dash and door bars?

Good grief, recently I've been finding myself agreeing with Grant in nine out every ten posts he makes. What is the world coming to?

FWIW, I agree with Grant.

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heymagic
A staright bar is stronger but room is at such a premium I would bend it out to the inner door skin.

Don't forget that also by bending Vee or crossed door bars out you get the added advantage that in a big side impact the bars must be compressed(trying to push the main hope backward and the front leg forward) before being stretched inward.

The cage I attached a picture of was possibly build too stiff, but did have about 2 feet of rear crumple zone in the shell behind the towers and 18" in front of the front towers.
It's a compromise - stiffness and lack of deformation on one hand good for multiple rollovers, and suspension effectiveness, etc not so good for sudden stop style accidents.
A bit softer which may yeild better in a big sudden stop absorbing engery but may collapse in multiply roll.

A really good cage may have multiple strenght zones but thats probably beyond the design skills or budget of your average clubbie.

Someone I won't speak for but knows what he speaks *cough* sfi *cough* RA Tech dir *cough*

does NOT like bent out "Nascar" door bars. His theory, my paraphrase: The straight bars are immediately in compression and tension, when taking a hit, they actually pull on the main hoop and A pillar leg. a bent bar, being longer than the distance between the two, has more "play" in it.

To me, it seem It doesn't really matter if the bends are in the horizontal or vertical plane. I don't know what the top, front of a x door bar is protecting? I like the sill bar/ straightish diagonal door bar, that's where the people are... I've seen a car w/o a sill bar take a stump in sill at the B pillar, Got clear to the driver's seat.

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hoche
Good grief, recently I've been finding myself agreeing with Grant in nine out every ten posts he makes. What is the world coming to?

FWIW, I agree with Grant.

Scary prospect!!!

I don't bend them outward for exact reason Tony stated.
I don't think something in the backstays is as important in these days of a main hoop X, though I am now doing the >< in the roof and rear stays as standard cage and have offered to fit the rear V for free in cages that chose to do a minimum spec cage.
I refuse to consider ANY deviations from FIA designs in cages I build as I figure the FIA design gives me a WORLD standard and some level of protection in case of a worst case scenario in a car I built.

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Grant Hughes

Grant, it is hard to tell in pictures but Ryan's door bar doesn't bend out to touch the b pillar at all?

I saw several cages at Ojibwe that bent out a bit in that area. Not into the door like NASCAR bar but just a bit. I think straight is better too and with the notoriously strong Subaru b pillar I feel that the single door bar and sill bar is the way I want to go. In the sedan that pillar is in a very good spot for protection. If I had a coupe I think I would be leaning more towards an x door bar with a sill bar.

This one on a Focus :

By bending them out I mean a small bend about 6 inches from the ends to slightly bump it towards the door skin, not road race or NASCAR style. A lot depends on the individual car, available room, size of the seats and competitors.

Here's an Apillar support that was installed at a cage builder that I cut out and fixed. A little bit of fore thought and a small bend can make a huge difference in inside room .

Space is at a premium in a DSM - even BEFORE the cage...

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heymagic
This one on a Focus :

By bending them out I mean a small bend about 6 inches from the ends to slightly bump it towards the door skin, not road race or NASCAR style.

IMHO, If you bend them at all, the very least you should do is make a gusset between the sill bar and the sill itself so that even if the tubes are not placed in tension right away, they absorb a lot of force in the crush process.

Also, don't forget to put cage padding at the bottom the the a-pillar (half lateral) area for your poor ankles, feet, and shins to strike as they flop about in a crash.

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bknblk2
does NOT like bent out "Nascar" door bars. His theory, my paraphrase: The straight bars are immediately in compression and tension, when taking a hit, they actually pull on the main hoop and A pillar leg. a bent bar, being longer than the distance between the two, has more "play" in it.

This may be appealling to our instinctive thinking, but leads to is mostly incorrect thinking, and I feel that it is about time that some discussion is done to get this out of circulation. A long noodly bent bar is certainly a big issue. And a single bend near the middle will behave to some degree as xyz has said IF and only if the impact is right at the bend.

