I know it sounds like a dumb question, but I only have a very vague understanding of what the difference is. My question is, how come body on frame is better for offloading vs. Unitized body?

I guess it would be a light comparo of the highlander/4runner.

In a word, strength. A truck built on frame rails is stronger than a body built out of a pan bottom. Think of your suspension attached to railroad rails versus cookie sheets and you will get an idea of the difference between a 4Runner and a Highlander. The cookie sheet will bend and flex, giving a softer ride and making a car lighter in weight, giving better fuel economy among other things. A truck built on rails does not flex much at all, weighs more, but is stronger and more able to pull heavy trailers and handle heavy loads, typically at the expense of fuel savings and ride comfort.

Neither one is inherently good or bad, just made for different uses and needs.

Unitized body, or uni-body the frame is an integral part of the body structure. There is no seperate frame from the body. The body is essentially where everything bolts up to, the engine and suspension. It does have reinforced parts, like the pillars, front and rear sections.

Body on frame, like it's name implies the body is mounted on top of a ladder frame. Frame construction is not all created equal though, some are fully boxed (like the 4Runner's), others are open C-channel and some the frames are hydroformed. Everything, the body, suspension components and engine are bolted up to the frame or the back bone of the vehicle.

The reason a body on frame is stronger than a uni-body is the extra backbone a body on frame vehicle has. The benefits of are a stronger frame for towing and off-roading. The downsides are it's heavier, thus it doesn't get as good gas mileage; and because everything is mounted on top of the frame, there are less interior room than a uni-body.

Another general disadvantage of body-on-frame design is crash safety. Although there are exceptions (4runner for instance), a body-on-frame SUV will have worst crash test scores than a unibody, if everything else being equal. The reason is that a unibody can manage crash forces better by distributing it as a whole unit.

If you want to see an "easy" comparison, go and look under a commercial truck (semi tractor or large straight truck) such as a Navistar, Freightliner, Kenworth, etc. Check out a trailer or even a school bus. I think you will see the ladder frame with both latitudinal and logitudinal support beams for handling a lot of weight.

After that, examine the undercarriage of most any minivan or smaller SUV. Extreme examples - but you'll get the point.

The 4Runner is indeed a "body on frame" design. The only passenger cars I know of that are "body on frame" are made by Ford: Crown Vic, Mercury Grand Marquis and Lincoln Town Car. There could be more, but I can't think of any off hand.

I may be a geezer, but I remember when most cars were indeed made that way. Unibody cars were in the minority, and consisted of the Chevy Nova, Ford Falcon, Chevy Vega, Chevy Citation, Dodge Aeries, etc., etc. during that pitiful automotive era we try to forget about.

The Ford cars are done that way for use as limo's etc. Makes the frame easier to stretch and extending the wheelbase.

Unibody was the minority then, because it's easier and cheaper to change the body and leave the frame alone. Now that computers are so much more powerful and software, it's easier to design a unibody in CAD and it's not as cost prohibitive as it once was.