able to work with regular steel hex nuts, depending on the extrusion type you're dealing with).

304 Stainless steel hardware is 150ksi, and four 1/4" diameter bolts at that strength could be used to hoist your truck off the ground with another Tacoma hanging from it and plenty of strength to spare. With a small diameter washer, a bolted joint located near the corner of the box will have its strength limited to the shear strength (Fsu) of the plastic multiplied by the thickness times the circumference of the washer (which is why strength varies directly with washer diameter).

As far as air pressure, that 12.5psf at 70 mph is the "dynamic pressure" of air moving at that speed, and would only be pushing directly at that force on an object which forced that airflow to stop. There will be a "point" (really a line across the width) of the box where that load applies to the box, but to figure the actual air load acting on the box there'd be a number of other variables involved. If the 12.5 psf pressure were somehow acting on the bottom face of the box (with no air pressure of any kind acting on the lid, which is more or less impossible at any speed your truck can go), that's still only 112 lb of force (assuming box outer dimensions of 26x50 inches) from which the weight of the box and its contents would be subtracted before becoming load into the bolts, so if the box and contents were weightless, that's 28 lbf at each of the four bolts, or about 1/8 of the strength of the joint if you're using 1/8" thick aluminum nuts with coarse thread steel bolts which you really shouldn't be.

 

all this nerdy talk I'm into! keep going with the engineering aspects!

 

Much farther into it and I'll really be into the deep water where the next step would be to post some FEM and CFD analysis results, and that level of detail is really not particularly useful if I don't know at least the approximate physical properties of the case shell material.

 

this is what my brain enjoys my wife absolutely gets super lost when my engineering family members show up and we start talking static loads and ratings on certain things.

 

Definitely overthinking things. You don't really need a full roll cage for a plastic box carrying camping gear. Just use your noodle; 1 bolt on every corner (4 total) with decently sized washers and its not going anywhere. 1 piece of flat bar to span the width on each end for extra insurance and its definitely not going anywhere.

 

also not wrong! sometimes over thinking things is unesessary

 

I do structures analysis for aerospace. At this point most of my career has been on fighter jets, but I have been lucky enough to spend some time working with the Space Shuttle, X-37, and James Webb Telescope. Other than actual nuts and bolts, I don't deal a lot with "rating" values, and the weakest fasteners we tend to use on stuff that's built to leave the ground (or the atmosphere) are the equivalent of something like Grade 12.

My brother does bio-med research focused around/associated with genetics, and he can lose me completely about 10 seconds into the answer to most of the questions I've tried to ask.

 

ah ha, my wife is a nurse, I work in a hospital doing Admin, my grand father was a structural engineer, and my mother was a drafter. at 8 I use to help my grandfather do survey work, and did it up until I was in my 20's for his company. most of my family thought I would be an engineer, and I went to college the first time for automotive engineering. then ended up with a degree in health care administration. but oh yeah. when they built my shop the owner of the company who actually built it was really suppressed about lots of the specs I wanted and said I needed.

 

I'm all for overthinking and doing as much research as you can to make an educated decision, but that usually happens prior to being 100% invested into the decision. This thread has been an interesting roller coaster for OP, that's for sure. I definitely understand the reasoning for coming around to entertaining roof top cases, and I have one myself, but if I were scared to death of things flying off my roof rack, I probably wouldn't have a roof rack to begin with. I always think its fun to think about the amount of wild stuff we all put on our roof racks (cases/tents/spare tires/shower systems/etc). Some of us add an absurd amount of weight. We take every precaution to ensure everything bolted down to the rack, security measures in place, and everything's overbuilt and heavy as hell. And then we rely on the factory nutserts in the roof of the vehicle to hold it all there. I've always wondered what the shear strength, or the amount of force needed (think rear-ending someone) it would take for a roof rack loaded down with shit to shear and fly off. Sorry OP, new fear unlocked!

 

Probably won’t shear off roof. Never heard of it. Plus in accidents at that point anything can happen to any car.
Engines fly out. They’re designed to.

ran out of room. Needed room. Got boxes. There is room now. No regrets.

the bolts that hold the rack to roof are stronger/thicker. I think. and there’s more of them.
It’s not RTT either.
companies rate their racks up to like 700-1000lbs. Boxes nowhere near that.

I don’t rear end people. That’s controllable. Getting T boned is less controllable. Hopefully no ones deciding to run a red into a lifted BoF truck with sliders.

not saying they have weight but look at cars with rails. Their crossbars don’t even use bolts. But clamps. From factory.

 

yeah that’s probably the easiest option, similar in difficulty to flat bar or wider washers

saw a guy did exactly that. He mounted his differently and had issue with them sliding offroad (backwards I think)

so he basically got a couple cargo hooks that bolt to t slot nuts
And put one on each outside corner of the box as kind of a “puzzle” to hold it where it is. Not down but just from sliding around.

I’ll look at the box more to see if there’s any useful exterior surface like that Roam has for the T clamps pulling down on it.

whatever’s not a big ratchet strap coming loose flailing around ugly interfering with lid open operation.

 

the parking garage won



and that’s without MaxTrax relocated to the lids…

 

I just realized OP started this thread 3 years ago with whats the point. Now has advanced to crashing into parking garages.

Strange things afoot at the Circle K.

 

Won’t be the last time. When I was a kid my dad opened the lid on a U-haul trailer at the entrance of a motel.