Audemars Piguet (AP) reinvents the chronograph with the Royal Oak “Jumbo” Extra-Thin Selfwinding Flying Tourbillon Chronograph RD#5. Inside the titanium and palladium BMG case – standing just 8.1 mm high – is the cal. 8100, a new take on the chronograph mechanism that employs spring-loaded racks to gather and store energy. The RD#5 is the fifth and final instalment in the “RD” series of watches that underline the impressive technical know-how of AP, particularly its Le Locle manufacture that evolved from Renaud & Papi (APRP).

Conceived with the goal of creating a chronograph that is extra slim and easy on the fingers, the RD#5 boasts exceptionally light and comfortable pusher feel, an instantaneously-jumping minutes counter, and an exceptionally slim profile (thanks in part to peripheral winding), all while remaining amenable to industrial-haute horlogerie manufacturing. It’s one of the most technically interesting debuts of 2025, and we explain AP’s new take on the chronograph in detail below.

The cal. 8100 with its unorthodox chronograph and peripheral rotor

Initial thoughts

Outwardly just another Royal Oak, with the same ideal proportions and class-leading bracelet as the iconic “Jumbo”, the RD#5 is actually one of the most innovative mechanical chronographs in recent years. Perhaps the only comparison for out-of-the-box chronograph construction is AgenGraphe that was unveiled almost a decade ago.

The watch was evidently developed with two goals in mind: thinness and tactility. It succeeded in both. Besides improving pusher feel, the RD#5 reduces the calibre’s height by removing a layer from the chronograph construction that is normally occupied by the hammers and heart cams and replacing them with racks; this layer-elimination approach is not unlike that taken for RD#2 perpetual calendar.

When activating the chronograph for the very first time, the sensation is expected and surprising. Giulio Papi, co-inventor of RD#5 (and one of the most important living watchmakers) claims the chronograph pushers are as light as the volume buttons of an iPhone volume buttons, which he cites as one of the inspirations behind the watch.

Mr Papi is right in terms of the tactile feel. Smaller than most pushers, the chronograph buttons of the RD#5 travel a tiny distance before engaging, doing away with the depth of travel that accompanies ordinary chronographs. At the same time, the feel is smooth and crisp, making it quite unlike any other chronograph on the market.

But as a chronograph enthusiast, most appealing to me is the instantaneous jumping minutes counter that is also employed by AP’s peers in Geneva and Glashütte.

The novelty of the RD#5’s movement also illustrates the impressive development of AP’s industrial-haute horlogerie manufacturing capability. Several examples of good design for manufacturability (DFM) are evident in its details, most obvious being the extensive use of clips rather than screws to secure the chronograph works. The many thin levers, springs, and linkages are ideal fodder for wire EDM.

Most importantly, Audemars Piguet’s patents note the deterministic nature of the RD#5, which doesn’t require tedious adjustment of spring tension and cam alignment. The result is a movement unabashedly designed for modern manufacturing, which I find interesting and appealing. In fact, cal. 8100 in the RD#4 has two fewer parts than AP’s rank-and-file automatic chronograph, the cal. 4401.

The movement styling is rather unusual for a Royal Oak, and instead takes after APRP movements found in Royal Oak Concepts and high end Richard Mille models. The extensive skeletonisation of the chronograph levers and linkages is, I assume, as much for functional weight reduction as aesthetics. A notable detail is the chronograph bridge that takes the form of a semi-octagon, a subtle reference to an obvious theme.

The RD#5 costs CHF260,000 before taxes, which isn’t all that bad for an impressively engineered movement with a tourbillon and inventive chronograph. But the value proposition of the watch also lies in its rarity. While this model anniversary model is limited to 150 pieces, I expect other models using the cal. 8100 will soon follow, and hopefully a non-tourbillon variant is in the cards.

The RD series

AP launched the Research and Development (RD) series in 2015 with the RD#1 Royal Oak Concept Supersonnerie, which transformed AP’s approach to repeaters and chiming watches. AP followed up with the RD#2 Royal Oak Selfwinding Perpetual Calendar Ultra-Thin in 2016, which was the thinnest automatic perpetual calendar at the time at only 6.3 mm in height.

Six years later, the RD#3 introduced a high-amplitude lever escapement within a flying tourbillon, which was reused in 2023’s Universelle RD#4, a user-friendly self-winding grand complication only 42 mm in diameter, named after an extra-complicated Louis-Elisee Piguet movement AP delivered to German watch brand Union during the late 19th century. With the RD#5, AP sets its sights on reinventing the chronograph mechanism, specifically, the reset function.

The regular production versions of each RD model, from RD#1 to RD#4

The high-amplitude tourbillon returns for a third time in the RD#5. The watch itself is a titanium Royal Oak with bezel, case back, and intermediate bracelet links in a palladium alloy-bulk metallic glass (BMG). AP used the material just recently in the Royal Oak perpetual calendar that was the swan song of the cal. 2120, as well as a handful of Royal Oak “Jumbo” models and special editions.

Palladium BMG is what it sounds like – a metal that is amorphous rather than crystalline. The result is greater rigidity and hardness compared to typical polycrystalline morphology of conventional alloys.

The dial is quintessential Royal Oak and finished in petite tapisserie, midnight blue PVD treatment, along with solid-gold “bathtub” hour markers and hands. Like other limited edition mades in 2025, the dial carries the stylised 150th anniversary signature that was inspired by the brand’s historical logo.

But the RD#5 is really all about the movement inside. The cal. 8100 contains the high-amplitude flying tourbillon, along with automatic winding by a peripheral rotor and function selector crown to toggle between winding and setting, a signature feature of many APRP movements used by Richard Mille. But, the focus here is the novel chronograph construction.

