Transferring power to the ground is an important function in any race car, with the driveshaft being the critical link that must handle massive amounts of torque. A quick search on the internet for “driveshaft breakage” will reveal many videos of cars spitting them out — plus a collection of still photos with pretzel-shaped driveshafts. A broken driveshaft can also lead to severe engine damage from over-revving.
There are many factors to be considered when selecting a driveshaft. They include the type of shaft material, diameter, thickness and length, yokes, and U-joints that are properly rated for the task at hand. It’s also important to factor horsepower and vehicle weight into the equation; a heavy stock-framed car will put much more strain on the driveshaft than a lighter tube-chassis car.
To get some expert advice we called on Mark Williams Enterprises, the Colorado-based manufacturer who offers no less than seven different driveshaft models, covering a range of applications from E.T. bracket cars to 2,500 horsepower Pro Mods. The company also has a custom-built torsional testing device that can exert up to 15,000 ft./lbs. torque in computer-controlled cycles. They know the limits of driveshaft materials and related components.
One important yardstick for measuring a shaft’s potential is its critical-speed rating. This is when the shaft’s natural frequency is the same as the rotating speed of the driveshaft. When they coincide, vibrations are multiplied and the shaft can go into “jump rope mode” and can shake to the point of driveshaft failure. There are three factors that determine a shaft’s critical speed: length, diameter, and the ratio of weight-to-material stiffness. And like a wind chime, a longer shaft has a lower frequency.
The length of a stock 1969 Camaro’s driveshaft is about 49-1/2-inches. For the sake of comparison, a 50-inch-long driveshaft made of 3-inch diameter mild steel has a critical speed rating of 6,050 rpm. One that’s made of 3.5-inch o.d. 4130 alloy steel is 7,110 rpm, and a 4-inch diameter 7075 aluminum shaft is good to 8,170 rpm. Carbon-fiber (3.75-inch o.d.) takes you to 9,260 rpm. M-W’s chart covers 13 different material/tube size combinations and goes from 44- to 60-inches.
To look at the chart another way, if your driveshaft is spinning at 6,500 rpm, the safe, maximum length of the 3-inch mild-steel driveshaft is 48-inches, a 3.5-inch 4130 shaft can be 52-inches long, and the 4-inch o.d. 7075 aluminum shaft is good for 56-inches. Carbon-fiber takes you just over a couple of inches more.
According to M-W’s Andrew Dickson, a large percentage of driveshaft failures — especially in heavier cars — can be attributed to the U-joints. Spicer 1310 series U-joints can be found in many OEM muscle car applications, but really can’t handle serious race car power. The cups are 1.062-inches in diameter and the overall width is 3.219-inches. That’s why M-W eschews their use and only manufactures driveshafts with beefier 1350 or 1480 series U-joints. The popular 1350-sized joints have 1.188-inch diameter cups and are 3.625-inches wide and rated some 50-percent stronger than 1310, while the 1480 series has 1.375-inch cups and is 4.180-inches wide and is some 40-percent stronger than the 1350. There’s also the consideration of solid U-joints versus greaseable types. For pure race cars, solid is the way to go.
There are also differences in the way U-joints are secured to the yoke. These range from a simple U-bolt or bearing strap to a sophisticated CNC-machined billet-steel cap and bolt or stud setup. Obviously, the latter is the strongest.
While we’re on the subject of U-joints, understand that they do have limitations on how much of an angle they can safely operate at. The recommended procedure is to make sure the centerline of the transmission output shaft and rearend pinion are parallel, with any difference in height taken up equally by both U-joints. Keeping the centerlines parallel throughout suspension travel would be ideal (think four-link) but often difficult with ladder bars and torque arm. The general rule of thumb is that operating angles should be 2-degrees or less and be within ½-degree of each other.
There are five basic materials used to build driveshafts: mild steel, 4130 chromoly, 6061 aluminum, 7075 aluminum, and carbon-fiber. Titanium has been tried, but the cost is prohibitive and shows little gains. In addition to the inherent strength of the material, tube thickness and o.d. are important factors. For example, increasing the diameter of a driveshaft from 3- to 4-inches will result in about an 80-percent increase in strength.
