Armin Strom Caliber Arf21

Armin Strom Caliber ARF21

Armin Strom Caliber Arf21

Manufacturer Armin Strom
Caliber Number ARF21
Based On
Armin Strom ARF15
Movement Type Mechanical, manual-wind
Total Parts
276 components
Beat Rate
25,200 vph (3.5Hz)
Lift Angle
Power Reserve
48 hours
Barrel Count
Anti-Shock Device
Hand Count 4
Functions Offset hours and minutes (towards 3:00); twin seconds subdials at 7:00 and 11:00
Country of Manufacture Switzerland, Swiss made
Known Models
Armin Strom Mirrored Force Resonance Manufacture Edition (Ref: ST22-RF.05 Blue, ST22-RF.20 Green, ST22-RF.75 First Edition)

The Armin Strom caliber ARF21 is a handwound mechanical watch movement found in the Mirrored Force Resonance line designed by a team lead by brand co-founder and master watchmaker Claude Greisler.

Caliber ARF21 is based on the original technology introduced by Armin Strom in 2016, and is considered to be an evolution to the original ARF15 and ARF16 calibers.

Although it may be confusing to some that the Biel/Bienne based independent brand refers to watches with this movement as “Manufacture Edition”, the cal. ARF21 was developed and manufactured fully in-house by Armin Strom.


Wikipedia describes resonance as:

“The phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillating force is applied at a resonant frequency of a dynamic system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies.” –Source

With regards to Armin Strom’s Resonance watches, they attempt to simplify it to:

“Two oscillating bodies in close proximity influence each other and eventually synchronize; this is a phenomenon of physics known as resonance. ARMIN STROM introduces the Mirrored Force Resonance, a resonating dual regulator developed for maximum precision with its whole captivating mechanism on full display dial side.” –Source

Resonance Clutch Spring:

The ARF21 features two independent regulation devices that are connected by Armin Strom’s patented Resonance Clutch Spring system. In the manufacturer’s own words:

“A physical phenomenon that has intrigued the greatest watchmakers for centuries yet remains exceedingly rare, the principle of resonance allows for a more regular timekeeping rate by synchronizing two balance wheels that oscillate in opposite directions at the same time. The manufacture’s patented Resonance Clutch Spring is able to reach and remain in a state of resonance faster and for longer than any other resonance system in watchmaking, resulting in a consistent rate that’s more impervious to shocks and positional rate variances.” –Source

There are dual balance wheels and hairsprings (located at 8:00 and 10:00), which are feed the independently running seconds indicators (7:00 and 11:00). The two seconds subdials can show whether the resonant regulating device is in sync.

Typically, resonance watches are not easy to regulate, but that is not true of the ARF21. In the case that the seconds hands become out of sync and the movement needs regulated, there is a pusher on the side of the watch at 2:00, allowing for the synchronization of the device, with a flyback style mechanism to reset the seconds to match each other again.

Armin Strom states that their Resonance Clutch Spring is recognized as “the quickest synchronizing and most reliable resonance watch mechanism ever made”.

Similar Caliber ARF21_ZG:

There is a sub-caliber ARF21_ZG which was featured in the Armin Strom Resonance Zeitgeist model, which was announced on December 15, 2021 in honor of the five-year anniversary of manufacturer’s resonance technology. The main difference between the standard ARF21 and ARF21_ZG is the overall finishing, as well as the exposed dial and skeletonized cutout behind the dual balance wheels.

Replacement Price:

Currently, watches with this caliber are priced at $63,000 USD.


Introduction of AS Resonance technology (December 20, 2016):

Mirrored Force Resonance (October 31, 2017):

Watches with this caliber:

As of 2023, there are only three Armin Strom timepieces powered by the calibre ARF21:

  • Mirrored Force Resonance First Edition (Ref: ST22-RF.75) – This gray dial model was introduced on May 24, 2022 with only 25 limited edition pieces available. It is currently listed as sold-out on their site.
  • Mirrored Force Resonance Manufacture Edition Blue (Ref: ST22-RF.05) – This is a blue dial version with 50 limited edition pieces created. This colorway was launched on February 28, 2023 and as of 5/23, it is still available.
  • Mirrored Force Resonance Manufacture Edition Green (Ref: ST22-RF.20) -This is the green dial variation that was released on April 22, 2023. 50 limited edition pieces are available, not sold-out as of 5/23.

Additional Resources:

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