These days, 40-inch tires seem like the norm, and it isn’t uncommon for people to install hydraulic-assist steering on their street-driven 4x4s (or full hydraulic on trail-only-buggies). Our ’06 LJ has 37-inch tires and the factory steering system definitely didn’t like to turn them when they were aired down on the trail. We assumed that we needed hydraulic-assist steering, but after talking to Tom Allen at PSC Motorsports he explained that might not be the case. PSC Motorsports offers a whole host of options for ’87-current Jeep Wranglers.
Given our tire size and the amount of road miles we drive, Allen recommended the company’s Big Bore XD replacement steering box. The Big Bore box is a bolt-on upgrade for ’03-’06 Wranglers that replaces the factory ZF steering box. A German steering box on a Jeep? You may recall that in 2003, Mercedes-Benz purchased Chrysler, and at that time they switched steering box suppliers from Delphi to ZF. “These are the same basic boxes that Mercedes put on passenger cars in the ’80s,” Allen revealed. So, PSC offers an all-new 8200 Series box with a larger-diameter piston to increase torque output and a ported internal valve skirt and custom-machined torsion bar.
We had initially suggested that we could perhaps use a fish scale to measure the steering effort necessary to turn the factory box and the new Big Bore XD box. “I can make you a box that is super easy to turn, but I don’t think you will like it on the road,” Allen cautioned. “That is a function of the torsion bar in the steering box, it changes the steering effort, but has zero effect on the output torque.” Just like in a suspension system, the thickness of the torsion bar in your steering box affects its resistance to twisting. The torsion bar in the steering box slides inside the input shaft and is twisted when the steering wheel is turned. PSC sent us two Big Bore XD steering boxes with different torsion bar thickness so we could test them for ourselves. And like Allen warned, the lighter steering effort seemed great at first blush in the parking lot, but at higher speeds the lack of road feel had us making constant corrections.
We also learned that “steering system” is an important term during our conversations with Allen. “There are a number of factors that will affect steering effort and strength,” he explained. “Yes, you can just add a big hydraulic ram, but that could also result in slow steering and cavitation if you just slap it on to your current steering system.” We added a CBR-style steering pump from PSC after installing the new Big Bore XD box to complete our steering system. The new pump is considerably quieter than our stock steering pump was when turning aired-down 37-inch tires in the rocks. Our new steering system works just as well on the street, too, and we love that it is scalable should we decide to add a hydraulic-assist ram in the future.
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