Shaftmaster’s 3.7L V6 Mustang Driveshaft Failures

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Shaftmasters 3.7L Mustang Driveshaft

Both Myself And Frank have had interesting discussions with Shaftmasters. We were talking about the reasons why the Shaftmasters had stopped selling aluminum drive shafts for 2011 – 2012 3.7L V6 mustangs. Here is a detailed breakdown by Frank (westcoastsc)

By way of introduction, I’m a Metallurgical Engineer for an aerospace company in southern California with over 34 years of experience, most of that spent on what is known as material failure analysis. This is an engineering process by which one determines root cause of a failure and corrective action to prevent re-occurrence of the failure.

During my discussion with Robert Eppic, he mentioned that there have been OEM drive shafts that have also failed. He also mentioned that all of the aluminum drive shaft failures happened during dyno testing. It would be interesting to find out what type of dynometer (Mustang or Dynojet, or other) was being used when the drive shafts failed. Bob also mentioned that the tube portion of the V6 driveshaft is longer than the tube used in the GT driveshaft. Length of a drive shaft, as we will see later, is a component of the resulting angular deflection. He also stated that 10 to 20 times more GT drive shafts had been sold, without any failures.

Mention was also made by Bob that setting the correct pinion angle is critical to the life of any driveshaft installation. He mentioned that at least one owner had incorrectly set his pinion angle, after lowering his car. Bob also stated that one driveshaft failure occurred after the exhaust system was replaced, but the driveshaft had not been removed for the welding operation. Typically, a “mig” (GMAW) welder is used to weld exhaust pipes by most muffler shops.

Bob made mention of two of the three Shaftmaster failures. One of the failures involved a customer whose driveshaft failed on a dyno at 7100 RPM. The second customer’s drive shaft failed at a speed of approximately 140 mph. This failure was the worst Bob had ever seen. He mentioned that the shaft broke as well as the yokes. Bob said that this same customer also failed an OEM drive shaft on the same dyno.

Unfortunately there are still more unknowns concerning 2011-2012 mustang v6 driveshaft failures, than there are knowns.

For example, we know that the material used to make a Shaftmaster drive shaft is made from 6061-T6 aluminum. Steel drive shafts are typically made from AISI 1040 or 1045 steel. Aftermarket drive shafts can be made from low alloy steels, such as AISI 4310 or exotic materials such as carbon graphite. These different materials of construction have advantages and disadvantages. One being the maximum revolutions per minute (RPM) they can withstand before failure occurs.

There is a Mark Williams chart for determining maximum RPM based on material of construction and shaft diameter (LINK) see below:

Driveshaft Tech and FAQ

Operating Angle

Operating angles in a driveshaft are the angles between the pinion, driveshaft and transmission centerlines. The optimal angle for any driveshaft to run at is 0 degrees, where many vibrational and frictional problems are non-existent. In order to minimize power loss and vibration in an offset configuration, the pinion centerline and the transmission centerline need to be parallel. In general, the largest angle for racing applications should 2 degrees and the centerlines should be parallel within 1/2 degree. With suspension movement the operating angle will increase, but should not exceed 15 degrees. If the centerlines are off too far, the u-joints travel at uneven operating velocities, causing vibration (this is the same problem induced by poorly phased end yokes). This vibration is hard to distinguish from an unbalanced driveshaft.

Critical Speed

Critical speed is the speed at which a spinning shaft will become unstable. This is one of the single largest factors in driveshaft selection. When the whirling frequency and the natural frequency coincide, any vibrations will be multiplied. So much that the shaft may self destruct. Another way to think of this is that if a shaft naturally vibrates at 130 times a second, and one point on the shaft passes through 0 degrees 130 times a second (7800 RPM) then the shaft has hit a critical speed. There are several ways to raise the critical speed of a driveshaft. You can make it lighter, stiffer, or increase diameter without increasing weight. This is the reason carbon fiber makes a good driveshaft, it is stiff and light and can be made to any diameter or wall thickness. Aluminum, while it has a very good critical speed is not quite as strong as steel. Steel, with good strength characteristics will have a lower critical speed.
As one can see for 6061 aluminum (and all other materials of construction), the length of the driveshaft dictates the maximum RPM that the driveshaft can safely be run at. The longer the tube portion of the drive shaft, the lower the maximum RPM the tube can tolerate before failure occurs. The one item missing from the above table is wall thickness. This also plays an important part in determining torsional stiffness and maximum life of a driveshaft.
We know that there were two different drive shaft diameters offered by Shaftmasters, 3-½” and 4” diameter shafts. All things being equal, there is approximately a 900 RPM difference in the maximum allowable RPM between a 3-½” and a 4” diameter driveshaft. The difference in maximum RPM decreases as the driveshaft length decreases. The lengths of both drive shafts, per Bob are over 43 inches. All failures of Shaftmaster drive shafts have been the 3-½” drive shafts. As I previously noted, Bob also mentioned that the V6 driveshaft is longer than the V8 driveshaft. Wall thickness of the 3-½” drive shaft was 0.125” and 0.083” for the 4” drive shaft.

