A basic spring bar makes a very simple trap. A thin, flexible metal, wood or bone bar is embedded in a floor one spear-length from a door that opens away from the bar. The butt of a spear is placed against the bar so that the bar flexes back as the door is closed. The door now maintains tension in the spring; opening the door causes the tension to be released, and the spear is thrust forward. There may also be some form of support between the bar and the spear head, to help keep the spear horizontal and guide it as it is thrust forward; to eliminate this, a menacing spike can be welded to the bar instead of using a spear. The spear or spike targets the person directly in front of the door and can be automatically dodged if the character is not surprised.
This kind of trigger isn't immediately visible unless it's in a lit room; in that case, looking under the door will reveal a shadow cast by the bar, at the very least. If an adventurer can slide a long, thin tool under the door and swish it back and forth, the adventurer can feel an obstacle; however, the tool used must be longer than the spike or weapon (spear, javelin, bolt, dart) used in the trap.
A spring bracket is my made-up name for a smaller version of the same kind of trigger. Unlike the large spring bar trigger, the spring bracket is placed so that the door opens towards the trigger. The bracket is placed near one edge of the door with one tine against the door, under slight tension. As the door is opened, it flexes the tine more and passes it, causing the tine to suddenly release. The tine is usually connected by string or a wire to another trigger, such as a crossbow trigger; triggering the spring bracket yanks the wire and triggers the crossbow.
A spring bracket, like a spring bar, can be detected by sliding a thin object between the door and its jamb, lintel, or sill. However, because it is much smaller and right next to the door, a knife or dagger is usually long enough to reach it.
Although I've described these in terms of a door, the same principal can of course be applied to lids of containers, or the spring bar can be used with some kind of catch which is released by another trigger. It's just that the door traps described are the simplest to visualize. When used to trap a door, usually there are two or three doors heading toward the same direction; one door is safe and untrapped, the other two are trapped and lead nowhere. Those who set the traps know which doors to open and which should remain shut.
When a simple spring bar with a spike on it is used in a chest or any other container, there are more options. Consider this diagram of a chest with a lid that has a rim. If the bar is placed perpendicular to the lid, the spikes can be held in place by the rim and will stab towards the lip of the chest if it is opened only a crack (Arrow A.) If the bar is parallel to the lid, the spikes stab upwards (Arrow B.) This arrangement can be detected if someone opens the lid a crack and slides a knife blade in to swish around, but this method may be dangerous for Type A.
Most other detection techniques will have no effect on these kinds of chest traps, although if a lid looks like a flat slab, but actually has a hollowed-out area to create a rim for a Type A trigger, tapping the lid in several places will detect the hollow area. Normally, an adventurer wouldn't disarm this kind of trap, either, but instead would try to avoid the path of any such trap, for example by standing behind an obstacle, or lifting the lid slightly with a long object.
A spiral spring with a catch can be used to launch a needle or dart in any direction, including through holes in the lid, front or sides, or from a trap located underneath a chest that has legs. Darts and needles have a longer range than a spike that is part of a spring trigger.
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