My thinking:

DC (defence class) is 10 + strength modifier + dexterity modifier, representing the character's defensive capabilities.

Armour has hit points equals to PHB AC values x 10. Medium armour has resistance to slashing damage. Heavy armour has resistance to slashing and bludgeoning damage. Full plate (a new armour costing more than plate) resists bludgeoning and piercing damage and gives the wearer immunity to slashing damage.

If an attacker beats your DC your armour takes the blow, suffering damage. A critical represents finding that special gap at the armpit or groin, and you take HP damage. Armour at 0hp offers no protection and is considered broken.

HP for characters limited to starting HP as normal then just CON modifier. Healing only by rest or divine magic. Hit Dice can be spent during a rest to repair armour that isn't broken, representing the mending of leather straps/padding/smoothing out dents etc.

These are my thoughts so far on a grittier AC system. Any comments? Yet to play test, planning on doing some this weekend.

While armor does get worn out, keep track of armor hp and player hp might be more bookkeeping than needed. 

Consider DC to be just 10 to 12 -ish and Dex mod.

Then instead of armor hp, make armor reduce damage taken based on what kind of armor it is. For instance, leather will reduce damage taken by 2 points per attack.

A decent suggestion, but I've never liked the idea of DR.

The idea here is to go grittier, more realistic? Changing the AC calculation and having armour work as a sack of hit points doesn't feel grittier.

What if instead of trying to reinvent the wheel you instead come up with some rules related armour's vulnerabilities.

Have armour give its usual AC values, but vulnerability to the type of damage that famously gets passed that armour? Say Leather Armour makes you generally better protected, by its AC bonus, but when a Piercing Damage attack gets through it gets through good. Likewise heavy Plate is going to protect against Bludgeoning Damage up to the point it crumples, so when a Bludgeoning attack gets through the AC, its going to hurt.

You could also add in that when an attack hits an armour type's weakness it damages the armour, reducing its AC by 1, making subsequent attacks of all types easier to hit until its repaired. Repair for leather armour could be as simple as oiling and repositioning layers during a short rest or could be as annoying and costly for plate armour as having to find a working forge to hammer out the crumples.

You could even throw in a couple of elemental vulnerabilities for certain armours. Leather armour is porous and takes a beating from acid damage, chainmail focuses a lot of metal around your vital organs, so avoid lightning damage, plate armour is already heavy and hot, fire damage is not going to help.

This sounds potentially fun, but I wonder how different from normal HP/armor this will end up feeling in the long run. Depending on a group's ability to carry items, someone could theoretically just keep multiple armors on hand and swap between fights and essentially get the benefits of a full rest (in terms of HP recovery) by simply spending 10 minutes or so swapping armor.

Good point, Transmorpher, but in my opinion if the characters were organised enough and brought horse, cart and hired hands to watch their stuff... why not? Their actual hit points wouldn't recover so enough critical hits would still kill them, and their class resources wouldn't replenish.

As DM I certainly wouldn't be letting a character carry round a spare set of armour on their person along with their other necessities.

Quote from TheCivilisedGoblin >>

As DM I certainly wouldn't be letting a character carry round a spare set of armour on their person along with their other necessities.

 
So a character with a bag of holding using it for their spare armour, or even a backpack if they have the strength - all of which is perfectly reasonable, RAW and realistic, you'd just outright say no because it's inconvenient for your homebrew rules?

Very glad you're not my DM.

--

Anyway, to the topic. You want realism but present an incredibly unrealistic thing. Most armours in real life - especially the weaker ones - don't nullify damage, they reduce it. Yeah, yeah, you don't like damage reduction, sure, but you can't have it both ways. Leathers and such do not just "stop" blades, they reduce the force of the attack - but stab somebody with leather armour and they still bleed, the only difference is the wound is not as deep as it would have been. Even chainmail can still allow blades through. Even if the blade doesn't go through the material the impact itself still damages you and while the material can lessen it, it does not negate it. Even a sword slash to somebody wearing plate armour still hurts from impact if strong enough.

In real life there does not exist any armour that "negates" damage. Even the best armour ever made in history, still has you be hurt from stabs and slashes if strong enough from impact alone - and strong enough isn't some superhuman level strength, a good warrior with a well-made weapon and a good arm is enough. It will cause you to bruise and be hurt and worn down. And bludgeoning damage is the most effective against armoured enemies, not the least. This is because no matter the material a solid blow from a weight war hammer sends shockwaves of impactul force into your body. You could have the best plated armour ever created, but a decent swing to the chest by a well made war hammer will break your ribs rather easily. The downside is such weapons are slower, heavier and more costly to make which is why swords/blades were better used - don't weigh you down as much, easier to swing repeatedly, and more versatile against different armour types.

