(Original Source) (Refinements)

GURPS handles flexible armors like mail as follows: the wearer suffers one point of “blunt trauma” damage per full 10 points of damage (5 points if crushing) that doesn't penetrate DR. In addition, some of these armors have a split DR score that offers full protection vs cutting and piercing damage, and reduced protection vs crushing and impaling damage.

That makes for a workable but quite limited model. Flexibility becomes a simple binary switch (though the flexible armors with split DR could be seen as a third “very flexible” configuration). It doesn't offer a good way to model armor that may completely stop an edge from penetrating yet lets a huge wallop whack the wearer. But if DR is less than 10 (or 5 vs crushing), the GURPS “blunt trauma” effect can't come into play anyway, making the “flexible” designation meaningless for almost all low-tech armors.

Oddities pop up, too. Even when DR is substantial enough to allow for the “blunt trauma” effect, a damage roll that goes just a little over DR and penetrates the armor can inflict less injury than a roll that's fully blocked by flexible DR. On the lower end of the DR scale, armor with the split DR characteristic can infamously let an attacker deal more damage to the wearer by striking with the flat of his edged weapon, using this makeshift club to force the armor's lower DR vs crushing.

There's no huge problem with all of the above; it plays fine if you don't look for the bugs. But for the interested, below is an alternative way to handle blunt trauma from armor deformation. It's more realistic and flexible in what it models and avoids the above oddities, yet is easy and GURPS-like as well.

In addition to DR that completely negates the impact of a blow, armor can offer a lesser form of protection that allows the impact of a blow through, but prevents its edge or point from penetrating the armor (and thus flesh). This property is measured in points like DR, and is referred to as Edge Protection (EP).

Subtract DR normally from basic hits. These hits are absorbed by armor. From there, apply three simple rules for EP:

1) If the weapon is crushing, there's no additional protection from EP. We're done.

2) If the weapon is edged (cutting, impaling, or piercing), any basic hits left over after DR, and up to EP, deal crushing damage. The weapon deforms the armor and hurts the wearer, but EP doesn't let the edge through.

3) Basic hits from an edged weapon that exceed DR + EP do penetrate the armor. Those basic hits affect flesh in their usual manner.

These rules replace the standard rules for flexible armor and blunt trauma. There's no special “flexible armor” designation under these rules; the combination of DR and EP models an armor's unique resistance to both deformation and penetration.

Example: You're wearing chain mail with cloth underneath, for DR 2, EP 5. You're attacked with an axe.

• Your DR absorbs a blow of 2 or less damage. You're not hurt.
• A 4-point blow, after DR, leaves 2 basic hits. But those hits don't penetrate EP 5; the mail is not cut. You take 2 points of crushing damage.
• A 13-point blow, after DR, leaves 11 basic hits. EP limits the next 5 points to crushing damage; the mail isn't cut yet. But the remaining 6 points break through the mail and cut you normally, for 9 damage after the cutting multiplier. You take 5 crushing + 9 cutting = total 14 points of damage.

If the weapon were a club instead of an axe, you'd just subtract 2 from damage for DR, and be done with it. EP would offer no special protection.

Normal armor divisors affect both DR and EP equally.

The benefit of EP to the armor wearer is obvious: edges hurt! A claymore swat to your mail-covered midsection may still knock the wind out of you, but that's a heck of a lot better than getting chopped into haggis. The effect is particularly important where sensitive parts are concerned, most notably impaling and piercing attacks to the vitals. A little EP there can save you a lot of injury!

EP's clear distinction between armor penetration and non-penetration can be useful to GMs, such as in determining whether a venomed weapon delivers its poison, or in implementing rules for accumulated armor damage and deterioration.

EP nicely simulates the ability of armor – even low-tech armor with modest DR – to keep its wearer in possession of his limbs, yet still get rudely battered by those axes and swords. And as shown below, EP allows for much greater customization of armor than DR alone does – up to and including magical armor that's “impenetrable”, yet leaves the wearer anything but invulnerable.

