Here is a quick method for changing how armor works that has been floating around in my head. I don’t claim any originality for this system. It was inspired by Combat Musings over at The Jovial Priest and St. Innocent of Alaska over at Blood of Prokopius.
This assumes a B/X substrate. It would play well with damage by hit die rules, I think.
First, instead of AC there is an evasion score. This is 10 + dexterity mod. This is how hard it is for a character or monster to be hit. You would need to improvise the evasion score for monsters, but I think that should be easy. Sample: goblin, 12; pixie, 17; dragon, 10.
Second, armor provides a damage reduction die. Light (leather) armor is d4, medium (chain) armor is d6, and heavy (plate) armor is d8. If you are wearing armor and take physical damage, you roll your armor die and reduce damage by that much. This is similar to “soak” rules in other games, but I think this implementation integrates nicely with other traditional D&D rules. Monsters also require improv DR scores. Sample: wolf, d4; bear, d6; dragon 2d6.
That’s the core of the system. Here are a few optional rules for added detail.
- Armor damage. If you ever take a full damage blow (e.g., 4 points from a d4 attack or 6 points from a d6 attack), you roll damage reduction as normal, but mark down a point next to your armor. When N such points have been accumulated (e.g., 8 points for heavy armor), the armor is degraded one step (so degraded plate armor would reduce damage by d6 after one level of degradation). Armor can be repaired, probably at half cost.
- Weapon versus AC. Using a weapon that is “good against” a particular armor drops the damage reduction die by one step. So, plate armor would only block 1d6 damage from a military pick. Firearms could ignore armor entirely or drop the damage reduction die by one or two steps, depending on how much influence you would like gunpowder to have on your setting.
- Bulky armor. Armor reduces evasion by one point per class. For example, plate armor would reduce evasion by 3, leather by 1.
- Armor competency. The damage reduction die is limited by class hit die. So magic-users can wear plate armor, but they still only get d4 damage reduction, and all evasion and encumbrance penalties still apply. Maybe there is some method to gain proficiency with armor for classes other than fighters? That’s beyond the scope of this post, though.
Edit: damage reduction numbers for d6-centric OD&D using 2DTH: light (leather) 2d6 take lowest, medium (chain) 1d6, heavy (plate) 2d6 take highest. Evasion calculated using the B/X dexterity modifier, though it would not apply to anything else.