CARBON FIBER DRIVESHAFTS

The High Tech Connection
by Bob Wallace
Ford High Performance - June 1995

When you build a high performance 5.0L Mustang, a race-prepped "Pony" or other type Ford, you spend lots of hours and tons of money on your engine, transmission, clutch (or torque converter), and rear end. Brakes, springs and shocks, and the right tires and wheels also get plenty of thought and dollars invested.

What about the driveshaft? All the horsepower in the world won't do you any good if you can't transfer it to the pavement smoothly and efficiently. And that humble tube that links the trans to the rear axle often proves to be the critical link. Hidden underneath, in the middle of the car, a driveshaft isn't a glamour item. About the only time that any thought is given to the 'shaft is if, or when it fails. If a driveshaft failure occurs at speed, the results can range from minor damage, to crashes and severe injuries to the driver. Every motorsport sanctioning body has requirements for safety equipment such as driveshaft hoops, specifically to keep a failed shaft from causing a catastrophe.

We're not implying that the owners of high performance Fords don't pay attention to all parts of their drivelines. Ford Motorsport sells literally thousands of their aluminum driveshafts for "Fox" mustangs every year. However, when we heard about driveshafts made of carbon fiber composites, we had to check into it.

Advanced Composite Products & Technology, in Huntington Beach, California, is a leader in her development and use of composite materials in applications as diverse as undersea to outer space, for both government and private industry. Since the mid-1980's, they have been manufacturing filament-wound carbon fiber driveshafts for numerous topflight racing teams in drag racing, road racing , and various forms of oval truck racing. Dan Gurney's All American Racers team started using ACPT's driveshafts in their IMSA GTO championship-winning Toyotas in 1987, after experiencing harmonic "whip" with metal shafts. The change to composite shafts eliminated the "whip" and allowed Gurney's drivers to run the engines almost 10% faster in all gears. The carbon fiber driveshafts are becoming commonplace in NHRA and IHRA Pro Stock racers, where the drivers have found they experience less wheel ship and tire shake than with conventional driveshafts.

Composite materials offer a multitude of advantages over metals in products ranging from sporting goods (golf clubs and tennis rackets) to aerospace (just about anything that is shot into orbit). The fatigue life of carbon fiber composites easily surpasses that of the toughest metals, and on average provides a weight reduction of approximately 50%. At the same time, the composite material dampens vibration 2 to 10 times faster than metal. A carbon fiber driveshaft in your ultra-high performance Ford offers the following benefits:

• Smoother power delivery to higher RPM
• Increased driver safety
• Reduced tire shake and vibration
• A substantial reduction in rotating weight and driveline inertia
• Less wear and tear on transmission and rear axle parts.

In other words, more power to the pavement, less vibration, and more safety.

