Real World Roof Weight Limits

[GaryS]

Hi, I'm planning on purchasing 2 Hullavators (or the like) to put 2 kayaks on the roof. I calculate that altogether, it would weight close to 200 lbs. I understand that the dynamic weight limit is 100 lbs with a moonroof. But how firm is that? I'm always very careful when I have stuff on the roof and the idea that I could not carry 2 kayaks would cause me to think about selling the car. I had 2 on my Mercury Mariner, no problem at all. What do you think?

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[tburner]

Hi, I'm planning on purchasing 2 Hullavators (or the like) to put 2 kayaks on the roof. I calculate that altogether, it would weight close to 200 lbs. I understand that the dynamic weight limit is 100 lbs with a moonroof. But how firm is that? I'm always very careful when I have stuff on the roof and the idea that I could not carry 2 kayaks would cause me to think about selling the car. I had 2 on my Mercury Mariner, no problem at all. What do you think?

The dynamic weight rating exists to limit poor driving characteristics due to the higher center of gravity resulting from tossing stuff up on the roof. Basically, Ford doesn't want you to drive home with a refrigerator up top only to roll over as you're whipping out of the Lowe's parking lot.

Don't drive like an asshole and you should be fine. It's been discussed a fair bit around here so if you want more opinions hit the search bar up top.

 

[GaryS]

The dynamic weight rating exists to limit poor driving characteristics due to the higher center of gravity resulting from tossing stuff up on the roof. Basically, Ford doesn't want you to drive home with a refrigerator up top only to roll over as you're whipping out of the Lowe's parking lot.

Don't drive like an asshole and you should be fine. It's been discussed a fair bit around here so if you want more opinions hit the search bar up top.

Thanks that's kind of what I suspected but wanted to make sure before I buy the racks, load up and go on a trip in May or June. I suspect I'll tie down the kayaks so they won't slide right off on a turn. j/k.

 

[Osco]

Yeah, think about those roof top tents. You know by now more than a few couples have slept up top together with both adding up to 300 plus.
Many surly have tested the SUV’s suspension up there by doing the Horizontal Bop !
Lash em down good and enjoy the sway:’P

 

[thekingprawn]

I saw in one video the guy showed the door sticker, and it said 800 pounds total cargo weight (people and stuff), and in certain parts of my county that's less than 3 people. I have a feeling Ford has an X4 or X5 safety factor built into these ratings for liability reasons.

 

[GaryS]

Yeah, think about those roof top tents. You know by now more than a few couples have slept up top together by now with both adding up to 300 plus.
Many by now have tested the SUV’s suspension up there by doing the Horizontal Bop !
Lash em down good and enjoy the sway:’P

Thanks. The static weight rating is, I think, 600 lbs. So a tent with people in it, even doing the tango, is within weight limits except if their doing it while the car is driving down the highway. OTOH I've seen racks with all kinds of equipment on the roof, which along with the rack must be kind of heavy.

 

[Mark S.]

Yeah, think about those roof top tents. You know by now more than a few couples have slept up top together by now with both adding up to 300 plus.

I believe the question is more about dynamic than static limits. The static (vehicle not in motion) limit is 600 lbs; the dynamic (vehicle in motion—as in driving, not bopping, heh) limit is 150 lbs without a moon roof, 100 lbs with. This has to do with the vehicle's dynamic center of gravity.

The two factors that affect dynamic (driving) CG are weight and distance from the vehicle's CG. If you add 500 lbs to the weight of the vehicle at its static (resting) CG the dynamic CG won't change. But move that 500 lbs away from the vehicle's static CG and dynamic CG moves with it. The farther away from the vehicle's static CG you move a load the more it will change the dynamic CG. Owing to its safari-style roof, the Bronco Sport is a tall vehicle (which is part of the reason fuel economy suffers compared to other vehicles in its class). That means any weight you add up there is far from the static CG and will have a greater effect on the dynamic CG.

