How many crossbars do I need?

Hi all,

Planning on a trip from Amsterdam to Dakar as service vehicle for Heroes Legend rally...!

Currently, my Autohome Columbus Rooftent is mounted on two Thule cross bars. According to the official Dutch roof tent dealer this is the best way, because the roof tent construction can follow the inevitable torsion of the car. He has come across damaged cars (okay, they were Land Rovers) where the body had been damaged because of the stiffness of the roof rack.
On the us website of auto home I actually found:

" Off highway use: These tents are made to withstand a lot of abuse, but we highly recommend, when mounting a tent for using gutter mount rack systems for off-highway use, that you employ THREE or more cross bars. This helps to distribute the load evenly and avoid damage to the vehicle under extreme conditions."

I can do the maths that 3 or more cross bars would distribute the load, but I'm not able to speak from experience about the effects of the torsion powers - and I would like it to stay that way...

Anyone can speak from experience?

Many thanks, Jurgen

i've run two with no issues.

Timely question, as I just picked up a newtome RTT, and currently have 2 Thule cross bars and was wondering if a couple more would be needed.
I think the Thule bars are rated for around 150 lbs each, so with the tent weighing approx 100 lbs, two adults at 320 lbs, assorted stuff, and probably 2 kids squeezing in at times, that's around 550 lbs. Slightly over the 300 ish pound rating.

Can't say if we own the same Thules, but mine are rated 220 lbs.

The tent is 110 lbs, I am 150 lbs, so as long as the 115 lbs wife will not outperform me, we should be fine...

I have an Autohome Columbus Carbon. I run mine with 3 Thule bars. More bars means less stress on the roof and gutters, less stress on the bars, and spreads out the stress on the tent too.

What would be the reason not to run 3 or more bars? An extra bar is cheap compared with the cost of the tent or the vehicle.

The theory is that the third (or fourth) cross bar prevents the tent to follow the inevitable torsion of the car in extreme cases, like off-roading, and will do more damage than good.

It is not that I know this, I'm just investigating what would be the best option.

I actually do own 4 Thule cross bars and it is not about the money at all.

Cheers, J.

I'm by no means an expert, but the load when sleeping is downward. A cross bar can provide this kind of support without the tent being bolted to the bar. The middle bar(s) could be there for support, but without bolting the RTT to the bars. that way, when travelling off road (with no one sleeping in the tent at that time!), the tent would be able to have some additional movement to allow it to twist off the middle bar(s) but be physically secured by the two end bars.

I move my Columbus RTT from a trailer to a roof rack depending on the type of trip I'm going on.
On the trailer it's mounted to two cross bars but there are four cross bars underneath the tent. (it just sits on the two middle bars not attached)
On the roof rack the tent covers the whole rack and is mounted at four points but rests on a flat surface.
I haven't seen any damage to tent or vehicle with either mounting type.

I'm by no means an expert, but the load when sleeping is downward. A cross bar can provide this kind of support without the tent being bolted to the bar. The middle bar(s) could be there for support, but without bolting the RTT to the bars. that way, when travelling off road (with no one sleeping in the tent at that time!), the tent would be able to have some additional movement to allow it to twist off the middle bar(s) but be physically secured by the two end bars.

The dealer also constructs custom roof racks, based on the same principle that the roofrack can follow the torsion powers.

Thanks again, J.

Thinking outloud here.....

how much can the roof "flex" or deflect in reality?

I mean the doors wouldn't fit or the windows would blow out with very little flex.

I'd probably have like 4 bars at least. But I'm a chicken....

I would put up four bars. On bad roads the forces on these RTT and roof carriers are tremendous and I have seen mounting posts bent and gutters ripped off. So the bar will be strong enough, but the mountings should be able to spread the loadpoints. The more the merrier.

The weight rating should be when the vehicle is moving and bumping around. So when stopping you could have higher weight, except if you are one very active sleeper.

three is fine. I have the same tent on an 80 series and it hold up perfectly to the horrible washboard roads in baja.
J

Three bars are recommended on any installation, due to the fact that during occupancy the flex on the lower platform is not as great and your comfort level is greater. As far as flex from the outside there should not be too much as the bodies of the vehicles are usually one piece. Good luck.

The theory is that the third (or fourth) cross bar prevents the tent to follow the inevitable torsion of the car in extreme cases, like off-roading, and will do more damage than good.

It is not that I know this, I'm just investigating what would be the best option.

Cheers, J.

You have to remember that many vehicles cross rough terrain with heavy duty roof racks that are much less flexible and are fixed in 8 places.

If you really wanted to be certain it could flex,you might want to look at mounting the camper on rubber.

Any vehicle that getting cracks from torsional stress is probably been driven too hard ,overloaded,or not had the tyres deflated on rough terrain.
Deflating the tyres takes enormous stress off the body and tyres are cheaper than vehicle bodies and most of the other components.

I had the Columbus Large on 4. Two were actually attached to the tent, two were just under there to support the weight of two adults and two kids.

Moody, now this is a consideration, suport but not connection Good.

I've used 2 crossbars on a maggi for about 10 years without issue. Having said that, I like the idea of an additional crossbar or two that are not attached to the tent for extra occupant support.

I've used 2 crossbars on a maggi for about 10 years without issue. Having said that, I like the idea of an additional crossbar or two that are not attached to the tent for extra occupant support.

Good point...

Not attaching them is actually not a good idea on harsh washboard. The tent will flex enough to bounce on the non-attached bars and it will go toktoktoktoktoktoktoktoktok...till you are insane.

FWIW I never found the vibration an issue...