Leaf springs can suffer from axle wrap. Under heavy acceleration, the rear springs twist up against the forward rotation of the tires. As the housing rotates, the leaf springs contort against their natural arch. The front of the leaf rises and allows the housing to rotate. When the leaf spring cannot wrap any more, it snaps back into its natural state and shocks the tires. Unfortunately, this often happens several times. Once the wrap and snap has occurred, it tends to become a cycle. The rear wheels jump up and down and the rear of the vehicle starts bouncing up and down. Hence you get the term: “wheel hop.” This results in a dangerous loss of control and so it must be remedied.
Trucks are more susceptible to this condition because beds are light and do not place much weight on the axle unless there is something in the bed. If you punch the gas and the back end starts hopping, you have to let up on the gas to stop it.
Installing traction bars is an effective fix for wheel hop. They bolt to the housing mount under the leaf pack and extend forward to the front leaf spring mount. As the housing tries to rotate upward on heavy acceleration, the bar contacts the front mount, locking the leaf spring in shape so that it can’t wrap, and thereby eliminating the effect of wheel hop. Different versions of these devices are available, but the principle is the same with each.
Watt’s Linkage Rear Suspension
When torque is applied to a leaf spring suspension design, it has a tendency to walk from side to side. This is due to the rubber bushings, the way the rear of the springs are mounted with two hanging shackles, and the multiple leaves. Leaf spring suspensions have certain drawbacks. When a leaf spring car enters a corner, the entire leaf pack compresses and can fan out slightly, just like a deck of cards.
In addition, leaf spring shackles and bushings can flex so suspension actuation is sloppy. As a result, handling deteriorates and it becomes difficult to negotiate corners at higher speeds.
One of the better solutions for this problem is a Watt’s linkage, which connects both sides’ leaf packs with an articulating arm in the center to keep the housing from wandering from side to side under heavy loads. These work extremely well and are often used in road race vehicles.
Originally designed by James Watt in 1784, the linkage was intended for the Watt steam engine. Also referred to as parallel motion, the Watt’s linkage prevents side-to-side movement by providing a solid location at the center of the vehicle. The propeller is mounted in the center of the vehicle on a bracket that is attached to the chassis. The two side arms connect to the propeller, which rotates as the suspension moves up and down. This allows free vertical movement, but positively locates the housing in the center of the car.
This is the more versatile form of rear suspension because it provides a lot of tuning latitude. A coil spring is not susceptible to axle wrap because every coil spring rear suspension uses solid bars to connect the housing to the chassis. Two main types of factory links are used with 10- and 12-bolt designs: trailing arms and the triangulated four-link.
Some 1960s and 1970s GM cars are not equipped with the common Chevy 10- or 12-bolt axle assemblies. This means that you need to positively identify the axle assembly before you begin work. The GM B-Body cars, such as the Impala and Caprice, used a version of the four-link system, but it is a little more complicated than the A- and G-Body versions. This 1962 Buick used an oddball differential that’s unrelated to the 10 or 12-bolt, but the suspension is the same. This one uses a panhard bar as well.
Trailing Arm Axle
A trailing arm is a long bar that runs from the forward portion of the chassis to the rear where the housing is mounted. Trailing arms are typically used in an underslung axle assembly. Trailing arm axles are commonly found on GMC and Chevy trucks built from 1960 through 1968.
This design works well because the trailing arms are quite long. Their front pivot point is in the center of the chassis, so it yields excellent handling characteristics. The drawback for this design is that coil springs simply cannot handle extreme loads.
For this reason, coil springs are not often used for the rear suspension of trucks. The factory fix was a half-leaf overload spring, but that did not solve the tramping, or wheel-hop, conditions until they converted to a leaf pack in the late 1960s.
Passenger cars from the early 1960s often had the trailing arm design too, including the GM X-frame B-Body cars (Impala, Biscayne, etc.), but these cars don’t have 10- or 12-bolt differentials.
The coil spring in most trailing arm designs is positioned on the trailing arm itself, so it connects to a spring pad on the frame. This can make for some awkward spring angles on the car versions, but trucks use longer springs that sit vertical. One of the nicer aspects of working on a trailing arm vehicle with the spring on the arm is that you don’t have to disconnect the suspension to remove the housing.
Panhard Bar Axle
The panhard bar is a long bar that connects one side of the housing to the opposite side of the chassis with only two bars and one chassis mount per bar. The trailing arm design needs to locate the housing to the center of the chassis, and the panhard bar connects to the housing in the center of the chassis. In stock form, this bar is solid and not adjustable. Adjustable upgrades are available from the aftermarket.
With a panhard bar, the vertical motion of the axle is more controlled; however, it swings to one side in a slight arc. This is due to the fact that the bar cannot change length; the housing must move along this arc to facilitate the vertical movement.
The longer the bar, the shallower the arc; therefore the less the side movement. Because of this, a panhard bar is much less effective on passenger cars than trucks. The bar is too short to effectively control the horizontal location and keep the tires from contacting the fender wells. General Motors installed these bars on many of its cars in the early 1960s, and they have lots of space between the tires and the wheel lips because of it.
In addition, you cannot correct the suspension geometry and reduce the arc of travel. The only way to do that is to remove the panhard bar and convert to a Watt’s linkage design.
Triangulated Four-Link Rear Suspension
When the muscle car era began, General Motors introduced its rear suspension version: the triangulated four-link. This design incorporates a pair of lower bars that run in-line with the chassis, and two very short upper arms that angle toward the center of the car at the top of the chassis. All four bars connect to different points on the housing. The lowers are on one plane while the uppers are on another plane. The goal is to maintain a singular vertical range of motion. It’s also designed to eliminate the possibility of the housing wrapping up under acceleration and to reduce side-to-side deflection with the angled upper bars.
The type of Chevy 10- and 12-bolt axle housing depends on the style of suspension. This GM A-Body frame uses the triangulated four-link system, so this housing fits only a GM A-Body car with this four-link system. This frame is also used on GM G-Body cars, though it is not a direct swap.
When a Chevy 10- or 12-bolt is pulled from a vehicle, it often looks like this but it may be even more greasy, grimy, and possibly rusty. Years of road grime pile up on the differential housing. So the first step is to strip