How can we estimate the capacity of our factory crossbars and then test it before wrecking our vehicle?
Will factory crossbars work for a rooftop tent? It seems like a lot of people say factory crossbars are crap. But is that really true? Could the real problem be that manufacturers just don’t want to deal with the issue of rooftop tents? It’s a small market, and they don’t care to tell us about the maximum static (parking) load of a rack. If something goes wrong then they’re liable. Better not to say anything. Now throw in the occasional mishap (see below) with factory crossbars:
“Dented roof = Nimble 9 [the 30lb bike] + Thule sidearm [bike mount] + Subaru airo crossbars [rated for 165lbs ]” – Source.
So a 30lb bike caused a roof dent with a factory rack rated for 165lbs. When you are given the weight limit for a rack it is always the dynamic weight limit (vehicle in motion) unless told otherwise. It’s a bike so the weight was not distributed. The crossbar didn’t just flex too much. It collapsed at one point causing a permanent bend in the metal.
Are we really seeing a flood of damage reports about the factory crossbars on the internet? I’m not seeing anything like that. It’s not just car manufacturers who have an occasional problem.
Things happen even to the best crossbars.
Factory crossbars worked for this guy on a rooftop tent:
[This is a 2002 GMC Envoy.]
Has anyone tried putting a roof top tent on the factory rails and cross bars?
From what I gather the dynamic weight rating is 220lbs based on the 2002 gmc envoys owners manual. https://my.gmc.com/content/dam/gmownerc … owners.pdf
I decided to try out the 4 factory cross bars. Used it once so far, so far so good.
He added two more factory crossbars for a total 4. The load on each crossbar would probably be no more than 150lbs (600lbs/4). That’s still more than the dynamic load limit of 110lbs per crossbar (220/2.) However, we know that the static limit (parking) is much more than the dynamic limit (driving). General rule for Thule racks is that the static limit is 3x the dynamic limit, but that’s pretty conservative. Typically Thule and Yakima racks with a weight limit of 165lbs have a static limit of 650lbs or a factor of 4x the dynamic limit. I am going to use the 4x factor for calculations here. Then test the results on wooden blocks.
For rooftop tents that fold, all your weight is not going directly to the roof rack. Some is being offloaded to the ladder. This is ignored in static load calculations. I could imagine that 25% of the weight is being offloaded to the ladder.
In the example above, 220lbs dynamic limit translates into 880lbs static limit. So two crossbars should be enough. But we would want to test this. Failure to test might mean we run into trouble if reality doesn’t match the calculations.
Assumed Static Load Limit (N Crossbars) = [(Dynamic Load Limit)/2]xNx4
Let’s say a vehicle roof rack has a dynamic load of 165lbs. That would be for two crossbars. Now we want to calculate that static load based on 2 and 3 crossbars with a 4x factor:
2 crossbars => [165/2]x2x4 = 660lbs
3 crossbars => [165/2]x3x4 = 990lbs
Let’s take a look at the Toyota 4Runner. It’s rooftop rack has a dynamic load limit of 120lbs.
Estimated Static Load Limit (With 2 crossbars) = [120/2]x2x4 = 480lbs
Estimated Static Load Limit (With 3 crossbars) = [120/2]x3x4 = 720lbs
Ideally, your static load limit should be at or greater than 650lbs. In this case, you probably should add another crossbar or consider aftermarket crossbars (rated 165lbs) instead.
Take a look at Yakima’s recommendation for rooftop racks:
Lower than 165lbs means your static load limit will be too low. You will need to beef up the limit with additional factory crossbars or aftermarket crossbars. Beefing up the roof rack is so you can increase the static load limit. It’s not a good idea to stray from the original dynamic load limit while the car is in motion.
If the original dynamic load limit is 120lbs but you beef up the rack to get to 165lbs, that doesn’t mean you can put 165lbs of tent on your vehicle. Put a 120lb tent on your vehicle for driving. When camped now you can load it up to 650lbs. The reason for this is center of gravity. Don’t raise your center of gravity higher than recommended while driving via the original dynamic load limit.
Yakima also says:
The only way to really know if factory crossbars are too flexible is to test them on some blocks of wood. That’s covered in the next section. If the weight of the tent is not greater than the dynamic load limit then you should be fine while driving.
Yakima is a rack company who happens to sell other things. It wants you to buy racks. Naturally it is going to tell you not to use the factory crossbars. If you’re reading that at a Yakima website then you’ll probably end up buying a Yakima rack.
Testing Your Configuration
When you don’t know what your crossbars can handle (static load limit) do the above calculations and test the results.
I would test your static load weight limit by putting TWO crossbars on two blocks of wood like they would go on a car, as far apart as possible while still fitting your rooftop tent. Place the rooftop tent on the crossbars. Put a piece of wood on top then add ONE adult. Film this with a phone. Note the deflection of the crossbars. Would the deflection cause the crossbars to hit the roof of the car? If not a problem then add another person (two people on top) and note the results again. If you barely fail, or you’re not sure then add a THIRD crossbar. Do you pass now? Otherwise buy new crossbars from somebody else, like Thule or Yakima.
Tepui does not recommend using factory crossbars for its rooftop tents:
How Do I Mount This Tent to My Car or SUV?
Check your manufacturer’s roof rack minimum weight carrying capacity to ensure it will support the weight of the tent. We do not recommend mounting directly to factory rack crossbars. We recommend aftermarket racks (such as Thule, Yakima or Rhino). When mounting to factory rack raised side rails make sure they are structurally supported and make sure aftermarket racks are positioned next to rail supports.
Why? I think the reason is that we don’t know the static load limit of the factory racks. But we already know that factory rails can handle huge amounts of weight when the car in not in motion. People regularly mount their rooftop tents to the factory rails without problem. I guess that’s why Tepui is only targeting the crossbars. Tepui doesn’t know what the factory crossbars can handle, so its punting the issue to aftermarket racks.
Rather than punt to the aftermarket why not estimate the static load limit and test the result?
Disclaimer: I am not responsible if something goes wrong. Make sure you test out everything. This article is merely giving you options to consider.