If you're planning to drive your hot rod or custom (and why wouldn't you?) you need to make sure the engine, transmission, third member, and driveshaft are all working together in harmony. The piece of this puzzle that would seem to be the simplest, as it has the least amount of moving parts, is the driveshaft. We had a pretty good idea how to set up a one-piece driveshaft, but the question of the proper method for a two-piece unit came up, which also brought up the question of just when a two-piece is necessary.
We decided to go to the experts and ask. Greg Frick at Inland Empire Driveline told us the following: "A driveshaft explodes at an rpm called critical speed. In theory, critical speed depends on shaft length, weight, diameter, and rpm. In the real world, critical speed is lowered by U-joint angles, shaft mounts, and even the engine's firing. Keeping safely away from critical speed affects decisions about driveshaft tube diameter and the decision to use a two-piece shaft set with the added support when bridging long spans. Some cars will require a two-piece driveshaft system because of overall length or because of obstacles that must be avoided. Long wheelbase cars, lowered cars, airbagged, and X-frame cars all fall into this category. While the '58-64 Chevrolet cars are the most common example, long classics that had torque tubes are some others.
"Two-piece shaft systems are the means used to reduce critical speed where long spans exist," Frick continues. "As described above, the critical speed of any shaft is a theoretical number reduced by both the physics at work in the shaft and by the uncertainties of the installation. While there are some guidelines showing recommended maximum shaft lengths at various rpm, judgment is required when these lengths are approached. While each vehicle is different and no hard and fast length rule applies to all cases, start asking questions when your driveshaft measures 51 inches or more."
The first major advantage involved with the selection of a two-piece driveshaft is diameter. As a driveshaft gets longer, you must increase the diameter to maintain its overall strength. Splitting a driveshaft into two smaller-diameter pieces comes in very handy where space is an issue (an almost certain concern that becomes more important when the car is lowered).
The two smaller-diameter units, beyond fitting much easier into limited space areas like X-members and driveshaft tunnels, also run much smoother than their larger-diameter counterparts. "A two-piece driveshaft is a great item when built and balanced correctly," says Denny Bringhurst of Denny's Driveshafts. Denny also reiterates that two-piece units will allow the driveshaft to maintain a higher critical speed, which is essentially a redline rpm maximum of a driveshaft. "As a driveshaft spins faster, the higher the chances of a slight imbalance causing a wobble, which can lead to failure and breakage within the driveline itself," Denny warns.