“If you run at critical speed you will have a driveshaft failure,” he adds. “The usual failure mode at critical speed is the driveshaft tube will unravel like a paper towel roll. It’s not good and the driveshaft will almost always take something more with it.

“The theoretical calculation for critical speed is pretty simple. We do critical speed calculations for customers all the time, and we’ve adjusted our calculation to be closer to the real world. You can also influence critical speed by how the driveshaft is built and where the balance weights are placed. We have done a lot of work with critical speed, our NASCAR customers have always really pushed us to get more out of our driveshafts than anyone else. We do see racers going through critical speed and then run above it, but that is still risky because each time you pass through critical speed you are fatiguing the driveshaft and that ultimately will cause a failure.”

One big advantage to using more expensive materials like aluminum or carbon-fiber is that the lighter weight allows a higher critical speed. Gorsuch warns, however, that it doesn’t matter how high the quality of your driveshaft if you don’t have a good tight fit with the transmission’s tailhousing as well as the rearend. “If the bushing in the transmission tailhousing is worn you can encounter a vibration,” he says. “This is due to the fact that the driveshaft is now spinning off axis. Because of this it is always a good idea to inspect the yokes and splines to ensure that they are in good shape.”

Aluminum and carbon-fiber driveshafts are quite a bit lighter than steel, but they can also be wider. How big is this tradeoff when considering the moment of inertia?

Acceleration is a big part of circle track racing, since you are trying to maximize how quickly you can accelerate twice every lap. Moment of inertia is how much force is required to spin something, and there are two factors that increase the moment of inertia: weight and diameter. So the question is essentially, does the increased diameter necessary in aluminum and carbon-fiber driveshafts negate the reduction in weight?

Raymond insists that the increase in diameter isn’t much of a tradeoff because of the drastic reduction in overall mass.

Gorsuch adds that you must consider all the materials used to construct the driveshaft. “Typically, a steel driveshaft will use steel yokes welded to the tube whereas carbon-fiber and aluminum shafts use aluminum weld yokes. The mass moment of inertia will be reduced in a carbon-fiber or aluminum driveshaft simply due to the yoke material change,” he explains. “Also, since carbon-fiber is less dense than steel and aluminum, the tube diameter can be increased with no moment of inertia increase. For example, when looking at just the tubing (no weld yokes), a 2.0 inch diameter steel tube with a wall thickness of 0.095 inch has the same moment of inertia as a 3.1-inch outer diameter carbon tube with a 0.125-inch wall thickness.

What is the practical lifespan of a driveshaft?

“Lifespan is all about what kind of conditions the driveshaft is being used in” Raymond says. “If you are able to keep the joint angles down to 0.5 to 3.0 degrees and not have to deal with any moisture you should be able to run the shaft for an entire season. We have Truck series teams that run several races on a single driveshaft and never do a thing in the way of maintenance. We have other teams only run them for three races and then send the shaft back for inspection, U-joint replacement and high-speed balance. Everyone rebalances their driveshaft after a U-joint change at the higher racing levels.

“Dents and scratches are all stress risers and will most likely be the location for a failure,” he continues. “The tricky part is understanding how close your application is to the elastic limit of the driveshaft. Some Dirt Late Model applications can run all year with a pretty serious dent or scratch if it is causing only a little vibration because of the change in balance.”

Things get a little different for the lifespan of a carbon-fiber shaft versus an aluminum shaft which can suffer from stress fatigue. “For a properly designed, high quality carbon-fiber shaft where the max operational stress is in the fibers, the advanced composite material is highly resistant to the general effects of fatigue and is an excellent substitute material for metals in high fatigue environments,” explains a tech at Quarter Master. “For example, aluminum shafts should be designed to operate at a maximum 30 percent stress level to avoid potential life-cycle fatigue failure. In contrast, the fatigue characteristics for carbon-fiber are far superior to all metals. Published data shows that a stress level of 50 percent, carbon-fiber can operate for 30 years at that sustained load level with a 0.999999 reliability factor. In other words, theoretically, the Quarter Master carbon-fiber shaft could operate continuously at 2,050 lb-ft for more than 30 years without failure.”

And if the unthinkable happens and a driveshaft failure does occur, a carbon-fiber is definitely safer for both driver and car versus either an aluminum or steel shaft. When a carbon tube fails, the matrix delaminates and shreds into fibers leaving very little mass to tear into the car. A steel or aluminum shaft, however, maintains all of its mass and in the event of failure becomes a deadly weapon whipping around just underneath the driver. That shaft can rip through a sheetmetal floorpan like a baseball bat through a sheet of newspaper, and the driver is next. Unfortunately when human flesh and bone tries to go head-to-head with quickly moving piece of aluminum or steel, the metal always wins.

Measure Correctly

Measuring your driveshaft correctly is just as important as having the correct material or the right balance. Running a driveshaft that is too long or too short can cause loads of problems. Your driveshaft slides in and out of the tailshaft of the transmission during normal suspension travel. In some of today’s higher-level dirt cars, the suspension moves quite a bit each lap. If there isn’t adequate room for the front yolk to slide in and out of the driveshaft, it can damage bearings and seals, the transmission itself, or in extreme cases, slip off the tailshaft.

That being said, ensure you measure correctly for your new driveshaft is critical. Every driveshaft manufacturer can you the specific way they like you to measure, but here are a few things to always remember.

Start At Ride Height

Having the car at ride height gives you a great starting point to measure from, as you are in the middle of the suspension travel. The rearend is hanging or compressed, the slip yolk may be too far in or out of the tailshaft housing.

Leave Some Room

Your driveshaft needs room to slip in and out of the transmission. Leaving about an inch of room for the yolk to slide into the tailshaft in most cases will be more than enough room to ensure the driveshaft doesn’t damage the transmission.

Check For Obstructions

Check while you’re measuring, but be sure there isn’t anything the driveshaft can come in contact with. Exhaust brackets, or any other clips or brackets can cut the driveshaft causing a failure. Also check for loose hoses or wires. With the driveshaft spinning at a high rate of speed, it is easy for fuel or brake lines, or wires to get tangled in the driveshaft. They can get ripped out of the car causing much larger issues!

Driveline Angles

While this is a topic for a story all it’s own, this is a great time to check and correct your driveline angles if they are off. Extreme angles can lead to premature U-joint wear and failure. Be sure the angles are within the recommended range.

The trick is to select the driveshaft that not only is allowed by the rules but is also the best option available for your application