Why a unibody?

[Nismo280zEd]

Well if you have the money to build a full chrom moly chassis go for it! 8)

But most of us on here can't afford carbon fiber body panels and chrom moly chassis tubing.

[Michael]

An idealization of a unibody car is a box, closed on all sides but hollow inside. Provided that every side of the box is closed, and loads are only applied at edges and corners, the structure is pretty stiff. But the advantage of the tube frame is that (1) now the box can have diagonals inside it, and (2) tubes can be placed strategically in anticipation of where the loads are applied, and what sort of loads they are. With that in mind, you can hack away metal that used to form the sides of the box, and still retain the same or greater strength. If the metal hacked away weighs more than the tubes that you added, the resulting structure of course weighs less – and vice versa.

 

In practice, the tube frame causes additional weight gain when trying to integrate the tubes with the unibody. Things like 6”x6” steel pads welded to the sheet metal, where tubes terminate – they’re pretty heavy. Also, the decision of where to remove sheet metal from the stock unibody is difficult. Cut away the wrong unibody components, and the structure can become severely compromised. And no one wants to cut the sheet metal forming the car’s exterior. Then there’s the issue of how to mount stock components, such as dashboards, for which the unibody has integrated mounts, but which a tube frame won’t. Make additional mounts = pick up more weight.

 

So I agree with Katman in the comparison of unibody vs. tube frame for true “stock” comfort and features. But if we change the perspective and instead compare street/strip cars or casual racers, or cars with severe modifications in other areas (such as engine swaps requiring custom mounts), the tube frame wins. The more radical the car, the bigger the tube frame’s advantage.

 

Regarding the NASCAR cars – over and over again I hear that those chasses aren’t nearly as stiff as they appear to be. Safety is the primary design goal, not stiffness. Nor are those frames light; 3400 lbs for a car with a small block, gutted interior and plastic/composite body panels sounds like an awful lot!

[RB26powered74zcar]

I have been pondering the "frame" thing. I'm new to the z-cars and sold my '70 vette project for some play money for my new z project. The vette was nice in that the body and frame was two separate pcs. Dont remember just exactly where I seen it, but someone was putting a vette suspension in their z-car and had a complete frame made. He cut out the entire firewall and floor of his car and welded the unibody to his new frame, and then planned on welding in a new floor and firewall. I havent looked into it enough but I would think if one is going to go to that extent, I would think you should be able to make some modifications the unibody where you could bolt it to the frame???

 

I think this maybe the pics you saw. I remember seeing this also, and would love to see a current updated pic...

This guy is a member here and has a few pics in his personal gallery...

TAKE A LOOK---http://www.hybridz.org/phpBB2/album_pic.php?pic_id=621

http://www.hybridz.org/phpBB2/album_personal.php?user_id=1438

V8ZRACER260Z

[katman]

An idealization of a unibody car is a box, closed on all sides but hollow inside. Provided that every side of the box is closed, and loads are only applied at edges and corners, the structure is pretty stiff. But the advantage of the tube frame is that (1) now the box can have diagonals inside it, and (2) tubes can be placed strategically in anticipation of where the loads are applied, and what sort of loads they are.

 

You're missing the concept of a "monocoque" structure. The panels carry shear loads which are in effect the diagonals of your tube frame truss. A monocoque, or unibody, will usually be lighter than a tube frame, which is why airplanes and real race cars aren't , like, tubular man. This is especially true when the loads are not simple.

[Michael]

A surface panel is not necessarily acting as a shear web. Consider again the box (cube) example: remove one of the six sides, and the formerly stiff box is now weak in torsion.

 

Katman, I have no experience designing competative race cars, but I've had the opportunity to dabble a little in airplane design here and there ... just a little. Monocoque structures are always reinforced with judicious selection of stringers, ribs and bulkheads. A stress-bearing skin and a tube structure underneath are complementary partners; they do not obviate one another.

 

It is unfortunate that in most amateur automotive racing classes, and many pro classes, the "body" is just a shell, carrying aerodynamic loads but not suspension or drivetrain loads. If, in a street-type car, the "body" is the stock metal unibody, then indeed the resulting "tube frame" chassis will probably weigh more than the original OEM product ... all else being equal etc. etc. But more careful design practice allows one to lighten the unibody while compensating for strength with tubes. Assuming that intrusion of the tubes into the cabin is acceptable.

