Avoid Getting the Shaft - Tech File

As racers, we spend a large percentage of our budgets on tires, engines, gears, brakes, and other equipment responsible for both powering and stopping our race cars. However, there is a constant struggle over weight and where it is placed on the car. It's better to be in a position to add weight than to struggle with having to remove excess weight. The simple matter is that removing weight is more expensive than adding it to the car. The issue is compounded if heavy spinning parts are involved (e.g., wheels, brake discs, flywheels, crankshafts, or driveshafts).

The driveshaft, in particular, has a very difficult job. It's responsible for transferring to the differential all of the power your engine develops. It has to do this while the suspension is going through its full range of movement, and it has to take up any angular irregularities in the range of travel. Also, it has to take up any misalignment between the tailshaft on the transmission and the differential.

Simple math tells us that a 311/42-inch driveshaft will be heavier than one of the same length that is 2 inches in diameter. The problem is that the 2-inch driveshaft will not be as stiff as the one with the larger diameter. Stiffness is a function of the diameter to the 4th power. That means that a 311/42-inch driveshaft is roughly 14 times as stiff as a 2-inch driveshaft. This assumes a common wall thickness. There are other considerations we need to think about:

* How stiff does the driveshaft need to be?* Will a 2-inch driveshaft work without breaking?* The 2-inch driveshaft will have a lower moment of inertia. (Translation: It will take less power to accelerate the smaller shaft, meaning that the 311/42-inch driveshaft will use more power to turn. The difference in power can be used to accelerate the car. On paper, this looks good.)

While these are all interesting conversation points, the essential fact is that most racers will opt for the lighter, smaller-diameter driveshaft instead of the larger OEM version if they are allowed to make a choice. If you have to select a driveshaft that weighs 12 pounds rather than one that weighs 33 pounds, the obvious choice is the lighter unit. If you have to add weight to the car to achieve racing weight, you can place the weight/ballast where you need it.

The issue is not if the racer needs the lighter shaft or if a thin-walled, larger diameter driveshaft would actually work better and be more durable. A discussion on this topic would make for an enjoyable evening of bench racing, with adult beverages, calculators, metallurgical data, and spirited debates about stiffness versus diameter and inertial deltas, but it would not be too productive in terms of direct benefit.

What is of great benefit to Saturday night racers is an explanation of how to maintain the driveshaft so that it never causes a DNF or a DNS. The method is fairly straightforward. Inspect the shaft for any obvious damage, such as dents, and any metal damage. This usually shows up after a serious crash or a minor wall strike. If the driveshaft has been balanced, make sure the weights don't become dislodged. It's also a good idea to check the welds on the driveshaft and to look for any signs of twisting, such as a difference in the driveshaft's diameter, metal pulling away from the welds, or wrinkles in the shaft. This is more common in high-horsepower cars that have a good bit of track grip.

Keep a close eye on the universal joints and keep them lubricated on a regular basis. Keep track of the hardware that holds the universal joints in the driveshaft and the hardware that holds the universal joint on the rear yoke. Grab the driveshaft, push it up and down, and pull it side to side. If there is any play in the universal joints, they need to be replaced.

If you have ever seen a car drop a driveshaft, you know it's not pretty-especially if it's a front universal joint. Can you say pole vault? This is less of an issue if the car has a driveshaft loop. But if the rear universal joint breaks, the driveshaft whipping around at the back of the car can cause collateral damage. At that point, replacing universal joints will seem like a free fix.

Something else you can do during your universal check is to look at the yoke in the transmission and make sure the seal in the tail stock of the transmission is not leaking. You also need to check the pinion seal on the rearend and look for any signs of leakage. If you discover leakage, replace the offending seal.

Driveshaft maintenance is fairly easy and pays huge dividends in reliability and performance on the track. The track is not where you want to find out that there is a problem in your drivetrain.

Take time away from the track to include driveshaft maintenance as a part of your routine. We'll be looking for you on the podium.