Carpet Base Setup

Tamiya TRF421X - High Grip Carpet Set-up

A team of fellow TRF421X racers and I have been putting in a lot of laps on carpet, experimenting with different settings and sharing our findings. So I thought this would be a good opportunity to share my current setup and, more importantly, explain some of the changes that have made a noticeable difference to the car's performance.
There have been a few particularly interesting discoveries, some of which go against what you might traditionally expect from a touring car setup.

As you can see, there are a few things that are quite different from more conventional setups.

Three changes in particular have made a real difference.

1 : A stiffer rear anti-roll bar
One of the biggest changes has been running a 1.3 mm rear anti-roll bar. For me, this has been transformational.

The car feels incredibly reactive throughout the entire corner, from entry through the apex and onto the exit. It also remains exceptionally flat, which gives it a very direct and precise feel.

Running a stiffer rear anti-roll bar relative to the front generally encourages the car to rotate, as it increases the proportion of lateral load transfer occurring at the rear axle. The potential downside is that this can reduce rear grip, particularly in fast corners where the tyres are already working hard.

This is where the spring balance becomes important. I've found that running softer rear springs is key to making this combination work.

Initially, I was using 2.65 rear springs, which worked very well, although the rear could occasionally become a little nervous in fast, tight corners when running at full throttle. I experimented with 2.70 springs, thinking that a little more stiffness might help lock the rear down. Interestingly, it had the opposite effect, and the car became much more prone to snap oversteer.

Moving down to 2.60 rear springs made a remarkable difference. The rear became much more stable, even through high-speed sweepers, while the car retained all the steering and responsiveness that I liked.

The softer springs help recover some of the rear mechanical grip and compliance, while the stiffer anti-roll bar continues to control roll. It's a combination that has worked exceptionally well for me.

Of course, other settings play a part. I'm running 450 cSt shock oil with four-hole pistons, and the damping influences how quickly the suspension responds to weight transfer. Changing the oil or piston configuration could alter the feel of this combination, particularly during rapid direction changes.

The important takeaway is that a stiffer rear anti-roll bar doesn't necessarily mean you have to live with a nervous rear end. Finding the right spring balance can make all the difference.

2 : Longer rear toe-control links
Another change that has made a noticeable difference to the feel of the car is moving the rear toe-control turnbuckles to the outer mounting holes.

This effectively creates longer rear toe-control links, changing the geometry of the rear suspension.

Interestingly, this was suggested to me by a mechanic (Thanks JK), who works on full-size cars, who felt that the longer arrangement offered some advantages.

The theory is quite straightforward. As the suspension moves up and down, the rear toe-control links travel through an arc. Changing their length and mounting position alters this movement and therefore the amount of toe change that can occur as the suspension compresses and extends.

Running a longer link can potentially reduce unwanted bump steer, helping the rear wheels maintain a more consistent alignment as the suspension works.

However, the actual effect depends on the relationship between the toe links, suspension arms and mounting points. A longer link isn't automatically better in every configuration. I've retained the same spacers and 2.5 degrees of static rear toe-in, so the change is primarily about how the suspension behaves dynamically rather than altering the basic rear alignment.

This becomes particularly relevant when the car is cornering hard. The outside rear suspension compresses and the inside extends, the geometry can introduce small changes in rear toe. Depending on their direction and magnitude, these changes can influence how stable or reactive the rear feels.

My aim is to have a rear end that remains predictable under load, particularly when using the relatively stiff rear anti-roll bar.

I haven't measured the exact bump-steer curve between the two mounting positions, so I can't definitively say how much toe change has been reduced. However, the longer-link arrangement has felt very good on track.

I've shared this change with several other TRF421X racers, and the feedback has generally been positive.

3. Less droop than you might expect
The third significant change is the amount of droop I'm running.

If you look at my setup sheet, you'll notice that the downstop values are quite high, meaning I'm actually running considerably less suspension droop than we would traditionally use. Many of us are accustomed to droop settings around the 5–6 mm range, so these figures might initially look a little unusual.

I've been using the MR33 v2 droop Gauges for some time, so I hadn't really appreciated just how far I'd moved away from my previous settings until I translated the measurements for the setup sheet!

The reasoning behind this change is largely related to the surface we're racing on. Carpet is generally very smooth, so I just wanted to explore things further. Reducing droop limits how far the suspension can extend as the chassis pitches and rolls. This can make the car feel more immediate and responsive, particularly during quick direction changes, and this is what I experienced. The TRF421X felt very direct, reacting quickly to steering inputs without excessive chassis movement.

There is, however, a compromise. With less droop, the suspension reaches its extension limit sooner. This can unload the inside wheels more abruptly during cornering, potentially making the car nervous if the rest of the setup isn't balanced correctly.

The stiffer rear anti-roll bar helps control roll and encourages rotation, while the softer rear springs provide the compliance needed to maintain rear grip. The reduced droop contributes to the sharp, reactive feel.

It's the combination of these settings, rather than any one adjustment in isolation, that seems to work so well.

I wouldn't necessarily recommend immediately copying my droop settings without considering your track conditions, but on smooth carpet they've certainly been an important part of making the TRF421X feel so responsive.

Other settings worth mentioning
There are a few other things I've been experimenting with that are worth highlighting.

Running a stiffer rear differential. I generally have 5,000, 7,500 and 10,000 cSt diffs ready for my touring cars, but at the moment I'm not using the 5,000 at all. If the grip is relatively low at the start of a race night, I may begin with the 7,500. However, once the grip comes up, I generally move to the 10,000.

I'm also using progressive 2.55–2.85 front springs. At one point I thought the car was grabbing too much in the corner. so I went a little stiffer. I think now I would be fine with 2.5-2.8 or just 2.6 linear at the front. I have just left these on as the car is great as is.

The front and rear suspension arm mounts remain largely at their kit settings other than no spacers under the front. I find this changes the feel of the car under braking, making it more flat.

I run theRcRacer Soft Front stiffener. (Free download link here) This just takes a slight edge of initial bite over a solid brace.

Like all setups, all of my findings are subjective. We all have our own driving styles, and what works brilliantly for one driver might not necessarily suit another. However, I can say that I've been sharing information with other TRF421X racers about the longer rear toe-control links and the stiffer rear anti-roll bar, and both changes have had a positive impact in most cases where they've been tried.

What I particularly like about my setup is that the car feels incredibly reactive without becoming difficult to drive. It has loads of steering, changes direction quickly and remains stable even through fast, demanding corners.

Rather than simply making everything stiffer to control the chassis on higher-grip carpet, I've found that using the anti-roll bars and droop to influence the car's response, while retaining softer rear springs for grip, has produced a really effective balance.

Enjoying RC Racing !
As an aside, I really cannot emphasise how much I'm enjoying racing at the moment. Things have been very stressful this year, and getting out racing has been a fantastic release. The Tamiya TRF421X has only helped make that experience even better for me :)