Driveshafts

If you break a driveshaft your car stops. The strongest engine, or the most sophisticated suspension, will not overcome a driveshaft failure. If you break a driveshaft you’re going home. A few of us may carry extra U-joints in the trailer but very few of us have extra driveshafts at the track.

Driveshaft Loops and Safety

Most race groups require some sort of driveshaft safety loop to prevent a broken driveshaft from injuring the driver. This is not the place to skimp on material or do the minimum you can get away with. The driveshaft loop should be substantial enough in thickness and mounting to contain the wild gyrations of the broken shaft and protect the adjoining flooring. While driveshaft failures are rare they can be catastrophic even if the car doesn't crash.


The driveshaft loop must be placed in a location that allows it to restrain the shaft yet at the same time allow for the easy removal of the shaft. A two-piece bolt-together loop can make driveshaft removal much easier. This loop is typically placed about two thirds of the length of the shaft from the rear U-joint and it’s large enough to allow the insertion of the shaft into the transmission while the rear axle is at full drop. A little thought expended here during the mock-up phase can save a lot of frustration on Saturday afternoon in the paddock.

U-Joints

You need to select a racing-quality U-joint that’s strong enough to survive a full season of hard use. Streetcars and low-horsepower cars typically use what is known as a 1310 series U-joint. Most of us have increased the horsepower of our cars. A lot of us have done to the extent that the stock U-joints are marginal. Heavier trucks and stock cars with high horsepower use the larger 1350 series part. You need to consider this series for your car.  

The 1310 series u-joint measures approximately 3 1/4 inches wide. The 1350 measures approximately 3 5/8 inches wide. The 1310 series can have cap diameters of 1 1/16 and/or 1 1/8 inch or a combination of both sizes. The 1350 series has a cap diameter of 1 3/16 inch. Also the body and journals are bigger than the 1310 or 1330.

The 1350 U-joint also has 45 percent larger bearing cups and a thicker cross section. While 1310-series U-joints are the most common choice you should really should consider using 1350-series joints. Remember though the 1350 is almost never found in production car driveshafts.

Changing to a larger-series U-joint is not a simple task. You can't just buy bigger joints. All the yokes have to match the desired joint size. While the front and rear u-joint can be of two different sizes remember that the driveshaft is only as strong as the smaller U-joint.

Crossover U-joints allow you to mate a larger (or smaller) U-joint to one another. For example, you buy a new driveshaft that comes with 1350 weld-in yokes, but if your car has 1310-sized yokes for the tranny and rear differential. A 1350-to-1310 U-joint will have a 1350 trunnion on one plane and a 1310 trunnion on the other plane. While this solves a problem it also creates another weakest link situation. Have you really made things stronger?

A racing-quality U-joint should not have a grease fitting since it won’t need periodic greasing. The hole for the grease fitting in a U-joint weakens the strength of the cross and becomes the a failure point for a highly stressed joint. Spicer type solid-body U-joints come lubed for life and do not have grease fittings. This makes them a little stronger.

A Balanced Driveshaft

A balanced driveshaft can transmit more horsepower to the rear wheels. Steve Raymond from DynoTech Engineering says "We have had several NASCAR teams tell us our driveshaft saves them 3 to 7 bhp at the wheels of their race cars. That's why balance and design are important and that’s why we manufacture shafts for about 85 to 90 percent of the NASCAR teams."

DynoTech Engineering suggests balancing a 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. Balancing is also importatnt because any vibrations that the driver feels will sap his confidence and reduce him to just driving around hoping he can stop before it blows up and he crashes.

Carbon Fiber Driveshafts

Carbon fiber driveshafts are actually a vintage part. Dan Gurney first used them back in 1987. They were able to raise the rpm limit from 8,000 to 9,000 rpm by switching to carbon fiber shaft. In 1988 Cars and Concepts were able to raise the red line on Tommy Kendall’s Beratta 10 percent just by installing a carbon fiber driveshaft. In both cases there was a severe harmonic vibration with the metal driveshaft that imposed an artificial rpm limit.

Carbon fiber shafts are also much safer than a steel or aluminum driveshaft. Unlike a metal driveshaft that fractures when the U-joint or yoke fails, a carbon fiber drive shaft will simply disintegrate leaving the chassis and driver safe from harm so both can race and win another day.

The driveshaft is a dynamic part of the drivetrain. It bends, twists and goes through a lot of movement. You really don’t think about, because it’s in a location where you seldom see it. Ignorance is bliss, so if you never think about the driveshaft you may never know until something breaks. Engines are fun to look at and everyone knows you need a robust transmission. Most of us equate the rear wheels and tires with the rear end, so the rear housing and gears get a lot of attention but the driveshaft, a component is usually never seen or touched during the weekend tends to be forgotten—until problems begin to occur, or worse, it fails.