Door Opening "X" Bars as Side Protection

[lateapex911]

I looked at it closely. The bar (as Greg described) sheared, as did all the other failure points, near to, but not at the welds. The door pillar/upper rocker area was pushed in nearly a foot. The cage had significant deformation at the cross car horizontal.

The points being made regarding the car deforming so that the energy being transferred to the driver is less are good ones. When we see a cage sheared, we get all amazed. In and of itself, the shearing isn't the end of the world, unless it happened at very low velocities. I'd GUESS this impact had a car that was initially traveling at 80, then locked them up, and hit in the 50MPH range, which is significant as Richie was not moving at all.

On the passenger side, I think "weaker" isn't bad, as it allows more deformation, which takes energy out of the driver impact. On the drivers side, such intrusion would be catastrophic. That said, I think a lower bar wouldn't be a bad idea, with a vertical bar linking it to the center of the X.


EDIT: I see Greg posted as I was writing, making my comments redundant. Oh well!

[tom91ita]

how are the "taco" gussets being formed for around a bar? i see the formed hole as more optional for further weight savings.

i think i will add some sheet metal to my driver door bars as some insurance. not for this type of incident but there are still suspension pieces, etc. that can penetrate the door/cage.

and my car/logbook is old enough that the plate rules do not apply, iirc.

edit: that went away, i guess. haven't looked lately cause i was not changing anything. here is an excerpt from 2008 GCR.

3. Mounting Plates
a. Mounting plates welded to the structure of the car shall not be less than .080 inches thick. The maximum area of each mounting plate in the American Sedan, Improved Touring, Showroom Stock, Spec Miata, and Touring classes shall be 144 square inches. Plates may be on multiple planes but shall not be greater than fifteen inches on any side.

[chuck baader]

I see the addition of door bars on both sides as extra intrusion protection. I designed my own cage and I like to be as far away from harm as possible. Attached are the pictures of both sid[IMG]file:///C:/Documents%20and%20Settings/Charles%20Baader/Desktop/TEMP/DSC_0027.JPG[/IMG]es of my car. Chuck

[Speed Raycer]

IThat said, I think a lower bar wouldn't be a bad idea, with a vertical bar linking it to the center of the X.

Possibility of becoming a spear in the same situation.

I've been toying with going with a 1 bend X lately. Single straight bar ala the traditional X then the second bar bent out around it, then gusseted together.

I have to wonder if the car had had a NASCAR setup on the side that took the hit and had the same results what the discussion would be.

Why isn't anyone bringing up 4130 as an option to the 1020 used? 4130 isn't all that much more expensive anymore. Maybe 1-1.50 pf

Edit... reason I bring up 4130 is because this seems to me to be a case of the materials properities being exceeded during the impact... not necessarily of the "design"

[tnord]

Possibility of becoming a spear in the same situation.

I've been toying with going with a 1 bend X lately. Single straight bar ala the traditional X then the second bar bent out around it, then gusseted together.

I have to wonder if the car had had a NASCAR setup on the side that took the hit and had the same results what the discussion would be.

Why isn't anyone bringing up 4130 as an option to the 1020 used? 4130 isn't all that much more expensive anymore. Maybe 1-1.50 pf

Scott -

is 4130 just a type of steel "alloy?" i'm not terribly knowledgeable in this area, but i am interested in this topic as i'll likely be building a new car this winter that should have plenty of room for extra weight in the cage.

is chromoly any stronger than the "regular" stuff? if you were to strategically place some stronger tubing (being chromoly or 4130 or whatever) that might cost and/or weigh more, where would you suggest that be used? the main hoop and the driver door bars perhaps?

[ddewhurst]

***The bar (as Greg described) sheared, as did all the other failure points, near to, but not at the welds.***

Very ^ interesting. Jake, how far would you remember the failures are with respect to the edge of the welds. Do we know what tube material was used & what weld process was used?

[chuck baader]

4130 has greater ultimate strength than DOM, but it brakes whereas DOM bends more before it brakes. Chuck

[RexRacer19]

4130 has greater ultimate strength than DOM, but it brakes whereas DOM bends more before it brakes. Chuck

I would add that the welding process would be more difficult as well. For this application, I think that TIG welding would be the only good way to do it to keep it from becoming too brittle at the joints.

[Speed Raycer]

(Random ramblings while I'm at work....)

First, I'm not a metalurgist but I did get some sleep last night
2nd, I'm a much bigger fan of 102x in wheel to wheel cages than I am of 4130 due to the nature of the two materials.

It doesn't surprise me at all that the Mild Steel material failed *away* from the welds. The impact exceeded the materials properties. The welds *when done properly (and it sounds like they were well done)* end up being stronger than the base material. Its the nature of the material.

4130's (chromoly) yield (bending) point is greater than 1018's (mild steel) yield point (75kpsi vs 70k). Ultimate strength is 95kpsi vs 82kpsi. It's tensile strength is higher which leads to the "break before bend" reputation. For the same tubing size and thickness, the two materials weigh the same.

There's a ton more work/time involved with using 4130. It likes to be welded slowly. Same joint, a MIG weld might take 2-3 minutes, a TIG joint can take 3x as long. The pre/post weld heat treatment process adds time. I won't get into the debate on whether or not the heat treatment is necessary at our required thickness' nor will I get into the TIG/MIG/OXY debate either. 4130 & 1020 shouldn't really be mixed as the materials "wash" together during the weld making wierd things happen to the material properties of the 1020 tube.

A good free read from our buddy Mr. Smith:
http://books.google.com/books?id=5a8...esult#PPA66,M1

I liken 4130/10XX to a Hershey bar. 4130 is a bar kept in the freezer. 1020 is one kept in a cabinet. Feel free to do the experiment on your own