 

Regardless, let's not argue about what's a monocoque chassis.

[Michael]

I overlooked in the above post.... replace the missing side of the box with a diagonal tube....

[Guest cbrunberg]

It seems that the end determines the means. The stock unibody chasis is well designed for a stock engine and street driving. However even factory built racecars with all their resources still add tubeing both for safety and shear. When you add the shock points that come from hard launches and or cornering and braking your considerations change dramatically. If you add to that a possible collision at high speed or a roll over...beer can. I don't know about you but I'm afraid of high up ill handling poor stopping soccer mom's in SUVs. I would prefer the best of both worlds approach. I think the frame connectors and a light cage are good. Also I've seen several of these cars add tube inside the rocker assy, appearently for both reasons (strength and safety). I'm not a competetive racer, just a joy rider. I like my car to go fast and turn and stop. A few extra pounds doesn't keep that from happening and gives some piece of mind.

 

Think of the engineering in a beer can. I can stand on an empty if I'm balanced but a small shift of my weight and it crushes or I can crush it with 2 fingers from the sides.

 

Carl

 

ps a little rust in the wrong place changes everything.

[katman]

Well now we're starting to deviate from the statement that got me started- that is that a tube frame is lighter than a unibody, to which I disagreed. If you're talking about penetrations due to crashes from racing, or additional suspension and engine loads beyond the original unibody design, well then yeah, you gotta add something. But I stand by my generalization that a shell or unibody will weigh less than a truss or tube frame for most road going apps.

 

Also correct as pointed out that practically all "monocoque's" are really semi monocoque in that they have reinforcing stringers and frames to keep the shear panels from buckling. So do unibody cars with their stamped in beads and welded in stiffeners here and there. My apologies for using the term monocoque when I should have said semi-monocoque, very good point. Replace the shear panels with diagonal tubes however and you'll probably weigh more. Regardless, if a tube frame car for normal road apps was lighter, everybody would be doing it. Some sort of exoskeleton or unibody will always be lighter. Even for road racing, the reason you see cages inside of unibody cars is because the rules mandate the use of the tub to begin with (SCCA production type racing, BTCC, etc.) or tube frame construction (NASCAR). In full on race classes the cage goes away in favor of a tub of some sort, LeMans GTP for example. Tubes are easier to analyze than shells so the cost to design is less.

 

Say, I dabble in airplane design occasionally too, and I can't remember the last time we designed one with a tube frame (World War I?).

 

So to summarize, can we agree that if we don't have a GTP budget we need to add some reinforcing to a stock unibody for some of our hybrid apps, but replacing the entire unibody with a tube frame right off the bat isn't necessarily the lightest way to go?

 

I believe Michael's summary to be correct- in the end a little tube work inside an existing unibody is the way to go.

[johnc]

Say, I dabble in airplane design occasionally too, and I can't remember the last time we designed one with a tube frame (World War I?).

 

You're on a 'dem commercial plane guys, huh? Lots and lots of aircraft are being built with tubular structures - think Ultralights.

[TimZ]

You're on a 'dem commercial plane guys, huh? Lots and lots of aircraft are being built with tubular structures - think Ultralights.

 

Being from Marietta, I would have guessed military.

 

I've noticed a couple of references to the tube frame/cage adding safety, so I thought I'd offer up a somewhat heretical opinion...

 

I'll buy into the idea of a full cage with a properly anchored multi point harness being safer, but only in a car that is a 100% track car.

 

In a car that sees time primarily on the street, I'm not convinced that this is the case. First off, a tube based cage (even if it's padded) makes the assumption that you will be wearing a helmet so that your skull won't get crushed against it. It also makes the assumption that your motion will be well-constrained by a properly anchored multi-point harness, so your neck won't get broken by it if you hit it at a weird angle. Neither of these are true on the street. I don't know anybody that wears a helmet in their car on the street (huge red flag to the cops ), and a racing harness is totally impractical on the street (if you've ever used one, you know what I mean).

