Mark's roll cage question list

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darkknight9
I <3 the funicular shape machine.

I like the shape and the sound. Hmmmm funicular..It has a woody sound fun-nic-ular heh hehh hmmmmm like


Double fun-inc-ular....
heh...heh heh [/perv laugh off]

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
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You can also see this function in the very shape of the tubing you're using to form the cage.....

(why NOT "square" tubing? John just esp'lain-ed with his fornic-u-lar MO-chine...)

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Parfois, on fait pas semblant!

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Nowhere.



Edited 1 time(s). Last edit at 01/23/2013 07:41PM by Creech.

Grant, the reason to add the lower bars below the backstays to the rear strut tower is twofold:
1) Any rearward force on the cage from a front impact is converted to a mostly downward vertical force on the strut tower and not just a mostly reaward force; strut towers have their strength in the vertical direction (not a rearward direction) as that is what they are designed to handle 100% of the time.
2) The reverse is true, in that shock loads up through the suspension into the towers is MUCH better taken by the lower added tube added to the backstay. A single backstay tube will flex up with upward shock loads to the strut area; the lower bar prevents this by immediately going into tension and preventing that movement.

It is immaterial whether the last 1000 cages had it or not; it works.

What kind of sight hole do I need on the little triangle gusset in this picture? Bites out of all three corners, a hole in the middle, or don't worry about it?



After all this talk my next cage is going to look like this...



Maybe there are so many bars in hopes of one of those welds holding during a crash. hahaha
ohhhh, who am I to talk.

Quote
john vanlandingham

Generally compression for a tube secured at the bottom like this II the force going V thataway..
And generally tension is for a tube like this secured at the bottom force going thisaway ^

doorbars being hit seems to me like its mainly perpendicular or nearly so, so bending force ----->ll seems the largest factor .
So an arch bent outwards has a shorter straight part, that's stronger. And the arched shape itself IS stronger---we've known that since pre-Roman times although they really went to town with arches, and the arched outward shape absorbs and disperses energy further away from the bodies in the car and that can't be bad...

Two things John

1) We don't see nice symmetrical arched tubes in rally cars because they would stick 2-3' out the side of the car. A straight tube with a single bend in the center making a 'V' or with two short bends at the end and a straight middle are not the same as an arch.

2) In order to be effective, the arch has to be 'anchored' to something that is effectively immovable. Even 1.75" tubing with additional backstays does not come close to meeting this requirement.

I am pretty sure Mike Hurst would back me up on my prescription.

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____________________________________________________________-

One. Class -- 2WD

www.quantumrallysport.com

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Quote
Josh Wimpey

Quote
john vanlandingham

Generally compression for a tube secured at the bottom like this II the force going V thataway..
And generally tension is for a tube like this secured at the bottom force going thisaway ^

doorbars being hit seems to me like its mainly perpendicular or nearly so, so bending force ----->ll seems the largest factor .
So an arch bent outwards has a shorter straight part, that's stronger. And the arched shape itself IS stronger---we've known that since pre-Roman times although they really went to town with arches, and the arched outward shape absorbs and disperses energy further away from the bodies in the car and that can't be bad...

Two things John

1) We don't see nice symmetrical arched tubes in rally cars because they would stick 2-3' out the side of the car. A straight tube with a single bend in the center making a 'V' or with two short bends at the end and a straight middle are not the same as an arch.

2) In order to be effective, the arch has to be 'anchored' to something that is effectively immovable. Even 1.75" tubing with additional backstays does not come close to meeting this requirement.

I am pretty sure Mike Hurst would back me up on my prescription.

Hurst is frequently flat wrong on things and will dogmatically argue and resort to all sorts of extra-"legal" means including obscenity filled emails with threats to "do everything I can to see you never rally again"..

His "interpretation" of FIA rules is a particularly error filled subject which some of us have gone so far as to email and call FIA Technical Commission, who were, to put it very mildly, astounded by the "reasoning"...and said so....

Thus, I would never suggest the idea that Hurst agrees as a strengthening of an argument.....

I have seen the argument over and over and over for more than 25 years, the reference to NASCAR was odd. They require their multiple bars because when a car has spun and is going to be hit by something , that something is something weighing about 3500lbs and going upwards of 180mph..
They want so time and time is gotten with distance and vice versa--before the person is smoooshed...

