Roof rack weight limits don't seem right

[franc]

Hi guys,

I've been doing some research on roof racks and roof top tents and I'm noticing the weight limits don't seem to come together.

Average limit for a full size rack on a large 4WD seems to be about 150kg give or take.

A roof top tent is about 60kg and two adults have to average at least 130kg. That's 40kg over the limit already.

If you're a big lad this could easily be 10kg or 20kg more, most people will want a few other things on their roof at least, a couple of jerry cans could easily be 40kg more.

That would be 100kg over with pretty standard gear.

What's the deal?

Obviously they build 'em strong than the stated limit, are we just ignoring the weight limits and relying on the excess strength?

[Gamerancher]

The roof of your vehicle is not really designed to "carry" weight. Older vehicles with gutters same deal, they're designed to run water off not carry loads.

[Bourt]

Good point by Gamerancher, I assume we're talking about a 4WD here which is designed to take weight on the roof. Answering on that basis...

You'll find it varies depending on how much brands are willing to claim/take responsibility for, but you'll find in most cases the limit is based on a static weight.

You've got to remember 100kg is not 100kg on a moving 4WD.

For examples sake, say you've got a box that weighs 100kg on the roof rack, even though your cargo would be strapped down to buggery remember the roof rack and box are two separate interacting objects.

When you drive into a dip in the road the 4WD drops and the box almost immediately follows. When you reach upward side of the dip the 4WD (not the box) starts to come up again, and for a fraction of a second the 4WD/rack is basically accelerating upwards into the box, while the box is dropping down onto the rack so the forces are multiplied massively.

For that fraction of a second the rack is effectively supporting hundreds of kilos of weight.

Once the 4WD is static (camped with the tent out) the roof/rack should comfortably support significantly more (everyone clambering into the tent) than the static weight limit.

Not promising anything and double check everything with your 4WD roof and rack specs but that would be the basic idea.

[ebr love]

Bourt has covered it.

Between the family and crew we've loaded up Prados, Land Cruisers, Patrols... all the usual contenders for off road travel. Never had a problem with 75kg - 100kg on the roof. Including walking around up there while positioning stuff.

Speaking only of 4WD's here which I'm sure are expecting some weight. Couldn't say for a regular car, probably what gamerancher has highlighted there is right.

[monbeg]

Worth a read...
https://outbackjoe.com/macho-divertisse ... top-tents/

[Gamerancher]

Loved the mathematics. Many moons ago I read an article about loading sh#t on the roof of a vehicle and it's effect on fuel economy. The conclusions were that the equivalent amount of weight could be "towed" for less fuel consumption.

[WayneO]

Most decent roof racks will allow you to carry up to 100kg's on the roof while traveling. Some of the better racks will allow for much for than this weight.
When it comes to Roof top tents and people in the tent, you are referring to the static weight and in most cases you can load upwards of 250kg's before you experience any trouble with the rack.
As for vehicle without gutters not being able to take weight, have a look at this https://www.facebook.com/Front-Runner-O ... 343059150/
and go to their load test.

[Cooper]

I had myroof top tent on a couple of vechicles. 80 series landcruiser and ford falcon ute with Flexi glass canopy. The 80 series had an ARB rack I didn't give weight a second thought other than I was conscious of having a bit of extra weight up high when 4WD driving. The falcon Flexi glass canopy has a load rating of 75kg per bar (150kg total) pretty sure I workout this figure was more rated to what you can carry driving along. I gave the roof top tent a full test with a couple of people a few times

[BBJ]

Just go easy on the pies before bed if you're in a roof top tent