adding to rollcage stiffness

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Many people add a "Petty bar" from the main hoop near the drivers head, down to the base of the leading edge of the passenger door. Good proven design, 'nuff said. But I'd like to leave space for an occasional passenger seat.
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I added a Petty bar to my cage, as well as a horizontal bar behind the dash. That noticably stiffened the car.

On the other hand, IMHO, I don't understand how a cage like this http://www.bimmerworld.com/E36%20cage/Mar2804.jpg could possibly be necessary when you can't attach the front strut towers, or really even the firewall, to the cage.

But I'm not out there winning the ARRC. So what do I know?
 
I got a nasty surprise one day driving an ITB Rabbit home from a race in Portland. It had an Autopower bolt-in and I had absentmindedly wrapped my hand around the A-pillar tube as we tooled along on I-5. We hit an exposed expansion joint and my fingers were pinched between the cage and A-pillar as the gap went from more than 1/2" to less than a finger thickness.

That's one brand of "not stiff" and the one that is addressed by adding a Petty bar here or a dash bar there.

On the other hand, the difference between a basic, six-point, welded-in cage and Stiff (big S) is a whole 'nother deal.

Geo was spot-on about a car actually having 5 springs - the fifth being a big, poorly damped, combination torsion bar/leaf spring that we call the chassis. It has a rate and a frequency just like the coils that we buy for each corner and, if its torsional rate is low enough, that becomes the movement in the entire system. As suspension spring rates get higher - and some IT cars are very high - the 5th spring comes into play earlier and earlier. Importantly, notice that I said 'undamped.'

Part of the point of my wacky door bars is to triangulate the HUGE bay imposed by the hole in the 2-door Golf shell intended to let people in and out of the back seat.

cage08.jpg


I would anticipated that, particularly with a modern front-drive car, the connection between the strut towers and the firewall is probably pretty good (relatively speaking): The connection between the front and rear suspension is not so there is a lot to be gained in terms of stiffness, in the back of the car. The structures that appear in the back of cars on which real engineering money gets spent (ETCC, BTCC come to mind) reinforces this belief in my mind.

It's a great exercise to model rollcage structures in your kitchen. Use those bamboo skewers from the grocery and a hot glue gun, mocking up cages in something like 2"=1' (so a 12" stick gets you 6' of rollcage pipe). Don't fuss over the joints, instead just poking each end in a blob of hot goo. Let it cool and twist away...

It becomes apparent VERY quickly what additional elements add stiffness and (qualitatively) how much.

I used to do a slightly more complex version of this activity with my junior high engineering class students. Their task was to design a spaceframe for a touring car, which we then tested for tosional stiffness (you'd be impressed with the numbers) and crash tested into a cinderblock wall.

m3chassis2.jpg


Have fun and don't put hot glue on your hand.

K
 
Originally posted by Knestis@Jan 13 2004, 01:41 PM
I think they are a really bad idea, FWIW, the cross-section of the structure being approximately the area of one tube at its narrowest point.

K
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Was just reviewing the GCR last night about the 7th/8th point rules and came across a blurb stating that the X is allowed in addition to rear braces. It was in one of the drawings showing the 8th points and giving the A-E discriptions (GCR's out in the shop or I'd give you the section #s)

{EDIT} Whoops... thats what I get for not tossing old copies of the GCR out! I just looked through the PDF file and the wording has been removed.
{/EDIT}
 
Back in the mid 90's i tested an auto power cage (bolt in) at road atlanta...I had a wheel depart the car (4 bolts on LR hub not secured properly *NOTE: when driving other peoples cars, tech them carefully) and went into the grass at t3 backwards with the trailing arm acting like a pogo stick. the car flipped landing RR corner first then alternating LF RR for 4 rotations. the cage was very ugly and i had always said "i would hate to crash this thing". The cage held up and I was uninjured. Inspection showed that the cage bent down 1 inch in the RR corner and about 2 inches on the LF corner. They are stronger than you may think and alot of engineering goes into them. I think the biggest danger to us as racers are the other things flying around the cockpit in a bad wreck. More injuries could be avoided if all the fancy things we put in the cockpit are bolted down more securely.
 
