9:1 puts more load on the motor compared to 10:1 , that 10:1 is already too high; AE recommends 12.85:1 FDR with 8.5T motor and AE recommendations are a bit overgeared on some surfaces. The more grippier the track, the more strain on the whole drivetrain.

Running 84 Spur with 19 Pinion on mine, as recommended by AE, on dirt runs flawless, motor temps never surpass 65°C (150°F), raced it on sunday on grass; same setup, motor temps were 100°C+ (220+°F).