What are the differences between a Unibody and a Full Frame vehicle? What are the characteristics that define each and the pro's and con's of each design?

 

This is just some general comments:

Unibody - also known as Monocoque.
It is where the frame of the vehicle is integrated into the body - doesn't have a seperate chassis.
Generally MUCH stiffer in torsion and bending.

Full Frame - also know as seperate chassis.
Has a seperate frame, which the body is attached to.
Generally, cheaper to manufacture and design, cheaper to repair, allows greater flexibility with body type, usually preferred by those carrying heavy loads, as they usually have a full length frame running the whole length of the body (although a lot of Unibody chassis' now have full length frame rails integrated into the body).

As for which is better, it depends on what you are doing, but I personally prefer Unibody for most applications, the handling, structural and safety improvements are fantastic.

Hope that helps!!!

 

Differences? In a nutshell, a Full Frame vehicle has a separate, detachable frame with all suspension and drivetrain components mounted on it. Remove the body, you've still got a vehicle of sorts. Unibody has no frame, per se. It usually has front and rear subframes, most of the time they are heavy sheetmetal stampings that are spotwelded to the body. You can't remove the body (not easily, anyway). Pros and cons? Depends on what you are looking for. A Full Frame vehicle is usually easier to work on, insofar as bodywork is concerned, but is generally heavier...but also, often safer(?) It usually has frame rails (on a passenger car, anyways) that come right out to the edge of the rocker panels. (this is known as a perimeter frame) and can provide some extra side-impact protection. Most stock car racers who build their cars out of street cars prefer Full Frames. A Unibody car can generally be built lighter than a Full Frame car, but not usually as strong. If it's a class that has a lot of contact between the cars, the Unibody car will usually not hold up as well.
So, there's my input, anyway. I'm sure I missed a few points, but never fear, someone else will be along soon to make additions and/or corrections. -TD

 

I see. So basically the Unibody has everything welded to it, is stiffer (has less flex), but the full frame is heavier, stronger, and better suited for towing capacity? So in theory, wouldn't the full frame vehicle survive better in a crash? I would think that the seperate chassis would act as a shock absorber of sorts in the event of a wreck. Any more light to shed on this?

 

The the unibody frame is integrated into the body, there essentially isn't a frame of sorts. All the supporting members are built into the body.

The reason a Unibody frame is better in a crash is because it can distribute the force of the crash all across the body.
With a full frame it has two large frame members, and cross members to take the force.

Imagine this:
Try hitting the wall with two chopsticks held in one hand. (Full Frame)
Now try hitting it with a stiff bit of cardboard. (Unibody)

Which one will absorb the shock better?

 

OK, somebodies gotta play devils advocate. There's one type of crash that a unibody is typically at a disadvantage- a T-Bone. Unfortunately, this is also the most common type of wreck that involves fatalities.

 

Good point Ernie.

Around here the usual crash that involves fatalities is a single car into (insert large hard object here).
Unibodies are far superior in such an event.

 

What's the usual large hard object? A Kangaroo? I suppose that would often cause a fatality for the Kangaroo, anyway. I don't suppose your Falcon would look too good with a 'roo guard... -TD

 

full frame vehicles are also supposed to be more comfortable, since they block out a lot of road harshness (the suspension is connected to the frame, which is separated from the body by rubber bushings).

 

A full frame vehicle is cheaper to repair up to a point. Since the full frame is stronger,, usually in most accidents all you need to repair is sheet metal. If you bend the frame though, you are in for big problems.

I had, notice I said had, a 96 Impala SS. This car is essentially the police cruiser in drag. It has a full frame and it is about 10% heavier/stronger than the run of the mill Caprice.

I was rear ended in a 4 car pile up in March. From external views, there wasn't that much damage, just grille, bumpers and hood(about $5K-$6K). All easily fixed or replaced. However, the frame was bulged in two spots over the left rear wheel. It was barely detectable. To a layman, you would never see the bulges. This was the kiss of death.

On a unibody, usually you can just put the car on a frame jig and pull the frame straight. With my full frame, you have to remove the frame in order to straighten it. You can't just pound or otherwise straighten the frame. The frame costs only $150 so just cheaper to replace. But, to repair/replace the frame, you have to remove everything attached to it including all the suspension, drivetrain and body. Lots of labor involved which resulted in my car being written off as a total. The car value was worth between $11K and $15K depending on condition etc. The insurance gave me a little over $13K for it so I fell that I was not one of those classic cases where the payoff wasn't the amount the car was worth. So essentially a bent $150 frame cost plus grille, hood bumpers cost more than replacement cost of the car.

The frame shop said that the full frame is one of those problems some people don't realize can work against them in some accidents. It is harder and more expensive to fix than a unibody if the frame gets bent.

So while I liked the ride and strength the full frame gave me, now I understand one of it's strengths is also a weakness.

Jim Henderson

 

Are there any pics around here to illustrate the two by chance?

 

Try this link for a lot of Unibody info. I didn't really read it through, but it looks to have some good information and pics/diagrams:

http://www.engin.brown.edu/courses/EN175/project%2001/peverill/UnibodyProject.htm

And here's a basic bare truck frame:

http://www.presskogyo.co.jp/english/product/automobile/pop_frame_02.html

-TD