The last item to consider is weight. Since weights of these drive shafts are not in either table, we can estimate the weight of a drive shaft tube by the following formula:

Density = Mass/Volume, or stated another way, Mass = Density x Volume.

The volume of a hollow shaft is: L * Pi* (OD2/4 –ID2/4).

Area of a hollow 3-½” x .125” t driveshaft            = 1.32 sq. inches.

Area of a hollow 4” x .083” t driveshaft     = 1.02 sq inches.

Volume for a 3-½”: x 43” long drive shaft = 56.8 cubic inches.

Volume of a 4” x 43” long drive shaft = 43.9 cubic inches.

Density of 6061 Aluminum is 0.098 lbs/in3,

Density of 4130 or carbon steel is 0.283 lbs/in3. This is about three times more dense than aluminum. To break even, from a weight standpoint, the volume would need to be 1/3 that of Aluminum (.042” wall).

Approximate weights of the aluminum drive shaft tubes are 6 lbs for the 3-½” drive shaft and 4 lbs for the 4” drive shaft. The yokes and other hardware make up the majority of the drive shaft weight.

The 4” diameter will weigh less and will be able to withstand higher RPMs than the 3-½” drive shaft. This has been proven by Shaftmasters, as they have had no failures for their 4” drive shafts.

The last thing Bob related to me was that the proposed new design would decrease the tube length by 4”. As you can see from the Mark Williams charts, a shorter drive shaft (done by increasing the lengths of the yokes) would be able to withstand higher RPMs.

Bob also made mention of the high heat that occurs underneath a car strapped to a dyno, and that this is an unusual condition. During drag racing or normal driving, air is moving underneath the chassis, so heat is not a problem. 6061-T6 aluminum is typically heat treated (aged) to the T6 condition by placing the tube into the furnace at 350º F. Operating temperatures which meet or exceed 350º F will tend to overage 6061 aluminum.

Normally 6061 aluminum in the T6 condition has an ultimate tensile strength of 45,000 psi (pounds per square inch) and a yield strength of 40,000 psi. Elongation can be as low as 12%. Shear strength is 30,000 psi. Torsional drive shaft failures are affected by the shear strength of the drive shaft material.

When we expose T6 aluminum to temperatures of 350º F, yield strength drops to 22,500 psi. This means that shear strength can drop to 16,785 psi (» YS * .75). This is approximately ½ (65%) of shear strength at room temperature. This can drop the maximum RPM, listed in the Mark Williams chart to 65% of the stated RPM values.

For the 3.5” shaft, this becomes 6900 RPMs. The 4” shaft has a maximum RPM of 7880 RPM.

These are the recommended RPMs that one could safely expose the drive shaft to. If we look at a typical horsepower curve for the V6 mustang, horsepower is still rising at 6800 RPM, especially for a supercharged car. It may be tempting to see how much horsepower we can squeeze out of the V6 mustang, but I would tell my dynotuner to limit my maximum RPMs to 6800 RPMs if I had a 3.5” aluminum drive shaft. In all cases, make sure your dyno tuner does not run several HP pulls back to back without allowing the engine and drive train to cool down, especially if you are running an aluminum drive shaft.

I can understand why the 3.5” diameter shafts are failing above 6900 RPMs. However, without a thorough failure analysis, this may not be the only cause for failure. There may be other contributing factors for failure. For example, a gouge on the drive shaft tube will reduce the maximum RPM since it creates a stress riser, which increases the stresses concentrated in the gouge. This is also a likely contributor, especially for the OEM drive shafts.

A 4” diameter aluminum drive shaft appears to be the way to go if weight and price are your primary concerns.  However, since the 4″ diameter shaft is a larger diameter than the 3-½” driveshaft, clearance issues between the drive shaft and adjacent parts could be a problem. The metal matrix composite or composite drive shafts cost more, but have higher stiffness and lower weights (due to lower densities) than even the aluminum drive shafts. The steel drive shafts weigh more, and offer similar to lower maximum RPM ratings, but are not affected by the low temperatures that reduce aluminum strength levels.

I would venture to guess that most of the aluminum drive shaft failures were due to operating environment. I cannot comment on the OEM drive shafts because I have not seen high magnification photographs of the fractured tubes. There is one OEM drive shaft failure for a V6 mustang that I did find, but it was not possible to discern from the photograph, the nature of the failure.

The problem was discovered and the V6 mustang driveshaft issue resolved.
 Installation of the V6 Mustang Aluminum Driveshaft