Coils vs. Leaf Springs | Early Classic Enterprises


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Coils vs. Leaf Springs

Publication Name: 
Classic Trucks
Publication Date: 
June, 2003
Sprint Time

GM Coils Vs. Leaf Springsand How to Build Stronger Arms

When it comes to '60-72 Chevy and GMC C-series trucks, there seems to be more myth than fact regarding the rear suspension systems. While some trucks came from the factory with rear leaf springs, others have coil springs and trailing arms. We've heard numerous stories over the years, and to help clear things up, we visited the experts at Early Classic Enterprises in Fresno California. Dave Clark and his crew have been building suspension components for the '60-72 GM trucks for close to a decade, and

their wealth of knowledge and experience is priceless.
We quizzed them in detail about the different chassis and suspension configurations, and here's a run down the facts: Prior to 1959, all GM light trucks were only available with a rear leaf spring suspension. Starting with the 1960 model year, General Motors completely redesigned their light truck line, and GM engineers came up with a simple, yet revolutionary trailing arm design.

Instead of the frame being supported by the parallel leaf assemblies, a variable rate coil spring mounted between a trailing arm and the frame supported the rear of the vehicle. The trailing arms ran forward and inboard to meet near the center of the truck. This same basic design has been incorporated into many different performance applications over the years, from NASCAR to desert racing. It is a simple, yet effective design that provides a comfortable ride, hauling capacity, and traction.

General Motors decided to build the Chevrolet line using the coils as the standard rear suspension. But, the uncertainty of just how popular the new suspension would be caused them to build the GMC line using the proven rear leaf design. A Chevy though, could be ordered with leafs, and a GMC with coils.

Many classic truck owners want to lower their vehicles, and the trailing arm rear suspension is an easier design to modify than the leaf. The ride height can be changed by simply replacing the coil springs and shocks with drop components available from numerous aftermarket suppliers.

While the leaf suspension can be slightly more difficult to lower, it does offer a more stable ride when hauling heavy or tall loads. GM engineers widened the rear spring width by seven inches overall by mounting the leaf springs on the outside of the frame---instead of underneath each frame rail like the coil setup. This wider mounting arrangement helps stabilize a taller center of gravity, such as a slide-in camper, not really anything most classic truck builders are concerned about. For those with leafs, ECE does offer drop leaf spring assemblies and extended rear shackles.

The trailing arms do have one drawback that needs to be addressed; they are known to corrode between the two stamped halves. Over time, they torque and flex causing the trailing arms to push apart and twist, resulting in failure. To help solve this problem, we now offers a trailing arm rebuild/upgrade kit to allow you to reinforce and strengthen your original trailing arms before it's too late.

For those truck owners with excessive horsepower (is there such a thing?), this is also a great upgrade to strengthen the rear suspension and help put that extra power to the ground.

We'll start by showing a few of the differences between the leaf spring and the trailing arm frames and a few of the key components needed to convert one to the other--- if so desired. One reason to convert to the trailing arms is to run a wider tire in the rear with a mini-tub. Because the leaf springs are placed outside the frame rail, they are just about as wide as the bed floor, but the trailing arms are inside the frame rails, which provide about five inches of extra room.

How To Steps and Images: 
Early Classic built this trick rolling chassis for display at events and in their showroom. They installed a coil spring/trailing arm setup on one side, and a leaf spring assembly on the opposite. This helps explain and compare the two different designs and shows how much wider the leaf springs setup is. Although the trailing arm does protrude outside the frame rail at the axle, a larger diameter wheel will go around it---allowing a wider wheel and tire combo.
A trailing arm frame will have these front trailing arm mounts, which you'll need to convert from a leaf spring.
If you find a bare frame in a salvage yard and it has these spring hangers riveted to the frame, then you know it was equipped with leaf springs. The longer rear shackle is one way to lower these trucks a couple of inches. Not shown here, but, another way to get a little more drop is to have a custom spring made with a reversed-eye, which rolls the spring over the top instead of from the bottom.
The same rearend can be used, but new spring saddles will be needed and will have to be repositioned. The trailing arm ones are installed at a slight angle, while the leaf spring ones are straight and closer to the brakes.
To stabilize the rear end laterally, GM used a panhard bar between the frame and rear end housing. When lowering the rear end, the panhard bar geometry changes depending on how low the suspension is dropped. ECE makes several different components to alleviate this problem, along with relocated upper and lower shock mounts to correct the shock angle.
If you've got the trailing arms and want to make them even better, here's the Early Classic rebuild kit which consists of two pair of upper and lower reinforcement plates, polygraphite bushings, mounting hardware, and new U-bolts (retail is $129). The installation does require welding, but this kit will give you a much stronger rear suspension capable of most anything you can throw at it.
After years of moisture and grime, the corrosion causes the stock trailing arms to expand apart, as shown on this trailing arm. While the new reinforcement plates will strengthen the stock arms, this example is probably too far gone to save.
The new reinforcement plates are shipped flat, but they will contour themselves into place when clamped down. Make certain that both the U-bolt and spring mounting bolt-holes are lined up correctly before welding.
The old trailing arm bushings can be removed in several ways. They can be pushed out using a press, or the outer metal sleeve can be heated until the rubber starts to smoke and then the center of the bushing can be pushed out. Either method will work, but the outer original metal sleeve must be reused in order to use the polygraphite bushings. We also carry the OEM rubber bushings, but they require pressing in and out.
The reinforcement plates can be stitch-welded, but a continuous weld will help seal out the elements from settling in behind the plates and starting new corrosion. The clamps help hold the plates tight against the surface of the trailing arm, allowing a nice, smooth weld.
The completed trailing arm is now much stronger than stock and will better withstand a high horsepower engine, as well as allow the truck to handle corners better.
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Company Name: 
Early Classic Enterprises
--, CA --
United States
(888) 777-0395
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