 

Second, adding a bunch of stiffness to the chassis with tube reinforcement can be good for handling, if done properly. However, you run a VERY good chance of screwing up the crush zones that are designed into the body (AFAIK, they were in fact doing this even in the 70s). The end result here could very easily be the exact opposite of what you thought you were getting, safety-wise.

 

I've been thinking about this for a while, and I'm not convinced that you are really any better off at all with a full cage on the street.

[johnc]

In many states a full roll cage is illegal on the street. Something about cracking your pumpkin on the roof bars.

[Pop N Wood]

Commercial auto makers do whatever is the easiest to fabricate. It sure is a lot easier to stamp out a bunch of sheet metal parts and spot weld them together then to bend, gusset and weld tubing. Hell, I saw one article where Ferrari used round tubing to fabricate a prototype, then switched to square tubing to ease "production".

 

As for helmets in a cage equipped car, you're kind of going over the edge. And as long as we are throwing out unsupported opinion, if you get in that serious of a wreck then my guess a bone stock, 30 year old Z would have been your coffin anyway. I am not convinced that helmets in a car add any more safety factor then helmets on a motorcycle. Which would you rather have, a crushed skull or broken neck?

 

I wouldn't read too much into laws making cages illegal. Maybe there is a sound reason for them, but chances are somebody just thought the law was good idea, or that cages incite people to drive wrecklessly.

 

But most of all crumple zones don't mean much if you get T boned by an SUV. In this case I would much prefer a couple of nice, stout door bars then the 20 gauge sheet metal on my 240.

[Guest cbrunberg]

My cage isn't accessable to my head. I would have to be dislodged from my seat or the roof would have to come down which if the roll bar is doing it's job shouldn't happen. I'm not an engineer, but I am a structural welder and have built several cages for circle track cars. They've been tested. Front, back, t-bone, upside down, into the wall, oblique, you name it I've seen the impact. I think the tubes helped. As for whether helmets help or not? I think they do. When I raced bikes as a kid I went through a bunch of them. I spained my neck a few times as the result of my head impacting something. I used to look as those old units on the wall and think, thank god I had it on.

 

Carl

[TimZ]

As for helmets in a cage equipped car, you're kind of going over the edge. And as long as we are throwing out unsupported opinion, if you get in that serious of a wreck then my guess a bone stock, 30 year old Z would have been your coffin anyway. I am not convinced that helmets in a car add any more safety factor then helmets on a motorcycle. Which would you rather have, a crushed skull or broken neck?

 

Not sure what you mean by 'unsupported opinion' - I thought I had supported this with pretty logical arguments. In fact, I would contend that while possibly not popular, this particular opinion was as well supported as anything in this thread.

 

Most full cages that I have seen have a forward hoop that follows the windshield and anchors up by the front of the doors. This is usually connected to the rest of the cage by a pair of bars that run along the top of the side window. Guess what - if you aren't properly strapped in (i.e., 5-point harness) and wearing a helmet, it won't take that much of an impact to whip your head into that bar.

 

Also, are you saying that helmets don't add much of a safety factor? Did I read that right? I'm sorry, but that's just silly.

 

Find me just one cage manufacturer that will say in writing that their cage is as safe (doesn't even have to say 'safer' - 'as safe' is just fine) as no cage for unhelmeted passengers wearing 3-point retractable belts, and I will retract my statmentment. Just one.

 

Seriously. Just one.

 

I guess my main point was to get people to think about what they are doing. Just because the racers do it on their track cars does not automatically make it suitable or even desirable for street cars. Safety systems designed for track cars make many assumptions about the environment that the vehicle operates in and the equipment that the driver is wearing/using. These assumptions are not always valid on the street.

 

Also, this is not to say that you couldn't design a cage that improves the stock vehicle's safety for 'normal' passengers - it's just that I have yet to see one for our cars designed that way.

[TimZ]

My cage isn't accessable to my head.

 

Well, I haven't seen your cage, but you might be surprised at what becomes accessible to your head in an accident.

 

Also, I'm not doubting the structural integrity of your cage, but I'll bet those circle track guys wear helmets. And they are probably using proper racing harnesses, which do a very effective job of limiting where your head can and can't go in an accident.