Again this is so self-evident about curves , arches, modified archs , force vectors, that I simply do not undrestand the flights of fantasy that I read in the "other side".
the tension/compression is the crux of the error, the door bars are primarily seeing bending -------->ll.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Quote
john vanlandingham

the tension/compression is the crux of the error, the door bars are primarily seeing bending -------->ll.

NO, only bent/arched door bars see 'bending' or whatever the hell force that is.... Straight bars see tension--- at least until something fails/moves upstream

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____________________________________________________________-

One. Class -- 2WD

www.quantumrallysport.com

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Mark..nibble a small notch at the front leg.

2nd cage pictured..horrible welds, horribe rear backstay location.

Now to the 'arcument' (groan..) arcs in building are very thick, not so much in a door bar. An arced door bar that takes a pinpoint hit from a tree will just collapse as quickly as any other, maybe quicker as the inpact is spread over a smaller area due to the arc. Loads need to be spread over the widest area possible .

Double V bars properly installed, or a Vbar joined to a sill bar will not hinge or twist as claimed. The key is a proper tickness plate welded on the inside. Instead picture the 2 Vs coming from the legs like saloon doors. They will stretch/crush/bend inwards but the hinging will be either on the front A pillar or main hoop evenly like a gate.

A conventional X is still only one bar thick in the middle unless the segmented bars are cut longer and overlapped and gusseted.

NASCAR bars are a huge grid, built on a perimeter frame out of 1.75 inch tubing and plated with sheet steel. They've also made big changes for 2013 http://www.macsmotorcitygarage.com/2012/09/11/mcg-exclusive-2013-nascar-roll-cage/ . They use pure mass to absorb damage from a theoretical flatish hit of a wall or car front. They shouldn't be in a discussion on rally cages.

Just for discussions sake we should remember that in all of our cars, except maybe a VW Beetle, the main hoop is effectively bowed out at the belt line as is the front hoop.

Cages in small cars are a compromise due to available room. Cages are meant to save the occupant not the car. Energy absorbtion is a good thing. There is no perfect design for a sport run in imperfect conditions.

There is a HUGE hole, around 14"x24" on most cars right next to your head with no protection. Drive accordingly.

Gene, I gotta agree with some of your points and disagree:
- Rear backstay location pictures is fine (the actual backstay loactions mind you). The lower tubes from main hoop floor mounts and the cross tube between towers and the W shaped bracing to the floor create 2 fixed points in space to which the backstays are attached that are much, much stronger than a typcial rear strut tower and carry the loads properly. The vertical strength of the towers to which these are attached is the weakest point. It uses a LOT of extra tubing but it IS strong and well spreads the loads. And the rear X brace mount location is OK. Now the 'floor stub's for the lower rear X brace ARE interesting.....
- Agreed on the double V door bars. The tendency for the vees to distort in rotatation is much less than for them to bend in, and to also bend the main and A pillars. But to work properly and transmit loads and make things bend as we prefer, the center joint HAS to be stronger than just a couple of weld beads, hence the point to strongly plate the inside of the joint.
- I am still working to analyze a simple X side bar; this jury is out.
- The idea of a NASCAR cage having not place in rally cage discussion does not make sense; any thing that can work has a place. And the idea of "They use pure mass to absorb damage..." is OK! ANY multiple tube side bar structure like the NASCAR cage will serve to keep the vertical part of the main hoop from distorting forward as much (from a side view) than a single tube attached at the center of the main hoop vertical. With the NASCAR side structure, more force is transmitted to the floor and backstay, rather than going into bending the main hoop vertical portion forward. This is good in that is lessens the inward bending of the side bars. The really big good thing about a NASCAR type cage is the the bending starts out further from you and thus you get more crush space. The possible bad thing is that the deceleration MAY be higher. But this is more dictated by the main hoop and A-pillar bending inward (energy absorption) than is recognized. And getting in and out can be a problem if the top bars is high.
- Any overly bowed side bar is an issue in a concentrated laod. But let's recognize that trees tend to be our enemy much, much more than pointy rocks. Saplings and stuff under 4" is not the issue. Trees with OD's of 6"-8" and up are the issue. So we are not really dealing with point loads. We are dealing with large diameter objects, larger diameter than the mandrels we use to bend tubing. So tube collapse due to points is not the big issue to worry about. A side bar tube with the bent ends kept short will work just like a straight tube.

