1. Track and Weather Conditions
Conditions at AMCA Apeldoorn were consistently dry throughout the weekend. Friday saw moderate grip with ambient temperatures around 20°C.
On Saturday, with temperatures rising to 20–22°C and a large number of drivers on track due to a concurrent Mugen Track Day (approx. 50–60 attendees), grip levels increased to medium-high. Sunday brought warmer conditions (22–25°C), with slightly lower grip levels than Saturday but still very usable. The track was in excellent condition all three days, offering a reliable testing environment.
2. Key Observations and Initial Challenges
At the beginning of the test sessions, the TRF421 was extremely difficult to drive. The rear end often broke loose at corner apexes in an unpredictable and uncontrollable way. By the end of the weekend, however, the car was transformed: stable, consistent, and confidence-inspiring.
One issue to address going forward is the unusual amount of damper leakage observed during the weekend. This will likely require replacing the O-rings before the next event.
3. Testing Process and Setup Evolution
Friday – Suspension Geometry and Rear-End Stability
3.1. Spacers under the Rear Lower Suspension Arms
The day began with tests focusing on the effect of spacer height under the rear lower suspension arms. Initially, 2mm spacers were used, which contributed to the rear instability noted earlier. Reducing the spacers to 1mm brought a clear improvement in rear-end control. Finally, removing the spacers altogether (0mm) significantly increased stability, making the car much easier to drive consistently.
3.2. Rear Chassis Stiffener (T-Brace)
Later that day, I tested a rear T-brace (Stiffy) supplied by TheRcRacer some time ago. The additional rear-end stiffness calmed the car down further but slightly reduced turn-in on corner entry. This trade-off is manageable and can be compensated for with other tuning options.
Saturday – Core Geometry and Chassis Flex
3.3. Camber, Caster, and Droop
I systematically explored a wide range of camber settings—up to 3° rear / 2.5° front—down to minimal values. The best compromise was found at 1.5° front and 2° rear, yielding a balanced mix of agility and rear traction.
Caster was tested at the rear only. For a track as open and flowing as Apeldoorn, a rear caster setting of -0.5° (corresponding to the longest wheelbase and maximum toe change under load) proved to be optimal.
As for droop, a front-to-rear difference of 0.8 to 1mm was effective. There may be further potential in increasing front droop slightly to improve corner-exit acceleration.
3.4. Top Deck Configurations
From past experience, I already suspected that the single-piece top deck would not be well-suited for outdoor conditions—it tends to overly restrict flex and reduce steering response.
This was confirmed once again during testing. The two-piece top deck, particularly the stiffer version provided by H2RD (many thanks for the ongoing support), delivered significantly better performance. For outdoor tracks, I would strongly recommend the two-piece variant.
3.5. Motor Mount Flex Settings
With help during one of the practice sessions, I tested different flex settings by adjusting the screw configuration at the motor mount. The most precise and stable handling came from the fully screwed-down setup—the stiffest option—eliminating vagueness during cornering and improving steering accuracy.
3.6. Front Anti-Roll Bar
Later in the day, the car exhibited some sluggishness during fast directional changes. To address this, I experimented with different front anti-roll bar stiffness levels. The stiffer bar significantly improved chassis response and made the car feel more agile through fast transitions.
Sunday – Differential and Spool Height Tuning
3.7. Rear Diff Height
Testing began with rear differential height adjustments. While the effects were relatively subtle, the second-lowest position offered the most consistent and forgiving behavior—likely in conjunction with the low ride height used.
3.8. Front Spool Height
Changing the front spool height brought dramatic differences in handling:- With the spool in the highest position, the car felt "pulled" through corners, similar to a front-wheel-drive car. This improved rotation but slightly hurt overall pace.
- With the spool in the lowest position, the car had excellent cornering speed and traction. It felt planted and composed—lap times and consistency both improved notably.
4. Summary and Key Takeaways
- Spacers under the rear lower suspension arms should only be used if the track has very high grip. In medium-grip conditions like those seen this weekend, running the arms as low as possible provided better traction and predictability.
- The two-piece top deck is clearly the superior option for outdoor conditions.
- A low front spool height significantly enhances drivability and performance.
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