Well, we’re now past mid- September and prospects for more testing this year is not looking great up here where I live, so it’s time for the yearly testing summary.

This summer’s testing was very focused on the Tamiya TRF421. That was always the plan, but the weather made sure it was even more centered on this car, as May and June were cold, windy and wet, and late July was super hot for several weeks – all which are conditions where I’d rather stay away from the track. In addition to the 421 testing, I also wanted to get a good reading on the FFV4, which I did, and then fully focused on the TRF421 towards the end of summer.

I think I managed 3 test sessions with the car in June, and maybe 4 in July, so August and early September were the busiest months by far. Overall I did around 100 runs, or about 1900 laps with the TRF421 this summer, so significantly less than the 150 runs  last summer, which shows the influence of the weather.

As I did a lot of tests with this car last year, it was already very well setup, and of course I started this summer where I left off last summer.

Because the car was so good, I chose to spend more time trying various directions rather than always just chasing instant improvements. This meant that even if some things were not an improvement directly, I tried to keep working around it to see if I could unlock more by making other changes.

As an example, I spent a lot of time running with high diffs and trying various things around that, even though I found last year that low diffs were better in the low-ish grip conditions I run. Another example is how I tried various split top deck setups, even though I found the one-piece best last year.

Early on I tested a lot of springs back-and-forth, and even though I still really like my (maybe quite unique) setup from last year with Axon HLS C2.15 (Green/Gold) front springs and SLS C2.6 (Blue) rears, in the end I settled on using the Xray XLP springs this year. After a lot of combinations tried, my favourite was always 2.4-2.7 fronts and 2.5-2.8 rears, so progressive springs F & R, something I had not run for a couple of years now, although it used to be my go-to setup on the TRF420.

This setup made the car easy to drive, hence consistant over 5 minutes, but still fast.

In combination with the springs, I also experimented a bit with anti-rollbars again, using a 1.2mm both front and rear for a while, but in the end I returned to the 1.1 front, 1.2 rear setup I preferred already last summer.

In the photos of the car so far, you will see it with low head ‘half-thread’ titanium screws for the top deck, while in later photos you will see the Tamiya/Square blue alu screws are back. This is because it was another thing I wanted to test, and I found the car had more grip and worked better with the alu screws so after that I always run them like usual.

Don’t underestimate the effect of screw choices.

Another thing tested early on was the ‘Stiffy Brace’ made by TheRcRacer, which I posted about back in April.

This worked as expected in calming down the rear end of the car making the rear more stable. A really useful option to have if you race on high-grip or in tricky conditions, but not really needed for the particular conditions I run my testing in.

Like already mentioned, I did a lot of tests with various top deck (essentially flex) setups.

While the ‘favourite flex setup‘ found last year in the end still turned out to be my favourite this year, I did also find my favourite flex setup with split top decks.

I tried various optional split upper decks available from other manufacturers as well, but the car worked the best for me with the Tamiya kit ones. The important bit was to add a 1mm brace at the rear though (see photo above), as this made the rear just that bit more stable for a perfect balance for me. Overall, playing with these braces of different thickness (both front and rear) is a really effective way to tune the balance of the car to the track.

When I run split top decks, I do not run the motor mount screw (bottom) in front of the spur, while I use this when running the one-piece  top deck.

As I already mentioned, at the end of testing, the once-piece was still the winner though as it gave that little more grip overall.

In photos up to this point you will have seen cut upper arms, while here the ‘standard’ upper arms are back.

I originally cut them last year, and you can read about that in last year’s report, but I always wanted to check this again and now ended up liking the original non-cut arms better.

For this year I wanted to take away the friction difference, so I made sure they were so free they would fall under their own weight (upper arms only). Basically I used a lighter to warm upp the thin rib before fitting them, and this worked well.

With the arms cut the car is a bit more lively, while the standard arms make the car more precise, and I liked this feeling more.

I spent most summer running high diffs front and rear, as this was one way I thought I might get more speed from the car, but in the end when I tried lowering them again, the car instantly had more grip and was just that little bit faster.

