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Full steel plate armour is seen as the most effective type of armour from the Medieval period, viewed by some as the pinical of medieval/early-renaissance armour. It’s rounded surfaces and layers of padding and chain mail underneath the plates reduced the wearer’s chance of injury drastically. It also allows for a high level of mobility and articulation without resticting the wearer too much.

In our own history, we have developed armour such as brigandine, lamellar, a coat of plates, segmented plates (like the lorica segmentata), gambeson, chain mail, we’ve even made armour out of wood and bone. However, none of these are quite as protective and mobile as a suit of full steel plate armour. Indeed, of these armours mentioned, some are more effective than others, almost in a hierachy.

With this in mind, in a world where multiple types of armour co-exist in the same place at the same time, why would people actively choose not to wear the most effective armour, such as full steel plate, if it was available?

For this question, assume that monetary cost is not a factor. The reason for this is that it is fairly obvious that people would not buy better armour if they could not afford it, so cost of manufacture and sale should not be factored into answers.

Also assume that the availability of resources is not a factor. Again, it is fairly obvious that if you lacked the resources to produce more effective armour, you would have to produce something less effective.

The best answers should cover why less effective armour would be chosen on both an individual level (such as an adventurer, mercenary or other form of lone traveller) and a militaristic level (such as an army or other large group of organised fighters).

Magic and monsters may factor into your decisions but this is not required for a good answer.

Comfort, individually

Plate mail is heavy and hot, which is why you don't want to wear it all the time. Instead, you'd strip down your armor while you weren't seriously expecting combat, leaving only the gambeson (cloth padding) or gambeson and chain mail. Much more comfortable, but now you're vulnerable.

Logistics, on an organization level

Let's say you've got an army. One of the important bits about your typical medieval army is how far they can march in a day for a given level of exhaustion. So your army isn't going to be wearing plate mail 24/7. If they're not carrying their armor on their persons, it has to be brought on carts. This means more carts, more draft animals to pull the carts, and more resources for those. It adds up. You also need more blacksmiths to help maintain the armor.

Training

First off, many medieval armies were levies/drafts, were they basically grab one out of every N men in an area. These are not professional soldiers and they would not know how to fight in plate mail, both because of weight and because of the vision impairment. It would take longer to train them in fighting in plate mail.

Don and doff

Plate mail is not easy to get into and out of. It usually required someone to help, and if your entire army is wearing plate mail, along with the gloves (that need to be secured) then you'll have problems.

This is also important because you can't always predict when combat will happen long enough in advance to get into armor. If you're sleeping, outside your armor (see comfort) then it will take you a bit to get into armor. In which time someone will probably stab and/or shoot you.

Trust

In modern times, we have these things called security clearances, because we don't trust everyone. You can't fly a fighter jet without passing a background check, nor will you be hired as an information security officer.

The same principle applies here: as a government, I don't want to give every Tom, Dick, and Harry armor that makes them very hard to put down. I don't trust them. That's reserved for my most loyal troops and bodyguards. With an equipment advantage, my palace guards and bodyguards can compensate for numerical or informational (i.e. ambush) disadvantages.

One answer is that the lesser armour type is lighter, and allows for more rapid troop movements and deployments. If said soldiers have to cross rivers or hike up mountains, lighter armour may be a necessity. Another answer is that full steel plate armor will become a lot hotter in warmer climates. This could be problematic an a desert environment. Another answer is that steel can rust. This might be an issue in a humid environment like a rain forrest.

Say we have a knight in full steel armor, fighting a guy in practically nothing. Certainly, the armor should be useful. However, fire and blunt weapons will both be worse for the knight, because dodging is presumably far harder. A skilled fighter also may opt for lighter armor to outmaneuver the knight, such as using a sword (armor blocks slash attacks), and stabbing joints with the faster and more accurate movement the light/no armor can allow.

Perhaps armor stands out. A knight in full metal must be a clear target to raiders than what appears to be a poor "unarmed" beggar/peasant.

Armor also is very annoying in terms of heat (protective fencing gear is bad enough with sweat, I'd hate to imagine 60 pounds of armor).

Traveling also would probably be bad in armor. I highly doubt armor is comfortable for normal situations, nor would I believe knights would always wear armor at all times (sounds like paranoia to me).

Another thing I'd like to note: Spartans would go into battle mostly naked with a shield, breastplate, and helmet. I don't have any idea of the accuracy to this statement, but this getup will already weigh more than 50 pounds in metal, explaining why minimal armor would be favorable.

As a final side note, there is something called the "murder stroke", slamming the hilt of a sword into a helmeted person's skull. Regardless of armor, the impact should still kill. If I face a swordsman of considerable skill, I personally would take the no-armor route and sprint full speed away from them, if I could.

Armor was chosen based on how good it was for its actual purpose. This is the same way modern armies use to choose their equipment.

As an example the rifles used in the world wars had more powerful cartridges and were expected to be accurate to greater distances than modern assault rifles. And you could get them in automatic versions even. But starting in the 30s (I think) more and more armies realized that most soldiers never or extremely rarely shoot beyond 150 to 200 meters. Or need the extra penetration that a full powered rifle cartridge gives.

This means that the rifles are not really better than an assault rifle in actual use. Equipping your common soldiers with those will give you no benefit over using cheaper and lighter assault rifles shooting weaker and cheaper ammunition.

Same logic applies with arming medieval armies with armor. A plate mail is heavy and expensive armor designed for repeatedly getting hit by heavy weapons without taking lethal damage. And it works very well.

But most medieval soldiers do not actually spend that much time getting repeatedly hit by heavy weapons just like most modern soldiers do not spend much time taking long range shots.

Ranged and light units are supposed to avoid taking heavy hits altogether. Equipping them with heavy armor would just make them slower and encourage them to do things they really should not be doing.

Even normal front line melee units where soldiers do get hit with heavy weapons do not really need soldiers to take repeated heavy hits. There are other soldiers behind him who can take the next hit. So such units are equipped with armor that keeps them from taking lethal or crippling injury and keeps them fit enough to fall back.

So who does plate mail make sense for?

Heavy cavalry and heavy infantry. These units are expected to smash thru enemy formations where falling back would leave you alone surrounded by enemies. Or to take a charge without needing to fall back. And it is much easier for the unit not to fall back when individual soldiers do not need to.

Officers in other units usually can use extra protection as having them injured makes other soldiers less effective. You can also have specific elite soldiers with better armor in a melee unit. Typically they would also act as NCOs. This is because they have better abilities or even weapons as other soldiers so having them injured actually makes the unit weaker.

Adventurers in modern RPGs or fantasy are an important group as well. They need all the protection they can carry. Since they are most familiar group of people to use medieval armor to modern people, this kind of skews how most people see armor.