EP protects less than full DR. As natural armor, treat EP as a -60% limitation on the cost of DR, which becomes 2 cp per point of EP.

DR represents resistance to any deformity at all; EP represents resistance to penetration but not deformity. The more flexible the armor, the more its protection will take the form of EP instead of DR. Tough, hard material, like a bronze chest plate, has high DR and low EP. Tough, flexible material, like mail armor, has low DR and high EP.

To rework existing stats, below are rules of thumb. Round values to the nearest integer (rounding up on 0.5). Keep in mind that a point of EP offers less protection than a point of full DR; reworked armor's total DR+EP should at least equal listed GURPS DR, to maintain overall level of protection.

However, any armor can vary considerably from the suggestions below, especially in EP.

These would be the standard armors given a “flexible” asterisk and a split DR score (higher score vs piercing and cutting, lower score vs others).

Set DR vs all attacks to the lower DR score. Then set EP equal to the higher DR plus half the lower – but halve EP vs impaling.

Example: Mail has DR 4/2. Use DR 2 vs all attacks, plus EP 5 (but EP 3 vs impaling).

These are armors with the “flexible” asterisk and a single DR score, like cloth and light leather.

Set DR to listed DR minus a quarter. Set EP equal to listed DR.

These are armors without the “flexible” asterisk, like scale and plate.

Set DR to listed DR minus a fifth. Set EP to a third of listed DR.

You can vary EP a lot more than DR, without making armor too powerful or too weak. Take mail armor: no matter how high-tech and tough the metal, it's a bit silly to imagine chain with a DR that repels heavy axes with no injury to the wearer. Yet it's easy to imagine quality chain failing to split, even as the wearer gets battered by the blows. In practice, cheap materials might subtract 1 or none from DR, but halve EP; high-quality materials might add 1 or none to DR, but offer double or better EP.

A great example of very high-quality armor material is the above-mentioned Dwarven mithril mail, which saved Frodo from what would have been a fatal stab in the vitals. The mail's extremely high EP allowed a big crushing blow to go through, but not the spear's point itself.

An Impenetrable Coat of Armor bequeathed by the gods might have no more DR than normal armor, but infinite EP. You can be bludgeoned to death wearing it – fairly easily so if it's flexible with low DR – but it won't be penetrated.

Make that a thin Shirt of Impenetrable Mono-silk, and you can have zero-DR armor that's of no value in a fist fight, yet will stop a sword from running you through.

On the other end of things, a theoretical very hard, strong, but brittle armor, perhaps ceramic, would have high DR and no EP. Damage above DR simply breaks through the armor.

Below are standard GURPS armor types, reworked with new DR and EP stats following the rules of thumb above.

Tweak those numbers as you like. Scale armor, for example, happens to get the worse of rounding on both DR and EP; boosting one of those by a point might be fair.

Also, it's uncertain how plate inserts for modern flexible armor should be handled. The added plates presumably make the armor anything but flexible where impact is concerned; perhaps it's best to add the plate DR to the lower of the original split DR score, and call that a single overall, non-flexible DR score with no EP.

The above covers everything you need to play with EP. Below are options for the interested.

EP can vary with attack type. The guidelines above already suggest halving EP vs impaling damage, for those flexible armors that GURPS designates as especially vulnerable to impaling. You may think of other worthy tweaks. As an extreme example, your Impenetrable Magic Armor's infinite EP may drop to a low value (even zero!) when faced with the All-Penetrating Magic Sword.

Likewise, detail-oriented GMs will find EP a good way to model weapon sharpness. A blunt edge might double EP, and a very sharp edge halve it. This nicely modifies weapons' armor-piercing ability without mucking up overall base damage or overplaying the role of DR.

EP is definitely useful against large and huge piercing attacks, protecting the target from these weapons' damage bonuses. EP will also save you from extra hurt vs small and regular piercing attacks to the vitals.

