Door bar construction

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heymagic
I'd still tun a sill bar btw, probably a 1.75 X2" or 3" box in 3/16".....did that on one of my cars and it later (different owner) soundly smacked a stump. Roll panel and door went pretty flat along the box tubing and that was that. Door still opened and closed...leaked a little wind but oh well.

In my thinking these days, not running a sill bar is about the worst omission in current cages that you can do.....well, unless you leave out the main hoop! The idea that the side bars can somehow be rigidly held 'taut' between the main hoop vetical tube in the back, and the lower part of the A-pillar tube in the front, goes out of the window (no pun intended) without a sill bar. Without a sill bar, the side structure formed by the main hoop, bar over the door and the A-pillar is an upside down U. If there is a side bar between that is hit, then there is NOTHING to keep the lower ends of the U from just pulling towards each other as the side bar bends inward; the stock sill has been totally crushed by the time the side bar starts deforming inward, and has become totally useless as a 4th side of the U to prevent that from happening. So the theory of the side bar(s) being held taut is just not real without a sill bar. We all know this happens from regular car experience; any side impact of a regular car in the door ends up with the A and B pillar sucked towards each other as the side parts are crushed in.

Sill bars are really essential, and they don't have to be 1.75" super heavy to perform the function of providing a stable framework for the side bars to work against. Heavy sill tubes DO work for stump hits; those occur more than you migth think.

If you really want the ultimate side protection, forget about x this or bent that, just run a sill bar, a diagonal ...and then another parallel diagonal... gasp! Not necessarily parallel, maybe three bars at a node low on the a pillar, then fanning out on the b pillar, low-middle-high. Stay away from x-games jumps if you're worried about not tying into the strut tower through the door "x"

On mine I could easily add a bar in between the sill and upper door bar if I wanted to. This pic is way old, circa 2001 when stock seats were still legal! Doesn't have the front lateral support bar either, and probably won't get one since I don't think this car will be rallied again. Not really serious about this, too much metal, overkill I think, but I'd rather run that than an "x" in the door....

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heymagic
Billy,
No constraints on bending the bars in this case. The Mini bars don't touch so the gusset will need to do a lot of work. I would have siamesed the bars and then a gusset over them.

I'd still tun a sill bar btw, probably a 1.75 X2" or 3" box in 3/16".....did that on one of my cars and it later (different owner) soundly smacked a stump. Roll panel and door went pretty flat along the box tubing and that was that. Door still opened and closed...leaked a little wind but oh well.

That's why I'm running a sill bar. I don't get how the WRC guys don't run one. Seems you hit a tree stump with the side (or a rock) and it can go under the bar and crush the seat. To me, the sill bar gives extra side intrusion and like starion887 said, keep the door from being a U and close it off to an O.

The sill bar subjuct, I had a conversation exactly about this a few months with Luigi Basile which is one of the senior engineers across the street at Prodrive and he said and I quote "Sill bar, why? The sills are the stiffest part of the car's mid section"

From what he said, if you want more side protection against trees the sill bars do very little, and "bring down the upper part of the cage." I'd say they ran similations on these things.

Ever wounder why a lot of the FIA cages dont have sill bars? Do "we" know more than "them"?

That being said, I have very little opinion on the subject aside from that NASTYCAR style bars are the safest because of the load distribution.

Hey Paul, Hope all is well there. Hoenstly, no, I don't think FIA rules reflect more kinowledge. I've been looking at their rules for almost 15 years on an off and some of the designs are OK, and others non-sensical. Examples: The roof brace design with a single bar in the center of the roof 'rectangle' running fore and aft. This does absolutely ZERO for what roof bracing needs to do: try to prevent 'squashing' in of the upper A pillar area in case of hitting the upper A pillar from the side. The only reason I have been able to imagine for adding that desing into the FIA rules was for cars like a Lancia Stratos or Renalut Alpine where that's the ONLY pace a bar can be put in the roof rectangle, and they had to conform to a rule.
Similarly, folks are fooling themsleves if they think that the .040" inch thick metal in clamshell gussets will do much; the thickness/area ratio makes it behave like sheet metal, not plating. Some tests run recently and on which there was some discussion on SS showed that.
I've spotted some other items where it seems like the FIA designs are the result of a factory team wanting to have things done a certain way, and they get it put in the rules so that they are covered. It is form before function in those cases; does not surprise me due to the heavy politicis in FIA. Similarly, things like bars added for bracing the suspension rather than for real safety; it gets it legalized but adds little to safety.
As for the sill being the strongest part, of course, any car guy/gal knows that. Strong enough for stump hits? No way; we've seen that over an over. I am not getting the 'pulling down' thinking; but I am open to explanations. It honestly sounds to me that the ProDrive guys is thinking in terms of optimizing wieght vs. critical areas of chassis stiffness for the suspension to work its best; a sill bar adds little to the overall chassis stiffness for competition purposes; so I get that, and guy in his position is paid to do THAT. But we are not talking WRC cars and maximized performance here; we are talking improved safety for what is pretty much hobby level racing.
The final thing that gets me is in side protection; the death of the co-driver a couple of years ago in the side hit showed that FIA is not very advanced at all. They had a similar scary case in Rally Germany 1-2 years before, that fortunately did not have any dire results, but they did nothing. NASCAR has done a better job over all, IMO. I'll agree with you pretty much that their side protection is better in all regards except for crush space and deceleration; the bowed tubes and the interconnecting tubes make the side bars act like one fairly rigid plane, and the bowed design places the end welds in compression, not tension, in the initial part of the impact.
Sorry for expresing opinions so strongly.
Regards, Mark B.

