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Discussion Starter #1
Ive read some conflicting information and am curious to what is actually required from main hoop back. I hear they have to be straight then see some people have a slight bend at top, other have a slight bend at bottom. Is there a direct requirement as to what bends can and can not be, angle, distnaces. Is this a grey area that people use. Do they need to be a certain angle, attach to a certain point behind the driver ( its within 5 inches of the top of the bar i believe) on the frame.

Also bars be added anywhete as long as they dont negatively effect the main parts, or specifically be told against.
Thanks
 

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The way I read the NHRA spec (for 8.50 & slower) the main hoop rear support bars must..........


(1) Have an installed angled back from main hoop of at least a 30* angle from vertical. This one can sometimes be kinda controversial. Lets say a straight rear bar is installed at the minimum 30* degree angle, that means that both the bar's C/L angle and it's force/load application angle are the same at 30*. But if you put an unsupported bend somewhere in that rear bar, then the bar's C/L angle and it's force/load application angle are no longer the same angle. So this means that even if the rear bar initially came off the main hoop at the 30* minimum angle, then an unsupported bend is introduced somewhere along it's length, that means the bar's actual force/load application angle could end up being less than a 30* angle from vertical. But most people that are putting bends in the rear bars usually end up with more than enough initial angle (off the main hoop) that the bar's force/load application angle usually still ends up being greater than 30*. The force/load application angle of any piece of tubing might not be something that the NHRA specs directly address. But IMO it's always something to think about when fabricating a tubular structure. Of course if the bend in a piece of tubing is supported it will make life much easier for that tubing since any unsupported bend is exactly where the tube will want to bend and/or flex when compression/extension forces are applied to the tubing.


Pic example of a rear bar with a force/load application angle of 48*.




(2) If two rear bars made of at least 1 - 5/8" OD x .118" wall (mild steel) or .083" wall ('moly) and can be any length. But can also be made of 1 - 1/2" OD x .118" wall (mild steel) or .058" wall ('moly) as long as they are no longer than 30". The must attach to the main hoop within 5" of the top/horizontal section of the main hoop.

(3) If four rear bars made of at least 1 - 3/8" OD x .118" wall (mild steel) or .049" wall ('moly). At least two of the bars must attach to the horizontal section of the main hoop.

(4) If six rear bars made of at least 1 - 1/4" OD x .118" wall (mild steel) or .049" wall ('moly). At least two of the bars must attach to the horizontal section of the main hoop.


(5 - reading between the lines) Any added bar/brace that is not a "specified/required" bar by the specs can usually be made of whatever OD & wall thickness tube you want to use.....usually.
 

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Discussion Starter #3
The though is rear bars for an el camino, as long as its over 30 degrees, it can have a bend at the rear glass to make it go further back to rear area, or run the bar 4 inches down at 30 degrees and attach frame rail then have a bar running from there below the glass to the rear upper shock mounts

Essentially adding a bar to the back of the cage like the fronts attach to the a piller bars.
 

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If I understand you correctly you want to find a legal way to run the rear bars under the rear glass instead of through the top of the glass so they are less visible/hidden?

If so you can theoretically do it as long as (A) the rear bars are installed at at least a 30* angle from vertical. For inspection reasons it might be a good idea to go with at least 33* (or even 35*) just to be safe. And (B) as long as the rear bars attach to the main hoop withing 5" of the top/horizontal section of the main hoop. If you can achieve both of these requirements without having to move the main hoop forward a bunch then you're good to go. If you have to move the main hoop forward a bunch to do this, and if any part of your helmet lands behind the main hoop, (short of installing a full F/C cage) you will have to at the very least add enough bars around the part of your helmet that would be sitting unprotected behind the main hoop.

IMO it's probably best to install the 2 main rear bars straight at the 30* angle (or again 33*-35* to be safe) and then branch off from there with 2 secondary bars back to the shock mounts (at whatever angle). The 2 secondary bars (running back to the shocks) probably wouldn't need to be made of the bigger 1 - 5/8" roll cage tubing. Something smaller like 1 - 1/4" OD would probably be more than big enough (example "A") for the secondary bars.

I'm not too crazy about the other possible method (2 real long bars with a bend) since it would need to be all bigger 1 - 5/8" OD tube. But it could probably still work OK as long as the bend is supported with additional bracing from the bend down to the frame (example "B").

Both methods would probably benefit from an additional brace each side (say 1" OD tube) from the main hoop to the point where the secondary bars attach to the rear bars (example "A"), or from the main hoop to the bend (example "B").

 

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Discussion Starter #5
That was actually my thought after my last message. Run bars down then a second set from main hoop at seat belt bar. Thanks.

Funny car cage being built behind main hoop probably more than i need but a cool concept. This will be strip and show, probably at more local shows, due to distance to track. Want to keep me safe, without the danger of the a funny car cage and my head
 

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I would also be concerned with rear mounting. For a leaf spring car, attach the bars to the rear spring shackle point. For a coil car, attach the bars to the upper spring seat.
 

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Discussion Starter #7
Coil overs. Figure have the bars follow window shape, or possibly push into the sail panels further out. Can the rear down bars go to outriggers or must be main rail.
 
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