HockeyFamily's interesting thread about his wife's accident here ) brought up some topics which I thought might deserve their own thread.

By the nature of the beast, a truck chassis (ladder frame) is stronger than a monocoque (unibody) chassis. Don't believe it, than look at one that has been taken apart (any body shop will be able to show you).

So, to make up for its inherent weakness, unibody vehicles are designed with a safety cell around the passenger compartment and 'crush zones' in front of and behind the safety cell.

The effect is that in even a minor collision, the unibody vehicle basically gives up it's own life to save the lives of its passengers.

It is not uncommon to see a unibody car that has been hit at low speeds (i.e. 35mph) on a front 1/4 panel, that has big ripples in the opposite rear 1/4 panel, and be a total loss.

It did what it was designed to do. A more expensive repair, in terms of vehicle, a less expensive repair in terms of human life.

When repairing unibody vehicles, body shops tend to slice and dice a lot. They can cut out whole 1/4 sections of the floorpan and weld new sections in.

In minor collisions, they usually just weld the 'horns' on that attach to the bumpers and crush first. It is still a 2 or 2 1/2 foot long part that gets cut out and then a fresh one welded back in.


Check this site where you can see how the guy sliced up and modifies his floorpan.

One other thing to note Troop. Almost every full frame vehicle made today has a unitized bolt-on-body-structure. So they are trying to get the best of both worlds. The days of removing the fender off a truck and being able to touch the side of the engine are gone. Now the fenders are just light sheetmetal, and when you remove them, there is an "upper unirail" and "apron" assembly, same as on a Camry or Cobolt.

Also please note that even when a panel is buckled from the transfer of energy, it can be properly repaired, and doesn't mean it has to be replaced. With the sophisticated measuring systems out there, frame machines can straighten unibody structure back to factory specs without too much difficulty.

When our Sable was hit the repair yard said it would be desirable for a rebuilder as only the rear portion was crushed. The care would be cut in half and the rear half of a car that had the front end totaled would be joined to it.

A neighbor did that to a couple of Tauruses he bought cheap. When it was done it looked sharp. He showed me underneath how to tell if this had been done to a car. He says you can make a good car this way but you will never match the finish of the factory welds.

Actually I-Car has rules and procedures for "clipping" cars. They actually have you weld about 13% more welds than the factory.

The welding I have seen done in the shop I work at looks as good as the factory's, after you put the seam sealer back on it.

It can be done, what you have to watch is the title and the VIN's.

Typically the car you buy that used to be two cars, now welded together, has a title branded as Salvaged.

It is branded for life, and wholesale value will be 50% of a comparable non-salvaged title car.

It also will not normally qualify for a loan.

Many brand now put the VIN on every body panel in a conspicuous spot, just so it is easy to tell if it is original, or factory-replaced as new.

Strong is not always better.

Another problem with the old frame style vehicle is that when you get one with a particularly strong frame - like say a 3/4 ton pickup, they can get too strong for a crash. Which was a bit of the discussion in HF car's accident.

You see a really strong frame doesn't give much - so the deceleration of the body inside the vehicle can be very great. On heavy trucks that hit a strong object at high speed, this can be deadly - the brain can literally crash inside the head and cause death.

It's a bit like holding an empty beer can and pounding on something as opposed to holding a coke bottle and pounding on something. As long as the coke bottle doesnt break, it will create a much larger shock on your hand - same thing as a frame vs a unibody.

OK, but I would rather be in the coke bottle when it pounds on the beer can than vice versa.

'specially an older coke bottle. My friend is having his dad's 53 chevy work truck redone at his body shop and the frame (from New Jersey) had NO rust.