I guess the mathematical question is how much extra damage do you take if you have -1 to your AC?
Yes, I believe if you answer this question then you should be able to come up with a balanced ruling. Maybe just use the monster builder averages and don't forget that you have increased chance of getting a crit.
I guess the mathematical question is how much extra damage do you take if you have -1 to your AC?I'm not sure you could. If you remove the AC bonus that armor provides then you are opening up a character to getting hit and crit a lot more often, which could easily overwhelm any DR, but if you keep the AC then you are effectively giving all the characters DR just 'cause.
Though that second wouldn't be terrible if you applied it to everyone, IMO.
But this is a function of the enemy's damage output, and not of the armor (or AC loss) itself.
A quick plot I made is telling me that level scaling is somewhat regular: for a monster of high accuracy and high damage, it is about 0.2 damage per level per point of AC; a bit more in the lower levels, a bit less in the high ones. So you could increase the DR with Armor Potency Runes (at levels 5, 11 and 17) by 1 point for each point of AC the armor would give, and you are set, more or less.The tricky part is that a monster with higher (or lower) damage would need a different adjustment to have a similar outcome: 0.25 points instead of 0.2 for extreme damage, 0.17 for moderate damage, 0.13 for low damage.
Monster accuracy also matters: for attacks with full MAP, keeping high damage as a standard, you need the same (0.2) reduction for extreme and high attack bonus, 1.8 for moerate attack bonus, and 1.4 for low bonus. The second attack in the round should meet a damage reduction that is between 20% and 50% lower than usual. The impact of different accuracy shrinks a bit in the higher levels, when monsters tend to hit better.
Let's settle, anyway, for the 'standard' DR scaling factor of 0.2 per level. We still need to decide how much basic DR should a mundane armor give. This varies a lot, and could really unbalance the game at low levels.
At level 1, when fighting an equal-level monster with high accuracy and damage, the magic number is between 0.5 and 0.6 per point of AC lost. But applying that, you would take more damage than usual from enemies with stronger offense, and less than usual from enemies with a worse one (moderate/low damage or accuracy). This is even more important when the monster's level is different than the PC's.
And more, the tankiness of your characters also matters: a Champion or a Monk will still have an higher AC, thus reducing the likeliness of critical hits, and DR will be stronger for them.
As a result of all this, changing armor to give DR instead of AC is a mess; and you have to keep the Dex cap, or it becomes even worse.
If I was forced to do it, I would go with something like this:
Padded Armor: DR 1
Leather Armor: DR 1
Studded Leather Armor: +1 AC, DR 1
Chain Shirt: +1 AC, DR 1
Hide Armor: DR 2
Scale Mail: DR 2
Chain Mail: +1 AC, DR 2
Breastplate: +1 AC, DR 2
Splint Mail: DR 3
Half Plate: DR 3
Full Plate: +1 AC, DR 3
Hellknight Plate: +1 AC, DR 3
Increase DR (but not AC) by the armor's base amount for each tier of potency rune.
This probably makes armor better than it currently is, expecially at low levels (with heavy armor you are almost immune to low-damage enemies); at the same time, upgrading your armor's potency is a much bigger step.
I think you'd have to tweak monster accuracy for this to work at all... At the highest end of the spectrum (+3 full plate), you are turning a "hits on a 7, crits on a 17" monster - which is a pretty common monster - into a "hits on a 2, crits on an 8" monster, which is a devastating swing in terms of the amount of damage the character is going to take.
Removing Dex cap makes that stat absolutely mandatory for everyone.
Giving heavy armor DR 10-12 at level 1 is absurdly strong.
Reducing accuracy may be a good way to go, but then armorless, high-Dex characters become very hard to hit and probably still superior.
I don't know. Maybe with enough DR on heavy armor to ignore normal hits and only actually get damage on crits, it may work.
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What about an idea like this: Keep the AC bonus as is, but the armor also provides some DR based on your proficiency rank tier x1 for light armor, x2 for medium armor, and x3 for heavy armor. So if you are only Trained in each armor, it just provides 2/4/6 DR. If you are an Expert, you have 4/8/12 DR, Master is 6/12/18 DR, and Legendary is 8/16/24 DR.
Now all this free bonus DR brings up the question of how to compensate Unarmored characters in some way (maybe apply the rank value as an +2/+4/+6/+8 AC bonus to better avoid being hit at all, or just treat Unarmored the same as Light Armor?
Lastly, if you are still using the "Damage to Shields" rules from the CRB RAW, does this now rule damage your armor just like it does shields? Wouldn't that cause you to go through suits of armor like they are toilet paper? (If you don't know about it, you can look up my rules for indestructible shields in my house rules doc)
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I was thinking of using the armor material's Hardness as DR (similar to Shield Block).
I was also thinking that Critical Hits bypass armor entirely, including any DR it has. The idea is they have got around the armor's defences and found a weakness.
I would leave AC alone.
The hardness for basic armor is found here:
Armor Statistics
Special materials and runes would affect the hardness of course.
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These are really interesting ways to go about an answer! For context the main reason my players want Armor as DR is because they do not like the "AC as abstraction" They like how other systems have a difference between getting hit and taking no damage and the attack missing.
Ah, but damage itself is an abstraction in games like this (except, of course, when it isn't): The fact that you get more HP as you level up clearly indicates there is more than just 'physical toughness' to this core concept (one of the things I kinda liked about the old Earthdawn game was that beyond the baseline HP you got at character creation, every HP you gained was actually you channeling magic into your body to toughen it up).
There have been many attempts to add 'Armor as DR' to d20 games over the last 20+ years (and probably many before then as well), but I don't think any of them have worked particularly well because you are messing with a 'core component' of the game:
- Just adding DR based on armor type and not removing anything tends to disproportionately buff heavy armor builds by reducing the damage they take: If you buff your foes damage to compensate, you just end up right back where you started except that lighter armor characters will die faster now.
- Replacing Armor's Item bonus with DR means the heavier armor you use, the more likely you are to be crit, which means you'll need a lot of DR to compensate. Overall, the -6 AC for Fullplate users will be really hard to balance.
- The need to adjust basically every creature the party ever fights for these rule changes can be a serious issue for GMs.
It does interest me a lot. Mostly because armour is abstracted away in the game, and it doesn't feel at all like the medival experience of armour. Which was - armour was really important in combat, but very inconvenient to wear out of combat.
To do it right would be a major change in the game. You would have to allow the AC of non and lightly armoured types to get higher. Just to allow the characters who choose not to wear armour to still be competitive.
So relative to what we have now, some typical values might be.
+1 AC for no armour
+0 AC +3DR for light armour
-1 AC +6DR for medium armour
-2 AC +9DR for heavy armour
Yes its a major change. You really need to critical to hurt someone in heavy armour -but that is more likely as he is easier to hit. I think it would add more tactics to the game.
The damage resistance would have to scale up as you level Probably add that amount of DR again when you add each extra striking rune. Call it a reinforcement rune. (It seems easier to do this that rescale all the hitpoints and damage in the game).
As it stands that is a net defensive bonus for everyone.
Then you would have to add in weapons that were armour piercing (against half armour value) plus a few spells that ignore armour.
You also want Runes to quick change into armour, or to make armour float, or rest comfortably in. Plus rules for endurance - maybe even Constitution minimums for armour.
Lots to do. You'd have to test it. It would be fun. But is it the same game?
Yeah I think so. Just another dimension added to it. I'd certainly like to see it as an offical variant.