So your armours just absorbing all damage and having between 110 to 180 hit points is absurd. It's further away from gritty realism not closer and therefore the very opposite of what you claim to want. It will also just be a lot less fun at lower levels unless you greatly change monster CRs and damage output, because you need to rework their ACs and damage otherwise your low level chars basically have 110+ damage absorbtion against creatures that deal like 5 or 10 damage per turn with no extra HP and similar or less AC/DC.

If you want gritty realism then you need to consider Damage Reduction type of things and only use armour HP to be reduced as well as character HP as the armour eventually wears out. You also need to really take into account the type of armour and the protection offered - armours like chain shirt only protect the torso, the arms and legs are not protected and now you need to add in other items like gauntlets, vambraces, pauldrons and greaves to define where the body is protected and the % chance of how often an attack will hit an unprotected area, since you can't rely on crits as that doesn't make sense, the more unprotected areas the greater the chance of striking there, so I guess crits are no longer "nat 20s" but will be based on a a stacking bonus by armour piece? So crit level is 0 (unarmoured) and there everything bypasses the armour, and this increases eventually to 20 (almost everything protected). That's going to be cumbersome for you and the players, but you want gritty realism so this would be necessary.

How much realism do you want? Because plate armour (which is already full plate by descripion, it's just not called such as an item name, hence "half-plate" and "plate") is very heavy and uncomfortable and in real life people spend only a couple of hours a day at most in full plate armour. It is only worn when entering a battle (going off to war) and for the rest of the time they would wearing lighter armour. Going too real would there mean having character constant taking things on and off and making Con checks to avoid Exhaustion levels for wearing it too much. And then the weight has to be factoring into running speeds, walking, and helmet designs have to consider visibility (full protection is great but that means your visibility is greatly reduced as most of your field of vision is obscured by metal - you basically lose all peripheral vision, this is why many helmets only cover half the face or only the sides/top/back).

Do you see where I'm going with this? The real world physics and complexities of combat and armour-creation is beyond the scope of a game, really. There's a reason why almost no game ever created matches with reality.

So I'd lessen up on "real world" thinking and work out how to create a new armour/damage system that isn't going to change too much (so it can be more easily applied to the monsters faced) and adds more fun. Not in a "gritty realism" thinking, just a way to maybe rethink it out of preference. Make it too complicated (realism) will be a balancing chore and force players to get bored from trying to track too many variables and circumstancial minor changes. Games don't think too much about realism for this very reason: it's not fun, and a game should be more fun than more real.

I'm also going to break down the current AC for you and perhaps you can see why it is what it is and what you could change to improve your combat without making things worse:

10 -- this isn't your character's defense, it's the attacker's aim/force. Getting 1-9 total basically means they aimed wrong and missed you or didn't throw enough weight to actually damage you.

+ worn armour -- worn armour overrides the 10 with a new base of 10 plus an amount of armour bonus dependent on the armour type. The first 10 is the same as normal but anything above that represents the armour's protection: the attack was against an area the armour protected it and was able to avoid the damage.

+ Dex -- this is your evasion, you moving out of the way of the attack. Heavier armours restrict this because the weight slows you down and is more difficult to move in.

+ Con -- generally only given from features like Barbarian's unarmoured defense it's basically shrugging off attacks through the toughness of your skin, resilience of your muscles, etc.

+ Wis -- generally only given from features like Monk's unarmoured defense it's representing the monk's efficiency in evasion by using their honed instincts.

So let's take Barbarian's Unarmoured Defense and it is 10, plus 3 dex and 3 con. The AC is 16.

If that barb was attacked and the attack roll was 9, then the attacker didn't aim properly and missed.

If that barb was attacked and the attack roll was 12, it was aimed correctly but the barb dodged out of the way.

If that barb was attacked and the attack roll was 14, it was aimed correctly and hit the barb but the barb's tough skin and natural resilience meant it did not take damage.

-

This is the sort of way AC works to factor what the attack is doing and what you are doing while considering what other defenses you have. It's enough to cover everything you need while not being so complicated that people are bogged down by numbers and variables and stuff. It's just enough realism to be immersive combat while streamlined enough to be not be encumbered by too much realism and all the numbers and tracking that would entail so you can focus on just playing. Additionally, it's fantastical enough to make you feel like a fantasy hero from movies and novels rather than the reality of being exhausted after a few swings or unable to run or swim or constantly switching armour and all the other inconveniences of wearing armour in real life.

The AC system is well designed for the purposes and very integrated into game balance. I do not recommend making big changes as it will have very large balancing problems throughout. I recommend starting with small changes and going from there rather than a complete revision.

Looking at that breakdown, I suppose the most realistic means of giving Armor HP of some kind would be to just to make it so that armor needs to be repaired regularly or it loses efficiency. Maybe give players the option to use a reaction to temporarily increase their AC at the cost of instantly dealing serious damage to their armor, sort of representing using their armor recklessly.

