Roll cage design loads? 1

[Schmeal]

I'm designing a roll over protection system for my personal pickup truck and would like to know what loads I need to design to.

I started with a CAD model for the connection points and packaging. I then modeled the design in Femap using line elements representative of the tubing cross-sections.

Now I'm stuck thinking of how to load the model to asses it. Are there automotive standards for roll over protection and if so what are they? This is not in my industry.

I started with the vehicle weight as a static load on the sides and top in which it is well under yield. I then assumed an impact at 40 mph with a 12 inch deflection which gave me a load of around 300,000 lbs. Using this as a static load overwhelms the structure. Can someone point me in the right direction?

 

[MikeHalloran]

The formal standards for ROPS are at sae.org. Bring your wallet.
Start by getting the index, which I think no longer fits on a CD.

According to an old friend who was a part-time steward at the Mt. Clemens, MI dirt track,
their local standard for roll cage assessment included vigorous application of a heavy sledge, anywhere and everywhere, and visual examination for yield, and especially weld cracks.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

I start with NHRA roll cage construction requirements. Probably SCCA and SCTA (Bonneville) has some too.

 

[DanEE]

I'm sure the references listed above will cover the question of strength of your attachment points, but for what it is worth, photo below shows result of a 4 point roll bar tied only to body sheet metal without appropriate body reinforcement or frame tie-in.

I witnessed this slow speed rollover on a rock face where the 4 point attached roll bar had two of the roll structure feet go through the floor to be stopped only by the leaf springs directly below. The failure mode was a combination of the 1/4" roll plate foot shear cutting through body sheet metal and body spot welds tearing apart. Driver was very lucky to have no injuries.

Ron Fournier's Metal Fabricators Handbook who has an extensive history in race and specialty car construction is somewhat dated, but a worthwhile read.

https://books.google.com/books/about/Metal_Fabricator_s_Handbook.html?id=qCmoaBtYjYkC

 

[Mr168]

For what its worth, the SCCA addresses sports racer and formula cars using the following criteria:

The roll bar must be capable of withstanding the following stress loading applied simultaneously to the top of the roll bar: 1.5 (X) laterally, 5.5 (X) longitudinally in both the fore and aft directions, and 7.5 (X) vertically, where (X) = the minimum weight of the car

 

[GregLocock]

Good to see some hard numbers. They seem very high to me, but if everyone works to the same rules, why not? I agree with the hammer method, that's how i test my welding, but it is not very helpful as an analytical design case!

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Schmeal]

Thank you for the replies.

The fabricator is a desert race guy and pointed me towards the SCORE rule book. They are very simplistic, with a main tube size specified by the vehicle weight. The max being 4k lbs. There are also general design guidelines, but these apply to a full cage, not just a functional roll bar.

http://score-international.com/raceinfo/wp-content/uploads/2016/01/General-Regulations.pdf

Mr168, I'll give your loading cases a whirl and see how my design does. I trust the SCCA would be able to spec out roll bar properly.

 

[SwinnyGG]

I would submit that a hammer is a pretty terrible way to evaluate a roll cage subject to impact from a multi-ton assembly.

You might be able to detect cracked welds from the sound you hear but any purpose other than that seems like a waste of time.

 

[Dave K]

Could refer to the FIA regs, granted the web page is hard to navigate, but they do have roll over protection guidelines, for various classes of cars, so may glean some info there.

FIA WORLD RALLY CHAMPIONSHIP

http://www.fia.com/regulation/category/119

-Dave

NX 9, Teamcenter 10