Cage design outside the box

[WHP]

Folks, frequent lurker, seldom poster here. I have spent countless hours researching via the search on this forum and have accumulated a lot of great ideas. However I would like some additional ideas for a roll cage. This car is an "early" 74 with zero rust issues, and will have a SBF engine with about 350 ft lbs torque at the crank. It will be a weekend street driven car with a passenger, once or twice a year track days, and a couple times a year drag strip evenings. Therefore I do not have to conform to any santioning rules. My goals are: chasis stiffening, increased safety without a helmet, minimal weight gain, not too difficult entry/exit. Also, combinations of other stiffening methods are ok, ie cage plus PDK braces or subframe connectors. My engine will be mounted to a square tube frame crossmember connecting across both frame rails behind the factory front crossmember. Let me hear your ideas on design and materials where rules don't limit design.

[Guest Mike]

WHP... my goals exactly!! I'll be frequenting this thread. Just to clarify... you won't be doing any racing other than straight-line, right? No hard cornering on the street either?

[johnc]

Unfortunately you will have to design the cage to fix inside the box (unibody). The most important reason to install a roll cage is for safety. Chassis stiffness is a distant second in importance to safety and, unless you're extending the roll cage forward of the firewall, there's not much you can do about chassis stiffness with a cage. The big issues a roll cage design must take into account are:

 

Roll over

Side impact

 

A roll cage exists to transfer impact loads throughout the chassis. Knowing this, you need to design the cage to minimuize the concentration of loads at an expected level of impact. For a mostly street driven car you probably won't see an impact at speeds greater then 80 mph. Even on the track you're looking at a top speed of 140 and a likely crash speed of under 80 mph. You don't need a BTCC, ALMS, or NASCAR style cage.

 

And don't ignore the sanctioning body rules. They have been develoepd from years and years of real world experience and are a very good primer on cage design.

[74_5.0L_Z]

I had similar design requirements.

 

 

I have more several images in my album.

 

http://album.hybridz.org/showgallery.php?cat=500&ppuser=7833

[JMortensen]

I'd take a good long look at 74_5.0L_Z's cage. He has a roll bar but no bars in front of the seat at the roofline around his head, then door bars which connect to a dash bar and that goes out to his tube frame front end. If you ignore the tube frame front end and consider running some bars from the dash bar out to the strut towers, then you'd have pretty much all of your requirements. No bars to smack your head on, significantly stiffer chassis, fairly easy to get in and out of.

 

http://album.hybridz.org/showgallery.php?cat=500&ppuser=7833

[JustinOlson]

Its really interesting that 74_5.0L_Z's cage doesn't have A pillar bars. I really like that on a street car as visibility would be better.

As Jon said, theres nothing to hit your head on. How much would you be giving up in terms of rigidity and roll over protect?

 

 

Justin

[JMortensen]

A cage done this way would actually be pretty similar to what you would find in a convertible race car that doesn't run a windshield (most of them), so shouldn't be giving up too much in terms of rollover protection. Usually the convertibles have a Petty bar which probably wouldn't be a good idea here if you wanted to carry passengers, but other than that should be pretty similar...

[WHP]

Thanks guys, keep the ideas coming. And yes Mike, I corner hard. I live and drive in the Blue Ridge Mtns. and hope to do track days at VIR someday. Because I drive in the mtns I am looking especially for rollover protection in case I go off road. What if I angle the main hoop just behind the dome light and brace it good?

[Guest Mike]

I'm just watching here:-D Jon and the others are the experts.

[JMortensen]

I'm still in the beginning stages of building my very first roll cage, so I'm no expert. I am trying really hard to think it through though...

 

This might help. This the guy who bent my hoop for my cage. His Miata cage may give you a few ideas: http://www.chaserace.com/cage/cage.html

[johnc]

A cage done this way would actually be pretty similar to what you would find in a convertible race car that doesn't run a windshield (most of them), so shouldn't be giving up too much in terms of rollover protection.

 

Kinda similar, but not really. The low hoop front bar style of cage used in open top, no windshield cars requires:

 

1. The front low hoop be as high as the cowl of the car.

2. A line drawn from the top of the main hoop and the top of the low front hoop should be higher then the driver's helmet when seated and strapped into the car.

3. Two additional braces that start no lower the 6" below the top of the main hoop and run forward connecting to the low front hoop no lower then 6" below the top of the hoop. These braces are in addition to the required double door bars.

4. Two additional braces that run forward starting no lower the 6" from the top of the low front hoop and ending at least at the firewall, preferrably at a frame member.

 

An example is here, although this type is no longer legal. The rear main hoop must run the full width of the car.

 

[JMortensen]

That means the one that I linked to is no longer legal either. Don't know how much that matters seing as how he said legality isn't an issue for him.

 

The similarity I was referring to was in the main hoop and then a smaller cowl height dash "hoop" that gets tied together with door bars and then through to the struts.

