Fiero space frame vs. Roll cage

bryson · Post by **bryson** » Sat Apr 29, 2006 11:40 pm

I have a two part question:
1) I'm trying to decide how the space frame compares to a roll cage, mainly in terms of chassis stiffness.
2) How would the chassis integrity be compromised if a roll cage was installed and attatched to the bottom of the A and B pillars, then most of the supports for the roof were cut out or swiss cheesed? If I installed a roll cage, I would want to cut out the crash bars in the doors, as well as any weight I can at the top of the car. Keep in mind -- I would run diagonal bars either through the rear window to the tops of the shock towers, or through the firewall to the tops of the frame rails. If I go through the window, I also might have a removable cross brace that goes from the top driver's side of the rear window to the passenger side shock tower, and vice versa. That way I could put it in for track days or autocrosses. How would adding a roll cage and removing roof structure impact chassis stiffness, and how close could I come to removing enough weight off of the chassis to negate the added weight of a cage?

The Dark Side of Will · Sun Apr 30, 2006 9:41 am

A properly designed roll cage will increase chassis stiffness dramatically.

Ask Doug Chase about weight. He's built a few.

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Apr 30, 2006 10:52 am

There really isn't much weight in the top of the car. If I understand it right, you are talking about the roof? Not much to remove unless you remove the whole thing and make it a convertible.
By connecting the strut bars and tieing them into a rollcage the rear window will have to come out or replace with lexan with the proper holes cut. It wouldn't really be much of a street car anymore in my opinion if you did this.

I think Dougs 4 point keeps everything inside the cabin or maybe its his 6 point. Not sure

I think doing a simple rollcage like a 4 point will help with rigidity, and if done with chromemoly tubing will save a good bit of weight when everything else from the doors that isn't needed is removed.

I concur with asking Doug like Will said

bryson · Post by **bryson** » Sun Apr 30, 2006 2:32 pm

I might shoot him an email or PM -- I was hoping he might see this thread. I don't think that making the rear window from lexan will take away from the streetability of the car, though. As long as you do a good job cutting out the holes for the bars to go through, and seal the holes to the bars (grommets, weatherstripping, that would take some planning), the car should behave pretty well as long as you don't pressure wash the window from the rear -- and you can't even pressure wash the stock side windows from the rear without it going into the interior.

I was talking about removing the majority of the roof -- basically making it a convertible but leaving the body panels on (and as little metal as possible to retain the weatherstripping and body mounting points). The roll cage would basically replace the chassis's A and B pillars and roof. The cage would not only be tied into the floor of the car, but also to the chassis underneath the pillars.

Series8217 · Post by **Series8217** » Sun Apr 30, 2006 9:18 pm

I don't think you can put "roll cage" and "streetability" in the same sentence. Roll cages are a no-no for a street car due to the dangers of making contact with them while not wearing a helmet.
I thought you were just building a race car..

bryson · Post by **bryson** » Sun Apr 30, 2006 9:28 pm

Series8217 wrote:I don't think you can put "roll cage" and "streetability" in the same sentence. Roll cages are a no-no for a street car due to the dangers of making contact with them while not wearing a helmet.
I thought you were just building a race car..

Yeah, I've heard that, but I would think that there is enough room in most cars to where a cage can be installed without a whole lot of risk. It doesn't look too dangerous here.

The door bars are the only thing that look like they could be potentially dangerous, but I would try to make them lower than in that cage.

DOHC GT · Post by **DOHC GT** » Mon May 01, 2006 12:39 am

Look at where that center bar is. If you got hit in the drivers side your head would go right into that bar, and without a helmet you would probably die. If you are thinking about a roll cage make the car a true racecar and not a street car.

Series8217 · Post by **Series8217** » Mon May 01, 2006 12:45 am

Yeah, that center bar will get ya good.
Also, the side bar shown there does not provide adequate protection for a side collision if you remove the factory door bars. You can't shave much weight from the doors and still make the car safe.. you'd have to add it all back in tubing anyway.

bryson · Post by **bryson** » Mon May 01, 2006 7:24 am

I wasn't planning on having that diagonal bar. Sorry I forgot to mention, I thought it was a given. I think I'll send Doug a PM to see what may be best for my needs. Maybe doing nothing at all would be the best option. It would surely be the most affordable :salute:

The Dark Side of Will · Mon May 01, 2006 10:23 am

The door bars (definitely) and diagonal bar (maybe) can be made removable. That way they wouldn't be in the car for daily driving, but would be available for track days. Beyond that, the roll bar and fixtures to attach the lower ends of the door bars and diagonal bar should be minimally intrusive/dangerous.

