Harley-Davidson Fat Boy vs RKS RZ125

The Harley-Davidson Fat Boy (2025) and the RKS RZ125 (2026) are compared with the same Moto Kıyas physics model under identical conditions: flat dry asphalt, no wind, a 75 kg rider and no passenger. The Harley-Davidson Fat Boy is a cruiser with 1,923 cc, 103.3 hp and 315 kg; the RKS RZ125 is a sport bike with 125 cc, 14.5 hp and unlisted weight. Power-to-weight with the rider: 264.8 hp/ton versus 63.8 hp/ton. On this measure the Harley-Davidson Fat Boy is 315% ahead.

In the simulation the Harley-Davidson Fat Boy covers 0-100 km/h (speedometer) in 3.54 s and the RKS RZ125 in 13.42 s; the Harley-Davidson Fat Boy is quicker by 9.87 s. In the 100-0 km/h brake test, braking distance without the 0.3 s reaction time is 49.1 m for the Harley-Davidson Fat Boy (ABS unknown, class assumption: no ABS) and 36.4 m for the RKS RZ125 (with ABS). The RKS RZ125 stops 12.7 m shorter. Calculated top speed: 201 km/h (estimated from power and drag) versus 125 km/h (estimated from power and drag).

All values are an estimated simulation, not road-test measurements. Technical data comes from the sources listed below; the model uses class-average calibration (no accepted model-specific road test) for both. Open the interactive app (currently in Turkish) to watch the run side by side and change the load.