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I'm replacing the bolt-in cage that's currently in my 78 SC (PCA E Prepared) with a custom fabricate cage in the next few months ...
I will certainly work very closely with my fabricator on the design, but I've given a lot of thought - and spent a lot of time reading others' thoughts - to what I think are the safest features.
But its officially the off-season for a lot of us, and we do need something to talk about - so I'm anxious to hear what everyone's current thoughts.
It may only be slightly better than a kindergartener's drawing, but here's what I'm thinking:
- Main hoop: X design cross bracing
- Driver's side: pyramidal X brace with gussets, connected to door sill; door sill bar; FIA bar; gusseted to A pillar if possible; anti-intrusion bars in footwell
- Passenger bar: Standard X brace, no foot intrusion or FIA bar
- Rear braces running from rear attachment point to center of main hoop; that same point is the origin point for the roof cross bracing
- No knee bar
- Transmission tunnel idea: I haven't seen this done before, but given that I don't want a knee bar, I wanted to think about how to tie the car together in the event of a side impact. Since the transmission tunnel is boxed, why not triangulate a bar from the main hoop attachment point on the sills, to the transmission tunnel itself. It might even make sense to triangulate a few more bars from the tunnel to the door sill beneath the driver's seat (and use that as a seat mounting point).
I think a knee bar is important. It will help transmit the load from a side impact to the other side of the car.
Also, I think it's a mistake to try and have the roof diagonal and the rear diagonal stays only go to the center of the main hoop. For maximum triangulation, you really want these to terminate at nodes (joints of other pipes).
Same with you "anti-foot intrusion" - that diagonal bar should terminate right where the door diagonal starts.
For the FIA rollover protection, why not include it on the passenger side? You're OK with crushing that side of the roof?
On your main hoop front view, is that horizontal bar supposed to be for the shoulder belts? If you're not putting a passenger seat in (no foot protection on that side) you don't need a bet bar on that side.
I think a knee bar is important. It will help transmit the load from a side impact to the other side of the car.
Two things changed my mind on knee bars: (1) There was an FEA analysis linked to on Pelican that showed the knee bar didn't significantly help in a side impact and (2) Porsche Motorsports does not include a knee bar in any of their cages. Given how cramped an older 911 is - and my own seating position preferences - I've decided the potential damage from the bar outweighs the benefit. I could be persuaded though ...
Originally Posted by Van
Also, I think it's a mistake to try and have the roof diagonal and the rear diagonal stays only go to the center of the main hoop. For maximum triangulation, you really want these to terminate at nodes (joints of other pipes).
I might have some head room issues (slider, sloping roofline) which would preclude two roof diagonals crossing. I spoke with a fabricator who was a big fan of this configuration, especially when paired with the rear hoop layout. Might be smart to add a few extra braces like this Boxster:
Originally Posted by Van
Same with you "anti-foot intrusion" - that diagonal bar should terminate right where the door diagonal starts.
Agreed - poor drawing
Originally Posted by Van
For the FIA rollover protection, why not include it on the passenger side? You're OK with crushing that side of the roof?
I could, but at a certain point weight (and cost) does become a factor. Everything is a series of compromises; it certainly would be safer.
Originally Posted by Van
On your main hoop front view, is that horizontal bar supposed to be for the shoulder belts? If you're not putting a passenger seat in (no foot protection on that side) you don't need a bet bar on that side.
Yes. I will still have a passenger seat (because of the rules). And I might even have an instructor or coach in the car in the future. But this is just another compromise- I will only have a passenger in the car a few times a year when the pace will likely be below 10/10ths and the environment will likely be restricted (passing, etc). I've always wanted to do the Targa Newfoundland though, so maybe I need to rethink the passenger side.
I'm in favor of safety and I have a nice custom cage.
But that being said, I think there is a fine line between "safe" and overkill. Don't forget that being able to see out the back of the car is safe too. It is also safe for your competitors to be able to see through your car from behind as well.
Those "FIA rollover" bars are not going to work in your old 911 car. It is very hard and awkward to exit our cars (with a cage) as it is. That FIA bar will make it close to impossible.
And I completely agree with Van's comments about triangulation and those bars in the roof and rear NOT terminating in the middle. Also about the "knee bar."
Two things changed my mind on knee bars: (1) There was an FEA analysis linked to on Pelican that showed the knee bar didn't significantly help in a side impact and (2) Porsche Motorsports does not include a knee bar in any of their cages. Given how cramped an older 911 is - and my own seating position preferences - I've decided the potential damage from the bar outweighs the benefit. I could be persuaded though ...