[johnc]

I would have to be dislodged from my seat or the roof would have to come down which if the roll bar is doing it's job shouldn't happen.

 

That's assuming there's no deformation of the floor structure, firewall, bulkhead, etc. A good friend of mine had a 1972 240Z with a full 12 point cage built by Dave Kent. When he put his 240Z end over end at over 130 mph his helmet hit the roof. He only suffered a mild concussion and attributes his survival to the design and fabrication of the cage by Dave, his Schroth 5 point harness, Cobra Imola 2 seat, and his Bell Kevlar full face helmet.

[katman]

You're on a 'dem commercial plane guys' date=' huh? Lots and lots of aircraft are being built with tubular structures - think Ultralights.[/quote']

 

Isn't an ultralight just a kite with a motor? Whoever equated Marietta with miltary was right, although I still get the occasional structures issue on an Electra (remember when airliners had propellers?) or L-1011.

 

Good discussion guys, valid points on both sides of the fence.

 

I'm usually critical of cages in street cars, mostly because of the implementation- not enough benefit in the typical design to overcome the additional weight, cabin intrusion, CG change, etc. IMHO. OTOH I wouldn't let my daughter drive an old 240Z without something else in the door, and all my street cars have had strut bars and/or some other sort of chassis reinforcement. My ITS cages have been successfully? crash tested but I wouldn't want (or feel like I need) one in a street car.

 

No easy answer but obviously enough knowledge on this board to keep decisions from being made in the dark.

[Pop N Wood]

Not sure what you mean by 'unsupported opinion' - I thought I had supported this with pretty logical arguments. In fact, I would contend that while possibly not popular, this particular opinion was as well supported as anything in this thread.

 

Sorry, seem to have upset you. Wasn't my intention. By “unsupported” I meant not supported by citing some type of reference to back up the claim. IMO, any sentence that begins with "I think" and doesn't end with a reference of some type is “unsupported”. Doesn’t mean you aren’t correct, just means your opinions are as much speculation as mine.

 

The point I am try to make is you guys are going to extremes. I knew a guy who wouldn’t wear steel toed shoes because he was afraid that something would pinch the steel down on his toes cutting them off. Yeah, a definite possibility, but since steel toe boots take something like 50 tons to crush you were going to lose your toes either way.

 

What you are doing by adding a cage is moving the weak link a little farther down the safety chain. There is growing concern among auto manufactures and safety experts regarding ankle and foot injuries from brake pedals. Seems like there is a growing rate of severe injuries in this area. After 100 years of automotive history, why are we just now starting to address this issue? Because the combination of mandatory seat belt use, air bags and increased safety standards means more people with these types of injuries are surviving these wrecks in the first place.

 

Now for the issue of helmets.

 

Also, are you saying that helmets don't add much of a safety factor? Did I read that right? I'm sorry, but that's just silly.

 

Oh, thanks for clearing that up for me. Now I see the errors in my logic.

 

I have never seen any statistics regarding helmets in a race car. I do know Dale Earnhardt died of a broken neck and not a cracked skull. Was the helmet a factor? Who knows, probably not. The G forces on his neck were so strong that the added weight of the helmet probably made no difference.

 

I have seen statistics for motorcycle helmets and believe me, the case for mandatory helmet laws is far from clear. Helmets bounce, and the early ones use to come too far down the back of the neck making a nice little wedge to snap the neck. But the biggest worry is that above 35 MPH your other injuries are often serious enough that the only difference a helmet makes is an “open casket vs. closed casket funeral” (to quote a biker friend of mine).

 

Cage manufacturers are just like any other manufacturer, they are afraid to say anything that will get them sued. In fact, the more safety warnings you place on a product, the more chances you have to say “we are not at fault because they weren’t following our directions”. If you believe what some members on this board say, most cages aren’t well engineered to begin with. What cage manufacturers say doesn’t mean much to me either way.

 

To be honest, if I were racing competitively, I would have a car with a full cage, a 5 point harness and I would be wearing a helmet. I would also have a fire suit, fuel cell, extinguisher and full safety crew yada yada yada.

 

But to think that by adding a properly designed cage you are making your car so dangerous that it would be unsafe to drive without a helmet…..