[JustinOlson]

How well does high density foam work to protect a person not in a race seat with full harness from the roll cage? Even the above example 74_5.0L_Z's cage could break your ribs if you had a hard enough side impact that threw you into the door bars.

 

 

Justin

[JMortensen]

Don't know, don't want to test it out either! If you're going to have door bars, race seat and belts is in my opinion mandatory.

[WHP]

Well I hit my head on a rollbar in my Jeep four wheeling when I was a teenager in the 70's and it felt like the padding was not there! I really would like to increase rigidity and get some roll over protection. Here is what I was thinking. If you can understand my description let me know how this might work. Run a heavy main hoop from the rocker/pillar junction angled back more than usual going up tied to the wheel well and going behind the dome light where the hatch begins. Brace it rearward with an X. Make a horizontal U from the main hoop for harness attachment and let it tie in to the strut towers too. Then from the main hoop at the wheel well, triangle more sharply down to the rocker beside the middle of the seat bottom. Where that door tube attaches to the rocker use a plate about 14" long to box the rocker (like the bracing on a Z convertable conversion). That would give some roll over protection, keep the hoop back away from my head and the door bars end up not very for forward so as to be minimal impact on the side of the body and allow easy access. Then I was going to reinforce the frame rails and tie them to the rear subframe similar to Bad Dog parts. Use a PDK style front brace to tie in firewall, strut towers, and front swaybar mounting points. My big concern is I have watched crash videos and with 3 pt street belts the head can travel pretty far around the cockpit. If the cage is going to increase danger I would rather just go the subframe buildup route.

[74_5.0L_Z]

I am in the process of replacing the stock seats with aluminum racing seats with the side bolsters. This in combination with padding should go a long way towards protecting my ribs.

 

Oh yeah, I do have 5-point harnesses in the car.

[JustinOlson]

What is the most comfortable non-reclining seat? Aluminum body or composite body? I really enjoy recaro SPG's in my friends car. Would that be the most comfortable daily driver race seat?

 

Regards,

Justin

[JMortensen]

Well I hit my head on a rollbar in my Jeep four wheeling when I was a teenager in the 70's and it felt like the padding was not there! I really would like to increase rigidity and get some roll over protection. Here is what I was thinking. If you can understand my description let me know how this might work. Run a heavy main hoop from the rocker/pillar junction angled back more than usual going up tied to the wheel well and going behind the dome light where the hatch begins.

Don't think this is possible unless you attach the main hoop really far forward, at which point it will probably interfere with the seat.

 

Brace it rearward with an X.

Diagonal in the main hoop is easier, lighter, and probably stronger.

 

Make a horizontal U from the main hoop for harness attachment and let it tie in to the strut towers too.

That's going to be one deep U for the harness bar. I'd go with a straight bar across the strut towers, then just do supports from the hoop to the bar or the tower right next to the bar instead.

 

Then from the main hoop at the wheel well, triangle more sharply down to the rocker beside the middle of the seat bottom. Where that door tube attaches to the rocker use a plate about 14" long to box the rocker (like the bracing on a Z convertable conversion). That would give some roll over protection, keep the hoop back away from my head and the door bars end up not very for forward so as to be minimal impact on the side of the body and allow easy access.

The angle here is going to be pretty weird, not sure if this is a good idea with the main hoop as laid back as you're trying to get it. The ends of the main hoop would be past the back of the seat, so you'd only be stretching this bar out another 18" or so farther than that.

 

Then I was going to reinforce the frame rails and tie them to the rear subframe similar to Bad Dog parts. Use a PDK style front brace to tie in firewall, strut towers, and front swaybar mounting points. My big concern is I have watched crash videos and with 3 pt street belts the head can travel pretty far around the cockpit. If the cage is going to increase danger I would rather just go the subframe buildup route.

This stuff all sounds fine.

 

I think your main problem is that the wheel wells are TOTALLY in the way for what you want to do, and that screws the rest of the whole idea up. It also appears that if you did this the main hoop might not be high enough to protect you from a rollover. As John pointed out earlier, the rule for convertibles is that you ahve to be able to draw a line from the hoop to the dash bar and have that line higher than your head. If you draw that line and your head is in the way, then you're basically relying on the roof of the car for your rollover protection, and the cage won't get involved until it's too late.

 

If you were to lay the hoop back so that it was behind the dome light bar, you'd have to kick the bottom part of the hoop really far forward like this:

 

Here's a quick sketch of what this would look like all together:

 

It just doesn't look right to me, and I think that making the main hoop stand more upright would give you a bunch of better options for the rest of it.

 

Here's a blank version of the chassis drawing if you want to try and sketch things out a bit.

[74_5.0L_Z]

My roll bar is angled back at a 10 degree angle, and is hard against the wheel well. This angle puts the roll bar parallel to the trailing edge of the front part of the quarter window.

 

[JustinOlson]

74_5.0L_Z, do you attach your harnesses to the rear horizontal bar that ties the strut tower together?