To mark its 20^th anniversary last year, Greubel Forsey unveiled the latest of its EWT (Experimental Watch Technology) inventions, the Nano-Foudroyante EWT, an extremely sophisticated chronograph tourbillon with a foudroyante display. Limited to just 11 pieces, the Nano-Foudroyante is the most compact watch ever produced by Greubel Forsey, measuring 37.9mm wide. It represents a series of firsts for Greubel Forsey: its first chronograph, its first flying tourbillon, and a horological first – the first-ever carriage-mounted foudroyante. The case, crafted from white gold and tantalum, also marks the brand’s first use of the highly demanding metal.

A foudroyante, or lightning seconds, is a complication in a watch that displays fractions of a second. While it is sometimes found in high-frequency chronographs, the two are distinct. A high-frequency chronograph improves precision by allowing for finer time measurement, whereas a true foudroyante is a dedicated hand that visibly jumps in discrete increments to indicate fractions of a second.

To say that the movement has a highly unusual construction is to say nothing at all. To start off, even though a foudroyante function is typically associated with a chronograph and is only engaged when the chronograph is activated, they are unrelated in the Nano-Foudroyante EWT. Instead, the foudroyante hand runs continuously in 1/6-second increments, corresponding to each half-oscillation of the 3 Hz balance.

This is because the chronograph is driven off the center wheel via an ad hoc gear train, which is visible on the back of the watch, while the tourbillon and by extension, the foudroyante are driven by the main gear train. The watch has a center chronograph seconds and a 60-minute totalizer at 9:00 while the running seconds, driven directly by the fourth wheel, is at 7:30 on the dial. Between 4:00 and 7:00 is the one-minute flying tourbillon, which doubles as an additional running seconds, and mounted on the cage is a tiny sub-dial for the foudroyante.

Leaving the chronograph aside for now, the challenge with a foudroyante is that it’s usually driven off the escape wheel, which is the wheel with the least torque. In this case, it’s further complicated by the presence of a tourbillon, as the same gear train that drives a parasitical cage must now also drive a foudroyante. Greubel Forsey accomplished this with so called nanomechanics. In this context, nano doesn’t refer to nanoscale components but rather to ultra-low energy consumption, measured in nanojoules. The Nano Foudroyante operates with just 16nJ per jump, which is an energy level so small that it is comparable to biological, molecular and microscopic physical processes. According to Greubel Forsey, it is a factor of 1,800 lower than a conventional foudroyante, which requires 30μJ (microjoules) per jump. This allows it to function within the escape wheel’s limited energy budget.

Tourbillon with a Cage-Mounted Foudroyante

The gear train from the escape wheel to the foudroyante is particularly interesting. The wheels have extremely fine teeth, which not only improve meshing efficiency and reduce backlash but also allow for a higher tooth count within a given pitch diameter. This enables the same gear ratio to be achieved with smaller wheels, reducing inertia and contributing to lower energy consumption. According to the patent, the tooth module is no more than 0.05mm, indicating a design optimized for smoother engagement and minimal frictional losses. While not explicitly mentioned in the announcement, the fabrication of such fine gearing was most likely achieved through LIGA, a microfabrication technique well-suited to producing high-precision, ultra-fine structures. Given the application and the requirements for wear resistance, low friction, and dimensional stability, the wheels were likely produced in nickel-phosphorus, a common LIGA material that is lightweight and known for its high hardness.

Mounted beneath the escape wheel and integral with its pinion is the drive wheel (21), which transmits motion through a set of transmission wheels (25 and 27) that are pivotally mounted on the cage. This gearing ultimately drives the foudroyante hand (23), which completes a full rotation per second. But to add to its complexity, the foudroyante sub-dial remains upright relative to the frame, even though it is mounted on the rotating tourbillon cage. This requires additional gearing that functions analogously to a differential, compensating for the motion of the cage to keep the dial at a fixed orientation. The tourbillon cage rotates clockwise at one revolution per minute, thus, to counteract this, the dial must rotate counterclockwise at the same rate to remain upright.

This compensation is achieved through a secondary gear train originating from transmission wheel (25), which is itself driven by the escape wheel. This gearing ensures that the orbital dial rotates counter to the cage at the same speed, maintaining a fixed orientation relative to the frame. With these additional gear trains mounted on the cage, the tourbillon comprises a staggering 142 parts. To further reduce inertia, the cage is made in titanium.