I'd guess anyone in the auto repair business has seen a bunch of bent, crushed door sills. Anything from turning too sharply in a parking garage to the butt head running a stop sign. They crush easily. How many old rally cars have crushed sills from big rock dings?

I suspect the boxed sill with the multiple bends has a lot of torsional strength, needed in a uni-body chassis. A side hit at 30-40mph from a tree, boulder or stump and they just crush away.

Sill bars add very little weight, effort or expense to a cage. It is a good place to anchor seats, belts or nets. If an impact was hard enough to blow thru the box sill, bend a sill bar and pull the cage down I wouldn't want to see the results without that sill bar...the seat would likely be folded in two. Either way the car is effectively ruined so I'd opt for a bit more protection.

I agree with Mark that the FIA isn't always perfect. Sometimes new ideas and trends enter the picture and things change. Not all designs that look good on paper work well when something stupid and unexpected happened. FIA does show the sill bar design as one of their door bar options. Most WRC cages are not FIA designs but some engineers wet dream of creativity..homolgation. They are accepted by the FIA with a stack of engineering papers saying it will survive a certain load in a certain area. This is why we don't accept 'styled' cages...it may work ok for ProDrive but Joe Blow put the bar at a 10* shallower angle and it doesn't work there.

I'm silly enough to like to put a transverse bar at the bottom of the main hoop also...

I heard one of the reasons why WRC cars had no sill bars, was this:

In the case of an off, if the sill bar was bent/damaged, the FIA would deem it as roll cage damage and would not allow the car to rejoin in SuperRally etc. However if the sill itself was punched up, and no cage damage the FIA would allow them to continue. Hence no Sill Bars.

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Chris

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heymagic

I'm silly enough to like to put a transverse bar at the bottom of the main hoop also...

I thought about it, but with an x, and the seat belt bars... Seemed over the top, plus there was no good way to tie a bar between the two hoop feet without fancy bending over the tunnel.

I think the bottom of my main hoop may be the strongest point in the galaxy.
Main hoop X, rear X tying it to strut towers, transverse bar tying sides together. Maybe overkill...

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Grant Hughes

Quote
NoCoast
I think the bottom of my main hoop may be the strongest point in the galaxy.
Main hoop X, rear X tying it to strut towers, transverse bar tying sides together. Maybe overkill...

Yeah the near flat X on the floor really isn't doing anything worth a damn. cut that shit out.

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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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I've thought about cutting it out but no real compelling reason to do so. Save a few pounds perhaps but whatever. I guess I could cut it out and replace it with tie ins to the rear beam and diff mounts instead.

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Grant Hughes

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NoCoast
I've thought about cutting it out but no real compelling reason to do so. Save a few pounds perhaps but whatever. I guess I could cut it out and replace it with tie ins to the rear beam and diff mounts instead.

Yeah, then those would be accomplishing something.

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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.

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BillyElliot

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heymagic
I agree with Mark. I'd guess the better plan would be to have the two bars be bent, run together for a foot or two then bend away to the terminating point. The parallell area gives more welding and plating, less twisting motion when hit and easier ingress/egress.

I think on the conventional race inspired X that if the x center was offset a few inches on each end instead of symetrical it might be a bit stronger.

I know Sean Murphy did this on my old VW when he re-did the door bars. How much can we legally have the bars bent to make contact? Because doing something like the Mini Prodive has would be an ideal situation to get the best of both worlds? Long contact between the two bent bars, low center for easy in/out of the car. But I agree, that an X is only good if you have the front shock towers tied into the cage.



Interesting thing I saw in the article I pulled the picture from... they don't seem to run a dash bar at all on this car from the 3D model.

http://www.eurocarnews.com/238/0/0/6895/3d-modelling-helped-prodrive-optimize-design/gallery-detail.html

Hey Guys, I'm new to this forum but I've been posting on Special Stage. I'm actually just about to do the door bars on my car so this thread is very interesting to me! I've been a long time lurker on this forum, and I like that there is a lot of discussion here...

