Next, take the slip yoke that will be used on the final driveshaft (or one that's just like it) and insert it into the transmission's tailhousing until it bottoms out. Pull it back out 3/4 to 1 inch--this is your safety margin to make sure you aren't banging the slip yoke into the transmission's output seal once the racing gets started. Now, measure the distance between the centers of the U-joints on the transmission and rearend.

Give that measurement to your driveshaft builder as well as the slip yoke. If you're using an old slip yoke and plan to have your builder purchase and use a new one, make sure he knows the exact brand and part number of the slip yoke you used to measure.

Judging Quality

Both Pozzuoli and Neal agree that two of the most important factors when it comes to making sure you have a good quality driveshaft are straightness (lack of runout) and a precision balance.

"The first step," Pozzuoli says, "is to make sure the tubing you use is as straight as possible. We buy only the top quality tubing from Alcoa for our aluminum driveshafts. You can only buy it in short lengths because they can only throw the laser out so far to check for straightness, but it makes a difference. But you can only make the tubing so straight right off the bat, so you have to go in and straighten it manually if necessary."

Most driveshaft manufacturers have developed their own processes for straightening tubing, so we won't spend time on exactly how it's done. But making sure the tubing is absolutely as straight as possible is important in order to make the balance job more precise.

Neal says Quarter Master technicians take pride in producing driveshafts that use only minimal balance weights, and their process for producing carbon-fiber driveshafts is so precise that they often require no weights at all.

One clue you can look for to gauge the quality of work done balancing the driveshaft is to look at how many weights have been attached to either end of the shaft. Pozzuoli says you should never see weights 180 degrees apart from each other on the same side of the shaft. This is a sign that the technician had trouble finding the right spot to add weights, because weights placed opposite each other essentially wipe each other out leaving you right back where you started.

"You also want to make sure your manufacturer uses high quality U-joints and yokes," Pozzuoli adds. "In my book, I've seen three levels of quality. You have Chinese, cheap American, and then top-shelf American. The top-of-the-line stuff really isn't that much more expensive than the cheap stuff, so I think it's always a good idea to get the quality U-joints, like Spicer or Dana."

Maintenance

Because driveshafts are highly stressed components of the race car, they should be inspected regularly to catch any potential problems. Check the U-joints for slop or slack. Any looseness will cause them to wear and potentially fail faster. Just be sure to get your hands on both the driveshaft and yoke when you check for slack, you don't want to confuse it for backlash in either the ring-and-pinion or the transmission for play in the U-joints.

If you do notice slack, the best fix is almost always to simply replace the U-joints. While you have them out, make sure to check the U-joint bores in the driveshaft and yoke. If they are oblong, worn or otherwise out of shape, you will need to have that repaired.

Next, visually check to make sure the yoke hasn't bottomed out on the transmission's tailshaft. If this has happened, you can have damage to both the driveshaft and the transmission. This can sometimes happen even when the driveshaft is sized properly if the car has been in an accident or raced on an extremely rough track.

Do not forget to visually inspect the tubing itself. Be on the lookout for dents, scratches or grooves cut into the tubing. Sometimes if the shaft is allowed to rub up against another component in the chassis, it can cut a groove in the tubing. This will significantly weaken the integrity of the tubing, so if you see this the drivehshaft will need to be repaired or replaced.

Finally, watch out for chains or rope limiting the travel of your rearend. "Unfortunately, I've seen this happen quite often," Neal says. "Teams won't have some way of limiting the downward travel of the rearend, so when they put the car up on jackstands the only thing that stops the rearend is when the driveshaft hits the safety hoop. This definitely isn't good for the driveshaft, and the fix is easy."

The Next Step

Carbon-fiber driveshafts have only gained popularity in short track racing in the last few years. But they are becoming very popular in the Dirt Late Model classes. Neal says that a quality carbon-fiber driveshaft can not only save 20 percent in weight versus an already lightweight aluminum driveshaft, but it also has several other benefits.

"The weight savings is nice," Neal says, "but that's almost minor compared to the other benefits. First, the critical speed of the carbon-fiber shaft is much higher than for either aluminum or steel. Critical speed is the maximum rpms that the driveshaft can safely spin. So if the critical speed is 9,000 rpm on an aluminum driveshaft, it will be closer to 12,000 on the carbon shaft.

"Also, the carbon-fiber driveshaft is safer. We put a coating on all of our carbon-fiber driveshafts that protects them from scratches, nicks or wear, so they are pretty durable. But if you ever have a failure the carbon weave just sort of shreds into its individual strands. So there's no mass there unlike an aluminum or steel shaft that can really tear through some sheetmetal and has been known to injure a driver."

Conclusion

Selecting and measuring for the right driveshaft is paramount to harnessing all of that horsepower your motor makes. It's a critical link in your drivetrain and one that you should not take for granted. Of course, a great driveshaft is only as good if it is properly aligned and we'll be tackling that in a future issue of Circle Track. Stay tuned.

SOURCE
Quarter Master
510 Telser Rd.
Lake Zurich
IL  60047
888-258-8241
www.quartermasterusa.com
The Axle Exchange
22 Commerce Rd
Fairfield
NJ  07004
973-808-2800
www.axle-exchange.com