Critical Speed Chart

Critical Speed is the RPM at which the driveshaft is computer projected to bend or whip. Exceeding Critical Speed can produce vibrations that can result in driveshaft failure.

The vibrations can also cause damage to the differential gears and bearings. The transmission extension housing, along with the torque converter and planetary gears can also be destroyed.

When you take a small piece of steel and bend it repeatedly, the stress will eventually break the steel. The same effect occurs on your complete driveline when no attention is paid to Critical Speed.

How to read the Critical Speed chart

Find your tube material and diameter in the left column, then read across to your driveshaft length. Length is measured center to center of the U-joints, not the raw tube length. The number where the two meet is the highest RPM that combination should ever see. A dash (—) means we don’t build that tube in that length. If your shaft falls between two columns, use the longer length — it’s the safer figure — and keep your driveshaft RPM under the number shown. In a 1:1 high gear the driveshaft turns at engine speed; in an overdrive gear it spins faster than the engine, so always check your highest gear.

What sets a driveshaft’s Critical Speed

Three things decide how fast a shaft can spin before it whips:

• Length — by far the biggest factor. Critical Speed falls off with roughly the square of length. Add about 10% to a shaft and its safe ceiling drops close to 17%. That is why long shafts — trucks, four-links, circle-track cars — are the ones that get into trouble first.
• Diameter. A larger-diameter tube is far stiffer for its weight, so stepping up from 3″ to 3½″ or 4″ raises the ceiling quickly — often by more than a longer shaft takes away.
• Material stiffness. Mild steel and chromoly share almost the same stiffness-to-weight, so a chromoly tube of the same size carries the same Critical Speed as mild steel — its advantage is torque capacity and a lighter wall, not more RPM. Aluminum and, especially, carbon fiber change the math: carbon fiber’s exceptional stiffness-to-weight is why it turns the highest RPM of any tube on the chart above.

What to do if your combination is over the limit

If your target RPM is close to or above the chart figure for your length, there are three proven ways to move the ceiling back up:

• Step up to a larger-diameter tube.
• Move to a stiffer, lighter material — aluminum or carbon fiber.
• Split a very long shaft into a two-piece assembly with a center support bearing. Halving the effective length of each section roughly quadruples its Critical Speed.

Not sure which way to go? That is what we do. Call PST at 1-727-442-1711 or send us your numbers and we’ll spec a custom driveshaft that stays safely under Critical Speed for your length, power and RPM — whether it’s for drag or circle track. New to this? Start with Drive Shafts 101.

Critical Speed: common questions

What is driveshaft Critical Speed?
It is the RPM at which a spinning shaft starts to bow and whip on its own axis, like a jump rope. Run past it and the vibration can destroy the shaft, the U-joints, and the transmission tailhousing and rear gears behind it.

What happens if I exceed it?
Expect a heavy vibration that gets worse with speed. Held there, the tube fatigues and eventually fails — the same way a piece of steel bent back and forth over and over finally breaks.

Does a bigger driveshaft raise Critical Speed?
Yes. A larger diameter is the fastest way to raise the ceiling, which is why we often answer a high-RPM, long-shaft application with a 3½″ or 4″ tube.

Is carbon fiber worth it for high RPM?
For long shafts turning big RPM, yes. As the chart shows, a carbon-fiber tube can run thousands of RPM higher than steel or aluminum of the same length.