A couple comments about this ... FEA analysis can be for stiffness and not necessarily resistance to crushing forces. Stiff does not equal resistance to crushing. All the effort in the doorway intrusion protection is useless if the whole side of the car pushes into you in a side impact ... If you're reading the same Pelican thread I recall (and commented on at the time), it didn't seem so conclusive to me that a knee bar isn't needed.
Also, if you're putting this car into an older Porsche, keep in mind that what Porsche does for newer, totally different platforms may not be comparable. The structural integrity of newer cars is on a different plane compared to older ones.
The new cars have essentially a rollcage built into the unibody with high grade metals going through the sills, doors, dash, and roof. While there isn't a bar in the cage, much of it is built into the chassis, unlike our old cars.
Given how cramped an older 911 is - and my own seating position preferences - I've decided the potential damage from a knee bar outweighs the benefit. I could be persuaded though ...
I think you should take out the dash and put the knee bar above the steering column.
I think you should take out the dash and put the knee bar above the steering column.
The A pillar bars come down well in front of the dash on an old 911. A bar above the steering column would be in no man's land.
Remove the decorative bumper/ash tray on the bottom of the dash and plaster the bar below the rest and immediately below the steering column (see pic).
I am only 5'8" and sit very far forward. And my knees are nowhere near the bar.
Why not swap the pyramid X bars on the driver's side for more traditional NASCAR bars? Then you can either have additional elements supporting the lower NASCAR bar to the sillplate or to a dedicated sill bar. This would also form part of a frame for mounting your seat to the cage instead of to the floor. This way you increase your side impact protection and if the car disintegrates around you then you and the seat are contained inside the cage and move with the cage. Watch your ruleset. Some consider this gusseting of the NASCAR bars to the sill an "attachment point" and some class limit the number of attachment points. If so then you use a sill tube and the only negative is the extra weight of that tube.
Jack, you should post some pics of your car. Having seen it in person, it is very well thought out by whoever built it. Ditto on Doug Crossman's cage in his 964.
If you PM me your email and a reminder on Friday I'll take comprehensive pic's of the cage I just put in my '88 (the car is still apart so should be able to get good coverage).
It is a very robust, just on the border of overbuilt, cromoly cage similar to what you envisioned....though I don't think it is PCA legal because I grabbed the rear suspension pick up points (just a stupid, stupid rule. The rear bulkhead of these cars has absolutely no business supporting the loads of a proper roll cage).
Why not swap the pyramid X bars on the driver's side for more traditional NASCAR bars? Then you can either have additional elements supporting the lower NASCAR bar to the sillplate or to a dedicated sill bar. This would also form part of a frame for mounting your seat to the cage instead of to the floor. This way you increase your side impact protection and if the car disintegrates around you then you and the seat are contained inside the cage and move with the cage. Watch your ruleset. Some consider this gusseting of the NASCAR bars to the sill an "attachment point" and some class limit the number of attachment points. If so then you use a sill tube and the only negative is the extra weight of that tube.
The seat mount is an interesting idea - part of what I was thinking by tying the tranny tunnel to the main hoop. Could you then create a structure to tie the sill to the tunnel along the drivers floor and mount the seat to those points? Doubt I'll do the investigating myself, but worth thinking about.
Personally, I prefer an X brace design to NASCAR bars when there isn't room or ability to the gut the doors and have the bars run most of the height up the door (as true NASCAR bars do).
"Pyramidal" is an overstatement; there isn't much room here, just enough for some elbow space!
The seat mount is an interesting idea - part of what I was thinking by tying the tranny tunnel to the main hoop. Could you then create a structure to tie the sill to the tunnel along the drivers floor and mount the seat to those points? Doubt I'll do the investigating myself, but worth thinking about.
Personally, I prefer an X brace design to NASCAR bars when there isn't room or ability to the gut the doors and have the bars run most of the height up the door (as true NASCAR bars do).
"Pyramidal" is an overstatement; there isn't much room here, just enough for some elbow space!
I understand. It is all a compromise only you can make with an informed mind. The thing I fear most is a driver's side impact. I make no compromises here. I gut the door if that's what it takes. I'm weird. I say you buy a seat first and build a cage to the seat. To hell with the car let it disintegrate around the cage of which my seat is mounted to the cage. I think this is especially true with newer cars that have aluminum chassis like Z06 or 360+ Ferraris. I see very poor designed bolted cages to these cars with seats bolted to the floors. How do you like to be killed by your cage when it rips out of the floor? We weld cages in steel cars for a reason. The safest cars have containment seats attached to the cage.
Is the egress of an older 911 so much worse than a 944? Just thinking in terms of FIA bar which we have. While it makes it a bit trickier to get in and out, it would add quite a bit in terms of structural integrity I would think?
10-20-2014, 01:41 PM
Rollcage discussion
