Door Opening "X" Bars as Side Protection

[Mike Guenther]

By your description of the force of impact, it sounds like the cage did a good job of absorbing the energy. Is there really a design that's practical in weight and material cost that can withstand any possible impact ? And how should it look after an incredible impact ? I wouldn't expect it to look like it did before the accident.

The bending and breaking is absorbing energy that would otherwise transfer to the driver. Had the bar broke upon a 20 mph impact, I would look to see if inferior materials were used. But at 80 mph, what would you expect ?

The design you show does appear to be a stronger way to make an X. I'm sure that at an 80 mph direct impact there would be some failure somewhere in that construction too.

[tnord]

my car had seen it's share of impacts, a couple pretty solid side ones as well.

it's not just miatas that impact much higher than the rocker panel, a VW got into my drivers door once, completely destroyed the door, got into the door bar, but left the rocker panel untouched.

as i get ready to possibly build another car, i'll be real interested in where this discussion goes. i'm no structural expert by any means, but isn't it a "rule" that the bigger the bend you put in a bar, the weaker it becomes at the point of the bend since you have to stretch/thin the metal to make the larger outside radius? with that in mind, the x-bar pictured above looks like a pretty big bend relative to the traditional nascar bars. the plates welded at the intersection counter this, as the bars are pushed inward the stress is placed upon 4 whole rows of welds, but is it enough? either way, i see the quality of the welds and material used in those plates to be the key to safety in that design.

travis
-who had a "traditional" x-bar on the passenger side, but nascar bar on the drivers side.

[raffaelli]

Greg, I was in T&S when Hunter’s car drove down pit lane. We were all laughing at the hay bail stuck to the front of the car, and then the air left the room as we realized the car was caved in and the door was against the driver. In a room full of jolly ladies, there were certainly a lot of holy shits going around.

I am glad to hear he is OK. I have always thought the glass in the door was a dumb idea. I have the X bar you describe with the three pieces. I had planned on added a diagonal across the roof and gussets elsewhere this winter. I will add an upgrade to the x bar also.

Good topic.

[RSTPerformance]

The thought of the passenger side collapsing is interesting, but we would never want this to happen on the drivers side... I am now thinking about what I should do to upgrade my drivers side bars...

Raymond

[Sandro]

My biggest fear has always been getting T-boned, mainly only the driver side, since I always thought I had crush space on the passenger side but from this example its pretty obvious thats not enough space.



My cage has the standard X plus a horizontal bar at the bottom, but as you mentioned due to the height of the miata the horizontal bar wouldn't have done anything. If I had a vertical bar on each side of the X, say about a foot from the center on either side, with taco gussets, how effective do you think that would be in a situation such as this? I think it would be fairly efficient because you would have two continuous bars(horizontal and one of the x bars) plus the two half bars of the x all having to bend in unison.

red = bars, blue = gussets

[DoubleXL240Z]

This is one of the three dimensional x braces that I did. It was then gusseted. It seems like what Greg was talking about.

[Knestis]

Thanks for bringing this up again, Greg. It's a big issue.

Submitted for consideration...





K

EDIT - Those bars project outward a couple of inches...

[tom91ita]

Sandro has excellent photos that basically shows typical configurations of both the nascar and the x style.

i have the nascar type on both sides and i think they go deeper into the door than Sandro is showing but that may just be perspective. mine go right up to the reinforcing bar in the door and you have to give a good slam to latch the door. i do not have as many cross braces as Sandro is showing. that looks quite well done.

with regards to the more bends, i think you need to look at how the hit takes place and what happens. i am not a structural engineer either but i know if if i put a 2x10 on its side, and i walk over it, it will bend a lot. if i put it on edge and walk on it, there is no deflection noticed.

when we bend a bar into the door, there is a lot of compressive forces that come into play when it is hit, for the x-bar, there is lot more tension forces taking place. i think the bent bars spread the force into the rest of the cage/hoop as well.

and for the extreme example, how much force would it take to "ben" a 1.5" bar that is 36" long when laying on its side vs. to compress it from its end?

now i will freely admit that i have no idea which is better at dissipating the force of impact to lessen the g's felt by the driver. Sandro's cage may be the best of both worlds. give up energy in all the space you can on the passenger side cause you can give up the space and lessen the g's felt by the driver. and where space is a major premium, you need the prevent intrusion on the driver side and know that you will feel more g's in the impact.

greg, thanks for a detailed description of what happened and let us know he was alright.

[dickita15]

We did a nascar style on both side though much less than Sando. The upper bar lines up perfectly with the stock anti intrusion bar.

[Sandro]

my bar goes all the way into the door and touches the door beam in a spot or two, have to slam the door to get it closed. Whole goal was to get the bars as far away from me as possible, allowing for the most crumple room.



I had considered doing the same on the passenger side, but thought I would have enough crumple room. Now I wish I at least had a little protrusion into the door, only for the sake of being able to remove the glass, for the reason Greg mentioned above.

[splats]

As an EX-circle tracker, I've had my share of T-bones & got hurt several times. And those were less than 60mph. Turn 1 at Sebring is a blind, 100mph turn. We built as strong door bars as we could. You can't see from the pics, but there are as many triangles in the bars as could get. All three horizonals are bent different. And the center bar will HOPEFULLY deflect front bumpers down. As with any cage, you can find a weak point with mine. If you see something that we overlooked, please let me know. When people talk about the cage adding weight, consider this..... my car is just 50lbs over the minimuim WITH 3/4 tank of gas (stock) & the spare tire still in the trunk. I'm glad that Richie is OK. But I'm supprised that no-one has gotten hurt because of building to the rules. If you can just open your door & STEP into your seat, IMO you don't have enough protection. I'm more worried about WALKING away from a crash than walking away with a placque. Sorry about the soapbox, but we just had this talk at our district meeting recently.