Tamiya TRF 421 Project

The seventh generation of the TRF touring car line began in July of 2024 with the 42384 TRF 421 chassis about two years after the previous generation (TRF 420X).  While the general chassis layout and suspension design had been very similar since the TRF 415, this version finally made significant changes by using A-shaped upper and lower suspension arms, eliminating the carbon shock towers entirely to result in even shorter dampers, and centrally locating the spur gear along the length of the chassis.  While the prior generations of the TRF touring car chassis had been nearly indistinguishable visually unless you knoew exactly what to look for, this one finally looks completely different and is easy to identify.  Many other changes (which involve virtually every part) are documented in my build journal below.  The chassis was updated to the 42391 TRF 421X in 2026 which mostly made changes to chassis stiffness and drive shafts but also introduced a lot of other refinements based on race feedback.  I present a detailed list of the changes to the new generation of the model in the table below.

TRF 421X

Aluminum lower deck instead of carbon Bulkheads now have guide pins Slightly shorter damper stays for more angled dampers Revised motor mount for 1.04mm lower motor with new belt roller Revised axles use ball bearings instead of plastic blades, new steel outdrive cups to match New rear chassis stiffener Upper arm mounts moved slightly inboard Suspension arms are now carbon filled instead of glass filled (stiffer) Stiffer default sway bars, fewer options in the box Shorter knuckle arms for revised Ackerman Slightly revised upper deck geometry with revised attachment hardware Revised bumper support New battery weight Updated cooling fan mount

There 42390 Aluminum Lower Deck was available for the TRF 421 separately as an option but did not come with the kit like the TRF 420X.  The 421X comes with an aluminum deck by default and there is no carbon option.

The TRF 421 is similar to but evolved from the TRF 420. Like all the TRF touring car chassis, this a twin belt driven 4WD model with carbon fiber chassis plates, loads of machined aluminum parts, and generally the best parts Tamiya could come up with including hardware and axles.  The general layout with the electronics on one side and the battery on the other is the same.  The real changes are all in the details and are explained much further in my build journal below.