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The Science Of Driveshafts

Is Your Drive Shaft Costing You Horsepower? We Show You What You Need to Know.

By Jefferson Bryant, Photography by Jefferson Bryant

When you hit the throttle, the engine spins the flywheel. The flywheel transfers energy to the transmission that is transferred to the differential, spinning the tires and putting the power to the ground. That's all there is to it, right? The driveshaft is only a link between the tranny and the rearend. The problem is this is the common view about the driveshaft-it's a simple link. The fact is, while you can't gain horsepower through the driveshaft, you can certainly lose it.

Anytime you increase the power output of a stock engine and subsequently increase the speed at which the engine operates, you need to look at driveshaft balance. Most factory driveshafts are balanced between 3,000 and 3,500 rpm. Spinning the driveshaft past that range can have a parasitic effect. Steve Raymond from DynoTech Engineering tells us, "We have had several NASCAR teams tell us our driveshaft saves them 3 to 7 hp at the wheels of their race cars. That's why balance and design are important and why we manufacture shafts for about 85 to 90 percent of the NASCAR teams."

DynoTech Engineering uses Balance Engineering driveshaft balancers, which are considered the best in balancing accuracy. DynoTech suggests balancing a performance driveshaft at a minimum of 5,000 rpm and as high as 7,500 rpm. This ensures a properly tuned driveshaft that reduces vibration and efficiently transmits power to the wheels.

Length And Diameter
Other than balance, the length and diameter of the driveshaft directly affect the performance of the unit. Critical speed is the rpm at which the driveshaft becomes unstable and begins to bend into an S shape. The longer and smaller (diameter) a driveshaft is, the lower its critical speed. Critical speed is felt as excessive vibration that could cause the unit to fail. To calculate the critical speed, the length, diameter, wall thickness, and material module of elasticity must be known. Then using the critical speed calculation formula (see the Speed sidebar), the numbers are plugged in and you will know the driveshaft's critical speed.

What the driveshaft is made of is just as important as its length and diameter. An OEM steel shaft is rated for no more than 350 lb-ft or 350 to 400 hp. For high-performance use, there are two types of steel used: DOM or drawn over mandrel seamless tubing and chrome-moly steel. DOM steel is better than OEM steel, handling up to 1,300 lb-ft and 1,000 to 1,300 hp and also has a higher critical speed. This is a good choice for any car that does not need a lightweight unit.

The step up from DOM steel would be chrome-moly, which is the strongest possible material and usually seen in Pro Stock cars. Chrome-moly steel tubing can be heat-treated as well, raising the torsional strength 22 percent and increasing the critical speed 19 percent. The problem with steel is it is heavy, which increases the load on the drivetrain.

Aluminum is probably the most common performance driveshaft material. The lightweight aluminum shaft reduces rotational mass, freeing up horsepower from the engine and reducing parasitic loss.

An aluminum driveshaft will support up to 900 lb-ft or 900 to 1,000 hp, making it a great lightweight choice for most muscle cars. However, it is not as strong as steel, so some custom driveshaft shops do not have twist guarantees on aluminum driveshafts.

By Jefferson Bryant
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