The Science of Choice: Gear Ratio & Telemetry Analysis in Rotax MAX Master (400 lbs Class) AMP track
Author: Eugene Panenko
Date: June 2025
Category: Performance Engineering / Karting / Rotax MAX Master
🏁 Why I Did This
When a racer and coach reaches for a sprocket, it's no longer about mechanics — it's about meaning. This is a battle between intuition, experience, and hard data. This article is my personal quest to discover truth between revs and seconds, between weight and elevation, between what “feels right” and what actually is.
📌 Why read to the end? Because this isn’t just telemetry — it’s a real race case, where every tooth on a sprocket decides overtakes, and every tenth lost is a lost position.
I write this both as a driver who feels the engine in his spine, and as a coach who must justify every choice to his students — not with words, but with lap times.
🔢 Introduction
In the Rotax MAX Master class, where kart + driver weight totals 400 lbs (181 kg), choosing the optimal final drive ratio is not a matter of preference but a requirement. The Rotax MAX Evo engine struggles to deliver torque at low RPM under heavy load. This drastically affects how well we can stay in the powerband, especially during exits from slow corners.
This study compares three sprocket combinations: 73/12, 75/12 (theoretical), and 78/12, using real on-track telemetry. RPM and GPS speed histograms were used alongside calculated engine power to determine real-world efficiency.
📊 Methodology
Data Acquisition
- Telemetry System: Mychron 6 with AIM Race Studio 3 (RPM & GPS histograms)
- Track Conditions: Dry with medium grip
- Driver: Rotax Master class (400 lbs total weight)
- Engine: Rotax MAX Evo 125 Senior, fresh piston, fully legal spec
Tested Final Drive Ratios:
All sessions were conducted in similar weather and fuel conditions.
📝 Analysis
RPM Distribution Comparison
- 73/12: Drops to ~7500 RPM on exits — well below optimal torque zone. But allows strong top-end speeds (66–70 mph)
- 78/12: Frequently revs above 13500 RPM — beyond peak power zone. Helps in tight corners but costs speed on straights
GPS Speed Distribution
- 73/12 spends more time above 62 mph (up to 33%)
- 78/12 clusters time in 52–62 mph, showing early over-revving and aerodynamic drag losses
Average Lap Power (Calculated)
Based on interpolated Rotax power curves:
High speed strength, weak off corners
Balanced — theoretical fastest lap
Strong exits, but fades at top-end
🧰 The Weight Factor: 400 lbs Rotax Reality
At 400 lbs combined weight, even 78/12 struggles to reach 14500–14800 RPM. No matter the ratio, the engine rarely reaches true top-end.
Conclusion: every setup has trade-offs
- 78/12: Great off the line but loses momentum up top
- 73/12: Weak at exits, even if speed is recovered later
🧩 Mid-Corner Superiority of 73/12
One key finding was that 73/12 significantly outperforms in the mid-lap high-speed section — specifically, after a set of S-corners leading into two full-throttle uphill right-handers.
- 78/12 revs climb into 12100–13000+ RPM — where Rotax power drops
- 73/12 remains in 11300–12000 — nearly perfect peak power range
Result: a consistent 0.4-second advantage for 73/12 in this section alone.
- Slow corners (two key ones) reveal the 73/12's weakness
- Rear straight includes a downhill → into uphill. With 73/12, resistance and weight stop RPM from rising. With 78/12, revs recover too slowly to reach usable top-end
Drafting during race telemetry showed up to 800–1000 extra RPM and speeds reaching 73 mph, solely from reduced frontal drag.
📌 Takeaway: There is no universal gearing answer for this track. Similar lap times arise from different gear strategies. It depends on:
We will continue testing and enter races using a compromise setup. Ultimately, the stopwatch is the final authority.
✅ Final Verdict: The Compromise
75/12 stands out as the smart middle ground:
- Restores needed torque in corner exits (7700–8150 RPM range)
- Maintains strong mid-high speed performance without over-revving
- Average calculated power just 0.1 kW behind 78/12 — more stable overall
Recommended Setup:
Run 75/12 in race trim. It balances powerband coverage across all sectors, reducing weaknesses at both ends of the track.
📌 Next Steps
- Run a training session on 75/12
- Log fresh telemetry: overlay throttle traces and sector deltas
- Measure RPM specifically on uphill exits
- Evaluate racecraft feel and traffic behavior
📈 Attachments:
- RPM & GPS Speed histograms
- Rotax MAX Evo power curve reference
- Speed distribution chart (73/12 vs 78/12)
