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The term “modular” usually has positive connotations. It can refer to something as basic as a chassis, which you plug various test or I/O cards into. Or it can even refer to something as complicated as a house that is built using individual room designs built in a factory, which are then trucked to the building site and assembled into a complete house. It’s also a word which often conceals—either intentionally or inadvertently—what is involved in a project.
The presumed benefits of modularity have been embraced over the years by automakers, among others, who use product-line “platforms” on which they build different vehicles. In nearly all these situations, the modularity is mostly at the body level and much less in the power train, using a single or closely related engine-transmission pairing, but with different body styles, interiors styles and finish.
But why stop there? I was intrigued when I saw a story about a highly publicized plan by Stellantis to use a highly modular automotive platform for next-generation vehicles. Dubbed “STLA Large,” this platform can accommodate many power-train options. (Note: Stellantis is the parent company of Chrysler, Peugeot, Fiat and many other well-known nameplates.)
The flexibility covers propulsion—front-drive, rear-drive, all-wheel drive and multi-energy—which are provided by a family of three scalable electric drive modules. That is a lot of modularity and flexibility, I thought.
By MRPeasy 05.01.2024
By Global Unichip Corp. 04.18.2024
This large platform is one of four different sizes they have developed, primarily—but not exclusively—for battery-electric vehicles (BEVs). These four modular STLA platforms (Small, Medium, Large and Frame) are engineered to be future proof and are inherently flexible in wheelbase, width, overhang, ride height and suspension design, according to Stellantis’ press release. They are also designed with provisions for future battery chemistries, including nickel- and cobalt-free batteries, as well as solid-state batteries.
STLA Large is designed and engineered as a native BEV platform with the option of 400 V and 800 V electric architectures. It can be configured in front-wheel-drive, rear-wheel-drive and all-wheel-drive layouts using transverse and longitudinal engine mounting configurations.
In addition to BEVs, the STLA Large platform also supports hybrid and internal combustion engine (ICE) propulsion systems, while the smaller ones are BEV only.
The STLA Large dimensional attributes are revealing, as indicated by their numbers (and they do so with too many significant figures):
- Overall length range: 187.6-201.8 inches (4.764-5.126 mm)
- Overall width range: 74.7-79.9 inches (1.897-2.030 mm)
- Wheelbase range: 113.0-121.1 inches (2.,870-3.075 mm)
- Ground clearance range: 5.5-11.3 inches (140-288 mm)
- Maximum tire diameter: 32.6 inches (858 mm)
All this modularity and associated flexibility seems like a very sensible idea. But then I started thinking about my own past encounters with modular design, both as a product designer and as a user. In many cases, the benefits and results were less than anticipated, primarily for two reasons:
- First, there is the issue of design optimization. A modular platform requires compromise on features and performance to accommodate the various permutations and arrangements within a common framework. Sometimes these compromises are minor or modest, but often they are not. So, instead of a “very good” non-modular design, you end up with a “sort-of good” modular one. The size of the gap between very good and sort-of good is hard to know without careful analysis.
- Second, there is the belief that modular means you can just drop in “unit B” to replace “unit A,” and do so with minimal additional changes to the platform, interconnections and supporting peripheral functions.
Once again, experience has shown that there are countless other less-visible changes you need to take to fully accommodate the drop-in alternative. The ripple effect of switching out even a single modest component can bring nasty surprises. For example:
- Weight and weight distribution will likely be different, so suspension will change.
- Different sensors are needed for each type of power train.
- Cabling, wire gauges and connectors will be very different.
I am sure a detailed review of the manufacturing bill of materials (BOM) and process will reveal many other hardware, inventory and production-process changes, in addition to the supposedly “painless” one of different software packages.
There is ample precedent for wondering and worrying about the overall impact on design, manufacturing, test, and in-use support when you attempt to modularize and build up in a logical manner—admittedly from a different context. My perspective is from the book “Apollo: The Race to the Moon” by Charles Murray and Catherine Bly Cox, the best book on the moon landing among the many I have read.
In a brief overview chapter, the authors point out that modular, stage-by-stage assembly of the rocket and capsule did not accomplish what people thought it did. The book notes that NASA project leaders realized this after a successful first-stage test, noting that “whenever you added a new stage, the ground support equipment was different, the checkout procedures were different, the hardware was different.”
The alternative strategy the Apollo project had to use for assembly and test was non-intuitive and called “all-up.” It meant that from the start, they would assemble the rocket in its entirety only in its final configuration, using subassemblies that had been individually tested, but not joined to others yet. They would then test the integrated rocket as a single completed system. This contrasts with building it up piece-by-piece and testing at each incremental build step, which at first seems the more logical way to do it.
I am sure the engineers and others at Stellantis have studied this idea very carefully before implementing it. It will certainly be interesting to see how manufacturability, performance, support and cost aspects work out. I still wonder how much of the modularity story is based on actual savings and how much is driven by the conceptual attractiveness of the story (especially to Wall Street) more than the reality. We will have to wait and see about that.
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