Belt Titan

WatchTime gets under the hood of the belt-driven Monaco V4, the first serially produced version of TAG Heuer’s 2004 concept watch.


+ Spectacular appearance

+ Innovative technology

+ Mechanisms are visible from the front.


— Sharp-edged case

— High rate deviation

— Difficult to read time with to-the-second accuracy.

Specs TAG Heuer’s Monaco V4

Manufacturer: TAG Heuer, Rue L-J. Chevrolet 6A, CH-2300 La Chaux-de-Fonds, Switzerland

Reference number: WAW2170.FC6261

Functions: Hours, minutes, seconds

Movement: Automatic, 28,800 vph, 48 jewels, Kif shock absorption, Glucydur balance, fine adjustment via index tail and eccentric screw, dimensions = 35 mm x 31.5 mm, height = 9.45 mm, 52-hour power reserve

Case: Platinum, sapphire crystal is non-reflective on both surfaces, caseback held in place by four screws, sapphire window in caseback, water resistant to 50 meters

Strap and clasp: Crocodile-skin strap, platinum folding clasp

The Italian term “bella machina,” or “beautiful machine,” is most often used to describe a well-designed, well-engineered sportscar, but it also applies to TAG Heuer’s Monaco V4, watch. The automotive comparison is apt, because the V4 is firmly rooted in the world of well-tuned motors. The “4” in its name refers to the number of “cylinders,” or barrels. The “V” alludes to the styling: as in an automobile’s V-engine, the barrels of the V4 are V-shaped and positioned to form a 26° angle toward each other. The watch’s most interesting technical aspect — its system of transferring power via drive belts — is directly influenced by automotive technology. And, of course, the Monaco model series has been linked to auto racing since its debut in 1969. It was named for the Monaco Grand Prix and Steve McQueen famously wore one in his 1971 racing film, Le Mans.

The three drive belts are visible from the front of the watch, as are the balance, several V-shaped bridges and the two wheels of the barrels, which are tipped at a 13° angle. The watch’s back is also unusual. There, the winding bar moves up and down, and the tipped barrels, with their drive belts, are visible to the right and left of the sliding bar.

When TAG Heuer introduced the Monaco V4 as a concept watch eight years ago, many wondered whether it could ever be developed into a fully functional and serially produced wrist-watch, but the company accomplished this feat in 2010, its 150th year. It produced a series of 150 pieces in honor of the anniversary. However, not all of the features that were incorporated into the concept watch could be transferred to the serially manufactured model. The tipped barrels and linear winding weight were preserved, but only five of the original 13 planned drive belts could be integrated into this new watch, and the belts are much thinner than they were on the original. Thirty-nine ball bearings were planned, but only 11 survive. And the engineers were unable to dispense entirely with ruby bearings and conventional gears, as they had originally intended.

Nevertheless, the

Monaco V4 represents a unique technological accomplishment. The biggest hurdle was creating a power system based on drive belts, something that had never before been achieved in a wristwatch. A specialist had to be found who could manufacture the belts in the needed minuscule dimensions, thinner than had ever been used before in any industry. The three belts in the gear train are only 0.25 mm wide and 0.07 mm thick, which means that each one is thinner than a human hair. The two belts for the barrels are about twice as wide and twice as thick, which enables them to pull 10 times more weight, up to 1.3 kilograms. All of the belts are made of an artificial material known as PEBA (poly-ether block amide), a thermoplastic elastomer sometimes used as a shock absorber in high-quality running shoes and basketball shoes, and also occasionally for the outer casing of electrical cables.

The primary advantages of using drive belts for the propulsion system are twofold: they require no maintenance and they convey force via a positive connection; that is, there’s no play between the teeth of the gears as there is in a conventional, gear-driven system. Furthermore, drive belts need no lubrication. Their high degree of efficiency (96 percent) is only slightly less than that achieved by a well-lubricated gear-to-pinion connection (99 percent). Furthermore, a drive belt’s efficiency doesn’t decline over time, unlike that of a gear-based system, whose efficiency wanes to 93 percent when lubrication begins wearing away.

SPECIAL feature: when two gears are connected by a drive belt, both wheels always turn in the same direction; two gears that directly mesh with one another must rotate in opposite directions. Since the balance of the Monaco V4 is reversed, its escape wheel already rotates clockwise, so no additional gear is needed to reverse the direction of rotation. However, to prevent the drive belts from slipping, the pinions for the drive belts had to be larger than the pinions in a conventional gear-to-pinion connection. On the other hand, space-related problems impose limits on the sizes of gears, so a drive-belt system needs twice as many gears as a conventional, gear-driven movement to accomplish the extreme reduction from the speedy oscillations of the escape wheel, through the fourth wheel, to the slow turning of the minute wheel. Although a greater number of gears is required, the individual gears can be located wherever is convenient, because there’s no functional limit to the length of a drive belt.

