As is in Errant, armour works like so.
- Armour gives you AP, essentially an extra pool of damage you can take before it gets to your HP. There are no to-hit rolls, so armour doesn't make you "harder to hit", except in the sense that HP is an abstraction that represents your ability to mitigate taking serious damage.
- Armour is piecemeal, so you can wear a helmet that gives you 2 AP and some gauntlets that give you 2 AP and end up with 4 AP total.
- When you're down to 0 AP, you can use an armour repair kit and repair your AP back to full.
- If you ever take max damage from a hit, a piece of armour you're wearing loses a point of Quality; at 0 Quality something is broken for good.
- Shields work a little differently, so as to create a mechanical distinction between a regular shield and a helmet. A shield instead Impairs incoming damage (Impair means to reduce the damage taken by a die step, so d8 -> d6). A regular shield Impairs 1 while a large shield impairs 2 (e.g. d8 > d4). While this makes shields more powerful for reducing damage, it also makes them more likely to break, because smaller dice size = higher chance of rolling max damage.
- You add a quarter of your max AP to target numbers for sneaking, climbing, swimming, squeezing, or balancing. Swimming while wearing chain or plate torso armour is going to cause exhaustion from hypothermia. Add half of your max AP to the target number for spell retention rolls.
I have a couple of problems with the armour rules as is.
- Once you have all your armour on, the piecemeal aspect is kind of lost as it all gets aggregated into a big blob of AP. It still comes up diegetically enough whether or not a character is wearing a helmet or gloves when interacting with environment, but still.
- Quality/AP disjunction: you track Quality for armour parts individually, but AP in aggregate. It can also be difficult to remember to take Quality damage, because unlike weapons which can take Quality damage when they're used, taking Quality damage to armour is passive.
- Which is the crux of the issue, is that armour is passive, and in general, it is harder and more fiddly to keep track of passive resources which deplete rather than active resources which are used (and less exciting).
So, here is my attempt to fix those problems.
- Every piece of armour has a number of Blocks. Each Block can Impair damage by 1, but the player has to describe how they use that armour to reduce the incoming damage (e.g. a helmet is helpful if rocks are falling on your head).
- You can use more than one Block at a time; this follows the normal rules for Impairment, where two instances of Impair 1 = Impair 2. Of course, taking a bigger hit with your armour means its more likely to lose Quality (if damage is reduced to 0, that still counts as taking max damage).
- You can only use one piece of armour to Block per attack or instance of damage.
- You can also use Blocks to negate non-damaging harmful effects that make sense (e.g. if a save or die poison needle trap is on a door handle, you can use a Block from your gauntlets).
- Total # of Blocks function the same as AP for physical checks and spell retention checks.