The variable inertia balance wheel solves a problem as old as mechanical regulation itself: how to correct oscillation frequency without touching the hairspring. The solution is to move mass. On the balance wheel rim — on the arms, spokes, or along the perimeter — small movable weights are mounted, often eccentric screws or gold blocks, that can be rotated or shifted to modify the wheel's moment of inertia. Increasing the radius of mass distribution lengthens the oscillation period; reducing it shortens it. Frequency is thus corrected with surgical precision, without disassembling the lever escapement or lengthening or shortening the hairspring.
The difference from a fixed balance wheel with a regulator is substantial. The regulator acts on the active length of the hairspring: it's quick, but introduces variations in the hairspring's center of gravity, with effects that change from position to position. The variable inertia balance wheel, by contrast, leaves the hairspring intact and works directly on the oscillator. The adjustment is more stable against orientation variations and, within certain limits, temperature variations.
In practical bench work, each manufacturer uses its own geometry. Rolex mounts Microstella — gold nuts adjustable with a dedicated screwdriver — on the caliber 3235 and derivatives. Patek Philippe, on certain calibers, uses asymmetrical-head screws. Moritz Grossmann adopts adjustment screws on a 14.2 mm balance wheel. David Candaux on the DC12 uses mean-time screws in gold with combined function: they regulate both the moment of inertia and the center of gravity. Vaucher, Bovet, Akhor: different approaches, same principle. What changes is the number of screws, the material — gold, brass, steel — and the type of seat, which can be threaded or friction-fitted.
The system works well when paired with free-sprung, that is, without a return spring on the balance wheel. In that case, regulation depends entirely on the inertia screws, and the system becomes more coherent: there are no parasitic forces introduced by the regulator. This is a choice that requires an experienced watchmaker, because every adjustment must be made with the movement in motion and verified across multiple positions.
One point rarely written about: the shock resistance of a variable inertia balance wheel depends critically on how the screws are fastened. If a screw loosens from an impact, the balance wheel loses equilibrium and the timekeeping is affected immediately. On the bench, I always check the fastening before any other assessment.