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.