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The Ins and Outs of Lock Bumping

The main point to note with all forms of lock bumping, including bumping with electric and manual pick guns is that the pins do not separate before the bottom pins reach the shear line.

In lock bumping there are two main constants:

 1] The length of the lower pins: This determines how much time the top pins spend crossing the shear line between the cylinder plug and the cylinder housing.

2] Manufacturing inaccuracies: Like with picking this causes one or more pins to bind under tension before others.

There are two main variables:

 1] Tension: This also helps to determine the speed / energy of pins crossing the shear line.

2] Strike force: This determines the initial speed / energy of each pin stack.

When the pins of a cylinder are bumped, the art is to balance tension with strike force:

This is so that the energy as the pins cross the shear line is only just sufficient to allow the bottom pins to either stop at the shear line or only just pass across the shear line. This is because of the misalignment of the top and bottom pin chambers.  It is at the shear line where the top and bottom pins start to separate. Why? Because the top pin is no longer affected by tension and the bottom pin if it passes the shear line does become affected.

After the bottom pins have stopped moving (they may be stopped at the shear line or across the shearline) the top pins carry on their movement upwards until the springs overcome them and drive them back downwards again. When they reach the shear line they are stopped by the overlapping pin chambers and any bottom pin that is across the shear line is pushed downwards clear of the shear line and the plug can then turn.

The tension normally required for successful lock bumping is very light. This is because a lot of energy is lost when the bottom pin is struck.

Tests show:

If a cylinder is loaded with just a single row of bottom pins and then bumped using no tension, the pins will hardly move due to loss of energy. The reason being is that the pins bounce off of the lower chamber walls, so losing most of their energy. This is the principle used by those manufacturers who incorporate short top pins to prevent lock bumping. In such instances these systems have two major flaws that I will not go into at this time. Another point to note is that if a top pin fails to reach the shear line when driven by a short pin, the short pin falls back (in rim cylinders), then, if it is bumped again without resetting, the pin will not have enough energy to move the top pin.

If the chambers are fully loaded with their top pins and springs and then bumped, the bottom and top pins will move upwards, together. The reason for this is that the bottom pin has been partially stabilised by the backpressure of the top pin and its spring therefore losing less energy.

Lock Bumping and the Laws of Motion

Newton’s 3rd Law of Motion is generally used to explain the act of lock bumping; but in actual fact all of Newton’s Laws can be applied to this lock breaking phenomena.

Isaac Newton’s Laws of Motion:

  1. An object in motion will remain in motion unless acted upon by a net force.
  2. Force equals mass multiplied by acceleration.
  3. To every action there is an equal and opposite reaction.

Newton’s 3rd Law: To every action there is an equal and opposite reaction

bumping pic 1

  • When the key is bumped into a lock, both pins move in a uniform direction and act as a solid.
  • If tension is applied to the cylinder, then the top pin has a force acting upon it.
  • The bottom pin has no force acting on it, other than that from being ‘bumped.’

Newton’s 1st Law: An object in motion will remain in motion unless acted upon by a net force

bumping pic 2

  • Although under spring pressure, the pins are still accelerating upwards.
  • When the bottom of the top pin and the top of the bottom pin reach the shear line, there is a change in forces acting on both pins.
  • The top pin has no force acting upon it other than the spring.
  • The bottom pin has the force of tension applied to it.
  • Therefore the bottom pin will decelerate and might stop at this point, the top pin will continue its movement upwards until the force of the spring stops its upward movement and reverses it.

If we assume for the moment that the bottom pin carried on its movement upwards, and was then acted upon by the force of the top pin moving downwards (due to the spring overcoming the upward movement) the result in energy when the both pins reach the shearline will be very low.

If we consider that the clearance between a pin and its chamber is 0.001 of an inch, when tension is applied, the core of the cylinder can rotate 0.001 of an inch for the top pin, plus 0.001 of an inch for the bottom pin. Therefore if the top pin with very low energy behind it tries to go past the shearline it will be stopped at the shearline, because of the difference in diameters of the chambers that have been formed.

But, why this didn’t happen when the pins were on their way up?

It is due to the energy transferred to the pins by the bump key which allows at least one of the pins to pass the shearline so aligning the chambers momentarily. Once the low energy top pin reaches the shearline the core of the cylinder can rotate.

It is on this basis that the system works. There are a lot more variables, but the outcome of it is that a pin normally comes to rest at the shearline on its way down.

To prove this I did some modifications to counter this effect, and the cylinder became bump proof.

bumping pic 3

  • We have reduced the diameter of the lower part of the top pin, so that it can enter the lower chamber, even though it is off-set.
  • The top pin will stop at the shearline, at its maximum diameter.
  • Therefore, the reduced diameter prevents rotation.

bumping pic 4