No matter how powerful your race engine, none of that awesome power is going to be making it to the rear wheels if your driveshaft can’t stand up to the abuse
When it comes to the driveshaft in your race car, many racers tend to file it away into the same category as the plumbing underneath your house, garbage collectors and exhaust fans in public restrooms -- you don’t think about any of them much until they stop working.
And for the most part, that’s OK. The trick is to select the driveshaft that not only is allowed by the rules but is also the best option available for your application. Now, if you drive a Pure Street car a high tech carbon-fiber driveshaft is probably unnecessary for your application and horsepower -- not to mention the fact that your tech man would probably throw you out if he ever bothered to stick his head underneath the car.
But just as putting a carbon-fiber driveshaft into a Pure Street that costs approximately the same amount as the engine isn’t the best idea in the world (or the best use of your hard-earned money), neither is trying to get by on a poorly balanced, shabbily made or un-rebuilt driveshaft out of a junkyard. You may save money up front but if a driveshaft decides to let go on the race track your repair costs will almost certainly include much more than just the cost of replace the shaft.
To help us all get a little more education on driveshaft tech, we contacted trusted technical minds at three companies involved in driveshafts for racing: Quarter Master, Dynotech Engineering and QA1. Now, if you’ve been racing or building race cars for a reasonable amount of time, you maybe thought you just caught us in the wrong. Yes, Quarter Master and Dynotech Engineering are trusted by winning racers nationwide for the quality driveshafts they produce, but QA1 has never offered a driveshaft. They are famous for rod ends, shocks, springs and a multitude of other chassis components, but they don’t offer a driveshaft.
Until now. The braintrust at QA1 has been working on developing their own carbon-fiber driveshaft for some time now, and by the time you read this they should either be announcing their new designs or will be very soon. Because the people at QA1 have at their disposal a ton of very up-to-date research developed while designing their new driveshaft, we asked them very nicely -- begged even -- if they would share some of it with our readers.
So we posted a series of questions to the tech department at Quarter Master, Steve Raymond at Dynotech Engineering Services and Travis Gorsuch QA1’s Director of Advanced Materials. Both QA1 and Quarter Master deal exclusively with carbon-fiber driveshafts, so their answers are specific to that material. Dynotech’s experience is more wide-ranging. Not only do they work with steel, aluminum and carbon-fiber driveshafts, they have also pioneered the super strong “hybrid” design, which is a carbon-fiber wrapped aluminum shaft. Following are some of the questions that we most often hear from racers, but we know there are plenty more questions you might have that we didn’t ask. If, after reading this article, you still have questions, all three manufacturers welcome them in their tech departments.
When the rulebook doesn’t require a specific material, are there any advantages of a steel driveshaft over aluminum, and aluminum over carbon-fiber?
By far the biggest factor after the requirements of your rulebook are the limitations of your racing budget. An aluminum racing driveshaft is going to be more expensive than a steel one and carbon-fiber is going to be more expensive than that.
But the old saying that you get what you pay for is true here. “One of the good things about good old-fashioned D.O.M. (drawn over mandrel) steel is that it is very tough,” Raymond says. “The steel can take a lot more abuse than any of the other shafts, but the aluminum and carbon-fiber shafts are also a lot lighter weight and as a result take less torque to rotate. You also get better response trying to spin the lighter weight driveshaft. We have been told that you can get 6 to 8 horsepower to the rear wheels just by doing a shaft change from steel to aluminum. All the materials are good and have their applications, it’s really a case of understanding what you are trying to do and how you wish to use the car.”
Is there a practical power limit for a driveshaft? If I’m racing on a high-grip surface, so I need to worry about shockwaves from banging on the throttle lap after lap weakening the shaft?
Almost all manufacturers will provide you a useable power range. Raymond says that almost any well-made steel or aluminum driveshaft should be able to handle up to 700 horsepower without any issues. That’s well beyond the power level of just about any race class except the Super classes and NASCAR Sprint Cup. Quarter Master rates it’s Ultimate Torsion driveshaft to up to and incredible 4,100 lb-ft of torque. But not matter what material you choose, regular maintenance will help prolong the useful life of any driveshaft. Quarter Master says that when done right you can use the carbon-fiber’s weave to create an inherent vibration damping characteristic. That’s because the weak link will almost always turn out to be the U-joints and bearings. Besides inspecting the various components of the driveshaft, you should regularly clean and rebuild the U-joints.
So how do I check the U-joints and the driveshaft in general for damage or wear?
“Your driveshaft should always be a maintenance item,” Raymond says. “It is good to check joint play, and make sure there is no cracking next to the welds. You should also make sure the driveshaft is straight, you can use a metal straightedge to check that the U-joints aren’t twisting. You will fatigue an aluminum shaft much faster than you will a steel shaft -- they just don’t have the same fatigue life.”
QA1’s Gorsuch also stressed the importance of regular inspection and maintenance, even when working with carbon-fiber. “In general, our driveshafts require no more maintenance than any OE steel driveshaft,” he says. “It is always a good idea to keep things clean and periodically inspect the universal joints and slip yokes to make sure that everything is tight. It is always a good idea to do a visual inspection of the shaft to ensure the tube end yokes have not been overtravelled and bound up on the rearend yoke. You will also want to inspect the surface of the carbon tube for any damage due to track debris. When we balance our driveshafts we actually remove material rather than trying to add weight onto the shaft. This eliminates the chance of the weight getting knocked off and the shaft potentially being out of balance. As of right now, we have not developed a torque limit. On-going testing will allow us to develop our standards so as to not “overbuild” the driveshafts, which diminishes any performance gain. Since we manufacture our carbon tubes we can adjust the design so that the limiting factor is the universal joint or tube yoke rather than the carbon tube.”
My engine builder is getting more rpm than ever before, is there a terminal velocity or maximum rpm?
The maximum rpm of a driveshaft is also known as its critical speed. Raymond says that any driveshaft’s critical speed is a factor of length, diameter, weight, and material. That’s why aluminum and carbon-fiber driveshafts, which have less inherent strength than steel, are typically a larger diameter than a steel driveshaft.