The safety factor is due to both the way a carbon fiber driveshaft is constructed, and that the composite shaft is designed to exceed any critical speed requirement, i.e. each shaft is computer-engineered for the specific horsepower, torque, and/or rpm requirements for the vehicle it'll be installed in. If there is a component failure, or the design limit is exceeded, causing the driveshaft to impact the safety hoop or frame, the shaft will shred and disintegrate, rather than possibly penetrate the car. The actual process of building a carbon fiber driveshaft begins with determining the requirements for the particular application. Those variables include length, shaft diameter (ranging from 2.5 inches to 6-inches), wall thickness, U-joint size (currently based on Spicer standard sizes), and end fitting material (aluminum or steel). The various diameter shafts are normally stocked, as are the more commonly used U-joint-sized end fittings. The shaft itself is the unique part. ACPT shafts are made of continuous filament wound carbon fiber. The actual forming is done on a mandrel, which has been treated with release agents. The forming is accomplished by a filament winding process. In the course of this operation, it is critical to get an even distribution of resin, with no dry spots, as dry fiber will have absolutely no torsional or bending strength. The "tows" of continuous filament carbon fibers, which are on the spools, are attached to one end of the mandrel to give the proper amount of tension on the fibers once the machine starts the winding process. The dry fiber goes through a resin bath, so that the fibers are impregnated with the epoxy resin. By the time the fiber reaches the mandrel, it's completely soaked with resin. The winding profile has been engineered to handle both the torque and the speed that these shafts may be subjected to. The specific application determines the angle at which the fibers are applied to the mandrel. After two full axial wraps of carbon fiber, a circumferential winding of E-glass (continuous filament fiberglass) is applied. This serves two purposes. It gives structural integrity in the hoop or circumference direction, and it pulls down and consolidates the composite lay up against the mandrel. The cross fiber winding also removes excess resin. Proper diameter is checked at this time with a Pi tape to determine if the resin-to-fiber ratio is correct. Depending on the design requirements of the specific shaft, this entire process will be repeated at least twice. During the final winding, serial number tags are placed at several locations on the tube; in the event of a failure ACPT can determine exactly when the driveshaft was made, by whom, and the batches of resin and fiber that were used. Once the winding is finished, the tube is baked in a curing oven and then can be removed from the mandrel. The tube can then be machined to the required length and the end fittings can be attached. This is a proprietary process, and all that ACPT will say about it is that the end fittings are bonded with an aerospace adhesive using the same technology that's used in fabricating aerospace composites. That's right, the ends are GLUED in place, and in every test that ACPT has conducted on their carbon fiber driveshafts, the first thing to fail is the U-joint!

The balance of the job is handled by ACPT's primary distributor, Inland Empire Drive Line Service, in Ontario, CA. Here the driveshaft is fitted with U-joints and yokes, and is balanced. The balancing act differs from that of a conventional shaft since weights can't be welded on. Instead of adding weight, material is ground off the end fittings directly opposite of where weight is needed, until the shaft is in perfect balance.

IN theory, a composite shaft offers a lot of benefits. Its lighter weight definitely cuts down the amount of rotating mass, which should help acceleration. The carbon fiber composite has a much higher vibration damping coefficient (the time it takes to reduce a vibration to zero) than steel or aluminum. That high damping ability means that the driveshaft absorbs shock form the driveline rather than transmitting it, which should give the tires a chance to stay stuck (or stick better) to the pavement, rather than spin. Also, the critical speed (the speed that the center of a shaft will begin to move or "whip" and destroy itself) of a composite shaft can be tailored to the specific application by varying the diameter of the tube, its wall thickness, and the fiber used. Steel and aluminum shafts, if of the same critical speed; a composite shaft has a much higher stiffness to density ration, and at the came dimensions as a metal shaft, will have a critical speed approximately 40% higher. The critical speed of a standard carbon fiber driveshaft for a 5.0L Mustang is over 10,000rpm! And that's driveshaft rpm (which you'll only see in high gear), not engine rpm. There isn't a Pro 5.0 Mustang around that'll ever get near that kind of driveshaft speed!

The stock steel driveshaft in a late model Mustang weighs slightly over 20 pounds, a Ford Motorsport aluminum shaft weighs 14 pounds. ACPT's replacement carbon fiber driveshaft for a Mustang weighs only 11 pounds. Due to an unusually rainy winter in Southern California, we've been unable to conduct a back-to-back comparison test of a composite shaft versus a stock steel driveshaft, but in normal driving in our supercharged 5.0L Mustang, our seat-of-the-pants impressions are that the car seems to run and accelerate smoother with the carbon fiber driveshaft than it did with the original steel OEM Mustang driveshaft.

If ACPT's experience with drag racers holds true, we expect to see an E.T. reduction of between .05 and.20 second, all else being equal. We're looking forward to testing it, and will report on the results in an upcoming issue. Meanwhile, follow along with us as we show you haw a carbon fiber driveshaft is built.

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Advanced Composite Products and Technology, Inc.
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