If you look closely at leaf spring NHRA Stockers or other stock suspension small tire cars, most have one thing in common: The suspension consists of some sort of traction device (Caltracs tend to be one of the most popular) usually hooked to either a mono leaf or a split mono leaf spring. With a leaf spring out back, bolting on some form of traction device is usually dirt simple. Some traction bars are easily adjusted. Some are not. Caltracs offer a reasonable range of adjustment. But what if the springs were also somehow adjustable? Today, that’s possible, and that’s the real story here.
So What’s the Story with “Split” Leaf Springs?
Calvert Racing (home of the CalTrac traction bar system), is the originator of the split mono leaf drag race spring. Calvert notes that its split mono leaf springs are designed to control and reduce excessive body separation in rear suspension and they can plant the rear tires without unloading. They’re also considerably lighter than factory leaf springs (especially multi-leaf designs). Some racers are claiming anywhere from 40 to 45 pounds in overall weight reduction when making the switch from multi-leafs to split mono leafs. And because of this, the weight reduced is un-sprung; in turn, allowing for more shock absorber control and in many cases, quicker elapsed times.
Split Mono Leaf Spring Design & Construction
Looking at their construction, the springs are designed with a parabolic rear half (the front half is not parabolic in terms of construction). According to Calvert: “the design distributes the main spring pressure point through the length of the half, reducing spring breakage.”
Let’s stop for a second and consider what a “parabolic leaf spring” really is:
Parabolic leaf springs are an evolution of a conventional taper leaf spring. Here (in the case of a Calvert spring), the rear segment leaf is thicker at one end, and then it tapers from the middle to the end, where it is thinner. With conventional leaf springs, the parabolic configuration tends to distribute stresses more evenly across the spring. This can result in a more comfortable ride in a conventional application, but with the high performance and race application the Calvert spring is geared toward, it makes the spring more efficient. Here, the number one task of the rear segment of the split mono leaf spring is to hold up the rear end of the car. It also sets ride height (more on this later).
The forward segment of the split mono leaf spring is what controls the launch of the car. That’s where the traction tuning is based. With a two-piece spring such as this, it is possible to have different spring rates or arc forward of the rear axle as well as behind it. As a result, it’s also possible to keep the front half of the spring soft so that it will work better with the traction device. Then ride height can be adjusted with the rear spring segment. Basically, if you’re not happy with the ride height of the car, you can purchase a new rear spring segment to adjust it.
In terms of additional construction, Calvert split mono leaf springs are manufactured with heavy duty U-shaped alignment clamps. They’re held together with Grade 8 hardware. Meanwhile the front spring eye bushing is solid aluminum and the rear spring eye bushing is manufactured from urethane.
You have plenty of options when it comes to split mono leaf springs, across a wide range of cars in an equally wide range of ride heights: stock, +1 and -1 are the basics (special orders are possible in some cases). In addition, it is also possible get the springs tailor-made with a unique spring rate (the rate ranges from approximately 200 to 225 pounds).
Can You Run These Springs on the Street?
One question that comes up is street use. Actually, that’s one of the basic concepts of the Cal Trac system. A number of Calvert Racing customers with leaf spring vehicles still use them as double duty machines (street and strip). If those cars were converted to something like a ladder bar or a four-link, the suspension hardware required to get the job done (panhard bar, anti-roll bar, watts links components, housing floaters and so on), tend to limit their usefulness on the street side of the equation. The complete CalTrac setup doesn’t have those issues (aside from some extra noise).
Clearly upon closer inspection, these springs are quite a bit different than the setups you’re likely accustomed to. Get a closer look in the photos below.