Originally posted by zracre@Aug 14 2005, 02:11 PM
Back in the mid 90's i tested an auto power cage (bolt in) at road atlanta...I had a wheel depart the car (4 bolts on LR hub not secured properly *NOTE: when driving other peoples cars, tech them carefully) and went into the grass at t3 backwards with the trailing arm acting like a pogo stick.  the car flipped landing RR corner first then alternating LF RR for 4 rotations.  the cage was very ugly and i had always said "i would hate to crash this thing".  The cage held up and I was uninjured.  Inspection showed that the cage bent down 1 inch in the RR corner and about 2 inches on the LF corner.  They are stronger than you may think and alot of engineering goes into them.  I think the biggest danger to us as racers are the other things flying around the cockpit in a bad wreck.  More injuries could be avoided if all the fancy things we put in the cockpit are bolted down more securely.
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Ditto what you said, Evan. I watched an SS Mitsubishi tumble down the hill from under the at Atlanta several years ago. The cars fire extinguisher came loose, and detonated making it an omni-directional missile that crashed through the rear window. I was really surprised that the SCCA didn't immediately require a different type of security for them, or at least make fire systems mandatory in all cars. Imagine what would've happened if the driver's body had taken the blow the rear window did. :o
 
Hey...time to put out "Blue Sky" thinking caps on...

The new IT cage rules require certain tubes be present, and that they be of certain sizes, depending on the race weight of the car. All well and good.

The new "option" the rules offer is that you can add as many bars within the stucture as you want, and they may be of any size you desire....

SO...........if I wanted to make the lightest cage possible, I would use the required tubing in the required areas, but I would be inclined to use alternative sizes for the rest.

What could I do and not get slapped with a protest that could stick..(under appeal)??

ANY size, right? How about aluminum tubes bolted in with turnbuckles?? Does the method need to remain the same for optional stuff?? Lots of pre "any size bar rule" cars used smaller bars as gussets, but called them "handles" to avoid the wrath of the tech inspector.

What else??
 
Originally posted by lateapex911@Aug 15 2005, 04:39 PM

ANY size, right?  How about aluminum tubes bolted in with turnbuckles?? Does the method need to remain the same for optional stuff??
What else??
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As long as that material meets the material specs (which I believe is specifically outlined as DOM or 4130) and the attachment point is welded 360 degrees and meets the AWS standard.

I'm thinking aluminum tubing and turnbuckles might not pass the test ;)
 
Hmmm..I should have read that bit more completely.

But...it would be really cool if we could add stuff in that manner, LOL.

I do want to do a camera mount that way.
 
ddewhurst wrote:
... within the following attachment is a cage feast for anyones eyes. If I was in your situation & had a substantial cage I would first check out the fastest cars (Herr Coello Dewey)cages. I would rather add weight to balance the car.

www.bimmerworld.com/cages/cagepics.htm
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They certainly were, and thanks for the link. Very informative. Slightly OT, but one caption mentions how the seat mounts are attached to the cage, which seems like a sound idea, but how does that happen without going beyond the allowed 8 points? It looks to me like the inner points of the seat mounts would be cage attachment points nine and ten. Or is this a rules nerd thing that I'm missing?

Thansk in advance for the education
 
Perhaps I wasn't clear enough - in the pictures provided it appears that the mounts for the seat are attached to cage on the driver's left side of the cage and to the transmission tunnel on the inside (driver's right) side. Would those be two extra points of attachment, or are am I missing something?
 
Originally posted by Wreckerboy@Aug 18 2005, 07:39 AM
Perhaps I wasn't clear enough - in the pictures provided it appears that the mounts for the seat are attached to cage on the driver's left side of the cage and to the transmission tunnel on the inside (driver's right) side. Would those be two extra points of attachment, or are am I missing something?
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Not legal in IT... that site is full of great ideas... just not all of them legal in our class.

Goofy rules (surprise) would rather us bolt to the 20 ga. floorpan.
 
Those pictures show two of the worse window net set up I've ever seen on a race car since SCCA outlawed bungie cords. Look like they were installed after the 5 min whistle in the hot pits.
 
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