Compared to drive-belt propulsion, which even the CAD construction programs used in the world of watchmaking cannot simulate, developing the 13°-tipped barrels and the linear winding was relatively simple. The winding weight weighs 12 grams and is made of tungsten, a rather heavy metal. This weight moves up and down along a track and is kept at its proper distance by a gear that meshes with its toothed underside. It winds the two pairs of barrels via two lateral, toothed racks, which are also tipped at a 13° angle. Power from the four barrels is brought together on the front at 12 o’clock: an ordinary toothed connection wouldn’t work with these tipped wheels, so the two 13°-tipped racks mesh with a conical gear that bears teeth cut at the correct angle.

TAG Heuer purchases the drive belts from a specialized supplier, but it makes the lion’s share of the movement’s other components itself. Each Monaco V4 watch is assembled by master watchmaker Denis Badin in La Chaux-de-Fonds, Switzerland. As with all of TAG Heuer’s watches, this one must pass in-house tests, which include a vibration test and a fall from a height of one meter. The watch must also continue to function properly at temperatures ranging from a frigid -10° C (14º F) to a scorching +60° C (140º F).

S exciting engine is matched by its high-tech design, with V-shaped bridges and a cleverly styled case boasting a curved, laterally faceted, sapphire crystal. The broad, thermally blued hands contrast well with the movement’s silver-colored components. However, the four applied indices, arranged somewhat haphazardly, provide only approximate orientation for reading the time, as does the only vaguely discernible minute circle. The hands glow in the dark, but the indices are not luminescent, which also detracts from the watch’s ability to accurately display the time. On the other hand, operating the Monaco V4 presents no challenges: its crown is easy to grip, turn and pull out. There’s only one extracted position, for setting the hands; unfortunately, the watch continues to run while you do so, which makes it more difficult to set it with to-the-second precision.

The platinum case, which TAG Heuer manufactures in its other Swiss workshop in Cornol, is cleverly designed and very cleanly processed: the curved sapphire crystal above the dial has two clearly visible facets that neatly transition into the flanks of the case, which offers an attractive contrast between its polished and satin-finished surfaces. The three sapphire windows on the back show great attention to detail, but the lower corners of the case are simply too sharp. Sooner or later, they’re sure to tear shirt cuffs and could even injure the watch’s wearer. In other areas, too, the watch’s case is uncomfortable to wear: the angled and slightly protruding windows on its back allow the watch to wiggle back and forth, and they have a tendency to press into the back of the wearer’s wrist.

By contrast, the supple, hand-sewn, crocodile-skin strap wraps closely and comfortably around the wrist. The strap can be securely closed and easily reopened with the practical folding clasp, which is made of platinum. This clasp’s clamping system allows the user to adjust the strap to any desired length, and its two safety buttons ensure that it never opens accidentally.

The movement is thick, measuring nearly a full centimeter from top to bottom. Its other dimensions are similarly large. We were pleased to see that this big movement amply fills its spacious case. The decision to install the barrels at an angle was most likely made for aesthetic rather than functional reasons, and only time will tell whether the drive-belt propulsion system and its ball bearings actually represent an improvement over more traditional systems. We would have liked to have seen a fine-adjustment mechanism that uses weights along the balance’s rim, which would have allowed the balance spring to “breathe” freely. The fine regulation system, via an index tail and an eccentric screw, seems inappropriate for a caliber of this stature.

The decorations, however, are perfectly suited to this movement. The edges are beveled and polished; the wheels and the heads of the screws are brightly polished. Satin-finished surfaces accentuate the high-tech design. And everything is beautifully visible from the front, which means you can look at all of it while wearing the watch on your wrist.

Unfortunately, the accuracy of the Monaco V4’s rate was far less impressive than its attractive finishing. TAG Heuer claims that this watch is accurate to within ±4 seconds per day, but our test model failed to uphold those standards: with an average daily deviation of +17.8 seconds, it had obviously been adjusted too far toward the “plus” side. The greatest deviation among the various positions was 13 seconds, a figure that remained narrowly below the announced maximum value of 15 seconds.

It goes without saying that a watch that required so much time and labor for its development will be expensive, but $80,000 is nonetheless quite a sizeable sum, especially for a brand like TAG Heuer, which is known for a much lower price range. Of course, it is impossible to predict how much this model might go up in value, especially since it is limited to 150 pieces, and there is nothing else like it — certainly nothing comparable for a lower price — on the market.

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