As Troy mentioned, the dynamic limit is about how the vehicle handles with a higher dynamic CG. To me, the real danger isn't under normal driving circumstances where the driver can just "be more careful," it's when you maneuver the car in an emergency, or under other than normal driving conditions (wind, off-road, etc.). Will you remember your vehicle is more prone to rollover when you swerve to avoid a sudden road hazard? How much will a cross-wind affect you with a large object on top, where it will have more leverage to push you around?

In response to the post about safety margins: We don't know if Ford included a "safety margin" when determining the rooftop dynamic weight limit. It's possible, but there's no data I'm aware of on which to base a definitive claim. The question is does that really matter?

Auto makers do sometimes design in safety margins for certain parts and assemblies, but they are usually based on mandates not fear of liability. Liability is avoided by meeting mandated testing standards, then setting and publishing use limits. In other words, if you experience a rollover accident with your Bronco Sport and you have more than the published dynamic weight limit strapped to the roof you will have curtailed your chances of a successful product liability lawsuit.

That aside, safety margins (when present) exist to account for the unexpected. There are several good reasons I can think of right off the top of my head not to encroach on a known safety margin involving rooftop loading--emergency maneuvering, load shifting, degradation of suspension components, etc.--but mainly I wouldn't do it for all the reasons I CAN'T think of. The fact that we don't really know if a safety margin even exists is even more reason not to push the envelope.

 

[GaryS]

I believe the question is more about dynamic than static limits. The static (vehicle not in motion) limit is 600 lbs; the dynamic (vehicle in motion—as in driving, not bopping, heh) limit is 150 lbs without a moon roof, 100 lbs with. This has to do with the vehicle's dynamic center of gravity.

The two factors that affect dynamic (driving) CG are weight and distance from the vehicle's CG. If you add 500 lbs to the weight of the vehicle at its static (resting) CG the dynamic CG won't change. But move that 500 lbs away from the vehicle's static CG and dynamic CG moves with it. The farther away from the vehicle's static CG you move a load the more it will change the dynamic CG. Owing to its safari-style roof, the Bronco Sport is a tall vehicle (which is part of the reason fuel economy suffers compared to other vehicles in its class). That means any weight you add up there is far from the static CG and will have a greater effect on the dynamic CG.

As Troy mentioned, the dynamic limit is about how the vehicle handles with a higher dynamic CG. To me, the real danger isn't under normal driving circumstances where the driver can just "be more careful," it's when you maneuver the car in an emergency, or under other than normal driving conditions (wind, off-road, etc.). Will you remember your vehicle is more prone to rollover when you swerve to avoid a sudden road hazard? How much will a cross-wind affect you with a large object on top, where it will have more leverage to push you around?

In response to the post about safety margins: We don't know if Ford included a "safety margin" when determining the rooftop dynamic weight limit. It's possible, but there's no data I'm aware of on which to base a definitive claim. The question is does that really matter?

Auto makers do sometimes design in safety margins for certain parts and assemblies, but they are usually based on mandates not fear of liability. Liability is avoided by meeting mandated testing standards, then setting and publishing use limits. In other words, if you experience a rollover accident with your Bronco Sport and you have more than the published dynamic weight limit strapped to the roof you will have curtailed your chances of a successful product liability lawsuit.

That aside, safety margins (when present) exist to account for the unexpected. There are several good reasons I can think of right off the top of my head not to encroach on a known safety margin involving rooftop loading--emergency maneuvering, load shifting, degradation of suspension components, etc.--but mainly I wouldn't do it for all the reasons I CAN'T think of. The fact that we don't really know if a safety margin even exists is even more reason not to push the envelope.

