Brakes & Suspension / Tech Articles

Suspended Animation: What Are Split Mono Leaf Springs & How Can They Help?

(Image/Wayne Scraba)

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.

Check out this photo of a stock production line Chevy mono leaf (on the bottom) and a CalTrac split mono leaf spring (above it). As you can see, it’s basically a two-piece mono leaf that’s joined in the middle. (Image/Wayne Scraba)
When viewed from the top, there’s not much difference between the springs, aside from the fact the pair of leafs are joined by way of two heavy duty U-shaped clamps. (Image/Wayne Scraba)
With a side view, you can get a better look at the large segment clamps. The springs do not install with any sort of rubber isolators either. (Image/Wayne Scraba)
Up close you can see that the clamps are actually riveted to each of the respective spring segments. To change a front or rear segment, clamp segment fasteners are loosened, and the nut on the centering pin is removed. This allows you to change the ride height of the car. (Image/Wayne Scraba)
If you take a close look at the rear segment, you can see that the leaf is tapered front to back and also in terms of thickness. (Image/Wayne Scraba)
(Image/Wayne Scraba)
At the rear, the springs include a set of polyurethane bushings pre-pressed into the spring. (Image/Wayne Scraba)
This photo shows the bushings installed in the Nova rear spring shackle. (Image/Wayne Scraba)
The front of the spring is fitted with a solid (billet) spring eye bushing. (Image/Wayne Scraba)
In this photo, you can see the front CalTrac traction bar segment has been pre-installed on the spring. In turn, the spring has been installed in the OEM Chevy front spring hanger. It must be installed this way in order to fit the car. (Image/Wayne Scraba)

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  1. Dennis A Martin says:

    Nice article. I have a 68 camaro, going to convert from factory 10 bolt rear to a 12 bolt or 9″ and considering the calvert set up.
    I have run a 3 leaf set up in this car since 1986. launches terrible now. it is now a tremec 5 speed Silver Sport conversion. This is more of a streetcar and It would do some cornering as well as straight line launches. How do these do with cornering? I assume the reference to the noise is the leafs slapping (every time you pull out or just on launches)?
    Thanks, your thoughts are appreciated. Dennis

    • Wayne Scraba says:

      Hello Dennis.

      There will be a little bit of noise and harshness from the Calvert springs, simply because they make use of a solid bushing at the front mounting point. The other source of noise is with the actual CalTrac bars. When the car encounters bumps on the street, it tends to rattle. If you examine a CalTrac setup, you’ll find there is a “load bolt” positioned on top of the spring. That’s the cause of the rattle. The solution is to cut a small section of hose and install it over the load bolt(s). The CalTracs will otherwise work fine on the street. They’re a well proven setup. Hope this helps!

  2. So these will be ok for autocross style racing also?

    • Dunn James says:

      Ok I’m in for the comments now too! I was considering doing a lot of work to convert the rear suspension to coilovers and a modified torque arm setup similar to F Body rear suspension but decided to give the CalTracs and their split mono leaf springs a try first. I like the idea of also being able to go around a corner somewhat fast as well as launching with traction. I feel like the concept of traction application with the CalTracs should work coming out of corners just as well as in a straight line but I’m interested in hearing what others might have to say about it as well. (And also I want to say in advance that I know a solid rear axle is going to be inherently less capable of good handling than independent suspension but if I’m comparing apples to apples then I want to know as compared to other leaf spring applications how these have worked out)

  3. Gerald Inman says:

    I like this suspension but I can’t get it to hook up on the trans brake with Hoosier 9in stocker tires at any rpm. I’ve a 1973 dodge dart with 416, 5.13 gears

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