Drivetrain Compatibility with RMU's 27t Raceface Cinch or SRAM 8 Bolt Chainrings

The dual idler system on the RMU Nighttrain 170 achieves universal rear drivetrain compatibility by physically decoupling the crankset from the rear shifting system.

How the Dual-Chain Layout Works

In a traditional mountain bike drivetrain, a single chain connects the crankset directly to the rear cassette and derailleur. This requires the front chainring and the rear components to share the exact same chain standards, which restricts mix-and-match compatibility between brands like SRAM and Shimano.

Dave Weagle’s patented dual-idler design on the Nighttrain 170 splits the drivetrain into two independent drives:

• The Primary Drive: A short, dedicated chain runs exclusively from the crankset to the dual idler assembly.

• The Secondary Drive: A completely separate chain runs from the dual idler back to the rear cassette and derailleur.

The Role of the RMU 27t Chainrings

Because the primary drive is mechanically isolated, it only needs to interface with the input cog of the idler system. RMU engineered specific 27t chainrings one designed for the RaceFace Cinch direct-mount standard and another for the SRAM 8-bolt interface.

As long as you mount one of these proprietary RMU 27t chainrings to your preferred RaceFace or SRAM crankset, the front chain loop operates perfectly. The front chain's only job is to drive the idler; it never interacts with the rear derailleur or the rear cassette.

Universal Rear Drivetrain Freedom

By confining the crankset and chainring requirements exclusively to the primary loop, the secondary drive is completely liberated. The output cog of the idler simply drives whatever secondary chain and derailleur you choose to install. This setup grants total freedom for the rear of the bike:

• Brand Agnostic: You can run a complete Shimano rear setup, a traditional SRAM Eagle mechanical system, or a wireless SRAM Transmission without worrying about how it pairs with the front crankset.

• Speed Agnostic: Whether you prefer an 11-speed, a 12-speed, or a specialized DH drivetrain, the independent rear loop will accommodate it seamlessly.

Ultimately, the dual idler system acts as a mechanical adapter. The RMU 27t chainrings handle the precise geometry and tension of the front loop, leaving the rider free to bolt any modern rear drivetrain onto the bike without facing traditional cross-compatibility limits.