Can My 4 Handle 80lb On Roof?

Hey everyone, I'm back from the dead! Sold my 98 4runner years back after I crashed, and have gotten a 2011 4 since.

I'm planning a business venture using my 4...going to buy cross bars to add to the stock rails on the roof. My idea is basically screw mounting a 2'x4' sign onto the cross bars, and the sign weighs about 80lbs.

I'm no engineer, but wondering if anyone here can say if this will be ok/safe or not? The sign will be run parallel with the roof, so long ways in order to "slice" through the wind as I drive no more than about 45 mph. I'm a bit concerned if there were strong sideways winds though, but maybe not as concerned as I should be?

I'm looking at these cross bars rated for 220lbs - https://www.amazon.com/2010-2023-Al...rd-Skiboard/dp/B0BRFQ2KJL?smid=A231UTS1W2L6O9

I could really use some insights here before I go spending money on the setup and committing to that kind of spend! Thanks ahead of time.

 

Not sure I understand what you’re intending to do. Why do you want to screw the sign onto the crossbars? Are you asking if it’s ok to use the sign as a roof rack. Or just carrying the sign as cargo on the cross bars?

 

Thanks for replying and asking. My intent is to essential mount a somewhat large, 80lb metal sign on top of my 4runner to display as a billboard of sorts to help promote local businesses.

So I figured I'd buy cross bars (only have rails at the moment) and drill holes to mount the sign directly onto those cross bars so the sign is vertical for display purposes. Unless that would be unsafe, which then I'd need to figure out a different way to mount it up top (I really want to avoid pulling a trailer behind just to display the sign).

 

The weight of the sign shouldn't be a problem. Here's a calculator the force generated by wind resistance, but you need some analysis to address leverage of that load on the roof. It doesn't look too bad, if it's secured properly. https://www.omnicalculator.com/physics/wind-load

 

Thank you! I figured I'd be in the clear so long as it's secured properly. Just second guessing myself I guess lol. Appreciate it, thanks.

 

FYI 2010-2013 Trail Editions have different roof rails. What trim is your 2011?

 

It's not weight of sign I would worry about-it's only 80lbs. It's the load the wind will place on the crossbars/roof while driving. It's essentially a 2'x4' sail. Will it withstand 5mph sidewind? Probably. 50mph? Not sure.

 

Ah ok. Agree with previous posts, the weight of the sign isn’t really a factor for the roof itself. Wind loads are what you have to worry about. The roof rack mounts may end up taking a lot of pulling force.

You’d have to also carefully look at how the sign itself is constructed, and consider if the sheet material itself is strong enough to survive with a crosswind gusts. Probably not, since sheet metal folds over under its own weight when the size is that big. So really, you would need to design a structure that will support the sheetmetal from folding and buckling.

In either case I wouldn’t drive around with it on top at road speeds, given the risks of a sign that decides to take flight. Flying sheet metal is actually pretty terrifying because you don’t know how it’s exactly going to hit something. Besides, if it hits another car, they’ll know exactly who to call

 

There are a number of manufacturers of vehicle rooftop 'billboards', from plastic to metal, ambient (daytime) visibility to internally illuminated to active LED displays. All of whom have designed means to attach directly to crossbars (not necessarily a 4Runner's in particular), straps with 'hooks' to drip rails (old school) to clips like those found on Thule, yakima...those companies have already done the engineering work to make these signs attach to a vehicle properly. I would take a close look at how these companies achieve this and mimic if you are designing up your own signage.

Word of caution, because the wind loading is more of a (safety) factor and not rooftop loading (lots of folks, in the thousands actually) have RTTs that weigh upwards of 200 lbs so static loading isn't the issue. It's the 'dynamic' loads that rooftop rack companies (and Toyota and other vehicle manufacturers) warn customers about. That being said, if you are considering a drill and tap method ("basically screw mounting") of attaching to a roof rack crossbar, thread depth (generally spec'd as nD where 'n' specifies how deep a threaded region needs to be in relationship to a fasteners diameter) needs to be your main focus. Too thin of a threaded region and you'll risk pulling a fastener out with little force...thread stripping also becomes a significant concern with too little thread present. General starting point would be to make sure a thread depth is at least equal to the diameter of the fastener that you are using...at a minimum. If the crossbar itself is made out of aluminum then you would do best to install thread inserts (helicoils for example). Rivnuts are another option for thin wall applications, thin as in sheet metal thin up to 1/4" or so. Basically on 4Runners this is close to the type of fasteners that Toyota uses for mounting points for the factory roof racks.

Here are images of a Rivnut installation on an interface plate that I'm working on. The plate is 1/8" 6061 T6 Aluminum but since I'm using 6mm fasteners the plate provides less than 1/2D thickness for necessary thread depth. First image is of the topside and the second image clearly shows how the Rivnut provides more than 1D thread engagement.

 

According to the calculator I linked, a 50 mph wind operating on 8 sq ft generates about 51 pounds of force.

 

2011 SR5 Limited (has leather heated seats and other typically Limited features).

Thanks for spelling that out for me lol the calculator you linked was cool but still kinda beyond my pea brain. Sounds reasonable.


I think I'll be sure to use the appropriately threaded and length of screws to fasten the sign (will look into the Rivnuts as mentioned above), and will probably have my welder neighbor help me spot weld some kind of A frame bars buttressed perpendicular between the sign itself and the cross bars to provide additional support should we have sideways winds like we sometimes experience where I live.

I appreciate the thoughts everyone, thank you!

 

I don't know if you have the sign already made, or you are still in the design phase, but this type of sign structure would seem to be stronger than one flat panel standing upright, especially in side winds. It would have more attachment points, making it structurally stouter:

 

Also, you might want to look at LFD Ruggedized crossbars. While pricier than the Amazon version linked above, they are designed to allow attaching items without having to drill, and I imagine are much stronger.

 

Thank you! Will def take a look at those for sure

 

Oh btw, the crossbars that you linked to are quite similar to the ones I used to retrofit onto my 21's basket rack. Strong enough and you should be fine with using the t-slot mounting method, just increase the number of fastening locations, not just four corners but also mid-way along the length (if possible) just to have a margin of safety.

 

Buy a trailer and tow the sign. Solves all your problems...

 

Will your insurance cover any damage to other people if your sign comes off @ 45 mph? Better make sure you're covered... "Anybody can sue anybody for anything..."