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Driveline Tech Tips

ALUMINUM or CHROMOLY???
When it comes time to purchase a driveshaft the first question is usually “Steel or Aluminum?” The answer to that question requires considering three other factors: horsepower, vehicle weight, and driveshaft length. Obviously, in drag racing, reducing weight is key. So, an aluminum shaft would seem the logical choice but horsepower and vehicle weight are critical factors in determining if an aluminum shaft can be used.

With the recent introduction of the 1480 series 7075 Aluminum Accu-Bonded shaft, virtually every car can take advantage of a lightweight aluminum shaft. This includes Pro Modified/Pro Extreme type cars. The 1350 series Accu-Bond driveshafts are perfect for all vehicles under Pro Mod horsepower levels, such as Pro Stock and Comp Eliminator. That being said, MW recommends aluminum shafts in extreme applications be visually inspected regularly and returned to their facility annually for a complete inspection. If having the lightest option is not a priority, a Chromoly driveshaft with either 1350 or 1480 series joints would give a lifetime of service.

An important consideration that cannot be overlooked is the critical speed limitations. As a shaft turns it will become dynamically unstable at a certain speed and create vibrations that can cause destruction. The shaft will go into a whirl or “jump rope” effect that causes imbalance. Factors influencing the critical speed of a shaft are its length, the diameter of the material, and its stiffness. In general the longer the shaft, the larger the tube diameter must be to have the required high RPM critical speed properties. By using lighter-weight materials that have good stiffness characteristics, the rpm capability is also increased. The practical thought process is that steel is the strongest and most cost effective material. The 7075 shafts are much stronger than the conventional 6061 shafts and have a slightly higher critical speed property. This is due to a thinner wall tube can be used while still greatly increasing the strength some 40-60% (depending on the wall thickness). A giant improvement is the bonding process verses welding. Any welding on 6061 shafts creates a zone around the weld that becomes weaker. The bonding process eliminates this problem. The Accu-Bonded shaft has the full strength of the material without sacrificing any strength. A carbon fiber shaft is also a solution for applications that require extremely long and high RPM cars. (Think a '55 Chevrolet with a short Powerglide and a small block Chevrolet motor.)

A comprehensive Critical Speed Chart that shows the RPM capabilities of various materials and sizes by shaft length is available online at: http://markwilliams.com/driveshafttech.aspx The tech staff at Mark Williams Enterprises has extensive experience in helping racers determine whether Aluminum or Chromoly Steel is best for their particular application and budget. Call 866-508-3374 for help finding what best suits your needs.