Roof Rack Safety 1

[drawoh]

I am being asked to do a safety check on some equipment mounted on top of an automobile roof rack. So far, I am seeing all sorts of websites by lawyers describing accidents in which their clients ran over stuff that was not secured properly to a rack. Is there any history of stuff becoming detached in the actual crash, and hitting someone?

I am accustomed to doing this analysis for stuff inside aircraft. Detached components are a hazard for anyone sitting in front of them. Where will the contents of a roof rack land when something is hit by a pickup truck?

So far, it looks like I need to write an installation procedure in which bolts are installed loosely. I would then provide a list of bolts to be tightened with a torque wrench, and then checked off.

--
JHG

 

[IRstuff]

Haven't had anything actually come off, but lots of stuff do wind up flopping around a bit.

I recall one instance on the 210 freeway back in the 80s(?) where a wheel came off a vehicle on one side of the freeway and it hit the windshield of a vehicle on the other side of the freeway and killed the driver.

I could certainly see bicycles, skis, surfboards as all possible projectiles from a crash. Anything within about a 70-ft radius is potentially at risk.

Most people don't know how to secure things, so the rack itself is almost secondary. Additionally, I don't own a torque wrench, so you'd have to come up with something that DIYers can manage.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[moon161]

I remember reading of a wheel that came off a truck on an elevated hiway, killing someone about a mile away in a parking lot.

 

[IRstuff]

Is that even physically possible?
Assuming a maximum height of 200 ft, the fall is 3.5 s, so even if object were traveling 100 mph horizontally, it would only travel a total of about 1050 ft.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[LionelHutz]

It's easily possible. The wheel wouldn't stop the first time it hit the ground.

 

[drawoh]

IRStuff,

This is test equipment, not a consumer product. We have torque wrenches. We have product that goes on top of trucks, but I am not responsible for it. The equipment is many orders of magnitude more expensive than a torque wrench. We can always supply one.

A number of people have been killed by flying truck wheels in the last twenty years or so, up here in the Toronto area. Salt on the highways in winter probably does not help. A wheel does not have to fly. If it breaks off the axle, it is oriented for rolling. This is how old cannon balls worked.

--
JHG

 

[GregLocock]

"Where will the contents of a roof rack land when something is hit by a pickup truck?"

Depends on whether the roof rack is on the target or the impactor. Guessing you mean roofrack on target, then an unsecured load on the roofrack will accelerate forward at a lesser or equal rate to the roof, and so the load may end up in the windscreen of the impactor.

Easy enough to model in MBD software, if you can find a representative crash pulse.

Cheers

Greg Locock


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[EdStainless]

IR, that is before it bounced and kept moving.
Look at youtube videos of race crashes and tires that end up 1,000 yards away. The bounce and roll a lot.

I had a roof rack once that a tiedown system. With straps that locked into specific locations and such.
But it was on a real truck.



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[MintJulep]

Might find something here. .

http://www.iihs.org/iihs/iihs-website-search?q=Roof rack

 

[truckandbus]

I've always used 20g decelerations to evaluate mounting systems on vehicle mounted components (CNG tanks on bus roofs) I guess you could apply that to the hold down system for the roof rack

 

[IRstuff]

A typical helicopter crash shock is 40 g. which is supposed to be survivable.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[DanEE]

quote LionelHutz It's easily possible. The wheel wouldn't stop the first time it hit the ground.

Ditto... witnessed it personally and too close to becoming a victim for my taste..... was some distance behind a car carrier hauling used vehicles.. The cable held spare tire broke loose off a pickup truck on the car carrier and I had to dodge the bouncing tire. On first bounce it was well over twice the height of a passenger vehicle and traveled down the highway in front of the traffic to my rear.

Even though I have one, I don't care to use the factory roof rack on my 4x4 due to further raising the vehicle CG and roll polar moment, and subsequent increased susceptibility to rollover in any accident avoidance maneuver.

As far as I'm concerned, since some vehicle spec allowable roof rack loads are close to the weight of a human being, the anchorages of the rack and contents ought to be comparable to the strength of seat belt anchorages to keep loads from flying off a roof rack vehicle and through the windshield of any other vehicle it were to hit in an accident.

 

[drawoh]

I've always used 20g decelerations...

This is a standard for appliances in aircraft. Maybe 40g is straight down.

--
JHG

 

[IRstuff]

40 g is the crash shock requirement from MIL-STD-810 Table 513.6-III for the forward acceleration during a aircraft crash. Helicopter was only 20 g forward.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529

 

[GregLocock]

If you are going to apply those sorts of forces you will probably need to reinforce the vehicle structure at the mounting points.

Cheers

Greg Locock


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[MikeHalloran]

If you look at the vehicle mounting points, you will realize why the typical auto roof rack is rated for < 200 lb, and certainly not for 20 g. You may want to tie into the drip rails or the pillars.



Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

GregLocock,

That is a good point. I am analyzing the stuff I am responsible for, and I am ignoring the rest.

These are not standard automobile roof racks. I am immediately interested in a frame on top of a pick-up truck. I think it was fabricated by a local welder. I don't see any weak points, but I have not looked carefully at it.

--
JHG

 

[3DDave]

Deformation is your friend. The more energy that can be put into bending and twisting, the less is left over for doing other damage.

You might also benefit from a probability analysis about the chances that such an event will occur. Perhaps contact your State Highway Patrol accident investigation division and ask if they have any information about how often this sort of crash happens and if they can tell anything about items that come loose.

 

[EdStainless]

You might look at rail systems such as aircraft seat track. The track and the anchors that lock into it are available with various ratings. I have seen loading test stands made using Ti 6-4 track and steel anchors.

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[truckandbus]

I don't know how far down the rabbit hole you want to go but the truck OEM probably has a body-builder manual that identifies pick-up points on the chassis for carrying load (most likely for plows and winches but you never know)