Free-sprung balance

There are two ways to make a watch run faster or slower. The common way is an index: a lever with two pins that grips the hairspring near its outer end. Move the index and you change the effective working length of the spring, which changes the rate. It is cheap, quick and adjustable with a screwdriver.

The other way is to leave the hairspring entirely alone and change the inertia of the balance wheel instead, by turning small screws or weights set into its rim. That is a free-sprung balance.

The physics is the same in both cases. The period of the oscillator depends on the moment of inertia of the wheel and the stiffness of the spring, so you can move the rate by adjusting either one. An index changes the stiffness by shortening or lengthening the active spring. A free-sprung balance changes the inertia: turn a weight inward, toward the axis, and the wheel becomes easier to swing, so the watch gains. Turn it outward and the watch loses. The hairspring itself is never touched after it leaves the factory.

How it is adjusted

The mechanism varies by maker. Rolex uses four Microstella nuts threaded onto studs on the inside of the rim, turned with a dedicated fork-shaped tool. Patek Philippe's Gyromax, patented around 1950, uses slotted weights that sit in openings in the rim and are rotated to move their mass in or out. Omega uses small screws in the rim on its 8800 and 8900 family. Grand Seiko went free-sprung with the 9SA5, while the older 9S65 keeps a conventional index.

Whatever the hardware, the procedure is the same and it is the reason for the price. Weights are moved in diametrically opposite pairs, because moving one alone throws the wheel out of poise and creates positional error that no amount of further regulation will remove. The balance is then re-poised and the watch re-timed in several positions on a timing machine. Beat error is corrected separately, by rotating the collet or moving the stud, not by touching the weights.

Why brands bother

An index regulator introduces a small error whenever the spring breathes against the pins, and the pins can shift under shock, which changes the rate. A free-sprung balance has nothing to shift. It holds its rate more stably over years and through knocks, and it lets the hairspring breathe concentrically, which improves isochronism.

The gap between the curb pins matters more than most owners realize. It has to be wide enough for the spring to move freely and narrow enough to control it, and that gap changes the effective spring length differently at high and at low amplitude. The result is a rate that drifts as the mainspring runs down. Remove the pins and that whole error class goes with them, which is why free-sprung calibers are the ones brands put their tightest published rates on.

Where you find it

It is a marker of a movement designed for long-term stability rather than cheap assembly. The Rolex 3235 pairs a free-sprung balance with a Parachrom hairspring and an overcoil. Omega's Master Chronometer calibers combine one with a silicon hairspring, and the METAS specification they are certified to is 0 to plus 5 seconds a day on the cased watch. You will not find one at the price of an ETA 2824-2, which uses an index and is the better design for a watch that needs to be adjustable by any watchmaker in twenty minutes.

Why it costs more

Regulating one is slow, skilled work, and the balance has to be poised again after any adjustment. It also needs the right tool: an independent who can regulate an index caliber in minutes may not have the fork for a set of Microstella nuts. A routine bench regulation on an index watch is an indicative $60 to $150 in the United States before sales tax, while a free-sprung caliber is more often handled as part of a full service, which runs from an indicative $350 to $600 at a mainstream Swiss brand center and $800 to $1,200 for Rolex. Those are indicative figures that move with the shop, the region and the caliber. See what a watch service costs.

Common confusions

Free-sprung does not mean more accurate on the day you buy it. A well-adjusted index caliber can leave the factory keeping better time than a free-sprung one. What the free-sprung design buys is stability: the rate you have in year five is closer to the rate you had in year one.

It is not the same thing as an overcoil, a silicon spring or a screwed balance. Those are independent choices that often appear together on the same expensive movement. Plenty of vintage watches have screws all around the rim purely for poising and temperature compensation while still being regulated by an index.

It does not mean the watch cannot be regulated. It means the adjustment is made at the balance rather than at the spring, takes longer and, in practice, is usually done by the brand's network rather than by the shop on the corner. Whether a given watch can be brought back on rate is answered in can a watch be regulated.