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August 1997 Willie's Workbench

Posted in Features on August 1, 1997
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There are several companies that are now selling "reverse shackle kits'' for Jeep CJs and YJs. I get quite a few inquiries on the subject as to the benefits of such a change. The question usually takes the form of, "If it's so great, didn't Jeep put the shackles at the rear of the front springs instead of up front, where they are now?''

Back when Jeep was winning the War, the shackles were at the rear, and they stayed that way on the later M38-A1s and the first couple of years of the CJ-5.

Why the switch from back to front? My theory is that Jeep was also using the old Ross steering setup, with a draglink to a bellcrank to a pair of split tie-rods. Lots of "monkey motion'' occurred as the suspension moved up and down, causing some self-steering motion referred to as "bump steer.'' Combined with the axle steering, induced by shackle movement as bumps are hit independently, the Jeep sometimes went where it wanted, not where you wanted to go. This might have been okay at convoy speeds, but in the hands of an untrained civilian at highway speeds, no way. So up front the shackles went.

In 1972, the Jeep people wised up, got ahold of some GM Saginaw boxes and mounted them with a single tie-rod and a far-reaching draglink setup that made them go down the road even straighter. It took Jeep engineers 30 years to make that change, and almost as long before they again made another change on the TJ.

So, why should the shackles be on the back half of the spring? Because the impact from compression is to the rear. When you hit a bump going forward, the axle wants to move rearward. With the shackle up front, the wheel actually has to move forward under spring compression. The easier you can make it on the spring, the better it can do its job, so ride quality greatly improves.

This reverse shackle trick isn't something new. Those who go fast in the dirt have been doing it for years. It must've been about 1970 when I first got smart and did it to a race Jeep. It took me about two sets of bent springs before I figured it out.

There is a downside to reverse shackles-perhaps it's why Jeep engineers kept 'em like they did. When doing some serious low- speed rock crawling, the forward movement of the tire as the spring compresses actually forces the tire into an object for better traction, allowing you to hop up onto steep stairsteps more easily. It's just something to think about before you make the change. Me? I'll take the reverse shackles for the gain in ride quality.

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