But EP leaves you no better off when regular piercing attacks hit non-vital targets – and it'll actually hurt you more when small piercing attacks strike non-vitals! (EP turns those hits into regular crushing damage for a x1 multiplier, replacing the usual x0.5 multiplier for small piercing attacks.)

That's not a flaw in the EP rules. GURPS rules that, compared to a blunt attack of a given force, a flesh-penetrating attack of the same force (i.e., same basic hits) can inflict the same damage (as with regular piercing) or even less damage (as with small piercing) than the non-penetrating equivalent. Whether this reasoning rings true or not, the above EP “oddity” is only following it to the letter.

It's up to you whether that's just as things should be, or a problem to be addressed. If the latter, you could rule that EP offers the lower of a crushing damage multiplier (x1) or the attack's innate multiplier (x0.5 for small piercing vs non-vitals). If nothing else, that keeps EP from making the wearer worse off when small calibers attack.

Using EP requires little extra work. But if you dislike even that, here's a simpler version:

Subtract DR from basic hits, as always.

If remaining damage is less than or equal to EP, then damage is crushing (per rules above).

But if remaining damage exceeds EP, treat all of it as penetrating, with its normal damage multiplier; don't bother adding the EP-blocked crushing damage to the EP-penetrating edged damage.

In other words, edged attacks that penetrate DR don't deliver a realistic combination of crushing and edged damage; rather, once DR is exceeded, a cutting blow is all crushing or all cutting, and an impaling blow is all crushing or all impaling, depending on whether remaining basic hits also exceed EP.

Example: You have mail with DR 2, EP 5.

DR absorbs an axe blow of 2 or less damage; you're not hurt.

A 4-point blow, after DR, leaves 2 basic hits, which don't penetrate EP 5 so the mail is not cut. Take 2 points of crushing damage.

A 13-point blow, after DR, leaves 11 basic hits. Those 11 points exceed EP; the mail is penetrated and EP has no effect at all. Treat as a normal 11-point cutting attack, inflicting 16 damage after the x1.5 multiplier.

Want the ultimate in armor realism? Use the full EP rules, with one addition:

For any and all basic hits absorbed by DR, apply 1/5 (round down) as blunt trauma damage, in addition to any damage suffered normally (i.e., by basic hits exceeding DR).

That's for any armor, flexible or rigid or in-between. When armor “absorbs” a hit, it's only spreading it out over a large area; a great big wallop, even spread over a large area, will still hurt! The net effect is that DR never completely dissipates basic hits; it only lessens them a lot.

Example: You wear a heavy steel corselet with DR 6 (and EP 2, though that's not relevant to this example).

Any time you're hit for 5 or more points of damage, from any impact, apply all DR and EP rules normally… but when you're finished, add 1 more point of blunt trauma, for the initial impact of the corselet slamming hard against you.

A thicker corselet with DR 12 offers even better protection – though 5 to 9 basic hits will still wallop you for 1 point of blunt trauma damage (even though DR was not exceeded), and 10 or more basic hits will inflict 2 points of blunt trauma damage (in addition to any hits that exceed DR).

With that said and done, there's yet one more variation possible: use both the above simplified and complexified rules. Yes, they work together fine, resulting in something similar to the default EP rules in complexity, just a bit different in results.

How about revised Toughness - protection (generally from muscular or otherwise stout flesh) that lessens crushing blows a lot, slashes less so, or a stab to the vitals very little. It's in some ways the opposite of EP. (Game terms: Toughness only subtracts from the basic hits of an impact, not the additional damage from edges, penetration and hit location.) Sounds like it'd be hard to juggle DR, EP, and this new Toughness, no?

Turns out it's really very simple: A point of EP turns a point of edged damage into crushing, and a point of Toughness absorbs that point of crushing damage. Voila, a point of Toughness and a point of EP have the same effect as a point of DR. Just treat the combination as DR.

