Shackles: Are yours doing their job?

Proper shackle angle on an Old Man Emu suspension

I wonder if Obadiah Elliot had any clue, when in 1804 he patented a system of stacked steel plates designed to smooth the ride of a carriage, that his invention would still be in use two centuries later.

To be sure, the leaf spring has been eclipsed in sophistication by coil, torsion, and air springs, yet its simplicity, ruggedness, and low cost keep it standard equipment on the rear axles of millions of pickups and four-wheel-drive vehicles, as well as on the axles of larger freight-hauling trucks.

It’s not so much the cost of the spring itself that makes leaf-spring suspension systems cheaper to manufacture—it has to do more with the fact that a leaf spring also comprises its own locating mechanism. A coil or air-sprung beam axle requires a leading or trailing arm (or multiples) to secure it fore and aft, and a transverse arm such as a Panhard rod to locate it side to side. The leaf spring does both all on its own. Additionally, the stress a leaf spring applies to the chassis is divided between its front and rear mounting points, while the perch of a coil spring has to take all the load, requiring sufficient reinforcement.

Perhaps the biggest disadvantage of the leaf spring—that is, in the common configuration with multiple leaves—is inter-leaf friction, which not only hinders springing action but can vary or increase as, for example, the leaves become rusty. Some manufacturers such as Old Man Emu address this with a nylon pad at the end of each leaf, which can be lubricated.

There’s one situation, incidentally, when that interleaf friction can be an advantage—if you blow a shock absorber (as we recently did on our Land Cruiser Troopy), inter-leaf friction attenuates the endless cycling (bouncing) that would otherwise occur. If you’ve ever driven a coil-sprung vehicle with bad (or no) shocks, you’ll know what I mean. 

Those of you with leaf springs at one or both ends of your vehicle likely have never given much thought to the shackles—those brackets that connect the free end of the spring to the chassis. But they perform a critical function, and their orientation can affect several aspects of suspension performance.

A leaf spring in its static position has a specified eye-to-eye length. When it flexes as the vehicle travels over a bump or through a hole, the spring “lengthens” or “shortens”—obviously it actually does neither; as it flexes the arch in the spring simply decreases or increases, changing the eye-to-eye distance. A leaf spring attached rigidly to the chassis at both ends could not flex at all, so the shackle travels through an arc to allow this. Clearly, then, you want the shackle oriented so it does this job as effectively as possible. 

Take a look at this diagram.

Ignore for a moment everything but the angle at which the shackle meets the spring at the eye. This shows that angle as 90 degrees to the datum line—a line drawn straight between the eyes of the spring. For most practical purposes we can think of this as essentially right angles to the spring itself—an easy orientation to ascertain visually. 

The most obvious and important result of this angle is that it lets the spring flex to its maximum extent both when compressed and extended. You can see that if the shackle were angled as in “A,” the spring could flex a lot downward (as the shackle pivots forward), but when compressed, the shackle would quickly bind against the chassis. Exactly the opposite is the case with the shackle at “B.” The spring has plenty of travel when compressed, but very little when extended. Another danger of a shackle angled as at “B” is that if the spring flexes too much the shackle can invert and lock itself against the chassis, completely immobilizing the spring.

You might also read or hear that the angle of the shackle can affect the ride quality of the spring—and this is where things get vague. 

Note that this diagram claims that a shackle oriented at “A” will stiffen the ride while a shackle at “B” will soften it. I could find no explanation as to the physics of this supposed effect. On the other hand, I found a source claiming exactly the opposite. This one noted that with the shackle at “B,” when the spring compresses the shackle has to travel slightly downward in its arc before rising to the rear, and this jacks the chassis slightly upward, exacerbating the effect of a bump. Makes sense.

Not finished, however. Yet another fellow, with experience setting up racing vehicles, argues adamantly that the shackle has no effect either way on ride quality unless it actually binds. He points out that no matter what, the force from the spring is virtually straight up and down at the axle; the slight fore and aft movement imparted from the pivoting shackle is indiscernable. (He uses this fact also to argue against shackle-reversal kits as a waste of money.)

While the ride question remains unresolved, there’s no doubt that proper a 90-degree shackle angle allows the spring to do its job through the maximum possible travel in both compression and extension.

Look at the opening photo, which shows the rear of my FJ40 and its Old Man Emu suspension. The shackle angle is, as one would expect from the company, spot on (the front is as well).

In contrast, look at the shackle angle on the front springs of our Troopy:

Much closer to “B” in the diagram, no? These springs were installed at a shop in Perth, Australia, after we took it in to have them diagnose a worrying clicking noise I could both hear and feel through the steering wheel, and which neither Graham Jackson nor I were able to diagnose in the field except to be pretty sure it was in the steering. But the shop diagnosed worn springs, so we let them replace both sides.

We picked up the vehicle the day before we were scheduled to containerize both our and Graham and Connie’s Troopy for shipping to Africa—and as I drove away from the shop the clicking was there as loud as ever. Some testy and hasty negotiating resulted in a refund of all our labor charges, but the springs stayed on. (After getting the vehicle home I disassembled the steering and found indeed that was where the noise was coming from—just loose bolts in the tilt mechanism.)

Examining the springs in Durban I realized they had too much arch, resulting in this poor shackle angle. Whatever you believe regarding shackle angle and ride quality, these springs also definitely ride more harshly then the previous set, so I’m on a mission to fix both issues.

The first and most obvious approach is to remove a leaf in the springs. This isn’t necessarily as simple as it sounds, because removing the wrong leaf could create stress risers in the remaining leaves and lead to breakage. (So-called “add-a-leaf” kits can do this as well.) However, it looked to me that removing the bottom leaf on these springs wouldn’t compromise the rest of the pack, and the bottom leaf was the only one not captured with a clamp (or rebound clip to give it its proper name). So I jacked up the front end, loosened the U-bolts, and pulled the bottom leaves.

Notice the near-total lack of a wear pattern on the tips of the leaves.

After tightening everything again, I took the Troopy for a drive to settle everything then examined the results. Note the shackle angle in the first Troopy photo, and compare it to the “after” photo below. 

If you’re thinking, “I don’t see the slightest difference,” congratulations. I don’t see one either. Clearly those bottom leaves are doing nothing at all—at least when the vehicle is static. They probably don’t provide any resistance until the spring is significantly compressed.

Since this is in no way an existential threat, I’m re-evaluating. I might still try removing another leaf, or I might just live with it for now—I certainly don’t intend to spring for new springs just yet . . .