Gravity knife
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A gravity knife is a knife which opens solely by the forces of inertia or gravity. Similar to a switchblade, they are made as side folding blades and OTF blades (out-the-front or telescoping). But instead of being spring driven, the knife is "flipped" out of the handle. Military gravity knives will lock open. Some English and civilian gravity knives do not mechanically lock open, instead containing a strong friction cam surface against the blade. Factory made gravity knives have various types of buttons, triggers or fulcrum levers, but are usually dual function to release the blade from both the open and closed positions.
The most recognized Gravity knife is the World War II Fallschirmjager-messer German military paratrooper model OTF that locked open and had a folding rigging spike [1] in case they had to cut themselves free from a parachute that had become tangled in a tree or other obstruction. This is a true gravity knife that requires the user to point the knife down to let gravity draw out the blade, it can be assisted with a fencing salute movement.
The military issued german gravity knife has evolved since the 1940's, with size and minor mechanical details. The bakelite handle trap door model, without spike is also produced in a civilian version with an all metal handle named the "silk ripper". (See OTF knife for silk ripper, gentleman's gravity knife and slider knife illustrations.) The civilian designed OTF gravity knives are of such small size they are not considered as weapons for military use, but are still considered to be restricted knives.
Some knives that will open by gravity, were not intended or advertised by the manufacturer as such. Knives that will lock open, but do not have any closed position lock device can be referred to as "False" Gravity blades. Knives such as a modified linerlock, will not lock a blade in both positions. Other false gravity knives or "flick knives" are folding stilettos made early in the 1960s. Immediately after switchblade prohibition in the U.S.A. these early folding stilettos did not have the pronounced blade heel (internal surface) in the pivot area allowing the longer 11 and 13 inch models to easily flick open. Modern (post 1965) folding stilettos have blade heels that are intentionally pointed and ratchet against the lockback mechanism preventing inertia opening. Other knives commonly mistaken for gravity knives include the automatic OTF knife [1], the switchblade [2] and the butterfly knife (or balisong),[3] the sliding knife and occasionally common folding knives (see penny knife).
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[edit] Mechanical design
The illustration at right, is of a second generation OTF German Fallschirmjager-messer. This model has a bakelite handle and side mounted trap-door dust cover. Next is a cross section of the trigger and sear hook, shown behind the blade tang, locking the blade out. The blade will have two pins, or pressed ball bearings that have a tight running fit in the groove within the handle track. These pins keep the blade aligned, and also being larger than the opening, prevent the blade from falling out of the handle. Some models have a blade notch at the point for the sear to engage the blade when closed. On this model, the sear hook spring pressure is enough to keep the blade in, resulting in a sleek blade design.
This illustration shows the internal parts of a 1960s Japan import folding knife. Collectors classify it as a "false gravity knife". Legal definitions say this as an illegal inertia knife. There is no spring. The knife will lock open but not closed. The toggle lever was designed to unlock to close the blade with a cam surface lifting the pin in the lockback bar. But with practice, the toggle will also lift the lockback allowing the blade to swing freely, and can be flicked open.
Some gravity knives are called "Conversions" or knives with after-market modifications. The knife at the right does not lock closed, and is a false gravity knife. It can still be opened manually without touching the toggle switch. The addition of the toggle, which is attached to an internal bellcrank-arm, will lift the lockback mechanism to allow the blade to swing easier. This knife mimics a rare device from the 1960s, thus allowing collectors to acquire an inexpensive facsimile.
Collectors have observed many trends in knife design that are resultant from legal rulings. Around 1965, New England local laws prohibited bayonet points with a sharp upper. A popular dealer simply ordered stilettos with clip-point blades starting a trend. And, Even though gravity knives were already outlawed, they also became available with clip points.
[edit] Legal Caution
In many jurisdictions a larger category of knives fall under the definition of gravity knives or switchblades, either explicitly or through any easily opening blade or use of a locking mechanism, because of a 1950s NY City law stating "any knife that is caused to open by means of button, lever, trip spring or switch, gravity or inertia opening and centrifugal force."
Collectors terminology and Legal definition may differ. Collectors use the term Gravity Knife (as a particular mechanism) to describe specific styles of knives that lock open and closed within a singular handle, excluding butterfly or balisong knives, assisted opening and/or liner locks. Lawyers use the term Gravity Knife for the general category of easy opening one handed knives without switchblade main springs. Leaving a big question, How "Easy" can an easy opening knife be ?
Recent NY rulings ( 01-10-08 )have determined that liner locks may be considered gravity knives. Court opinions are often justified by the arresting officer, upon finding those individuals carrying has done something else to cause reasonable suspicion or questionable intent.
But, It can be argued that the ruling does not follow the legal principle that all penal statutes must be interpreted narrowly. Liner locks are not manufactured to be inertia opening, since the lock mechanism is always in contact with the blade, causing drag and slow opening. It is only old worn knives, or knives modified with loose blade pivots that will behave in this manner. It would be more truthful saying Liner locks can be easily converted. Putting the crime on the individual, rather than Liner lock owners at Large. Other city rulings have adopted the term " blade tension bias toward the closed position" in an effort to identify legal knives. Still other city statutes refer to the terms "primarily a weapon with no useful purpose".
An example of Legal definition as less current than Collector Terminology. Assisted Opening Knives contain a spring blade that only cycles once the blade is pushed partially open with a thumb stud, past an internal "ball detent". This type of knife is very fast opening. But, Since the knife appears to resist gravity or inertia, and requires thumb pressure to open manually, they seem temporarily exempt from "gravity" classification. Thumb studs were not mentioned in 1950's statutes, laws are not recently updated, court rulings are not widely publicized. Another name for this type of knife is a "Torsion Bar" knife, named after the internal mechanism, the bias for the folding blade to remain closed is limited to around 30 degrees, then the torsion bar will flex and the bias is shifted to swing the blade open.
Legal terminology battles over the method of opening has little to do with a knife's speed and lethality. Many knife collectors sarcastically ask, does term inertia knife term apply only if you hold the handle when opening ? Since a great majority of regular folding knives can be inertia opened by grasping the blade and using the handle weight to overcome any bias to remain closed. Opening this way results in a knife held by the blade ready for throwing, or the knife can be readily flipped to a fencing grip.
[edit] Alternatives
Since the laws always begin with "be caused to OPEN by means of . . ." there are a few models of knives built to ridicule the wording. Some knives have springs set up in reverse, to cause the knife to snap shut, or bias closed, after manually opening. Removal of the spring will create a gravity knife.
Legitimate collectors of gravity knives in the U.S. do not carry "silk ripper" paratrooper or gravity knives, since the collectors value of such knives far outweighs their utility value. Given that a large number of knives originated since 1970 are designed to be easily opened one-handed (often by means of a thumb-hole or thumb-stud), the practical advantages of the gravity knife have been rendered obsolete.
[edit] Paratrooper Knife
In addition to the German gravity knife shown above, the term Paratrooper Knife may also refer to an automatic switchblade clip point jack-knife with optional folding cutting hook.
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