Example: You have Toughness 3, and put on DR 2, EP 4 armor. Toughness and EP combine to become DR 3 with EP 1 left over. Treat the total as DR 5, EP 1.

If you instead wear DR 1, EP 1 armor, Toughness and EP combine to become DR 1 with Toughness 2 left over. Add this DR to the armor's DR: net DR 2, Toughness 2.

Let Toughness be a -40% limitation on the cost of DR, which makes it 3 CP per point. Thus, a point of Toughness and a point of EP add up to a point of DR in both cost and effect.

In practice, DR and EP alone are probably as much detail as many players will want. But I'll note that with EP to stop edges from penetrating, Toughness to represent absorption of blunt force, and DR to combine the effects of both, you can model just about any armor effect with great realism.

In this earlier post, I mentioned that I'd been answering email questions about my Edge Protection (EP) rules for armor in GURPS. For those with an interest in EP, here's a paraphrasing of my correspondent's questions (in quotes) and my replies:

In the table converting classic DR to DR+EP, mail still has reduced efficiency vs. impaling. This is surely a 3rd ed leftover, no? After re-checking, yes, it is a 3e holdover. In 4e, mail has no special vulnerability vs impaling (or piercing, for that matter). I should change the the article's stats for mail to match:

Mail: GURPS 4/2*; new DR 2; EP 5; Notes: DR 2, EP 3 vs impaling

should be

Mail: GURPS 4/2*; new DR 2; EP 5; Notes: none

That sets DR low vs any damage type, with EP to keep the edges out. It fixes the problem of GURPS setting DR lower vs crushing than vs other damage types; as the EP article points out, that odd setup means you can hurt a mail-clad target more by making your sword blunt!

(Incidentally, an explanation I'd heard offered for mail's special vulnerability to impaling in 3e is that a weapon's point can slip between and break the mail rings. Apparently, 4e doesn't present this as a special vulnerability. I leave the fact of the matter to the armor specialists out there; I'm happy with whichever they claim is realistic. What's important is simply that DR not be set lower vs crushing than vs other damage types; that produces the above strange results.)

Digging deeper into simulation: What if one wanted to model GURPS mail more realistically, but without using EP rules? Maybe the answer lies in a better implementation of GURPS' flexible armor rules. Say mail had DR 4 vs all attacks, but flexible armor were defined as allowing 1 point of blunt trauma damage per full 2 points of damage absorbed by DR. Then a 4-point blow would be “stopped” by mail's DR 4, but still cause 2 points of blunt trauma damage, whether the weapon were a club, sword, or spear. That seems sensible to me, and it preserves the property of mail being “weak” vs crushing damage: it's “weak” vs crushing because the wearer took as much damage from the club as from the sword or spear, something that would not happen if the edged weapons actually broke the armor. I think that's much more realistic than current rules, under which the wearer mystically takes more damage from the club! Realistically, impaling should be a better penetrator. As an engineer I seem to remember that the more force you concentrate on the smaller area, the more likely you are to penetrate. So shouldn't all armour have less EP vs. impaling? Sure, I agree. I haven't done so, out of simplicity - and out of possible problems with combat balance - but some modest effect could be modeled, and shouldn't mess up combat balance.

Halving all EP vs impaling would be simple. It may be too drastic, though; after all, that's the effect you'd get from a super-weapon with an armor divisor of 2. So you could use -25%, -33%, whatever works.

But on that topic, by concentrating force into small areas, edges and points could (or should?) arguably decrease both DR and EP. In short, one could argue that all sharp weapons should carry some level of armor divisor, applicable to both DR and EP. I haven't done so, simply because GURPS doesn't generically reduce DR for edges. But it could be done.

For example, just making stuff up: Reduce DR and EP by 10% for cutting weapons, 20% for impaling. Further decrease each by 10% for Fine weapons, 20% for Very Fine. Further decrease mail DR and EP by 20% vs impaling, if following the 3e-style explanation about points breaking mail links.

