Inside Smith Racecraft’s Advanced Hi-Tech Assassin Traction Bar, Part 1

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Bolt-on leaf spring traction devices have been around for what seems like forever. There have been plenty of different traction bar arrangements built and manufactured over the years. Some were good. Some were not so good. Fair enough, but time doesn’t stand still, either.

First let’s spin the clock back sixty or so years: The bar of choice was the round tube Traction Master. It had a mount that was affixed to the bottom of the leaf spring (at the axle tube), with a section of tubing that stretched forward to the front spring eye. From there, though, the front bracket had to be welded to the chassis. They couldn’t be tuned.

The next innovation was the slapper bar. Several manufacturers built (and some still build) them. Most know what these are – they’re a square tube or rectangular bar that replaced the lower spring perch. From there, the bar tubing reached forward with a rubber snubber that (initially) came in contact with the spring just aft of the forward eye. They used a special “J” bolt that helped anchor the bar to the rear axle housing. Tuning consisted of cutting the rubber snubber or shimming it upward. You could also tune the bars with spring wedges in order to alter the pinion angle. Racers soon found that the bars worked better when they were stretched a bit on the nose so that the snubber actually made contact with the spring eye. Manufacturers of some of the later examples claim their models are physically longer and actually make contact with the eye.

Today, slapper bars are still in use. They work reasonably well, but you really don’t have a lot of tuning options. We should also point out that by the mid-seventies, everyone wanted slapper bars. The manufacturers paid attention and, as a result, there were all sorts of inexpensive examples on the market that really didn’t work. Some were even chrome plated. Consider them cosmetic. Some bolt-on ladder bars surfaced at about the same time. They simply didn’t work because with the configuration, it was difficult to absorb and control the forces present when big power was applied. Place them in the cosmetic file too.

Next up came the C-E Slide-A-Link bar. Here, the design is such that it bolts on to the housing like a slapper bar. At the front of the spring, a pivot point is added. A pivot point mount plate installs inside the front leaf spring pocket and clamps down on the leaf spring. In between is a sliding link. The system works much like the original Traction Master, but the actual traction “tube” can’t bind because the sliding link allows it to move fore and aft. Essentially, what this does is allow the front segment of the spring to remain stiff under power (much like an old Mopar Super Stock spring). They offer limited adjustment: Free travel and pre-load adjustments are made on the vehicle by adjusting the jackscrew at the rear of the bar.

In roughly the same time span as the sliding link bar, a unique traction bar came out of California (CalTrac). We’ve discussed them at length in the past, but the basics are that it too has a rear mount that replaces the lower rear spring perch. It then incorporates a section of round tubing that goes forward to the front section of the leaf spring. From here, a rocker mount (anchored to the forward spring eye) works an actual rocker assembly containing a large roller. That roller clamps down on the front segment of the spring as power is applied, which in turn applies force to the top of the leaf spring. The bars are pre-load adjustable (by shortening or lengthening the tubular link) and in some configurations, they have two positions that allow for moving the front of the bar up or down.

As you can see, traction bar technology moved forward (sometimes at a snail’s pace, but forward nonetheless). Some of the bars had (and some cases, still have) limited adjustment and, to be perfectly honest, some have earned a reputation as spring breakers. The latest (and perhaps biggest) move forward in bolt-on traction bar technology is the Smith Racecraft Assassin. It doesn’t have any of the negatives of the other bars and it has a ton of positives. What it is, is the most adjustable bolt-on bar in the business. It’s also an extremely high quality bar that incorporates a unique adjustable rocker that directly contacts the front spring eye (more on this later in the series).

In the photos that follow, we’ll show you the basics of assembling a set of Assassin traction bars. And in the segments that follow, we’ll give you some background on Smith Racecraft (you might be surprised), as well as a better idea of how the bars work along with the steps required to adjust them. Tuning a bolt-on bar has never been easier. Follow along. Smith Racecraft’s Assassins are without question the most advanced bolt-on traction bars ever built.

When unboxed, this is what the Assassin traction bars look like. Everything is over-the-top in terms of quality, from the choice of materials to the fabrication to the fasteners.

Smith Racecraft incorporates extra heavy-duty rod ends in their traction bars. An oversize shank rod end (extra HD) is most often built by installing an insert one size smaller in the body of a rod end. Here, a 1/2-inch X 5/8-inch rod end will always demonstrate higher load capacities than a 5/8-inch X 5/8-inch rod end, assuming both are built with similar materials and specifications. The reason is due to the amount of body material found around the insert. Typically, this is the rod end format for something like a Pro Stock car.

It’s a good practice to always use anti-seize compound on the threads of rod ends. You don’t have to slather it on. Instead, a thin coat works well. In the end, you’ll appreciate the forethought when it comes time to adjust your bars.

Here’s an assembled Smith Racecraft Assassin link. It looks exactly like the lower link of a four link. One end is conventional right hand thread, the other left hand thread, allowing you to establish the length without removing the link from the car. FYI, Smith includes a tubing adapter with a hex on one end. That’s a big bonus when working on the suspension.

Image courtesy Smith Racecraft.

There are two common spring eye configurations you’ll run into with a leaf. One is conventional and the other is called a “Berlin” eye. As you can see in this illustration, the type of eye dictates the location of the rocker assembly in the Smith Racecraft traction bar.

If anything, Smith Racecraft does too good of a job when it comes to powder coating their components! When you’re adjusting the rocker and moving it down, it’s a good idea to remove the powder coating. A Dremel with a sanding roll does a quick job of it.

Here’s the re-assembled rocker, set up for a conventional leaf spring eye. Note the Grade 8 hardware used in the assembly. We’ll dig deeper into it in the next segment.

This is the assembled Smith Racecraft Assassin traction bar. As we mentioned in the text, the quality is over-the-top. In the issues that follow, we’ll discuss how easy it is to set up. Watch for it.