Getting the Shaft
Spin to win with carbon fiber
by Richard Holdener
High-Tech Performance - March 1997

Performance is a funny thing. When the discussion turns to improving a vehicle’s acceleration capabilities, the first thing to come to mind is adding horsepower.. This makes sense, since acceleration is merely a function of horsepower vs. weight: the more power you have to accelerate a given weight, the better it will accelerate. Though adding horsepower is a surefire method of making your car faster, the flip side of the equation is also true. Removing weight from your car has the same effect as adding horsepower; after all, the engine now has much less mass to get moving. This simple acceleration equation should come as no surprise to any enthusiast. Anyone who has ever taken a trip to the strip will attest to the fact that a light car with a big engine will out-accelerate a heavy car with a big engine. The intermediate-sized muscle cars were a good example of this. Now there’s a way to improve both sides of the equation by adding hp and removing weight while adding safety.

Recently, we came across a company that offers carbon-fiber driveshafts for car applications. Like most of the other supercool technologies, composite materials such as carbon fiber come to us via the aerospace industry. The merits of a material that is simultaneously lightweight and ultra-strong should be a difficult proposition. If you’re building a component that needs to be strong, no problem; just whip it up out of stainless steel. If you need it light, try aluminum. Unfortunately, the steel would be heavy, and the aluminum might not be strong enough for the intended application. That’s where space-age composites like carbon fiber come into play. The one downside is cost. Much like exotic alloys such as titanium, the trade-off is having to pay the bill. The high cost is one reason it takes so long for the high-tech stuff to trickle down from the aerospace industry to us regular Joes here on terra firma.

To learn more about carbon-fiber driveshafts, we ventured over to Advanced Composite Products and Technology (ACPT) in Huntington Beach, Calif. A quick tour through the impressive facility revealed that ACPT incorporates carbon fiber into a whole bunch more than just driveshafts. Naturally, the aerospace industry was well represented, and though we would have liked sneaking a peek at some of the top-secret items, we were there to find out more about performance driveshaft. Although the uninitiated might only see the obvious lightweight advantage offered by a carbon-fiber driveshaft, there is areally a great deal more to it than simple weight reduction. Along with being roughly 50 percent lighter than a steel unit, carbon-fiber driveshafts offer greater fatigue life, superior vibration damping and a much higher critical speed. The most obvious benefit of carbon fiber is its weight. Unlike other weight-reduction measures, such as fiberglass hoods or ditching the A/C, carbon-fiber driveshafts actually aid acceleration, and deceleration, in two ways.

The most obvious is by reducing the car’s overall weight. In the case of our test vehicle, a 1995 Z28, the composite shaft was a full 10 lbs. lighter than the stock steel shaft. The second plus is a reduction in reciprocating weight. Light underdrive pulleys, the carbon-fiber driveshaft reduces the amount of drag that the engine sees as it tries to gain rpm. Though any lightweight component will not add horsepower to an engine in the same way a set of performance cylinder heads will with their added airflow, it will increase the output by reducing the amount of power that is taken away from the engine. That extra power can now be used to accelerate the car.

A glance at the driveshaft comparison chart will reveal that carbon-fiber shafts are both lighter and stronger than comparable shafts of steel, aluminum or even titanium. In fact, the bending stiffness of carbon fiber is nearly three times that of titanium. Although some experts are under the impression that carbon-fiber driveshafts are actually too stiff - a driveshaft needs to absorb some of the shock load from the engine before transmitting it to the drive wheels - a perfectly rigid shaft would shock load the wheels so violently that almost no traction would be available. This shaft stiffness is called torsional spring rate and is measured in in.-lbs. per degree of twist. The carbon-fiber driveshaft has a lower torsional spring rate with higher absorption qualities that either steel or the larger 3.0-inch-diameter aluminum shaft.

Strength and weight are important, but the real reason behind testing the carbon-fiber driveshaft is safety. Believe it or not, the stock steel driveshaft in a late-model F-body is not ideally suited for high rpm. According to the calculations given by ACPT, the steel driveshaft used behind the LT1-equipped F-bodies had a critical speed that is below the car’s absolute potential. GM has even recognized this shortcoming and is in the process of rectifying the situation. It seems that with the stock transmission, the rear end gearing and tire size, the stock steel driveshafts reach critical shaft speed somewhere near 153 mph, a speed that even a bone-stock F-body can see. Though 95 percent of the Z28 drivers will never buy the tach in fifth gear, 5 percent of owners will. Besides, what good is having an LT1 and a T-56 all wrapped in a nice aero package if the driveshaft lets go before you can reach top speed? Here’s more bad news. Are you one of the perhaps hundreds of enthusiasts who swapped out the stock 3.45 gears for a set of 4.10s? You’re going to reach the critical driveshaft speed much sooner, as low as 130 mph in Fifth and about 150 mph in Sixth.

