Every dirt track racer’s goal is to optimize “bite” and stay at the head of the pack. As such, suspension adjustments play an important role in week-to-week success. And for sure, torsion bars are at the heart of the matter. The torsion bar is part of a Sprint Car’s suspension, which twists in response to the up-and-down motion of the wheels, absorbing their vertical movement.
For some insights into this key, but often taken-for-granted part of a race car, we turned to Brian Skipper, Mechanical Engineer and president of Sway-A-Way Inc., a highly respected suspension components manufacturer who has been making torsion bars for more than 30 years.
The Golden Rule Of Torsion Bars
“The most important thing to remember is that torsion bars should never be twisted in the opposite direction after they’ve been run,” said Skipper. “A sprint car or dirt modified torsion bar should always be run on the corner it was designed for, although it can be run on the opposite corner in a ‘X’ pattern on a sprint car,” Skipper added. “Flipping the bar does not change the corner it can be used on. You can flip it end-for-end on the same corner or cross it.” Sway-A-Way pre set Torsion Bars are clearly labeled from the factory “left front/right rear” or “right front/left rear” to keep them organized.
Determining Spring Rate
The actual spring rate is determined by the length and diameter of the bar, with the arm length also a factor in relative wheel rates. Of course, there are both solid and tubular-style torsion bars on the market, so the size of the bore (typically .750-inch), and the outside diameter must be taken into consideration. Most manufacturers of tubular bars, increase the diameter in the middle of the bar so that it has the same spring rate as a solid bar. The only advantage to a tubular bar is weight savings, typically about 60-percent the weight of a solid bar of the same rate. Tubular or gun-drilled bars do not “respond quicker” as some rumors in the market would suggest.
Generally speaking, Sprint Cars are a bit more sensitive to spring rates than Dirt Modifieds, so Sway-A-Way’s Sprint Car torsion bars are 30-inches long and come in rates of 1000, 1015, 1025, 1050, and 1060 on the high end, and 900 to 975 increments on the low end. Dirt Modified bars are typically 29-inches long and have incremental increases in diameter of .025-inch from .900 to 1.050.
Remember, a tubular bar will have a larger outside diameter than the same rate solid bar so it has the same rate as the solid bar. A 975 spring rate tubular bar will measure larger in diameter than the same rate solid bar which will measures .975-inch. Sway-A-Way machines a circlip groove in each end of the bar for a simple, inexpensive, and effective arm-retention feature required by many race promoters.
Pre-Stressing Torsion Bars
As a function of physics and metallurgy, torsion bars will “take a set” over time, affecting the performance of the bar. They will not have the same spring rate forever. Accordingly, the life expectancy of a torsion bar can vary. However, to optimize performance, Sway-A-Way pre-sets its torsion bars in a special computer-controlled device that essentially twists it to a predetermined setting. This greatly increases the life of the bar, and eliminates the typical settling that is seen with bars that are not pre-set.
The Optimum Setup
There’s obviously art and science involved in “reading” a track and being able to set up a car to take advantage of prevailing conditions. All the power in the world won’t help unless it can be efficiently put to the ground.
Having an arsenal of arms and torsion bars of various spring rates will greatly help in tuning the car. Most Sprinters rely on torsion bar arms between 14 and 18-inches in length.
Proper Maintenance Helps
Successful racers will remove the torsion bars after each race, leaving the torsion stop on to aid in re-assembly. Clean the grease off the bar and inside of the bushing. Reapply a coating of quality lubricant to the bar and bushing.
While some racers may advocate that a torsion bar is only good for about 20 races, Sway-A-Way’s skipper says, “The life of a bar is determined by many factors and a set number of races is very arbitrary. Pre-setting a torsion bar definitely increases the life, eliminates the re-setting of a new bar after it has been run, and provides a more consistent spring rate throughout its life.