Another idea is to split Crushing damage into two types. Call one “Blunt”, for fists, falling, flat side of sword, wooden club, stones. Call the other “Bludgeoning”, perhaps, for spiked and flaged maces and flails, the non-spiked end of a warhammer/poll axe or -hammer etc. Both types of Crushing jar you around, with the same Knockdown or KO effects, but while “Blunt” mostly bruises, or breaks bones in bad cases, “Bludgeoning” tends to break the skin, creating nasty wounds, almost like the hack of an axe (but not like the cut of a sword). Bludgeoning should be quite good at penetrating armour. Bludgeoning sounds like something between crushing and cutting, both in effect and in explanation (i.e., it concentrates force into a smaller area than standard Crushing, but not as small an area as Cutting).

I tried this long ago with HERO system. HERO uses very binary damage types: either Normal for fists, clubs, etc., or Killing for edges, points, etc. I wanted to allow for something in between, for exactly the types of weapons described above: maces with flanges, weapons with short spikes, and so on. (After all, these had to have been added to weapons for a good reason! Yet it's not just a matter of weapons, either; I was also thinking of the difference between hitting your head on the corner of a table, vs on the flat side.)

My “middle type” of damage didn't fit well with HERO, as the system actually its two damage types in different ways. In GURPS, there are a couple of ways to go. One “cheap” method is to just raise basic damage from Bludgeoning by 1, which has the effect of boosting both armor penetration and damage vs flesh by a small, flat amount. There are existing rules precedents, like the +1 basic damage for a shield spike (which still remains Crushing, “spike” notwithstanding). Think of it as a sort of Fine designation for Crushing weapons. In 4e, Fine doesn't add to blunt weapon damage, but why not, I say? A Fine crushing weapon could mean flanges, short spikes, or other “semi-sharp” projections. (Or better yet, let's separate this form consideration from overall quality rules; just name the configuration Flanged, which adds +1 basic hit, +X% to weapon cost (or +$Y for a shield spike), and can be added to a blunt weapon of any quality level. It's perfect for maces, flails, war clubs, etc.)

But that's one limited idea. Instead of that, it's also theoretically possible to expand the GURPS edge rules to incorporate flanges etc. in a more generic way. Set a continuum of edge effects allowing for any degree of “edge”. If Cutting has the effects x1.5 damage vs flesh and (if you like) -X% DR, -Y% EP, and if Impaling has the stats x2 damage and -A% DR, -B% EP, then Bludgeoning could have stats like x1.2 damage, -N% DR, -M% EP. And then there could be some level between Bludgeoning and Cutting (sharp flanges?). And some level between Cutting and Impaling (maybe that's where Cutting Fine should be, replacing the current Fine rules?). And so on. I think your message suggests such a thing.

There's a concern, though. The above is fine for just thinking about edge “sharpness” alone, but weapon shape is important too, especially for certain special effects. That is, Cutting is more than just an edge of certain sharpness; it usually also assumes a flat blade, allowing (for example) cutting through a limb. Whereas a thick mace, even with sharp flanges that approach Cutting in damage mod, couldn't zing through a thick object in the same way. Likewise, it's easy to imagine a cutting edge with stats in between Cutting and Impaling, maybe even so amazingly sharp it approaches the same x2 damage mod as impaling - but could it also receive Impaling's x3 damage mod vs vitals? That effect relies on a long, thin point going deep into vitals, which isn't what even a very sharp swung cutting blade could always be assumed to do.

So, there's more to consider than just edge-based damage mod, DR mod, and EP mod; at least some of the effect of GURPS' blunt/cutting/impaling distinction seems to reflect overall weapon shape, not just edge sharpness. Combine that with considerations of playability (a x1.2 damage multiplier? Bleah.), and I don't know how far anyone would want to go in getting weapon/armor interactions just right in GURPS. But for the interested, there are plenty of ideas to chase down.