What exactly happens when a driveshaft reaches critical speed? Well, the critical speed of the driveshaft is based on a number of different parameters, the modulus of the shaft’s material, the tubing diameter, wall thickness and length all combine to determine the critical speed. The longer the shaft or smaller the tubing diameter, the lower the critical speed.

Regardless of the size, every driveshaft has a critical speed. When a driveshaft is accelerated, it resonates at a certain frequency. The frequency of a carbon-fiber driveshaft is different from the frequency of a steel or aluminum shaft of the same length. The resonation causes the shaft to vibrate. When the vibration is accompanied by rotation, the center-most point of the shaft distorts. The centripetal force from the rotation of the shaft accentuates the shaft distortion until it eventually snaps. The result of a driveshaft failure can be as simple as the broken piece dropping onto the ground, or as catastrophic as the front portion of the shaft digging into the ground and pole-vaulting the back half of the car into the air. Having a portion of driveshaft whipping around under the car at better than 600 rpm can do a great deal of damage to the undercarriage, the inside and even the occupants.

Another positive feature of the carbon-fiber driveshaft is that the internal makeup causes the shaft to virtually disintegrate when a failure does occur. Though a carbon-fiber driveshaft will take a lot more transmitted power and rotational speed, in the event that it does break, the odds of it taking out the whole underside of the car are very remote. This feature makes carbon-fiber shafts very desirable in racing where shaft failure is more common. In fact, the ACPT carbon-fiber driveshafts have been successfully tested in a number of different forms of racing including NHRA and IHRA Pro Stockers, SCAA, IMSA and late-model stock cars.

The guys at ACPT were even able to measure a performance increase on a chassis dyno. The reasoning behind any power improvement from the carbon-fiber shaft is that the lighter weight reduces the reciprocating weight that the engine has to accelerate. Whereas a dyno pull would likely not show any improvement, acceleration test should. Since we were running our tests on one of the new Dynojet chassis dynos, we thought the dyno might detect the difference between a carbon-fiber shaft and a heavy stock steel unit.

Before getting to the dyno results, we should take a closer look at how the lightweight driveshaft will affect horsepower. A large portion of the power an engine puts out is absorbed by its internal rotating components, accessories and drivetrain. Friction is responsible for a portion of the losses, but what about the rotational losses? Any rotating object offers resistance to a change in its rotational speed. It takes power to start spinning and power to stop it. Once it’s spinning, the resistance drops off, as it is much easier to keep it spinning than to start or stop it. The resistance is known as moment of inertia (MOI). The MOI of an object is determined by the diameter and the mass of the object. Increasing either the outside diameter or the weight of the object will increase the moment of inertia. The larger the moment of inertia, the more power it takes to start or stop the object.

The formula for moment of inertia is MOI = 1/2 x Mass x Radius2 As is evident from the formula, increasing the diameter has a much grater effect on the MOI than increasing the weight. Doubling a component’s weight will only double the MOI value, while doubling the diameter will quadruple the MOI value. Although the steel driveshaft is slightly smaller in diameter than the carbon-fiber driveshaft, the ACPT shaft is only half the weight. How does that affect our driveshaft comparison? The stock steel driveshaft is 2.5 inches in diameter and weighs roughly 19 lbs. Our calculations give the steel driveshaft an MOI of 14.84. Since the carbon-fiber shaft has an MOI of only 12.10, it will take a bit less power to accelerate.

The dyno tests were run at nearby Jackson Racing, in Westminster, Calif. What better way to determine exactly how much a performance bolt-on was worth that to run a couple of back-to-back tests? The dyno also makes tuning a breeze. Our test vehicle was a 1995 Z28 equipped with a Powerdyne 5-psi supercharger, Crane ignition amplifier and a Borla Exhaust. The Z28 was taken to Jackson Racing with the stock steel driveshaft and put to the test. A number of pulls were made to test the repeatability of the supercharged engine. The output was pretty impressive considering the blower was only putting out 5 psi at 5000 rpm. The blown LT1 pumped out a maximum of 332 rear wheel hp and 336 lbs.-ft. of torque.

A quick drive back to ACPT headquarters, a couple of 7/16 wrenches and out came the steel driveshaft. The shaft change took all of 12 minutes, in the parking lot, on their backs. Back at Jackson Racing the hammer was dropped, and behold, the entire hp curve was elevated. Though peak numbers had only improved by 4 hp, this was really a function of the test rpm. The engine was only taken to 6000 rpm in both cases,; it never reached a power peak, it just kept on pulling. A look at the difference between the two curves will reveal that the carbon-fiber shaft improved the power everywhere, right from the bottom all the way to redline. The largest difference was 16 hp, measured at 5250 rpm where the stock shaft netted 312 hp and the ACPT registered 328 hp. I guess technology really works!

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