That was quite an essay, Cyber. I'll read it properly later but from an initial scan I'm detecting some strong (and blunt) opinions. I'm not claiming to want ultra-realism, just want to try something I think will work better than current AC. Also hadn't considered things like bag of holding, but that's why I've posted here. Helpful folk like you can bring these things up for me 

Ok, have now read your full reply, Cyber.

I disagree. The current AC system for me doesn't cut the mustard and I'm seeking something that makes it "feel" better. I understand how it works and why it is the way it is. I've been holding off major revisions for a while with the thought the current system is "good enough", but I'm trying something else now.

Your arguments about damage negation are sensible, but you also point out the complexity of a very realistic system and what a pain in the bum it would be. I'm trying something to approximate the feel of an active defence combined with armour. As I said initially I haven't play tested this yet and may throw out the entire system.

As for carrying capacity (and considering that i don't want ultra realism), carrying a full set of armour in your pack doesn't seem right to me. In my mind you'd probably have enough materials to repair what you have (assuming a mundane pack) to repair rather than replace. Of course for magical items this will change. A bag of holding means you may be able to carry a full set of plate armor if it can fit in the bag. I'm fine with that too on reflection.

I guess I'm trying to say... chill. This is to air an idea and get feedback. I appreciate the time you took to give your opinion though I will say you could dial back on your tone a bit, it came across quite stand-offish.

Quote from Cyb3rM1nd >>

 

How much realism do you want? Because plate armour (which is already full plate by descripion, it's just not called such as an item name, hence "half-plate" and "plate") is very heavy and uncomfortable and in real life people spend only a couple of hours a day at most in full plate armour. It is only worn when entering a battle (going off to war) and for the rest of the time they would wearing lighter armour. Going too real would there mean having character constant taking things on and off and making Con checks to avoid Exhaustion levels for wearing it too much. And then the weight has to be factoring into running speeds, walking, and helmet designs have to consider visibility (full protection is great but that means your visibility is greatly reduced as most of your field of vision is obscured by metal - you basically lose all peripheral vision, this is why many helmets only cover half the face or only the sides/top/back).

I agree with you 100% but I think you over estimate how much plate armor encumbers you.  Real, fitted plate armor really doesn't effect you near as much as you think.  I would suggest you want this video from a youtuber named shad Here.  Visibility (in terms of seeing) yes, sure that has always been a problem in one way or another throughout history.  Most all helmets are a balance between offering visibility and protection.  So it's not just plate armor that has this problem, it depends on the design of the helmet even if your wearing leather armor.  Yes people didn't run around in full plate all the time, but that didn't mean you couldn't necessarily, it just wasn't super comfortable.  I always pictured it like how our soldiers wear there ballistic armor today.  If you in the field were you might encounter the enemy, yea, your in armor, maybe for hours, because not dying is really perforable.  If full plate armor restricted you like you suggest, no one would wear it, ever.  Also I would think that in a world with monsters around every turn,  it would be way more common than it was historically to wear armor more.  We are adding a major force in the world, it will have an effect.  There is the point that full plate was usually deployed on horse back but, even on foot it's not that big of a deal.  You could march in plate armor and not die of exhaustion.  It's approximately equivalent in weight to a combat load in a ruck sack, I've marched pretty far with a full ruck sack.  Others have marched way further, you can survive it.  It would limit how much else you could carry, I could see that for sure, but you could walk and not keel over in 60 ft dead.  If you fit and used to it, your be fine.

Borrowed material from HERO games:

Hit Location Table - roll 3d6 after a hit is scored. Obviously in this case AC has to be calculated in a different manner.

                                        Damage x                Called Shot

3-5     Head                          x2                           -8

6        Hands                       x1/2                        -6

7-8     Arms                        x1/2                        -5

9      Shoulders                  x1                           -5

10-11   Chest                    x1                           -3

12    Stomach                  x1                           -7

13    Vitals                        x2                           -8

14     Thighs                     x1                           -4

15-16    Legs                    x1/2                        -6

17-18    Feet                     x1/2                        -8

So whether you have full chest and arm protection might matter if the guy hits you in the leg instead. The target might have different types of armor on different locations (or none) so Damage Reduction from those differing types will come into play.

I just see all these rules as being very complicated.   If I feel I need / want to have software to keep track of hit / damage depending on weapon type, damage type, armor type, armor state, roll value, etc., I'd just make it a video game.  I don't see it working well at the table unless you make support software that does all the number crunching then your just making a video game you play as a role playing game when your not in combat.  I think lots of people hated THAC0 that tried to do a similar thing, it was slow and hard to understand.  I guess you could just build the software to do that, memory serves 4th edition wanted to do that basically (the website I don't think ever launched).  As some point your not just reinventing the wheel, your making the most complicated wheel in history.  I don't know why you would do that.

