Ask Away! with Jeff Smith: Examining Common Causes of Driveshaft Failure

I recently went with my buddy to chassis dyno test his LS-powered Nova and toward the end of the first test the car spit the driveshaft out and it went spinning across the floor of the shop! What causes this to happen? I’ve heard this happening much more often on the chassis dyno than on the street or even on the drag strip.

J.P.

Jeff Smith: We’ve also witnessed this a couple of times after watching hundreds of chassis dyno pulls and the result is always violent but luckily no one has ever been injured. To get some straight answers, we called Andy Wicks, owner of DynoTune USA in Watertown, SD. Andy has literally run thousands of dyno pulls in his career as a tuner. “I’ve only had five driveshafts break in all those tests,” Andy says “and all of them were in 300 to 400 horsepower cars.”

According to Andy, in the majority of his experiences, the problems arise from modified cars where either an engine or trans swap has required a different length driveshaft. He says that the person who did the swap was able to make the driveshaft work despite the incorrect length. In the case of a car with a short driveshaft, under load there is insufficient slip yoke engagement into the transmission output shaft. This allows the slip yoke to twist and bind. The moment that happens, it breaks the u-joint which allows the driveshaft to begin flopping around. This usually destroys the transmission extension housing and then in addition to all the broken parts, there is gear oil or ATF all over as well.

Worse yet, that broken driveshaft often continues to spin at engine or rear axle speed which beats up the car’s floorpan, causing major damage that is expensive to repair. Pinion angle can also play a part in this situation, especially if the car is equipped with leaf-springs since they allow the pinion to travel in a greater arc compared to coil spring cars under load. This spring wrap-up ca cause major pinion angle changes. If the pinion angle exceeds the 3.0 degree included angle between the engine/trans and the pinion, this can also bind up the u-joint and potentially cause a failure. In Andy’s experience, broken driveshafts were traced back to either worn components like u-joints and/or poor installation technique.

Another issue worth discussing is something called driveshaft critical speed. This is a situation where the driveshaft is spinning at a very high rpm–5,000 to 6,000 rpm. The spinning shaft creates a frequency. At a given rpm, this frequency will cause the driveshaft to become unstable, vibrate, and eventually cause it to bend into an S-shape. This shortens its overall length and can cause the shaft to bind the slip yoke on the output shaft. That’s really bad news when that happens at 6,000 rpm. This is more than enough stored energy to break into the interior and perhaps injure the driver. There is a complex formula that driveshaft manufactures use to compute driveshaft critical speed that we won’t get into here. However, we can go over the three factors that affect critical speed: overall length, shaft diameter, and material.

There are three ways to increase a shaft’s critical speed. Make the driveshaft shorter, increase its shaft diameter, or change the shaft material. There are multiple combinations of these three variables that create a given shaft’s critical speed. We’ve included an abbreviated chart originally created by Mark Williams that illustrates this point.

As you can see from the chart, the best way to increase critical speed is to shorten the shaft. If this isn’t possible, then changing diameter is the next best move followed by a material change. Note how just changing to a carbon fiber driveshaft radically increases the critical speed. Of course, a carbon fiber driveshaft is also really expensive, so you need to take that into consideration as well.

If you look at these numbers closely, you can see that it’s entirely possible for even a relatively mild street car to hit a driveshaft critical speed. Let’s take a 500-horsepower big block ’66 Chevelle with a 28-inch tall tire and a 4.10:1 rear gear. We’ve simulated this package on our Quarter, Pro drag strip simulation program and it reports that this combination would run 11.11 at 120.5 miles-per-hour. We simulated the package with no torque converter slippage so that our engine speed would be 1:1 with the driveshaft. The simulation reports that the engine is spinning between 5,750 and 5,950 rpm for one full second as it clears the traps in the quarter mile. A stock length Chevelle driveshaft measures roughly 59 inches in length. If we plug a 3-inch mild steel driveshaft into the chart below at 59 inches, we can see that it will hit its critical speed roughly at 4,900 rpm. On our simulation, that occurs almost exactly at the 1/8^th mile mark.

This is bad because the car is not accelerating that quickly at that point and its possible for the driveshaft to hit its critical speed for a long enough period of time to cause it to fail.

Another variable that unfortunately we don’t have room to go into in detail is the driveshaft operating angle mentioned earlier. This essentially is the difference between the engine and trans angle and the pinion angle. These two components need to both be in parallel angles when under load so that the u-joints are spinning within their intended operating angles. If not, this will negatively affect the driveshaft critical speed. If this gets you to thinking about both driveshaft operating angles and the condition, length, and material of your driveshaft then our work here is done and perhaps you won’t suffer the same fate as your buddy.