Is a vehicle's payload related to structural construction? (Unibody vs Body-on frame)

[framedetective]

People,

I'd like to know if the construction method implemented (Body-on-frame or Unibody) on a vehicle affects its payload or carrying capacity?

This question is related with my previous post, but like to focus on this particular aspect specifically.

Thanks for any provided information.

Cheers.

[curtis73]

In many ways, yes, but payload and GVW are political/legal distinctions that have very little to do with actual weight capacity.

Pragmatically speaking, either one could be engineered either way. Some frame cars can't carry as much as some unibody cars, but in general unibody cars were designed to absorb energy and sacrifice themselves while geometrically supporting the car's weight. Frames are designed to carry loads with predictable yield qualities.

In general, the unibody cars on the market are less capable of carrying heavy loads safely than their framed counterparts.

[MagicRat]

Originally Posted by curtis73

In many ways, yes, but payload and GVW are political/legal distinctions that have very little to do with actual weight capacity.

Pragmatically speaking, either one could be engineered either way. Some frame cars can't carry as much as some unibody cars, but in general unibody cars were designed to absorb energy and sacrifice themselves while geometrically supporting the car's weight. Frames are designed to carry loads with predictable yield qualities.

In general, the unibody cars on the market are less capable of carrying heavy loads safely than their framed counterparts.

Well put!

To elaborate, it is generally less expensive to design and manufacture a body-on-frame vehicle with an emphasis on load carrying capacity.
For example, almost all mid and full size pick up trucks are body on frame.
It would be possible to make them all unit body, but this is simply a more expensive option.

There are exceptions. The Honda Ridgeline pick up is unit body, but this vehicle was based on the existing Honda Pilot SUV chassis and was not an all-new design. Subsequently, the Ridgeline sacrifices some payload capacity to the frame-type truck competitors in exhange for greater riding comfort and handling.

The only other exception I know of is the full size GM vans, manufactured from 1970 - 1995. All these vans were technically unit body, yet could carry as much weight as any other full size light truck. However, they had substantial 'frame rails' which, unlike other trucks were welded to the rest of the structure. Such large frame rails are never seen on most other unit body designs.
Also, these vans were replaced with a body - on - frame design in part for cost reasons.

[jonathanwang]

In my opinion ,the car with the body on frame is more expensive and difficult to mamufacture,the car with unibody is cheaper .

[MagicRat]

Originally Posted by jonathanwang

In my opinion ,the car with the body on frame is more expensive and difficult to mamufacture,the car with unibody is cheaper .

There is evidence to the contrary for this for larger vehicles.
The complex structure and structural welding, complexity of assembly etc make the manufacture of larger, heavy load carrying unit body vehicles more expensive than body-on-frame.
For example: According to Automobile Magazine's article on the Lincoln Mark III, Ford eliminated the unit body design and went back to body-on-frame for their Thunderbirds and all their Lincolns in 1966- 1969 specifically due to manufacturing cost reasons.

Of course, car design is affected by many many considerations, but over the decades, cost of manufacture is a major consideration. All manufacturers seek to lower costs wherever possible. As we see, the trend is of smaller cars virtually all became unit body, yet most light trucks remain body-on-frame is a good indication of the cheapest manufacturing methods.

[curtis73]

Framedetective; think of it this way. Let's say you have two tables, one foot apart. Take a wad of aluminum foil and fold it into a foot-long bridge and put it between the tables. Now make another bridge with a foot of square aluminum tubing. Let's assume they both weigh the same, meaning they have the same amount of material in them.

The foil bridge gets its strength by using paper-thin folded pieces all carefully aligned in geometric directions that give it rigidity. The tubing gets its strength because it is a solid piece of tubing.

Now let's take an 11" piece of 2x4 to simulate our payload. Lay it on top of the foil bridge. Chances are it will support the load since it is evenly distributed. The 2x4 is contacting the foil at thousands of points and the whole "unibody" is sharing the load. Same will go for the tubing bridge; the 2x4 will be supported. Now take that same payload and stand it on end in the middle of each bridge. The foil bridge will fail but the piece of tubing won't.

Now picture that with a unibody truck and a frame truck; let's use the Honda Ridgeline and the Ford F150 as examples. Imagine putting a 5th wheel hitch in the F150. It will handle the extreme weight because you are putting the weight on a solid, rigid frame. The F150 doesn't care if your payload is a 5th wheel trailer, a load of 5 people, or a bed full of gravel. Now put that same 5th wheel hitch in a Ridgeline. You are attaching the hitch to the surface of a crumpled ball of foil. The chances it will accept single-point loads as well as the F150 are slim.

I'm of course just using examples. The Ridgeline is a fine truck and its not a crumpled ball of foil, but I'm just trying to give you an idea of how its harder to engineer payload capacity into unibody vehicles. If you want to put a hitch in an F150, you bolt it to the frame and you're done. The frame distributes all the stress to the entire vehicle. In a unibody car, the frame gets its rigidity from the fact that the entire structure IS the frame. You would have to make sure that the sheetmetal onto which you bolt a hitch is geometrically linked to every support piece in the vehicle, otherwise you'll just rip out the flimsy sheet metal.