Mark, we ain't disagreeing much...
The backstays above are terminated on a tube rising from the shock mount. The further up the mounting the more the tube will bend. The longer the mount tube the more leverage the backstay will have to bend or rip it from its base. The actual cage/backstay doesn't have a proper plated mount. TheIt gets to be more of a 45* bent backstay like an old Jeep would have. Allt eh rulles pretty much indicate backstays must be straight with no bends, that above cage has a 'bend' welded into the backstay. Stuff like this is a problem because we don't have an esablished procedure or dimensions to use. This kind of ceativity is what gets cages a do over rather than a log book. While I seriously doubt the cage pictured is a huge concern function wise it is a concern on interpretation. Would that same configuration be allowed if the mounting tube came straight up from the floor? No, so where does the line of acceptance stop? thats my concern.

I have no issues with slightly bent door bars. I agree with the trade-off between tension strength and crush space. NASCAR door bars are totally different than rally car door bars and most people don't seem to realize that. They seem to think that 2 parallel door bars with an upright or two are NASCAR door bars. Forget about the frame, the plating, the number of bars or the fact there is no door or window on a NASCAR. I wish rally cars were big enough inside to have NASCAR door bars and proper seats. What people on rally forums call NASCAR door bars are more of a SCCA road race configuration.

from the major crashes i have seen working for rally worx, #93 bob olson's 2009 black river stages rock crash being the worst (you tube it if ya dont know), the weakest point of the cage is the lack of bracing between the front cage legs, at the bottom of the door openings. in car #93 the floor pan squished and contorted, and the car got a good deal narrower between the front legs, the other one, which we have no footage of, dick rockrorh's sti (we didnt build that cage) that he centered on a tree at LSPR, at full throtle, also heaved the floor pan up and in, between the front feet AND the main hoop. (his harness bar, which was between the rear strut towers, bent foreward 3-4 inches from the crash too, belts were way to long buth thats a diffrent point)

neither of these cages included a "dash bar" or front strut tower tie ins, in the cage, they did have straight door X's that did not bend, and both had "John's" rear lower strut bar.

Dick broke a toe due to the floor crumpling, and bob's injuries were from his HANS, and co-driver got knocked out by the map light.

the cages did their job. BUT now we add a dash bar between the front legs, and actully continue it to the chassis its self, we tie the front struts to the cage level with the top of the door X. we have yet to add the fire wall X that crosses thru from leg to oposite tower as i dont see it aiding much more over the dash bar.

WE have become a huge fan of over kill with cages, even An extra "$500 in tube, gussets and time" or an added 20 lbs of weight in safety is well worth it, as a cage builder, i suggest EXTRA and educate as to why.. i DO NOT want some one severly injured, or dead in one of my caged cars cuz they wanted to save $100, or $500.. If i feel its needed, i PUT it in and eat the $30 in tube, and 1 hour of work..

i agree with John about how arched are stronger in design, but as Josh pointed out a true tube arch in a car isnt going to happen, our next best option is Tri-angulation.
BUT what do i know...i just have a Non-Destructive testing degree and inspect structural welds for a living...

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I be sorry as a muthafucka I did, still sorry I did n' hustled ta peep what tha fuck I holla'd a lil' bit better, or at least try to.



Edited 1 time(s). Last edit at 01/29/2013 09:14AM by mke723.

Thanks Mike for the insight. After my very large side impact with a tree, on the 2nd car I added a sill bar below the door X, added the "Head wing" bars from the main hoop to the fwd lateral, and went from the bottom of the main hoop up to the rear strut tower tie in. Both cars had tie in bars to the front strut tower. No dash bar, but I wish I would have requested one. When you look at the cage completed with no other interior in the car, that area does look like a huge hole.

Fortunately on a Derik Nelson cage, everything fits so tight to the shell it is stiched to the shell in 20 or more locations. The way the firewall is designed on the Merkur, it is a "L" shape with a 1.5' shelf, so it seems it would be much stiffer in a side impact that a conventional firewall.