Critical Speed is an engineering calculation based on tube size, tube thickness, material and length. This calculation determines the RPM at which a given shaft enters its Critical Speed zone. At this speed, the shaft is likely to experience a severe harmonic vibration that will make the shaft try to leave the vehicle. Sustained operation at or above this Critical Speed means certain and rapid failure of the shaft will occur.

    It is important that a racing driveshaft be specified to ensure that its Critical Speed is above the normal operating RPM range of the vehicle. For example, a Late Model 2-barrel car operating below 7000 RPM can safely use a 3.0" shaft. But a Hooters Pro Cup Late Model operating near 8000 RPM cannot.                  

       Critical Speed  3.0" vs 3.5" vs 4.0" Tube O.D.