The beauty of this innovation is that it’s not just mechanically intriguing and complex to execute but a visual spectacle as well; the steady rotation of the tourbillon is contrasted with the foudroyante’s frantic jumps, yet look closer, and you’ll see the sub-dial itself rotating counterclockwise.

An Advanced Vertical Clutch

 The chronograph is also highly unusual in the way it is powered and constructed. The pusher at 2:00 is to start and stop the chronograph while the pusher at 3:00 is for reset and flyback. Typically, a chronograph train is driven off the fourth wheel in a movement, regardless of the clutch system used. However, given the demands placed on the gear train – powering a tourbillon, a foudroyante, and an orbital sub-dial – it is far more sensible to draw power from the center wheel. As it is driven directly by the barrel, the centre wheel delivers higher torque. Visible through a cutout on the bridge plate is the auxiliary train used to drive the vertical clutch, which ultimately drives the chronograph seconds wheel.

In a standard vertical clutch, the input wheel is constantly driven by the finishing gear, while the output wheel, which powers the chronograph, is coupled axially via a clutch disc that transmits power through friction. A spring presses the clutch disc against the input wheel to maintain contact. The clamp lever serves as the actuator, lifting the clutch disc to disengage the chronograph and lowering it to re-engage power transmission. This seeks to eliminate the jerky start of a horizontal clutch. However, construction clearances in a vertical clutch assembly can lead to unintended movement when the chronograph is disengaged. During reset, when the reset hammers strike the heart cams, the output wheel of the chronograph rotates rapidly back to zero. This, combined with friction in the loosely mounted components of the clutch, can induce unintended backward rotation of the input wheel. As a result, when the clutch is re-engaged, any play introduced by this reverse movement must first be taken up before the chronograph restarts, leading to an unwanted delay.

To eliminate hesitation, the mechanism integrates a braking wheel that is coaxial with the input wheel and remains in controlled frictional contact with it. According to the patent, when the chronograph is reset, a brake lever engages this braking wheel, momentarily locking it. This prevents unintended reverse movement of the input wheel caused by friction in the clutch assembly. Since the auxiliary train terminates at the input wheel, stabilizing the input wheel during reset also ensures that backlash within the auxiliary train is already taken up before the clutch re-engages. Holding the input wheel steady during reset indirectly preloads the entire transmission, eliminating any further play that could cause hesitation or unwanted movements when the chronograph starts.

The braking system is directly controlled by the column wheel. When the reset pusher is pressed, a click or finger (visible under a bridge near the crown) advances the column wheel, triggering two simultaneous actions: the reset hammers drop onto the heart cams, instantly returning the chronograph hands to zero, while the column wheel pivots the brake lever to engage the braking wheel. When the brake is engaged, the braking wheel is locked in place, which stabilizes an intermediate ring and, by friction, the input wheel, preventing unintended movement of the input wheel during reset.

The braking wheel has extremely fine teeth to minimize engagement forces and improve braking precision. Crucially, even if the brake remains engaged, the finishing gear continues running normally, as the braking wheel is only in frictional contact with the input wheel. This ensures that the braking mechanism stabilizes the input wheel without interfering with the rest of the watch.

Beyond Mechanics

The movement showcases the finest hand-finishing in the business. The circular chronograph bridge – supporting the clutch, chronograph seconds, and minute recording wheels – has a frosted finish and features cutouts that showcase an abundance of anglage and crisp internal angles.

The ad hoc gear train, located one level below, is supported by steel bridge that has been black-polished and likewise, features numerous angles. Screw heads are black polished, and each jewel is set in its own gold chaton. The chronograph seconds wheel bears the brand’s signature micro-engraved inscriptions.

Tech Specs

Movement: Manually wound, 24-hour power reserve (with the chronograph running continuously); 3 Hz (21,000 vph)
Functions: Hours, minutes and small seconds, chronograph with 60-minute totaliser; tourbillon with foudroyante display
Case: 37.9mm x 10.49mm high, 18K white gold with tantalum case back and bezel; water-resistant to 30m
Dial: Multi-level in gold, rhodium-coloured, engraved and black lacquered hour-ring and minute-circle with a visual opening for the tourbillon; Small seconds and chronograph minutes counter in gold; polish bevelled foudroyante
Strap: Non-animal material, hand-sewn with white gold pin buckle, hand-engraved GF logo
Availability: Limited to 11 pieces
Price: CHF 465,000