TheRcRacer Tamiya TRF421 arm stiffeners (Free Download)

After a lot of laps with the Tamiya TRF421 over the last year, I can confidently say it’s the best TRF yet. That said, the racer in me is always looking for small tweaks that might unlock that extra bit of performance.

One thing several of us TRF drivers have noticed is that on very high-grip tracks, the car can sometimes feel a little numb mid-corner. It’s not a deal-breaker by any means, but at times it can feel like it’s holding you back from extracting the absolute maximum pace.

After some investigation, it appears the upper arms can flex front-to-back under load. This flex effectively increases caster as the car dives into the corner, which in turn can make the steering feel slightly muted or unresponsive.

We’ve been waiting for Tamiya to release an optional stiffer carbon-plastic upper arm set, but that hasn’t arrived… yet 😉. One alternative is to go aftermarket and run a turnbuckle upper arm setup on the TRF421. While this does increase stiffness, you lose one of the biggest advantages of solid upper arms: simplicity. After any decent knock, turnbuckles will move, regardless of brand or model, meaning constant checking and readjustment.

With that in mind, I wanted a simple, fit-and-forget solution.

The result is a set of small 3D-printed upper-arm stiffeners that attach to the underside of the standard upper arms.

Simply download the files from the TheRcRacer Thingiverse page (click link here), then print them yourself, or ask a friend to use 100% infill.

There are two files, left and right (assuming the arms are facing forward). Each stiffener has thin bonding tabs and a thicker central section that fits snugly between the two upper struts of the original Tamiya arm.

Before gluing, slide the stiffener under the arm to check the fit and confirm it sits correctly in position.

Once you’re happy with the placement, grab your superglue. I always use a gel-type CA, as it allows a little more compliance and tends to work better on plastic parts. Apply glue to the tabs, slide the stiffener into position on the arm, and hold it briefly, bonding takes just a few seconds.

Repeat this for the other side and your arms are done.

I’ve now run these stiffened arms at multiple events and they’ve performed extremely well. The car is noticeably more responsive on corner entry in high-grip conditions. Personally, I like to run these stiffened arms on the front, while using cut arms on the rear. For me, that balance works really well. If grip is especially high and I want even quicker rotation, I’ll stiffen the rear arms too.

The superglue has held the stiffeners perfectly, even after some fairly heavy impacts. As a side note, they should also help prevent upper-arm warping during medium-to-high impact, although that wasn’t something I personally struggled with on the standard arms.

Have fun, and let me know how you get on with them 🙂

Disclaimer and thanks

I have invested my own time and money in these parts for the Tamiya community, at no profit for myself. These designs are always helped via feedback from other racers. So please let's share feedback and set-ups and I can try to refine or design other parts in the future :) Please do not sell these for profit, The design is copyright TheRcRacer.

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