Let's face it, the driveshaft isn't exactly the sexiest part on your race car--or your personal car, or even a combination of the two for that matter. But considering that it's impossible to move all that torque produced by that expensive race engine of yours to the rear wheels without it, the humble driveshaft is pretty necessary.
Even on the most complex, high-end race cars, the driveshaft's form and function have both been whittled down into their simplest essence: a hollow tube with universal joints on either end to provide a solid mechanical connection between the output shaft of the transmission and the pinion on the rearend. It's as simple as that. When the driveshaft is right and doing its job correctly, it's practically noise-free, vibration-free, spinning with minimal drain on the torque the engine is sending to the rear wheels and out of the way so that the driver can concentrate on his job. Driveshafts are sort of like the offensive linemen in a football game, you only notice them when they screw up.
And when they do screw up, a driveshaft failure all too often ends up being a big deal. A vibration can quickly turn into early wear in your transmission or rearend, or a broken shaft that leaves you stranded on the racetrack. Worst of all, broken driveshafts have been known to break through the safety hoop and tear into the driver's compartment, injuring the driver.
To help make sure this doesn't happen to you, we spoke with two leaders in the industry, Mark Pozzuoli of The Axle Exchange and Jeff Neal of Quarter Master, about the latest developments in driveshaft technology when it comes to racing, how to spot a poorly made driveshaft and even maintenance tips.
Although the grade of materials may vary widely, all racing driveshafts are constructed from one of three materials: steel, aluminum, or carbon fiber. What you choose will depend primarily on the restrictions in your rulebook and the restrictions on your checkbook.
Quarter Master’s carbon-fiber...
Quarter Master’s carbon-fiber driveshaft is top of the line when it comes to driveshafts for short track racing. It’s approximately 20 percent lighter than an equivalent aluminum driveshaft, features a special coating to protect the carbon-fiber tube from scratches and other forms of damage, and is made so precisely most don’t even require balancing weights.
Most Street Stock-level classes mandate a "stock style" driveshaft, which essentially means it has to be constructed from the same material as the OEM shaft. In most cases this means an old-school steel driveshaft. But just because you can't upgrade to lighter weight aluminum, that doesn't mean you can't improve your performance by ditching the stock shaft. A high-quality, racing grade driveshaft--even if it is constructed from steel--will usually be lighter than the stock unit and almost certainly will be balanced better and exhibit less runout than a stock driveshaft. In plain English, "runout" equates to straightness. Less runout means a truer driveshaft that will balance better and introduce fewer damaging vibrations into your driveline.
Racing also introduces completely new levels of stress on the driveshaft. An OEM shaft that performed well in a street application where the rpm levels rarely ever exceed 5,000, may not hold up as well in circle track racing where the rpms can reach 7,500 or more for extended periods. A small imbalance becomes a big one when the rpms go up, as do any weaknesses. Racing also introduces shocks to the driveshaft that it never sees on the street. Any time the wheels break loose and then gain traction again it can send a shockwave through the rearend and through the driveline all the way up to the crankshaft.
That's a major reason why Neal cautions against having your local driveshaft shop prepare a driveshaft for you if it isn't already involved in racing. "A lot of times with race cars you will change the rearend or transmission so the driveshaft will need to be changed," he says. "If you just go to your local shop and have them cut the shaft and resize it, they may be able to do the work up to the quality required for racing, or they may not. If all they do is street work and aren't involved in racing, you need to be careful."
The next step up in the materials hierarchy is aluminum. Aluminum driveshafts are significantly lighter than equivalent steel and the lowered rotating weight means the entire driveline is able to spin up faster, making for a race car that accelerates out of the turns harder. Aluminum driveshafts are quite popular in racing because they offer a real performance advantage over steel shafts without being too much more expensive.
At the top of the line are carbon-fiber driveshafts. When done right, carbon-fiber driveshafts can be incredibly light. But they are expensive, not only because carbon fiber is expensive but also because a carbon-fiber driveshaft is more difficult to manufacture. For example, with a steel driveshaft, the ends can simply be welded in place. But that's not possible with carbon fiber. Instead, the ends must be bonded to the tube.
Purchasing a driveshaft custom-made to your application isn't difficult, but it is important to get it right because an improperly sized driveshaft can cause real problems. Too short and the slip yoke can come all the way out of the transmission and leave you stranded on the track. Too long and the end of the driveshaft can be smashed into the transmission causing expensive damage.
To measure for proper driveshaft length, begin by making sure the car is at ride height. You will need to be able to crawl around underneath it, so getting the car on wheelstands is a good idea. Just make sure it isn't raised with jackstands underneath the framerails so that the suspension is in full droop. Obviously, the rearend and transmission you'll be racing have to be in place and fully assembled.
When we needed a new driveshaft...
When we needed a new driveshaft in the Circle Track Dirt Late Model, The Axle Exchange was able to provide a driveshaft custom-built to the length we needed with just a one-day turnaround. We also like, as you can see from the stickers on the protective tubing, that the product is entirely made in the USA. Owner Mike Pozzuoli recommends allowing The Axle Exhange to install even the U-joints so that he can ensure everything is fitted together properly for minimum trouble down the road.
When the driveshaft is installed,...
When the driveshaft is installed, make sure you have between 3/4 to 1 inch of free play before the slider yoke bottoms out on the transmission’s output shaft with the car at ride height.
Different transmissions use...
Different transmissions use different size output shafts. Make sure you give your driveshaft manufacturer an accurate spline count for the yoke because they can vary, as you can see here.