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Driveshaft critical speed calculator

Every driveshaft has a speed where it starts to whip and vibrate. This finds it from your tube size and length, and translates it into the top speed your gearing allows.

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Driveshaft tube
Vehicle

Critical speed

–
Rotating speed where the shaft starts to whip.
Safe operating limit
–
Top speed at that limit
–
Inside diameter
–

Longer means slower

Critical speed falls with the square of length, so a longer driveshaft loses safe rpm quickly. Going to a larger diameter tube is the usual fix.

The formula

Critical speed (rpm) = 4,705,000 × √(OD² + ID²) ÷ length²

This is the standard formula for a steel tube, with diameters and length in inches. Aluminum's stiffness-to-weight ratio is close to steel's, so the result is similar for aluminum tubes of the same size. Carbon fiber shafts differ and should use the manufacturer's rating.

Driveshaft speed equals wheel speed times the axle ratio, so numerically higher gears and smaller tires spin the shaft faster at a given road speed. Staying well below critical speed, commonly at 65 to 75% of it, leaves room for balance and u-joint condition.

Frequently asked questions

What actually happens at the critical speed?

The driveshaft begins to whip and vibrate like a jump rope at that rotational speed, which can cause serious vibration, driveline damage, or in extreme cases failure — it's a resonance point you want a real safety margin below, not right up against.

Does a shorter driveshaft always have a higher critical speed?

Yes — critical speed drops sharply as length increases (the formula divides by length squared), so a shorter shaft can spin considerably faster before reaching the same whip point as a longer one of the same diameter.

Why do both outside and inside diameter matter?

A tube's stiffness comes from its cross-section, and both diameters together determine that — a larger diameter tube (even with the same wall thickness) is meaningfully stiffer and pushes the critical speed higher than a smaller-diameter one.

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