BUT, if there are 2 shallow bends and they are made near the ends, like a NASCAR bar with shallow end bends, then the central part of the bar goes into tension as the impact starts EXACLTY LIKE A STRAIGHT BAR. And even the end bent sections go into tension, not compression. So, please guys, let us start looking at this as it really is; there has been too much misinformation on this topic for too long.

One of the other misconceptions is that the straight bar under tension somehow keeps the tree or whatever from intruding as far. The actual thing that will happen is that the side bar will bend in very far because all of that tension force will bend the long vertical span of the main hoop to which it is welded, towards the front of the car. This tension force can also bend the A-pillar section to which it is attached towards the rear of the car (but to a lesser degree with the A-pillar suports that we now add).

This bending of the vertical cage sections will happen as these long unsupported vertical tube spans can bend 1/2" to 1" under the amount of tension force developed on the side bar(s) in a heavy crash. And there is a big price to pay for this bending of the vertical cage spans: A 1" movement of the main hoop and A-pillar bars towards each other will result in a 5" or 6" inward bending of the side bar for the typical lengths of side bars employed in rally cars.

Don't believe this? Then look at/think about a few side impacts in standard passenger cars in the door openings. You will see in every case that the A and B pillars are pulled towards each other; this is not just due to the force of impact 'wrapping' the car around the pole or tree due to inertia of the parts not in the immediate impact area; the bending of the car's sill, floor and roof pulls the pillars towards each other. This is exactly the same thing that will happen with any side bar exerting tension on the main hoop and A pillar vertical spans.

This also says why it is so important to have a sill bar. The sill bar attaches low on the vertical spans of the main hoop and A pillar. This makes it a lot harder (maybe by a factor of 3-5; that's just an educated guesstimate) for the sill bar to pull the main hoop and A pillar towards each other. So in a side impact where the angle of the car is such that the side bar and sill bar are hit at approximately the same time, the sill bar really does most of the work to keep the tree or rock from intruding. And the sloped bar helps some in the same way: at least one end is low and well anchored to a spot that won't move much. However, that does not change the forward bending of the main hoop to where the back end of the sloped side bar attaches.

Which brings up another advantage of the NASCAR bent style side bar. In a side impact with the side bar a couple of inches further out than the sill bar, it will allow a progression of cage elements to come into action as the crash progresses. After the door panel and stock sill absorb what energy they can, the outwardly bent side bar will next start to absorb energy, and then the cage sill bar will come into play. This is the type of progression of car and cage element crushing that we want.

One question to ask is how to prevent the main hoop vertical span from being bent towards the front by the impact tension on the side bar. One way is to connect a tube between the section on the main hoop vertical section where the side bar attaches back to where the backstay anchors. Braces to the front strut towers do the same thing for the A-pillar. This will really stiffen the open vertical spans of the main hoop. (Edit to add: And this situation is also helped to a smaller degree by the diagonals that are commonly placed between the main hoop vertical spans and the half-laterals over the door openings by the occupants heads.)

The other way (and better way) is to use multiple side bars in the door opening. One bar's tension cannot now bend the main hoop or A-pillar vertical spans towards each other as much, since the other bars above or below are restraining the veritical spans from bending forward or backward The X-bars can do this to a lesser degree. But this is one principle behind the progression of NASCAR side bars design to 3 parallel horizontal bars with interconnections.

Guys, think this out a bit and I think you will start to see the light on this. As long as the NASCAR style bar is not taken to silly extremes, it offers the advantages that we seek in crush space over a straight side bar with no downside. There is a reason that this has been a successful design for decades, and we in the rally world have been scoffing at this with insufficient understanding, IMO.

If some force/stress diagrams are helpful, then maybe I can eventually find time to sketch and post some.

Regards,
Mark B.



Edited 3 time(s). Last edit at 09/01/2013 10:55AM by starion887.

After racing in Chumpcar, I think one big reason against NASCAR bars is the poor execution potential. Negative bends to go around the door bar, weird ladder bars, etc.
I know I spent many nights planning and thinking through my NASCAR door bars to allow them to work and for the door to still open and close with stock latch system.

By negative bend, I mean like this. Though I think he plated and welded the bar to the b-pillar in this one and he was rushed and was talking of doing them over.


Here's what mine look like before adding in the attachments down to the sill bar.

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Grant Hughes

The red car door bars will fold in at the top and concievably pin or crush the driver.

Grants set look much better.