I am looking for OSR rules that use armour to soak damage rather than make harder to hit - anyone have those?
This is a perennial and interesting question, so rather than clog up the comments, I'll put my thoughts here. It's not my intention to discourage pursuits along this line, and I've included a few different ideas. Hope this helps in some way.
The easiest way to turn armor into a more ablative system probably involves incorporating something like DR (Damage Resistance from the 3e era). Typically, one would assign some DR values to each traditional armor type, but probably still preserve the Shield's AC bonus (as most of the defense provided does still seem to fit as purely deflection).
Just in case, here's what DR is: Any damage inflicted by an opponent is simply reduced by the amount of DR that the armor has, possibly reducing it to 0. So you make a little table (these values are only examples, and are problematic for reasons outlined later).
I still love the old “Shields Will Be Splintered Rule” for homing in on the Shield “Soaking” damage as part of player choice (here's how it works: If a Character would take damage, they instead can sacrifice their shield and take no damage, the Shield is rendered useless). You could do this with all armor if you wanted, and have it all without any fiddly subsystems, but then combat turns into strip poker as character armor just falls off with each hit :).
Traditional AC just becomes the base for the system your using (potentially modified by Dexterity, or Shield, depending on the rules system you're going with). “Hits” become more frequent (since there is a “lower” effective AC all around) and subtracting damage for every hit makes inflicted damage lessen. So a “to-Hit” roll that doesn't meet AC is a “Miss” while damage is reduced for each successful “hit” meaning that a particular hit was “Soaked” in this case.
A good question to ask at this stage (before we dive in any further) is: What is your reasoning for wanting this change? I know a lot of people are dissatisfied with the Abstract Nature of D&D combat, but it's useful to think about what the perceived problem you're trying to fix actually is.
The most common desire is to try and graft on more simulation and realism onto the existing abstract combat system, and on the surface, ablative armor rules seem to stress that a hit "connects" instead of “wearing armor makes the character harder to hit.” But it's important to re-contextualize the line of thinking here.
Remember, a “hit” doesn't necessarily mean “connecting blow.” All a “hit” really means in game terms is the opportunity to inflict hit point damage. And don't even get me started on the abstract nature of hit points :).
Given the longish duration of D&D combat rounds, a “to-Hit roll” is pretty difficult to justify as a single swing. It's probably more akin to a series of strikes, feints, parries, etc. All looking for that perfect opportunity to “deal damage.” The “to-Hit roll” just tells you if that “perfect opportunity” came up due to a combination of luck, skill, the environment, and so forth. It doesn't even have to come from the weapon, it could be a knee to the groin, a bash with a shield, anything that impairs the opponents ability to fight for another round (in the case of eliminating that final and crucial hit point).
If the problem you are trying to address is the general perceived "whiffiness" of roll, miss, roll, miss, roll, miss, or if you need a subsystem that can synergize nicely with weapon breakage/armor upkeep rules, then ablative armor can still seem like the route to solve this. But again, it isn't necessarily the best route to go (it does add complexity for example, and has it's own issues that I try to outline below).
D&D's abstract combat really is a “feature” and not necessarily a “bug.” DM Narration can accomplish the same thing as DR with no additional systems:
Gragach the Clumsy is a Goblin, and rolls a 10, which is not enough to “hit” Haglef the Mantled, who is wearing Platemail. It's very easy to categorize this as a “miss” which is a convenient antonymn for the word “hit,” but not necessarily what happened here. The narration of combat can fix this.
So you narrate "The Goblin's spear point glances off Haglef's breastplate" (see it's technically not truly ablative protection here anyway, it's not "soaking damage" it is actually making the character "harder to hit" in the sense that no HP damage is inflicted, you could just as easily say “The Goblin's crude bronze spearhead just isn't hard enough to penetrate the solid Steel of Haglef's Breastplate” for a more ablative interpretation, but the end result is the same) instead of a series of variations on "The Goblin Misses. Next."
Here's another example (it's easier to apply to melee exchanges, as Ranged Weapons have a built in “You Missed.” excuse that doesn't strain most players' and DM's suspension of disbelief too much. Trebego the Yellow stabs an ensorcelled Haglef (still clad in Plate with a Dagger). He hits, and does 4 damage. Under the abstract system, we just narrate and figure out what happened “Trebego's blade squirms it's way between the plates that gird Haglef's shoulder, managing to pierce flesh and glancing off the bone.” There is nothing intrinsic about Haglef's armor that protects him from the possibility of this occurring. Trebego just got lucky, or really knows how to stick it to Fighters from his underhanded training.
Ablative Armor's intention is generally to create combat situations where there are a lot of “hits” but little “damage.” This is already built into the system as it stands (it doesn't matter how they don't deal damage really), if you just use a looser reading of the definition of a “hit.”
We still try though. We want a sword slashing a Chainmail Gambeson to not manage to penetrate the links (it's the whole point of that kind of armor after all!). So we keep coming back to Ablative Armor to solve this problem, but it in itself creates it's own subset of new problems.
If you decide to go with Ablative Armor, it might be a good idea to make any DR values somewhat variable, either through the application of randomness (assign DR a die value instead of a static number) or perhaps incorporate some simple rules for certain weapons being more/less effective against different armor types (A Piercing Pick is deadly against Chainmail, A hefty Maul is just more effective at battering someone in Plate Armor, than trying to slash at them etc). It might also help to think long and hard about hit location, because most armor types are mixtures of different things.
Variable DR like this is useful in the situation that the enemy is simply not able to inflict damage on a foe due to their attacks. For example: If you assign Plate Armor a DR of 7, and a Goblin has a Spear that only does 1d6, the possibility of that Goblin inflicting damage on the a character wearing Plate Armor is nonexistent, no matter how well the Goblin rolls. The side effect though is this adds another roll to each “successful hit” and more rolls to arbitrate combat tend to slow down play ever so slightly (which adds up with each successful “hit”).
You're also going to have to work through most existing monsters in most systems and assign them an arbitrary DR value and a new AC as well. Is their AC high because they're quick and difficult to strike or extremely skilled or lucky? Or is their AC high because they can be hit easily, but are harder to damage? It's a lot of work, but some find this kind of tinkering fun :).
The final issue is one of de-protagonization, that is: it just isn't always fun for Players. No one likes to see their critical hit and high damage roll (“I hit him! Finally!”) reduced to 0 just because the monster has high DR. It makes an otherwise wonderful roll meaningless, and snatches this sense of victory out of the player's head and hands far too often. It chips away a the FUN. Combat becomes a “mother-may-I” situation, and in some cases combat drags on because Players just can't beat the high DR, either due to bad Damage Rolls or insufficient weapons.
Hope this helps a little bit with your quest. I wrangled with this “problem” for ages, but no solution I found or homebrewed really addressed the issue without adding more rolls and complexity. I think re-enforcing the abstract nature of combat (ie: it's not trying to be “real,” there are no “hits” and “misses” in the normal sense, just times when damage is dealt, and times when it is not) and achieving the desired sense of verisimilitude through narration eventually got me to stop worrying and learn to love it as is :). Doesn't mean I'll ever stop tinkering though :).