In the search of quicker elapsed times and ultimately race wins and a championship, we rebuilt our in-house project car, BlownZ, from the ground up over the winter. If you’ve been following the build, you’ve likely seen that we updated our chassis to 25.3 specs, which included a new roll cage and virtually an entirely new front and rear half. The team also installed fiberglass doors, Optic Armor windows and a new front clip to remove as much weight as possible from our ProCharged Camaro. One area that’s often overlooked for a potential weight savings is the driveshaft of the car, and as you’ll see within this article, by choosing a carbon fiber or aluminum driveshaft, you can easily remove anywhere from 6-8 pounds of rotating weight from the driveline.
Late Model Engines built an all-new 400 cubic inch LSX based engine with the newly released F-1X ProCharger that is capable of developing close to 1,800 horsepower, so we were in need of some expert guidance in terms of driveshaft selection. For that, we connected with Mark Williams of Mark Williams Enterprises to learn the important factors to consider when selecting a high horsepower driveshaft.
Initially, our team was expecting to order a custom carbon fiber driveshaft for our project car, but Williams was able to convince us that an aluminum shaft might be better for our long term project goals. And within this text, we’re going to highlight these very reasons as we take a closer look at Mark Williams’ driveshafts, the types of applications they’re best suited for, and how you can go about electing the proper part.
Aluminum and the Critical Speed of a Driveshaft
So why did we opt for aluminum, as Williams’ suggestion?
“There are essentially four critical factors in selecting a driveshaft for a race car application: the horsepower of the car, the weight of the car, the driveshaft length and the driveshaft RPM,” Williams explained. “While there are other important factors to consider when selecting a high performance driveshaft, such as driveshaft weight and diameter, these are the critical factors that must be taken into consideration first.
“A car with a long driveshaft using an engine that turns high rpm’s can get into a situation where the driveshaft speed matches the critical speed of the driveshaft, which is when resonant vibrations could ultimately destroy the shaft,” Williams went on to explain.
Mark Williams Enterprises explains critical speed on their website as follows: 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 rotating frequency and the natural frequency coincide, any vibrations will be multiplied.
The critical speed of the driveshaft can be increased by increasing the diameter of the driveshaft or using a lighter material such as 7075 aluminum or carbon fiber. Typically, a driveshaft that’s longer than 43-inches is a good candidate for using a larger diameter tube or selecting a lighter weight material for the driveshaft.
Mark Williams explains, “A larger diameter shaft always has better critical speed properties which is why driveshafts are typically offered in multiple sizes. If you have a longer shaft and you have room in the car for a large diameter, it is always preferred to install a larger diameter shaft since it has better critical speed properties.”
The horsepower and torque of the car is also a critical factor in the selection of a high performance driveshaft. While many driveshaft materials can support the power level of a typical race car, only carbon fiber, 7075 aluminum and 4130 chromoly should be considered for high-horsepower applications, according to Williams.
MW Enterprises offers custom driveshafts in a wide variety of materials, such as mild steel, 4130 chromoly, 6061 aluminum, 7075 aluminum, and carbon fiber. In drag racing applications, only 4130 chromoly, 7075 aluminum and carbon fiber are typically used due to the strength requirements of these applications.
The 7075 aluminum and carbon fiber driveshaft utilize billet 7075 aluminum end yokes that are secured in place using a patented bonding process known as Accubond. According to Williams, “The Accubond process is so incredibly strong that you will actually fail a 1350 series U-joint before it will fail any part of the tube.” Currently, the 7075 bonded aluminum driveshaft is the most popular driveshafts offered at MW Enterprises.
BlownZ's 7075 Aluminum Driveshaft
- Rear End: 1350 U-Joint
- TH400 / 1.375″ output shaft diameter / 32 spline / roller bearing in extension housing / 1350 u-joint
- #39004 Slip Yoke, Chevy T-400 32 Spline 1350
- # 39550 Driveshaft 4″ 7075 Bonded Aluminum, 1350
- #39010 U-Bolt Kit, 1350 Joint W/ 12 Point Nuts
- Measurement E – 49-9/16-inch (49.56”)
- Measurement F – 52-13/16-inch (52.81”)
- Measurement C – n/a (using Mark Williams Yoke for Turbo 400)
The 4130 chromoly driveshafts feature billet 4130 yokes that are precision TIG welded in place. For a drag racer on a budget, the MW 4130 chromoly driveshaft is an economical alternative that will withstand the abuse of nitrous and power adders. The downside to a chromoly driveshaft is the weight.
Initially when our team spoke with Williams, we assumed that a carbon fiber driveshaft would be ideal for our application. Carbon fiber driveshafts are the lightest of any standard material used for driveshaft construction, which helps with the critical speed. The material is also incredibly strong. But as Williams preferred the 7075 aluminum driveshaft over the carbon fiber driveshaft for our application, because while carbon fiber driveshafts can be excellent driveshafts, there are a few drawbacks that customers should be aware of prior to using one.