Quote from Shadow_Guard >>

I just see all these rules as being very complicated.  ....  I don't know why you would do that.

 Which is why I consistently recommend Fantasy HERO for those that want more crunch in their game.

It isn't any more complicated than D&D, it just has more detail.

I would like to point out that this gives the 1st-level Forge / War domain cleric (with chain mail armor)  ONE HUNDRED AND SIXTY extra Hit Points. This is ridiculously overpowered.

However, I think the current system is a bit oversimplified. I am working on a system with two separate AC's, one based on DEX and one based on armor. In the current system, for example, the Monk has a high AC because they can more easily dodge attacks, and the Fire Giant has a high attack bonus because it is extremely strong. Realistically, the Giant would rarely hit the monk, but in DND, it hits about 65% of the time.

Personally I'd like armor to have some kind of exhaustion mechanics.  Have a reason to take it off and not wear it all the time in combat.  The biggest hindrance to heavy armor in real life is exhaustion.

Main issue I'm having with coming up with that is figuring out what to do with classes like barbarian and monk that have that unarmored AC bonus, since neither would suffer the mechanics faults...  Like, yeah plate is awesome, and it should be, and no, it should not be limiting mobility much.  However, it is also exhausting, and any prolonged battle will see even the most badass warrior exhausted and barely able to stand, let alone swing his sword.

I think armor should be badass, the heavier the better, and other abilities shouldn't replicate it as well as wearing armor, but it should come with serious drawbacks, especially if dumb enough to try wearing it everywhere and do prolonged strenuous activities within it.

Also I think shields should offer a bonus that changes based upon the level of armor you are wearing, the less armor the more bonus, and the more armor the less of a bonus as it's doing, as the less actual protection it's offering compared to the armor you are wearing.  Something like, +2 with heavy, +3 with medium, +4 with light, +5 with none.  This also gives more room for varying shield types than straight +2 bonus does.  Like a buckler might be +0 with heavy, +1 with medium, +1 with light, and +2 with none, but allow you to use that hand for various activities or something, and come with light armor proficiency.  Just musings...

Quote from VondalIronblood >>

However, I think the current system is a bit oversimplified. I am working on a system with two separate AC's, one based on DEX and one based on armor. In the current system, for example, the Monk has a high AC because they can more easily dodge attacks, and the Fire Giant has a high attack bonus because it is extremely strong. Realistically, the Giant would rarely hit the monk, but in DND, it hits about 65% of the time.

Quote from Ardenwolf >>

Personally I'd like armor to have some kind of exhaustion mechanics.  Have a reason to take it off and not wear it all the time in combat.  The biggest hindrance to heavy armor in real life is exhaustion.

Also I think shields should offer a bonus that changes based upon the level of armor you are wearing, the less armor the more bonus, and the more armor the less of a bonus as it's doing, as the less actual protection it's offering compared to the armor you are wearing.  Something like, +2 with heavy, +3 with medium, +4 with light, +5 with none.  This also gives more room for varying shield types than straight +2 bonus does.  Like a buckler might be +0 with heavy, +1 with medium, +1 with light, and +2 with none, but allow you to use that hand for various activities or something, and come with light armor proficiency.  Just musings...

The shield rules makes tons of sense.  The less armor you have on is directly proportional to how useful the shield actually is to defending you from harm.

When you're wearing full plate, a shield barely does anything your armor isn't already doing, but if you have no armor, the shield has increased your chances of survival significantly over no shield.  The simple act of having a shield can increase an unarmored persons chance of survival significantly, while in all honesty a person in full plate is better off barely increases their chance of survival significantly at all.  Realistically anyone wielding a two handed weapon with no armor into battle is insane, but a person in full plate can do so relatively safely, meanwhile a person with no armor can increase their survival chances significantly, even while traveling long distances, and cheaply, just by using a shield.

The rules should very much promote lesser armored characters using a shield, and armored characters being more willing to go without.  A flat plus 2 bonus does not properly represent how much more useful a shield is the less armored you are, over the more armored you are.

Hold on. Something like +5 AC unarmoured seems like it might be very abused by say... barbarians. Who would at first level, if they had a shield, be able to attain an Armour Class of about 20. This is assuming you have a +3 in Con and +2 in Dex. At higher levels, lets say... fourth level when you get an ability score improvement, this would become about 22 AC. I understand your reasoning for it, but the whole purpose of your shield is that it is moved and prevents people from hitting your armour, or the places not protected by your armour. Getting hit on your armour is much worse than taking a hit to your shield.

Obviously such drastic rule changes would have to affect barbarians and such.  In fact the barbarian unarmored AC would probably no longer work with a shield as it would be unnecessary at this point.  Like I said, barbarians screw up any armor rule changes.