Another factor to consider is the weight of the driveshaft since there are performance benefits to reducing the rotating mass, as well as overall car weight. For the sake of comparison, a steel 3.5-inch o.d. driveshaft (with an M-W Turbo 400 yoke) tips the scale at 20.6 lbs. A similar-sized driveshaft made of 7075 aluminum weights but 13.7 lbs. Ergo, it will take about 33-percent less power to accelerate the driveshaft to operating RPM, with that savings going to propel the vehicle.
The U-Joint Replacement?
The latest development in driveshaft technology from Mark Williams Enterprises involves using CV joints in place of U-joints. They are primarily intended for use in cars with the transmission positioned close to the rear end (like Dragsters, Altereds, and Funny Cars). There are a number of advantages to this, the foremost being the CV joint’s ability to operate efficiently at a much greater angle — important with four-link suspensions. It’s also less prone to vibration. M-W’s CV shaft package, available for “shorty” Powerglide transmissions with 27 or 32-spline output shafts, consists of a tail housing, driveshaft, and pinion yoke. And while the CV setup is substantially more expensive than a conventional driveshaft, it can prevent costly U-joint failures.
Mark Williams Enterprises has featured SFI 43.1-approved driveshafts since the specification’s inception. SFI’s test procedures include applying a 2,500 lb. load, reversing it to 1,000 lbs. and performing 25 such cycles at a rate of at least 3-5 cycles per minute. Any torsional angular deformation of the shaft is subject to certification disqualification. So are signs of buckling or cracks. Whichever class of car you run, you can be assured that an SFI 43.1 certified driveshaft will serve you well.
Another criterion is cost. As you might imagine, a carbon-fiber driveshaft is likely “overkill” in a 12-second bracket car, as it is typically more than double the cost of a steel unit.
So, let’s take a run-through in what choices you have. The least expensive driveshafts are made from mild steel. Stepping up to a more expensive alloy, 4130 chromoly gives you added strength and allows for thinner wall thickness — thusly saving weight. Aluminum is lighter yet, with many manufacturers offering 6061-alloy shafts. Additionally, M-W offers driveshafts made of 7075-alloy aluminum, which is some 30-percent stronger than 6061 and allows the shafts to have thinner walls and save weight. At the top-end of the scale, we have carbon fiber, which is the lightest and has the best critical speed. But it is also the most expensive. Over the years, there have been racers with custom titanium driveshafts, but they are extremely expensive and comparable in weight to carbon fiber. Many racers opt for the 7075 series, as it’s an excellent balance of weight, strength, critical speed, and cost.
Yokes are an important part of the equation, too. There are the end (or “weld”) yokes on the shaft plus transmission and pinion yokes. The end yokes range from standard Spicer 1350 models to CNC-machined, forged 4130 steel, and either forged or billet 7075 yokes that are used on SFI 43-1 rated shafts. In addition to welding the more economical shafts, a proprietary process called Accu-Bond™ is used for upper-end aluminum and carbon-fiber shafts. This patented (USPS 7,485045 B2) bonding process has proven to be significantly stronger than welding. It is also crucial that the end yokes are identically “clocked” to eliminate any phasing issues. Mark Williams utilizes special precision fixtures when assembling the shafts to ensure accurate phasing.
M-W’s transmission yokes are machined from forged 4340-alloy steel and splined to fit all popular OEM and aftermarket transmissions — including units with a special diameter and heat-treat when used in conjunction with a roller-bearing tail housing. Also available are “rapid release” yokes with removable caps that facilitate removing the driveshaft at the transmission rather than the pinion. The yokes are equipped with an O-ring seal to prevent any fluid seepage.
Pinion yokes are similarly manufactured from 4340 alloy forgings and splined to fit all popular rearends. M-W also makes billet-aluminum pinion yokes.
The final touch is, of course, balancing. M-W dynamically balances each driveshaft in-house to G30 specs at actual operating speeds to assure smooth operation at high RPM.
Now that you’re aware of what’s components and procedures are involved in building competition-quality driveshafts, you can set about getting what’s best suited to your application and budget.