There are at least three potential down sides to using a carbon fiber driveshaft, according to Williams. First, the exterior surface of a carbon fiber driveshaft can be damaged from rocks and debris striking it or when the shaft is removed from a car, thus causing a potential stress-riser in the material that could lead to a potential failure.
As a second point that closely follows the first, when a carbon fiber driveshaft fails, the carbon material breaks violently into many pieces, which could be dispersed on the track. And finally, a carbon fiber driveshaft experiences a “wind-up” in the shaft when the material is tested to high levels of torque, which is visible during torque bench testing. Williams explains, “We don’t feel that having the material wind-up is a good property in a driveshaft for very high horsepower applications, which is why we prefer 7075 aluminum driveshafts whenever possible.”
Mark Williams Enterprises has a state-of-the art torque test bench in-house at their Colorado facility that allows them to test driveshafts, bonding materials and u-joints to the point of failure. This test bench is crucial when developing new driveshaft materials and it’s also used for SFI Spec 43.1 certifications that are required in many of the professional drag racing applications. The SFI driveshafts to have to be recertified every two years, which can be done using their torque test bench.
It’s through the use of tools and technology such as this that the engineers at Mark Williams are able to learn more about the operation and the limitations of specific materials and designs to better cater their products, be they aluminum or carbon fiber, to the proper applications.
High Speed Balancing
All Mark Williams driveshafts are high speed balanced according to their intended application using the actual yokes and u-joints for the application. The balancing machine is capable of spin testing shafts up to 10,000 rpm, but most shafts are balanced around 6,000 rpm, as this is a critical rpm for many applications. Depending on how fast the driveshaft is going to run in the car determines how they balance the driveshaft.
Chromoly driveshafts are balanced by welding weights to the outside of the tube. The amount of weight needed is determined by the high speed balancing machine. Carbon fiber and 7075 bonded aluminum driveshafts, meanwhile, are balanced by drilling and tapping the yokes so small weights can be screwed into the shaft. Since the balancing weight is added to the billet yoke, the structural integrity of the driveshaft tube is unaffected.
“Another advantage of the bonded aluminum driveshaft is that there’s no distortion from welding, so the shafts are incredibly true when they’re balanced on the driveshaft balancing machine,” says Williams. “Typically, driveshafts have a run-out of less than 0.010-inches, but the aluminum driveshafts are even more precise than that, which means the driveshaft requires very little balancing weight to be added.”
The alignment angle of the driveshaft is very important when setting up a racecar, and is a critical factor that many racers miss. Williams explains, “The most important factor for driveshaft alignment is the pinion angle. The imaginary line for the engine and transmission to the pinion need to be parallel, although most cars are offset to the side. This is critical for the driveshaft u-joints that are running at the same velocity and for a smooth-running driveline.” If the driveshaft is too far out of line, it increases the parasitic losses of the driveline, thus robbing additional power from the engine.
Once our project car was nearing completion at PMR Race Cars, we were able to take some critical dimensions so we would be able to order our custom driveshaft from Mark Williams Enterprises. The chassis of the car was set up to race ride height and all of the necessary weight was added to the vehicle to simulate the ride height at full race weight. This is an important step since the vehicle ride height can change once all of the fluids and parts are added to the car.
The rearend was also set to the proper pinion angle so the distance measurements from the pinion seal to the rear of the transmission would be correct. If you look on Mark Williams’ website, they provide you with this same information on how to properly measure driveshaft distances using their techniques. In an effort to eliminate confusion, they also include a graphic to illustrate the different locations to take a measurement on your car. Below is the information that we included with our order to MW Enterprises.
Many racers typically don’t think about the driveshaft as a potential area on the car to cut weight, but in actuality, it’s a cost effective way to drop 6-10 pounds from the car without sacrificing performance. By using a carbon fiber or 7075 aluminum driveshaft, you can easily bolt-in a part that not only saves you weight, but can be many times stronger than the driveshaft you removed from the car.
Since a driveshaft is one of the last items that will be purchased for a race vehicle, many racers typically do not put a lot of thought into this critical part of the driveline. The old adage ‘you get what you pay for’ really applies to custom driveshafts. While it might be possible to save a little by going with a local driveshaft builder, they typically would not have the years of expertise and in-house capabilities that Mark Williams Enterprises has at their disposal. When building a drag race vehicle, it pays to invest in a high quality driveshaft that won’t leave you stranded during eliminations that would most certainly ruin your weekend.
We decided to use the Mark Williams 7075 aluminum driveshaft for our project car BlownZ because it was an easy way to drop weight in the car, while still having a driveshaft that’s capable of handling over 1,800 horsepower. Since MW Enterprises has decades of experience building custom driveshaft, we knew that we would be getting a part that wouldn’t let us down.