Schuyler wrote:Balanced pin stacks (drivers of different sizes to match the different sizes of the key pins) aren't about spring compression, but protect against decoding attacks. There are some systems that are balanced as a rule, like BEST, and they have additional advantages.
Also - considering the balanced stacks and abnormal pins, I'd say this was custom pinned. The key was likely custom cut as well. It could even be as simple as someone using the wrong chart when they were pinning the lock, then following through with the same measurements when cutting the key.
I may well be wrong, though.
Most of the references that I've read cite balanced or compensated drivers as being to prevent overlifting attacks where a comb pick is used to push all the pins both bottom and top into the bible or to pop the spring cover off (Of course compensated drivers would be of little use in preventing the spring cover from popping off). However, I suspect that it would be a mistake to say that there is only one purpose for balancing the drivers and we should talk about the effects of balanced drivers instead.
One effect is to compress the springs. In a lock where the drivers are not balanced, the short stacks would have springs extended when at rest while the tall stacks would have significantly more compression. The more compressed springs exert more of an initial force but may become more accustomed to being compressed and hence exert less of a peak force. It would be more correct to say that balanced drivers are to avoid compressing some springs less than others not to avoid compressing the springs more. Also in an unbalanced lock, there is more room for the pins of a short stack to move whereas a tall stack has the minimal amount of travel available while still allowing a key to enter and exit the keyway. The question is are there any advantages to any of this to a particular attack.
In terms of decoding where one measures the travel of the stack in order to estimate the bottom pin size from height of the pinstack, balanced drivers would present uniform pinstack heights regardless of the bottom pin size so definitely it helps there. With overlifting attacks involving lifting the bottom pins into the bible, balanced drivers ensure that there is no room for the bottom pin to be in the bible without popping the spring cover (note the spring cover can be held in place with a zip tie). In terms of a lock bump attack, the balanced driver pins are of differing masses, the initial spring force is greater and there is less pin travel available; this might make the lock more bump resistant but is in no means proof against a bump attack. There was an article by Billy Edwards of Master Lock that stated that stronger springs were ineffective against bump attacks and that high speed video lead them in the direction of shaping the bottom of the top pin. This presumably means shaping the bottom to either re-enter the plug quickly during a bump attack or to not leave the plug during a bump attack proved to be the key to their bumpstop product. Indeed the article mentioned that their first successes were with a shouldered pin where the bottom of the top pin was a smaller diameter and the pin chamber in the bible was such as to float that pin with the bottom inside the plug but the driver not being allowed to drop completely within the plug. The floating concept is that the bottom pin is not initially in contact with the top driver pin so must travel to it before transferring energy and hence is less effective in transferring that energy but the smaller diameter could also facilitate plug re-entry. Their first refinement was to taper the bottom of their driver pin and forgo overboring the bible pin chamber allowing the natural differences between the plug chamber and the bible chamber to float the pin, again although floating is the cited mechanism to resist bumping, it could also be that plug re-entry was facilitated. If plug re-entry does play a part then the added spring force could be a benefit, also balanced drivers may be done by utilizing select bottom pins as top pins which would then present a beveled, rounded or pointed bottom facilitating plug re-entry hence reports that balanced drivers helped resist bumps. Of course using bottom pins as top pins also means a much broader range of key cuts can camber that top pin back up into the bible so I wouldn't suggest such a use in more than one chamber.
I think we shouldn't simply discount balanced drivers because we heard or thought it was just for one particular purpose but rather consider whether or not the effects of balanced drivers would help with an attack of concern.
I would be interested to see what the shape of the bottom of those top pins are especially in the chamber where the bottom pin calipered out as 0.268" instead of 0.277". If the balanced driver pins were really bottom pins re-purposed as driver pins then it would be possible to use a slightly undersized bottom pin and still have the lock function as the shear would just cam up the top pin so long as the shear hit the bevel, rounded or pointed part of the pin. This could give you clue as to whether it's a factory pinning or a custom pinning. If the majority of the top pins are repurposed bottom pins then I would say that they were used out of ignorance but if only one or two were repurposed bottom pins then the custom pinning might've been by someone particularly clever. Also, drivers needn't be balanced by selecting appropriately sized pins, it can also be done by placing master pins above the driver pin (the above is important), so long as the lowest pin in a driver stack is as long as a standard driver pin then the master pins will never enter the plug and hence having the driver be a stack of pins doesn't create shear lines that can be exploited. Indeed it may help with the Newton's Cradle effect as much of the driver stack will remain stationary while the outer most pins bounce. The thinner master wafers may also jam the pinstack when the lock is under a violent attack like bumping or pick gunning. I would be tempted to put in a Schlage T top driver pin in upside down and then stack master pins on top to balance one pinstack in order to offer bump resistance. The T shape should facilitate plug re-entry without allowing for the driver pin to be cammed up off the shear and the multiple master pins should cause some havoc during bumping. The T pin should present a false set point as well.
