The 'AC vs damage soak' argument was mentioned in another thread - and the mere mention of it made my head near-explode with urges to derail, so I have started this one.
Firstly - apologies if this was covered at great length in another thread - I could not find it (maybe in the forum's past life?).
Now - my opinion on this.
Damage Soak and AC are two mechanics that are different in a fundamental way - they are not doing the same thing. You can't compare them. I would classify AC as a mechanic in the same category as an active defence (like in GURPS - yes - in this respect
D&D is like GURPS).
Damage soak is reducing damage done by a flat number which does not scale with the particular attack - whereas AC is effectively reducing damage over time by a percentage. This means these two mechanics cause damage over time to scale in very different ways.
Extremely simplified example:AC:
> Hero has a basic to hit TN of 11 (50% chance to hit)
> Goblin's AC increases this to-hit target to 16 (25% chance to hit)
> sans other bonuses - Hero rolls a d20 and will hit 25% of the time.
> If a hit will do 20 damage - thats 5 damage per round on average.
DS:
> Attackers Basic hit TN is 11 (50% chance to hit)
> Goblin is wearing armour that will soak 10 damage
> If a hit does 20 damage - thats 5 damage per round on average. The same as AC.
If we up the damage to 30 -
AC:
> Basic TN 11 (50% chance)
> after goblin's AC the TN is 16 (25% chance)
> 30 damage, once every 4 rounds is 7.5 damage per round on average.
DS:
> TN 11 (50% chance to hit)
> Goblin's armour soaks 10 damage
> 20 damage (after soak of 10) - once every 2 rounds, is 10 damage on average.
And if we up the damage to 300 -
AC:
> Basic TN 11 (50% chance)
> after goblin's AC the TN is 16 (25% chance)
> 300 damage, once every 4 rounds is 75 damage per round
DS:
> TN 11 (50% chance to hit)
> Goblin's armour soaks 10 damage
> 290 damage (after soak) - once every 2 rounds, is 145 damage per round.
Big difference with scaling there.
Now you COULD just give a psuedo damage soak as a % and be done with it - this is mathematically identical but instead of hitting less - you 'soak' a % of the damage - but thats no fun
Instead, you sub out AC for an active defence roll -
AC:
> Basic TN 11 (50% chance)
> after goblin's AC the TN is 16 (25% chance)
> 300 damage, once every 4 rounds is 75 damage per round
Active defence:
> Basic TN to hit of 11 (50% chance)
> Goblin has a dodge roll of 11 - (50% chance)
> So Hero hits once every two rounds, and goblin dodges these hits 50% of the time, that's a damaging hit once every 4 rounds.... and 75 damage per round on average (with the exact same frequency as AC
)
The way these probabilties interact to produce the result is slightly different - and I would personally prefer the active defense roll as the second die roll causes your results revert to the mean twice as quickly (less random).
Then - if you want damage soak - add that too. So AC is not 'instead of' damage soak - it is 'instead of' active defense. You could add damage soak to a game that uses AC without issue (or use the 'psuedo-soak' metioned above - but that's really just mathsturbation).
Whew! Got it all out.
I feel better now
tl:dr -
AC is not comparable to damage soak - so if you dislike armour class, consider using an active defense roll instead.