In my recent post about magic-users and armor, there was one option for doing strict class-based AC (that is, all fighters have AC 3, all magic-users have AC 9, and so forth). One downside of that system is that other than gear with special enchantments it doesn’t allow for much in the way of advancement (which is a big part of what makes D&D work as a game).
Here’s another idea that works with OD&D hit dice to address that concern. Take the combined hit dice value (for example, HD 5+1 = 6) and subtract that from 9 (or add it to 10 if using ascending AC). That is the characters armor class (improved further by one if using a shield). This gives fighters the best AC, but also allows gradual progression. Following the same example, a character with 5+1 HD has an AC of 3 . This is sort of the defensive equivalent of using hit dice as attack bonus. AC should also be capped, depending on the desired campaign power curve (if I was using this for Vaults of Pahvelorn, the best AC from hit dice would be 3, or 2 with a shield, because danger should always remain). Really, the term “AC” here becomes a bit vestigial; it’s really more of a defence stat, but continuing to call it AC probably helps from a UI perspective, given that it works exactly the same as AC.
If you wanted to preserve some mechanical effect from armor, maybe allow it to add a bonus to the death saving throw† (light = +1, medium = +2, and heavy = +3). This further reinforces the idea that HP is a mixture of elan and resolve, and that there are no potentially telling blows until HP have been exhausted. The downside of armor would be an encumbrance style penalty to ability checks, non-death saving throws, and escape rolls.
† Rather than deal with negative HP or have death occur at 0 HP automatically, I allow PCs to make a death save. If this is passed, the character is unconscious. If it is failed, the character is slain. This is one of my favorite house rules, and probably deserves its own post so that it can be linked to directly.
I’ve been using a hit-dice vs hit-dice combat table for a while now. I talk about it here:
One advantage of this method is that you can use hit dice to determine to-hit chances and then use the same chart as level vs level for things like pickpocketing and stealth rolls or even use it as level vs DC for fixed difficulty tasks like picking locks or removing traps.
I add armor saves to avoid damage with the caveat that generally only the PCs will have them to avoid bogging down the game.
That’s a really intriguing system. I’ve been moving away from table lookups in favor of 3E-style bonus and target numbers (at least in my experimental designs; I’m still using an attack matrix in my OD&D game for legacy/familiarity reasons) because they require fewer tools during play, but that is a pretty elegant formulation (and I like the symmetry of attack the monster but defend against the monster where players can roll dice for both sides).
I’ve recently been pleased with a more limited form of defence save (shields can do it against missiles and quarterstaves can do it against melee attacks) because that doesn’t come up as often during the game.
Have you considered static damage reduction for armor, rather than a save? In the style of Warhammer FRP or Apocalypse World? That approach has seemed very attractive to me recently.
There’s no reason you couldn’t switch my table for d20+HD vs a DC of 11+HD. I just like tables.
I played Runequest for years and years and it uses DR instead of saves. One thing I found with DR-based systems is that they greatly encourage the PCs to all use two-handed weapons, as a DR of 4 (for example) will completely negate a dagger but only take 1/2 of the hits from a great axe. A save mechanic will reduce all weapons by the same ratio.
Regarding DR, I use d6 weapon damage for all weapons (well, also 2d6 take highest, for some two-handed weapons, but that’s the same range). But yeah, you’re right regarding variable weapon damage.