ALWAYS wrap roll bars/cages in dense foam padding.

Mach10 · Post by **Mach10** » Mon May 01, 2006 11:54 am

Would it be a safe assumption to make that a roll-cage will interfere with the engineered crumple zones of a car?

I can't see the safety benefit to a roll-cage on the street. Race-cars have racing buckets, a 5 or 6-point harness, and you're wearing a helmet and (likely) a cervical collar. All the crash energy is taken by the chassis, and you depend on your personal safety gear to disperse that energy.

On the street, the car has to be as "flexible" as possible in terms of safety, and protect you as equally as possible in the event of all kinds of crashes.

Stiffening the chassis in my humble opinion will defeat the purpose; you'll be exerting that much more force on the 3-point belt. And the Fiero bucket won't be helping you much.

I could be off. Anyone with a Ba. Mech Eng. want to chime in? :scratch:

The Dark Side of Will · Mon May 01, 2006 12:05 pm

As long as the cage is comfined to the passenger compartment and possibly the engine compartment, there will be no detrimental loss of crumple zones. The passenger compartment needs to be a rigid shell that prevents intrusion into occupant space. The rest of the car can be a crumple zone.

Mach10 · Post by **Mach10** » Mon May 01, 2006 12:15 pm

So the crumple zones would be situated within the axle lines? Or would they include them?

I'm assuming of course that the rollcages would be tied into those supports, the strut towers, or the front crossmember mounting points.

The point about hitting the bars in a collision is still valid;

3-point harnesses don't keep you square in the seat in a collision. I'd be nervous of any rigid surface inside the car without something that stops me from moving in ANY direction in a collision.

Of course, if your body is planted in the seat, you still have that big bowling ball bobbling around on your shoulders.

It's all about the tradeoffs, isn't it? 3-points are nice, easy, and quick to use. How many people want to dick around with a 6-point to get groceries?

bryson · Post by **bryson** » Mon May 01, 2006 12:36 pm

I was kind of envisioning a 6 point cage -- basically a roll bar behind the seats with 2 bars going back through the window to the rear shock towers, and door bars that meet with the front hoop in the footwells. The front and rear hood would be connected as far up into the corners as I can get -- I don't mind cutting up the headliner to put them more out of the way. I plan on re-doing a lot of the interior anyway, and if I replace the roof structure with the roll cage, I can put it even more out of the way.

The Dark Side of Will · Mon May 01, 2006 2:51 pm

Everything forward of the front firewall and everything rearward of the rear firewall is a crumple zone. What's in between should not crumple.

Since engine's don't squash well, the engine compartment back to the strut towers could be made stiffer with minimal affect on crashworthiness.

Chase Race · Post by **Chase Race** » Mon May 01, 2006 7:23 pm

Figure about 100 - 110 lbs for the addition of a full cage. This is about 60' of 1.5" x 0.120" mild steel tubing plus mounting plates and stuff.

As far as roof integrity, I've seen a lot of cars with cages get upside down and end up with the roof skin form fitted around the cage while the cage remains unbent. I would guess that if you install a cage and remove or gut the roof you could still end up with a net increase in chassis stiffness. I haven't done any math to verify this, though.

Lexan rear window is easy. Do it in three pieces like this:

As long as you don't aim the pressure washer at the crack you'll be fine.

bryson wrote:Yeah, I've heard that, but I would think that there is enough room in most cars to where a cage can be installed without a whole lot of risk.

Seat belts and human bodies stretch under high G-forces. Be careful and pad everything even remotely close.

bryson wrote:The door bars are the only thing that look like they could be potentially dangerous, but I would try to make them lower than in that cage.

You can do that if you have a non-Fiero seat, but even then you might want to remove them for street driving. They're pretty close to you. Again, pad them well.