The car definitely stalls up more in the middle of the corners on low diffs, but the extra grip makes up for it on the track I run on, which lacks any big, fast and flowing corners really.

I’m sure the balance on this choice could/would swing on a faster and higher grip track.

I also should have got the 0.5 diff holders to test.

Above, (and in the following photots) the car is photographed now after the season.

You will notice an aluminium chassis on the car – the TRF #42290 from Tamiya, and this was the last major item I tested.

Testing that on our quite low-grip track was really interesting. Up until this I had only used the kit standard carbon chassis, and I did not expect the alu chassis to be any good, but I was surprised at how good it actually was.

To start with the car was setup the same (with roll centers corrected to make up for the 0.25mm thinner chassis), and it was not very far behind on times. After some small adjustments – like moving to the split top decks and removing some screws to increase flex, it was even closer on pace.

Still, it was always a bit behind compared to the carbon chassis, and between these two I will always go for the carbon one on this track. But I can see many conditions where you would go for the alu chassis, as it is very logical and straightforward to drive.

As for flex, my favourite when running the alu chassis was split top decks, without the rear brace, and with two screws removed front and rear. Ideally, I think you would need softer top decks for these conditions.

Another thing I tried during the season was damper pistons with the holes drilled to 1.2mm, but back-to- backing these a couple of time I felt more connected to the car using the standard pistons – still turned upside down like last year to avoid the stickiness at full droop.

Last year I run an RC Maker (now INOV8) rear body mount on the Twister body, but this year I decided to use the kit vertical posts with Axon holders on the posts. Not really sure what is my preference here, and I would need to do a proper comparison with the INOV8 ultra-lite horisontal mounts. Vertical is easier to use, but the car felt good with the RC Maker mounts, so I might need to re-visit this.

I once again run the Corsatec 5000 shorty batteries (together with CTX-10 RS ESC) just like last summer.

I had wished Tamiya would release ballast shorty holders like seen on Akio Sobue’s car at the JMRCA TC Champs this year, but I had to run the standard holders and use my old tungsten ballast weights again instead. I also run a 20g weight plate under the battery.

Late season I removed the two 5g weights under the front outdrives and placed the weight in the servo/front of battery area (not seen here since I removed that weight when running the heavier alu chassis).

The TRF421 run reliably throughout the summer, and I don’t think I broke any parts at all.

I rebuilt the dampers now and then of course, as well as the gear diff (do remember to do this every once in a while!), and I replaced the driveshaft caps (blades) once front and rear, and the car is still remarkably fresh after two outdoor seasons and 250 packs through it.

After two seasons, I still like the TRF421 as much as I did from the first time I drove the car.

It’s a car that always works, and is very easy in every way. Easy to work on and easy to drive. Oh, and it’s beautiful! 😍

After this time, it’s also obvious that this isn’t just how I feel about it, as time and time again people who have some experience and have tried the car comments how well it works.

When you add the fact that it’s a also a very competitive car at the highest level, as Akio Sobue has proved twice at the TITC now, it’s a car I can only recommend more people to try.

Above you see not the most consistent, but still the fastest run done this year with the TRF421.

Below, the track used for all testing this year as I did not go to any other track for the first time in many years!

As I went back on several things at the end of the season (high diffs, split top decks), the setup I ended up with this year is not so different compared to last year, but there are many small differences, which worked their way into the car as I spent late summer fine tuning everything.

Just like last year, testing was again done on the excellent Rush Vr3 32X preglued tyres (High Precision/Type A Yellow – part# RU0862) , and these contine to impress me. I also still run a standard Twister body as it is my favourite.

It will be interesting to see what one will be running next summer, and I do hope there is something new from Tamiya when that time arrives. At least some new parts, as that is the one thing I will criticise with the TRF421 – the fact that the only option released for the car from Tamiya since its release is the alu chassis. But I will write another aticle on this later.

Thanks for reading, and hope you found something interesting in here.