Disclosure: I work for General Motors (on software, not as a design engineer), this is solely my own opinion and experience.
An automotive assembly line consists of many "footprints" chained together. Each footprint is scheduled to have the same amount of work (in seconds). That work rate is also called the line rate, often expressed in jobs per hour. So 70 jph could also be expressed as 51.4 seconds per footprint.
Working backwards from the time you drive the finished vehicle off the end of the line, a truck assembly plant works like this:
v Engine line -------------
Final Line <-- Chassis (frame) -------------
^ Trim Line (body)--------Paint Shop <------ Body Shop <--- Stamped Metal
graph TD
subgraph Chassis Shop
Fr[Frame Line] -->EM
EM[Engine Marriage] -->C
C[Chassis Line] --> BM
BM[Body Marriage] -->F
F[Final Line]
end
subgraph Engine
E[Engine Line] --> EM
end
subgraph Body Assembly
B[Body Shop] --> P[Paint Shop]
P --> T[Trim Shop]
T --> BM
end
So setting the engine on the frame must not take longer than 51.4 seconds, and setting the body on the frame also must not take longer than 51.4 seconds. (Along with all of the other ~~1,000 assembly footprints)
As a matter of simple evolution, no design that takes longer will be produced, because it does not survive the first round of selection: Assembly.
Designs that survive Assembly may eventually need Maintenance, but by then the genes have already been passed on.
Now you may say that Engineers are smart enough to fix both problems, but I don't believe in Intelligent Design.
Snark aside - maintenance falls under "residual value", and "residual value" may be a luxury that the automaker cannot afford.
*** To be clear, I am using 51.4 seconds as a stand-in exemplar for many other requirements in the assembly process.
Show me the outcome, and I will describe the incentives ~ Inverse Munger
Automotive design engineers have to satisfy MANY criteria, the following items entail LISTs of requirements, not a single req.
- the part must function (includes failure modes)
- the part must be manufacturable (make the part itself)
- the part must be assembl-able (it has to fit with the parts around it)
- ... be lightweight, environmentally friendly, comply with government requirements
-
- then, to actually exist:
- the part will be prototyped in a pre-production environment
- a component plant will be set up to manufacture that part
- the manufactured part will be tested to ensure that it still meets the design criteria
Now, once you have all the parts you need to make a vehicle, you can design the vehicle so that all those parts work together.
If you find out that you have to change the design of a part while you are designing the vehicle, this pushes back your project completion date. This makes the spreadsheets upset, and the spreadsheets take it out on your stock price.
----
Ok so "why don't they just" make the design better for the next model year?
Because those engineers have been assigned to another project! Also, design changes incur costs, which pushes out the program pay-off period.
You amortize Research and Design costs over the actual vehicles produced. Those vehicles have to be competitive in the market - price per value.
The Value Proposition for maintain-ability/repair-ability is not readily apparent at time of purchase.
This is why "residual value" is a luxury. No one gets to pick all of their own success criteria.
~~~ Again, this is all just like my opinion.