What you're saying makes sense and it's obviously not what I want to hear but I would say that a 'safety margin' would matter. For instance if there's a fairly large margin I'd know that if I drive fairly slowly leaving a lot of room I'd be confident that I'd be OK. If there is none or very little I'd be biting my nails each time I load up the roof and the stress wouldn't be worth it so I'd probably either get a trailer (though I have no place to put one) or a different car altogether. Yecch! With my previous car I was completely unaware and had new crossbars and 2 kayaks and never had any issue even on the highway at 75 mph. The car always felt rock solid. I think the Merc Mariner I had was maybe 2 inches lower and I recently looked it up and it also had a 100 lb "maximum load" (however I was under the impression at the time that that was due to the OEM crossbars not the dynamic rating. The manual did not specify). I thank you for your detailed response.

 

[Meanderthal]

One more thing to consider in the "dynamic" category is the acceleration forces. That also includes deceleration. So if you have 150 lbs on the rack and have to slam on the brakes, it will approach 1g in braking. That would put almost 150 lbs forward on the rack. Stepping on the gas pedal will give you much less than 1g in this vehicle. Another form of acceleration is hitting a bump at highway speeds. Not sure what the "g" load would be given the suspension and all other variables but a 1g acceleration could also be possible. So that 150 lb load now becomes 300 lb.

The affects on handling and possible rollover are another form of acceleration. So cornering will put increased loads on the crossbars but to the sides.

 

[Mark S.]

What you're saying makes sense and it's obviously not what I want to hear but I would say that a 'safety margin' would matter. For instance if there's a fairly large margin I'd know that if I drive fairly slowly leaving a lot of room I'd be confident that I'd be OK. If there is none or very little I'd be biting my nails each time I load up the roof and the stress wouldn't be worth it so I'd probably either get a trailer (though I have no place to put one) or a different car altogether. Yecch!

Unless you have data how would you know? I don't think Ford is going to share that kind of data with an owner.

I thank you for your detailed response.

You're welcome. A final thought: I've owned a lot of different cars over my nearly five decades of driving. It's been my experience that using or modifying a vehicle to do something it wasn't designed to do is almost always a recipe for disappointment. You're better off buying a car whose specs meet your needs than hoping for the best with one that doesn't. Good luck.

 

[nateG533]

A couple other questions for the group here....Really appreciated your explanation above @Mark S.

1) Would a lift kit (HRG 1.5" kit seems to be common on these forums) affect dynamic weight limits, either negatively or positively?
2) Would a "zombie guard" mount on a roof platform help to distribute exterior storage weight in way to help avoid safety concerns with the 100-150lb dynamic limit on the roof?

 

[Benanza]

It also has to do with the force of the wind pushing down on the object like added weight. However a kayak shape may reduce that wind friction. You could also just lift a kayak up there without having a hullavator do it for you

 

[Elegance]

That dynamic roof weight is not something I would play with. We drive a literal box - Not the most aerodynamic thing on the market even with nothing on the roof at all. You can drive as carefully as you want, but if a pedestrian jumps out in front of you and you need to swerve or brake hard, you'll find out pretty quickly why that weight limit exists.

I'm about to drive from NC back to my home in AZ. We bought a bunch of goodies during our 4 months in NC, so I bought roof bars and a cargo rack to just load some of our clothes up top and save interior space. The rack itself is 35 pounds, and I'm still loading less than 70 pounds of junk up there even without a moonroof (150 pound dymanic capacity). You just never know what the road is going to throw at you, especially going that far on the interstate like I am. Those numbers exist for your safety and the integrity of the vehicle.

 

[maximumwang]

I load up my roof fairly often with lumber. I'm pretty sure I've come close or slightly exceeded the dynamic load limit a few times. I drive very carefully when I do. Not so much about speed, but about avoiding pot holes. When you hit a ridge or pothole at 30mph, the forces acting on your roof are going to be multiplied.

 

[Froggy]

When the ford bronco sport came out, there where failures of the sun roof, do to thermodynamics. With them failing with just heat, I would not exceed the load on roof. Imho.

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