full frame v. unibody

[zx2guy]

i want to hear some thoughts over this. and if you have any info to add either way ild like to hear. and for extra bonus points: can you turn a unibody into a full frame? and how?

[9ball]

If you've got a unibody car, why would you want a full frame? Unibody cuts weight by quite a bit, and body stiffness isn't compromised at all. If you want to get crazy with a welder, convert to tubular spaceframe.

[zx2guy]

this was brought about by my uncle, i have a 73 cougar convertable. and man you should see the door jams in turns, on lifts, and just accellerating and braking(lots of variations). i know i can add subframe connectors, but i dont really know id that will help with body flex.

[Carbon Fibre]

it'd help immensly

[Carbon Fibre]

The 80's Mopar can kick old cars' asses in all fields cause of unibody construction.

[curtis73]

Uhh... first of all, let's not turn this into a brand war. Its also ironic that you mention the 80s Mopars... Known by fact to be the absolute least rigid of any unibody ever designed for mass production. Need I remind you of the K-car and how its doors sometimes wouldn't close if you weren't parked on level ground? How about the problems they had with the Shelby Daytonas ripping the front stampings apart with a mere 224 hp. At least if you're going to post absolutes, make them right first.

There are no absolutes with this discussion. I've seen cars of both designs twist like wet noodles on the road. All you can do is make generalizations which I can safely do after working in the industry for four major automotive manufacturers. Unibody cars were initially designed for simplicity and cost. They told the consumer that it was for performance and safety, but the truth is when unibody cars became the mainstream for FWD cars in the early 80s, they were not the greatest on either category.

So, let the generalizations commence:

Unibody
-cheaper to build (fewer materials, more automated assembly)
-lighter
-more expensive to repair; more likely to be a "disposable" vehicle in the event of collision damage.

Generally the idea of a unibody car is to take thin, non-structural material and form it in a geometric shape that provides stiffness. Its much like the geodesic dome idea. By using thin light material you can make incredibly strong structures. The bad news is that if you compromise that structure, its nearly impossible to get it back exactly like the billion-dollar computers and robots originally built it. The effect of damage has a lot to do with what was damaged, how it was formed, and the accessibility the repair person has to the damage. Did you ever have a dent in a fender or soda can that would pop back and forth in and out? Once you've removed the static tension from the original, sometimes its never the same. A properly designed unibody structure is intensely rigid, and offers very little compromises. A poorly designed unibody is a very short-lived entity. Its own flexibility begins to fatigue its own structure. Two good examples of this are the 80s K-cars from Mopar and the 3rd-gen F-bodies from GM. Over their lives (even without collision damage) they turn themselves into wet noodles. Cracked "frame rail" stampings and stress fractures are common in well under 100k miles. Contrast that with two of history's best unibodies; the E30 BMW and the Porsche 928. Intensely rigid, well built, and often they maintain their integrity for half a million miles.

Body on Frame

The modular design of the body on frame car makes it easy to repair collision damage. A frame doesn't get its strength from a geometric shape as much as it does from its thickness, weight, and heft. If you bend a frame, you can return it to very close to its original shape and it will be just as strong as it was before. Its not reliant upon a close-tolerance geometry like a unibody. The body itself offers some rigidity, but again, its not based on careful and critical geometries to provide its support. In a severe collision, the worst that will happen is that multiple panels need to be replaced and the frame straightened. Proper placement of the new panels is aesthetic only.

zx2guy... You can put a full-frame under a unibody car and if done well will provide great help. You're basically taking the idea of subframe connectors to the next level. Many times unibody cars (especially FWD) have a front subframe that houses the engine and suspension. This subframe bolts to the unibody, so it can only be as rigid as the six or so bolts that hold it in. Adding a frame (often done with 2x3" thin wall tubing or similar) that follows back under the floor pan helps by spreading the loads out over the entire car. When the car corners, it wants to twist the unibody. Connecting the front subframe to more of the car helps spread out the load resisting flex.

Typically the first step is to add a tube chassis cage to the car. It ties multiple points of the front subframe to the rest of the car and it does the most help. It ties the back of the car to the front using a series of tubes in a geometric "dome" shape. But the base of this dome is only as rigid as the stock floor panels that tie them together. Adding two frame rails (in effect making it a full frame) that tie in the front of that cage with the back is the ultimate.

Of course, it depends on what you're looking to do. I personally usually prefer frame cars. I know that if I buy a classic car to restore, I can replace any component and not have it compromise the strength of the result. A rusted frame is no problem. You can either get another frame or weld the one you have. An excessively rusted floor pan is not cause for concern on a Bel Air, but on a unibody like a 70 Charger it can be a nightmare with unpredictable rigidity. I also tow alot. When I bolt a hitch to a frame, I know that it distributes the weight over a piece of 900-lb solid channel steel all the way to the front bumper. With a unibody car, you're bolting a hitch to one of the hundreds of critically geometric pieces that distribute weight.

So, for cars like my E30, I prefer the unibody since its rigid and light. For cars like my Impala SS I prefer the frame because its rigid and heavy

[Carbon Fibre]

You're right, Curtis. But a frame is heavier, and is less torsionally rigid. I think it's a better idea to spread the load out over a large area, and since the body's already there, wasting weight, well...

[zx2guy]

the main reason i ask is my 73 cougar... well... ild like it to stay stock... as much as possible, i dont want to add a cage... for the obvious reason of stock appearance. but the way it is i want more strength so when i take a corner the door jams wont rub, or cause any issue. a friend thought of the idea of cutting out the subframes and cut into the body> for mounting reasons, and use a modified frame off of a mustang... or torino. since you can but frames for them in aftermarket places.