I've got a question, in the FIA rules, they mention that the door bars should be as high as possible while still being less than half the door aperture height at the connections.... Isn't this subjective to a persons definition to "as high as possible"? looking at the Mini door bars, they seem rather low.


Also, for the comment on thin gussets, doesn't the strength come from the bend mostly? its the outer extremities of a structure that effect its resistance to bending more than thickness, this can be seen in I beams used for structural, the real strength (for what its designed to do) comes from the width of the flanges. This is what gives it strength. What thickness of gussets are people on here using? I'd rather go with recommendation on experience regardless, but I still can see that the minimum cant be that weak.

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Used to be verbage in the FIA regarding door bars not hindering ingress or egress needlessly, buit that is gone. Yes the heigth is a bit open in judgement. I think the Mini door bars are just fine where they are.

My personal opinion is the door bars, if crushed inward, should hit the seat and not over the seat. If they hit the seat bolster then the thigh is kinda sorta protected and you get shoved over. If the bar is higher than the bolster you can easily be pinned in the car. This has happened.

Gussets, I think most are 3mm thinck. They are fine and add a lot to the joints. I believe Mark, myself and others like to see a thicker gusset at the door X to prevent tearing . The door bar X gussets aren't really loaded in the right plane for a thin gusset to work the best. A thicker gusset on the inside has the tubing compressed into it and the the tearing/stretching starts. A gusset on the outside bends inward without adding a lot of strength....I think

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PeteNaz
Also, for the comment on thin gussets, doesn't the strength come from the bend mostly? its the outer extremities of a structure that effect its resistance to bending more than thickness, this can be seen in I beams used for structural, the real strength (for what its designed to do) comes from the width of the flanges. This is what gives it strength. What thickness of gussets are people on here using? I'd rather go with recommendation on experience regardless, but I still can see that the minimum cant be that weak.

The strength of an I beam is shared to a hugely varying degree between the vertical section and the flanges depending on profile, with narrow I beams having almost all the strength in the vertical section and very, very little in the flanges. In any case, the flanges do 2 other importnat things: rigidly hold the vertical section perfectly straight so that is can take the load and not twist or warp which would lead to collapse; and help spread uneven loads along the vertical section to that irregularly loaded beams still perform well.
You are right that most of the strength in a clamshell gusset is in the outer section, but you need to understand the loading and failure of that vs a plate gusset.
Imagine a horizontal tube running left and right and a tube welded vertically to it, with a gusset in the rigth hand corner junction, and a force acting on the veritcal tube pushing to the left while the horizontal tube is rigidly held; this force is acting to put the gusset in tension. The gusset is NOT uniformly loaded because the first thing serious that happens as the forces increases is the joint distorts in a way that there is essentially no force in the corner of the gusset at the intersection, and most of the force concentrating on the 'free edge' of the gusset. For the clamshell, you essentially end up with an almost flat strip of thin sheel metal (the curved part of the clamshell) in tension at a 45 degree angle to the tubes trying to keep the vertical tube from pushing over; the triangular sides of the clamshell almost dissappear. A strip of sheet metal is going to tear apart pretty easily as a classic tension failure of a strip of material, and continue into the sides.
Now look at the triangular gusset: the force concentrates along the free edge and tries to initiate a tear down through the gusset. If the edge is resonably free from notches and irrgularities, it turns out (due to laws of physics that I can't explain) that starting such a tear into a free edge with tension force along that edge takes a whole, whole lot more force than tearing a narrow flat strip with classic tension loading. So the inception of the tear in the plate gusset takes a lot more force, and once torn, it provides more resistance to continued tearing.
A force pushing the tube in the other way (gusset in compression) has different things going on but the results are similar in favor of the plate.
As for side bars, the gussets don't get loaded like this to the degree that we worry about a lot; they act to the keep the tubes from being separated with possibly (probably) wildly different side-impact loads on the tubes being joined. In that case, it is really important for the welds to be good. The issues I see with .040 thin clamshells is the thickness of the material being able resist tearing, and uniformity of the welds. Any welder knows that welding a thin edge to a continuous thicker plate is difficult; for good penetratoin of both with little undercutting, 80-90% of the heat has to go to the plate with very little going to the free, thin edge. This is hard for most of us who do not weld professionally. You can add a lot of rod into the weld but you will still end up with irrgular welds unless you are really good; the worst is a lot of thinning of the thin edge along its length, which means the thin plate will just tear away under any force. Also, I think that the weld penetration into the tubing will be much better for a thicker single flat plate.
Using a single plate is quite superior IMO for the simple fact that welding a .095 or .120 plate's free edge to the same thickness of tube will weld far better with better tube penetratoin of the weld, and you simply have more meat in the plate to resist any tearing of material or welds in this scenario.