Strength versus weight is a debate that many racers have with themselves when looking at parts, and a driveshaft is one of those components where the strength versus weight discussion can really heat up.
We decided to go the strength route and worked with Strange Engineering to select the right chrome-moly driveshaft for our Project Number Cruncher F-body Pontiac Firebird
You need to be able to go rounds if you want to win in bracket racing, and you can’t do that if you keep breaking parts. After talking with JC Cascio of Strange Engineering, it made sense to go with chrome-moly as the material of choice for the driveshaft for our bracket and index-racing project machine. The added strength the chrome-moly driveshaft provides over the steel part the car came with made it an easy choice. We weren’t super concerned with weight, so that ruled out a carbon-fiber or aluminum for our application, making this the right fit.
Chrome Moly Driveshaft Tech
Chrome-moly tubing isn’t considered an exotic material in the world of drag racing — it’s the foundation for most tube-chassis racecars. The strength of chrome-moly is what makes it a great material choice for a driveshaft, but Strange likes to take things a step further and use seamless tubing.
“DOM tubing has a welded seam — it doesn’t happen often, but if it fails it will be at that seam. Seamless tubing is stronger and that’s why we use it,” Cascio explains. “The material we use for our driveshafts is heat-treated to ensure maximum durability and longevity.”
The material you select for a driveshaft is important, but what’s equally as important is the diameter of the shaft itself. Most racers are going to need either a 3-or 3 ½-inch chrome-moly driveshaft based on their application. There are several different factors that play into how this diameter is determined for a particular vehicle.
Strange recommended that Project Number Cruncher would need a 3-inch driveshaft. While the amount of horsepower our SAMTech-built 427 made did play a small part in the process, it wasn’t the biggest factor. Cascio talks about how Strange figures out the driveshaft diameter a customer will need.
“The main thing we look at is the RPM range the engine is going to be spinning at when the vehicle goes through the finish line traps. You have to take into account what’s known as the critical speed of a driveshaft — the critical speed of is when you get to a certain RPM, depending on the length of the shaft, where you start to get distortion in the shaft, almost like a jump rope effect. That will translate into vibration and potential failure. We use the RPM range to calculate the critical spend and assist the customer in selecting the right driveshaft. ”
The physical space you’re working with under a vehicle is another thing that will play a part in driveshaft diameter selection. You’ll be taking into account the size of the driveshaft tunnel, whether there’s an exhaust system that needs to be taken into consideration, and what size driveshaft safety loop you’re going to use. You might have to make adjustments to your vehicle to ensure the best driveshaft for your application can be used.
A driveshaft experiences some extreme loads and is being spun pretty hard about a foot from the driver inside a vehicle and that’s why Strange offers SFI-rated driveshafts for those who need them for a specific class, or just want that extra peace of mind.
“We send sample shafts to a testing facility where they’re load-tested for SFI rating purposes. They put a load on them and try to twist them apart. They relax the machine so they can check if the driveshaft is still straight, that’s how they determine if it’s within SFI specs. It’s usually a much higher torque level than what they’ll ever see in the real world,” Cascio states.
Getting The Right Driveshaft
A driveshaft isn’t going to be a one-size-fits all type of part — chances are your specific application will need a custom driveshaft. Strange has made the process of ordering a custom part simple by providing customers with an easy-to-use order form, with illustrations that show where you need to measure and provides the dimension definitions to answer any questions you might have.
One of the most critical measurements to make when ordering any custom driveshaft is from the pinion yoke on the rearend to the transmission itself.
“We like to see the centerline of the rear U-joint to the seal of the transmission measurement. How far the output shaft sticks out of the seal is important, because you don’t want the transmission yoke bottoming out into the seal. You also don’t want the output shaft bottoming out into the inside of the yoke where the freeze plug would be. There needs to be room for suspension travel, as well…we like to see 7/8- to 1-inch in its full range so it doesn’t bind or bottom out,” Cascio explains.
When you’re working your way through the process of measuring for a custom driveshaft there are a few things you’ll want to keep in mind to avoid any potential issues. According to Cascio, people can get into trouble when measuring for a driveshaft by using different parts than what will be a part of the final build. Even using a different pinon yoke during mock-up can throw off how a driveshaft will fit after you’ve measured for it.
The other thing to keep in mind when measuring for a driveshaft is how the car is sitting.
“The most important thing we recommend is to take all the measurements with the vehicle at its desired ride height. Any vehicle with a suspension will have a certain amount of suspension travel — the suspension travels on an arch, so if you measure with the suspension hanging down you won’t get an accurate measurement versus if the car is at ride height,” Cascio says.
Now, there’s something else you need to account for when ordering a driveshaft: the U-joint and yoke. It’s great to have an ultra-strong chrome-moly driveshaft, but if the parts that connect it to your rearend and transmission are weak it won’t do you any good.
“All of our driveshafts come with a 1350 series solid U-joint. It doesn’t have a grease fitting because that’s a failure point…the solid joint is just stronger. If a smaller U-joint is required, we have a line of crossover U-joints that are 1350 on the driveshaft end, then a smaller size that goes into a factory slip yoke or pinon yoke. That does create a weak link, so we recommend that customers upgrade to a 1350 U-joint whenever possible,” Cascio states.
Yoke selection should be based on how much horsepower you’re making, the weight of the vehicle, and the type of transmission you’re using. Strange recommends that if your engine is making more than 750 horsepower you should look at a chrome-moly transmission yoke. If your vehicle is on the heavier side, uses a manual transmission, or leaves the starting line aggressively, you’ll want to think about a chrome-moly yoke, too.
A driveshaft can be a part that you never have to worry about or something that makes your racing life a living nightmare. A chrome-moly driveshaft will provide you with great reliability and can be custom-made for any application. Make sure you follow Project Number Cruncher right here to see what we’ve done to the car and what’s to come!