Editor's Note: When we commissioned this article from Bob Gosch (better known as "Bob the Driveshaft Guy" in NASCAR shops), we had no idea how complicated this seemingly simple part was. It is critical that the part match the application in many different ways. Here he explains why in Part One of a two-part analysis that every racer should heed.
The driveshaft of the modern racing stock car is one of the most critical links in transferring the power of the engine through the drivetrain to the track. If your driveshaft fails, you have failed. Because it is usually one of the last parts assembled to the car, racers in a hurry tend to forget that great care should be taken when specifying a new shaft.
During the racing season, the driveshaft system should receive regular inspection and its components should be replaced on a firm schedule to ensure reliability. A driveshaft failure on the track will inevitably result in a wrecked race car and possible injury to yourself, your fellow racers, and your wallet.
1. DRIVESHAFT
Operating Environment The racing driveshaft operates in a confined space with no cooling provisions while surrounded by the hottest components of the race car. Inches from the track, the driveshaft is fully exposed to debris from other cars, chunks of rubber, and red-hot sparks of metal from dragging chassis parts. The driveshaft is always rotating whenever the car is moving and spends most of its life at the highest rpm the engine can sustain.
We need to pay attention to the clearances of the driveshaft to other components under the car. Setting the car at ride-height configuration and moving the rear end to midturn position can help us align the driveshaft and to make sure there is no interference with other parts of the car.
2. TUNNEL CLEARANCE
When a race car chassis is built, the fabricator creates a tunnel in the floorpan to accommodate the driveshaft. Based on his experience, he allows enough volume in the tunnel for the shaft to operate. But he normally does not know what major components will eventually be used in which combination.
Older chassis are often converted to different brands, and transmissions are routinely changed. Assuming that the driveshaft will always have sufficient clearance to operate properly is wishful thinking-it must always be checked.
The more complicated the type of car, the more equipment exists that can interfere with the driveshaft. In a properly designed driveshaft, all of the parts needed for your type of racing fit together with no conflicts. This means all of the components can be removed for servicing without disrupting the other parts.
3. OBSTRUCTIONS
When you are constructing a new race car, always mock-up the location of the major components first and inspect the tunnel area for interferences or obstructions to the driveshaft zone. The underside of the tunnel should be as smooth as possible with no protruding fasteners.
NASCAR's Car of Tomorrow incorporates a full tunnel design to surround the driveshaft. This piece is composed of heavy 0.125-inch-thick steel with reinforced ends of a much heavier solid steel part. This will virtually eliminate damage to the car and injury to the driver should the driveshaft break.
4. OPERATING CLEARANCES
There must be adequate clearances for the driveshaft to follow the movement of the rear axle. Travel the rear axle assembly through its range of motion and check for any resulting obstruction to the shaft. Now is the best time to correct any potential problems by repositioning smaller components or re-contouring floor protrusions.
It is a common practice to run hard lines for engine oil transfer along the underside of the driveshaft tunnel. These lines not only introduce extra heat into the tunnel, but also become a serious limitation to driveshaft size and a potential disaster if the driveshaft fails and tears them open. Likewise, a large oil tank snuggled up next to the driveshaft for weight distribution reasons is a needless risk.
This rear hoop has a neat design and can be easily removed for servicing the driveshaft. It is made of thick steel strap and bolted with two bolts on each side to the chassis.
5. DRIVESHAFT LOOP DESIGN
All sanctioning bodies require some form of driveshaft safety loop to prevent a flailing, broken driveshaft from injuring the driver or leaving the car. But few car builders give much thought to the magnitude of the potential damage. This is not the place to skimp on material or do the minimum allowed. 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 and oil tank. While driveshaft failures are rare, they can be catastrophic even if the car doesn't crash as a result.