A light touch

Adolphe Nicole, a Swiss immigrant living in London and founder of Nicole Nielsen, a movement maker best known for supplying English watchmakers, invented the modern form of the the chronograph around 1840, creating the first mechanism able to be started and stopped independently of the movement – unlike a stopwatch – and instantaneously zeroed using a hammer and heart cam with a single stable position.

Upscaled scan of figure five from Adolphe Nicole’s 1844 patent. Image – British patent 10.348

And since then, most of the industry has considered the chronograph reset a solved problem, usually only seeking other solutions when it is convenient, such as in some retrograde chronographs.

Normally, the energy used to reset a chronograph enters the watch through the buttons. On many, pressing the start/stop button cocks the reset hammer, which is held in place by a pin (red, in the image below), which is then released when the reset button is depressed.

Pressing the start button is like pulling back a crossbow string, requiring significant force, while pressing the reset button is as effortless as pulling the trigger. Alternatively, the reset force can come from the reset button itself, in which case the reset button is particularly heavy.

The start, stop and reset cycle of Patek Philippe cal Ch 29-535, designed by Pierre-Maurice Rochat. Image – EP2241945B1

The RD#5 is decidedly not a normal chronograph, though it appears like one at first, with sweep chronograph seconds and typical 30-minute and 12-hour totalisers.

Rather than relying on energy from outside the watch, the RD#5 uses three spring-loaded racks to gather and store the energy from the mainspring as the chronograph runs, which is used to reset the chronograph or advance the hour and minute totalisers.

Conceptually, this is the same as the ingenious AgenGraphe, which uses snail cams and a spring-loaded hammer-like probe to the same effect, but executed in an entirely different manner.

Each of the three racks

Reinventing the chronograph reset

As the titanium chronograph seconds wheel (light blue, in the image below) turns counterclockwise, a pinion on it drives the counterweighted seconds rack (teal) to the left, while the weaker return spring (orange) fights to push the rack to the right.

As the seconds rack is dragged leftward it pulls the linkage (gray, 50) back until the minutes advance prawl (pink) falls behind a tooth of the minute recorder wheel.

A simplified representation of the RD#5’s chronograph works as designed by Giulio Papi and Julien Martel. Image – EP4555385A1

As mentioned before, the weak return spring is fighting against the pinion, which has the stronger force of the mainspring behind it. This also replaces the typical anti-stutter spring. When the clutch is disengaged (and the chronograph is paused) a red brake prevents the wheel from moving. If not for the red brake, the return spring would win out, and would turn the seconds wheel back all the way to zero, which is the key to the reset mechanism.

The seconds pinion is missing two teeth and another is cut down so when the chronograph seconds hand completes a full lap of the dial the rack is free of the pinion’s influence, and the return spring sends the seconds rack flying to the right. Note that the chronograph seconds hand continues moving forward throughout this sequence, as it is not a retrograde chronograph.

This causes the linkage to jerk forward, advancing the minutes counter instantaneously. Then, the pinion reengages with the rack and the process repeats. A similar arrangement centred on the minutes counter advances the hour recorder hand in the opposite manner, gradually inching the hour recording wheel forward before snapping back every thirty minutes. Like the chronograph seconds hand, neither the minutes nor the hours counters are retrograde displays.

Start. Stop.

The chronograph uses an odd vertical-horizontal hybrid coupling system, the chronograph driving wheel and clutch wheel are always meshed, and the clutch wheel has a small amount of freedom to move vertically within its pivot.

When you stop the chronograph, a lever (teal, in the image below) pushes the clutch wheel up and away from the chronograph seconds wheel. This lever backs off when reengaging the chronograph, and the chronograph tension spring wheel depresses the clutch wheel back into contact with the chronograph seconds wheel. The clutch wheel teeth are bevelled on the underside for smooth engagement.

Pressing the start/stop button turns the yellow column wheel, which engages and disengages the clutch via the purple linkages. The purple linkage also includes the spring for the chronograph seconds brake (red). When the chronograph is stopped, spring tension increases to hold the wheel in place, and relaxes when running.

Reset and flyback

When the reset button is depressed, a lever pushes the light blue reset carriage (my term, not AP’s) counterclockwise, but is stopped by a blocker (orange). Further pressing the reset button shifts this orange blocker, resulting in a tactile click and allowing the reset carriage to turn slightly, pushing against the teal lever, disengaging the clutch wheel from the chronograph seconds wheel in the center.

Pins on the reset carriage lift the brakes (red) from the chronograph seconds, minutes, and hour wheels, and the chronograph minutes and hour advance pawls (pink) allowing all three displays to return to zero action. If you reset the chronograph while running, it will immediately restart once you release the reset button, making it a proper flyback chronograph.

Key facts and price

Audemars Piguet Royal Oak “Jumbo” Extra-Thin Selfwinding Flying Tourbillon Chronograph RD#5
Ref. 26545XT.OO.1240XT.01

Diameter: 39 mm
Height: 8.1 mm
Material: Titanium and palladium 500 BMG
Crystal: Sapphire
Water resistance: 20 m

Movement: Cal. 8100
Functions: Flying tourbillon, chronograph
Winding: Automatic
Frequency: 21,600 beats per hour (3 Hz)
Power reserve: 72 hours

Strap: Titanium bracelet with palladium 500 BMG intermediary links

Limited edition: 150 pieces
Availability: At AP boutiques and AP Houses
Price: CHF260,000 before taxes (equivalent to US$325,000)

For more, visit Audemarspiguet.com.