Armor as DR Isn't Worth the Work

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Many people feel that the armor system in D&D doesn't make sense, and should work like damage reduction. I believe the armor system works fine for an abstraction-based game like D&D. In this long-overdue rant I'll explain that while you can create an armor-as-DR system, it is a lot of work that involves significant change to the game. I'm not saying it can't be done, I'm just saying it'll take a lot of work ... more work than it's worth to most people.
    Many thanks to Zero, Aelryinth, LStyer, GarenThal, Artanis Aeldarunde, Thirdwizard, Sacremas, and the other people who participated in the message board discussion about this topic, it helped me figure out my answers and come up with questions and DR armor system needs to address. You'll note that this rant tends to sound like a dialogue ... that's why.
    Note: I have not read Unearthed Arcana, so I don't know the details of its armor-as-DR system. It may be perfect and address all of the concerns I point out here. If so, great.

Let's establish some givens for this argument.

Given #1: Just as the AC system has a wide range of AC values for its armors, a DR is going to need a wide range of DR values, otherwise the armors all end up similar.

Given #2: If you have a wide range of DR values for armor, it means that some weapons that don't normally do a lot of damage (such as the dagger) won't be able to hurt high-DR foes unless they crit. For example, if you give full plate DR 8, it means a person with a dagger can't ever hurt someone in full plate (barring Strength and other bonuses to damage), when historically that wasn't true at all ... it was hard, but a skilled person can find the gaps in the armor

Given #3: To have heavy armor more effective than light armor, either heavy armor's DR has to be very high (which brings up the problem of Given #2) or light armor's DR is going to be so low as to be negligible.

Given #4: The AC system already models how well armor absorbs damage. Wearing full plate is eight points of AC better than not wearing armor at all; that +8 AC translates to 40% of the attacks that would hit the unarmed guy do no damage to the guy in full plate. In effect, the AC system gives you an all-or-nothing percentage-based damage-absorbing value, kind of like improved evasion.

    (1) Some armors have the same AC but different max Dex bonus (MDB), armor check penalties (ACP), and so on; a DR system could do the same thing.
    (2) and (3) Both of these problems are solved by creating a Penetration stat for each weapon. Spears have a good penatration value, maces don't, and this reflects that some weapons are better against certain armors.
    (4) You can also model this in the DR system by using the defensive roll option in the DNG (the defender rolls vs. the attacker).

    (1) True, you can make the armors different by giving them different secondary stats, but the primary effect of armor is its AC (or DR). Fighter with Dex 10 doesn't care that chainmail has a MDB of +2 instead of breastplate's +3, all he cares about is that they both provide +5 to AC and the chainmail is 50 gp cheaper. Having an AC range from +1 to +8 gives a designer a lot of variety when designing armor, and he can model armors for the special-case characters (like the high-Dex rogue who's willing to buy weaker armor if it lets her use more of her Dex bonus to AC with the net effect of higher AC than she'd get with heavier armor) because the designer has a wider range of numbers to play with. When building a DR system, you either have to choose a low range of numbers (which introduces the problem of Given #1) or a high range of numbers (which introduces the problem of Given #2)
    (2) and (3) Yes, you can model that with penetration, but that means you now have to determine an appropriate and balanced Penetration stat for every single weapon in the game. Plus every natural weapon (claw, bite, spike, sting, etc.). It can be done, but that's a lot of work.
    (4) This is an irrelevant argument because the AC system already has this as an option; it's neither supporting nor undermining your DR armor position: it's a wash.

    (2) and (3) Yes, you can look at it that way, but because natural attacks fall into the same three categories that weapon attacks do (bludgeoning, piercing, and slashing) you can just categorize them that way and use the default Penetration values for those three categories: bludgeoning = low Penetration (0-1), slashing = decent penetration (2-4), piercing = high penetration (5-6). The only except is crossbows that have even higher penetration.

    The above rebuttal means you're already willing to make an abstraction and say that "all weapons of type X have approximately this level of Penetration," yet the complaint is that the AC system is not realistic because it is too much of an abstraction. Either you're willing to do the work to make the system accurate and balanced, or you're not.
    Also, you're admitting that some weapons, like crossbows, are going to have penetration values outside weapons for their category. Apparently some weapons are worth the extra work to make them right in the DR system, but entire categories of weapons (natural weapons) aren't worth that work.
    This classification also doesn't address non-weapon kinds of damage. Is constriction damage bludgeoning? A strong creature ends up crushing your armor against you, which should ignore the effects of DR to some extent. Which also brings up the question of "If my armor is absorbing this damage, how much damage can it absorb before it's loses some of its protective value?" (True, the AC system doesn't address that either, but the AC system at least allows for the abstraction that the armor is turning the weapon away rather than saying it absorbs the damage ... particularly when the DR system has a weapon stat called Penetration which implies that the armor is being pierced).
    Some armor types were designed to be very good against certain forms of attack (like designed to withstand arrows and crossbow bolts rather than swords) because the people who wore it always faced foes of a particular type. How do you build that into the system? Does armor have DR and a Penetration-Resistance stat against certain types of weapons? Or do weapons of that type get an additional Penetration bonus against that kind of armor? So chainmail has DR 4, but arrows, bolts, and stilettos have penetration +2 against chainmail over and above their normal penetration?  
    What about multiple-damage-type weapons, like a morningstar that deals bludgeoning and piercing damage? Does it have two different Penetration values? How do crits affect Penetration?
    How to enhancement bonuses on weapons affect Penetration? How do enhancement bonuses on armor and shields affect DR? How do armor-adding feats like Two-Weapon Defense affect this system?
    What about spells that provide AC? Do they provide DR, and if so how do they interact with actual armor?
    What do shields do in this system? Do they add to your DR? Or add to your defensive roll? Or are they a first-line-of-defense DR that you have to get through or past to get to the armor DR?
    What about helpless opponents? Can I ignore the armor (and thus the DR) of a helpless creature because I can aim for a gap in the armor? But what if the armor is from a spell like mage armor, there's no gap to aim for, what happens if I try a coup de grace on them?
     How does armor DR interact with a creature with natural DR? For example, what happens when you use a morningstar to attack a skeleton (DR 5/bludgeoning) wearing full plate (DR 5)? Do you use the piercing aspect's Penetration to get through the armor and then do little damage against the skeleton's DR/bludgeoning, or do you use the bludgeoning aspect's penetration and just hope to do enough damage to get through the armor?
    What about natural armor? What about creatures that already have natural armor and DR? Creatures with bypassable DR like high-level barbarians?
    Does this system change how a touch attack works? Does it change how an attack-roll spell (whether a normal attack or a touch attack) works? Do you now need to classify and give Penetration values for every attack-roll spell in the game?

    That's a lot of questions. Is it proof that armor DR doesn't belong in the game? No, but that wasn't my intent (note the title of this rant, Armor as DR Isn't Worth the Work).
    You can introduce armor DR into the game. It will take a lot of work to make it balanced. It will have an effect on other aspects of the game, some of which will suprise you because you hadn't thought of them. If you're willing to accept those things, then go for it, make up a variant system where armor gives DR. I'd rather accept the abstraction of AC, it works well enough for my tastes and I don't have to spend any more time making it work.
    (I'm not against the idea of armor as DR, but I think to fully incorporate it into D&D requires a lot of metasystem thinking--about how this sort of system change affects every aspect of the game--like the 3E team did for every significant change in the new rule system. I don't have the time to do that, and most of the people who do have the time to do that don't have enough deep knowledge of 3E to do it right.)