The armor component for the Vehicle Construction Kit needs rewriting.
Firstly, the “undersized penalty” for Armor Class is inconsistent with the “average AC” use for overall vehicle AC. One armor component should give the same overall AC as two one-step armor components, or three two-step undersized armor components, etc. So the undersized penalty is gone. Instead overall AC is the average AC of the best three slots, and a slot has the average AC of the components within it (using AC 11 for non-armor components.
Hopefully people won’t find this complex, unless you find division harder than subtraction. You can always ignore undersized armor completely.
Secondly, I’ve made the assumption that a vehicle’s armor also represents its reinforcing material. So, the reinforcement component is gone and armor now grants extra hit points. A vehicle’s base hit points is reduced accordingly, so you can have “fragile object” vehicles with no armor. Hit points are proportional to the cube-root of a vehicles mass, so you add up all the extra hit points of all the armor components and apply a multiplier (yes, table will be provided). Undersized armor can also be “armor plating” that has reduced hit points but retains its AC.
Thirdly, costs have been adjusted. Adamantine is now solid adamantine, not “adamantine-plated whatever”. It’s 6000 gp for a mass i component now (from 200 gp). You make your own “plating” by using an undersized component. That’s the point of more expensive armors: you can use less of it to achieve the same protection as a cheaper armor.
Now with three values – AC, Hit Points and Cost – armor can be more versatile. Bronze isn’t as hard as iron but is stronger and more expensive. Darkwood has twice the hit points of wood. Glass and ceramics are hard but brittle. You can create composite materials that average the values of its layers.
Behind The Curtain: Despite D&D being heavily abstracted, I’m not fond of just pulling numbers out of the air. I’ve tried to do a bit of research into real material properties. To my fault, I like to come up with formulas that convert real properties into D&D statistics.
An armor’s extra hit points is one-tenth of the material’s ultimate tensile strength (in MPa), since I reason that this is roughly the vehicle’s resilience to the various impacts it might take. This is 6 for wood and 18 for iron. Happily this seems to give about the correct total hit points for my test vehicles (rowboat, apparatus of kwalish, sailing ship and warship). I’ve set the upper limit, adamantine, at 90, the same as titanium: a 2-ton vehicle with full solid adamantine armor has 840 hit points but will cost a minimum of 18,000 gp. May need to reduce this? (to be fair, it would be a very rare magic item).
AC for metal is… wait for it… 12.5 + 3 times the common logarithm of the material’s hardness (in kilograms-force per square millimeter ) in the Vicker’s hardness test. This gives the following ACs: Lead (14), Silver/Gold (16), Bronze (18) and Iron (19). At the very top of the scale, if diamond could magically be transmuted into armor plates, it would have an AC of 24. I don’t know about you, but I think that formula gives convincing results.