Rear Suspension And Chassis Tuning - Bringing Up The Rear

Ladder Bars
Mention ladder bars and nose-high gassers most likely spring to mind, but there are two types of ladder bars. Sure, the first type is usually seen on race or Pro Street cars and consists of a pair of triangulated bars that have a single mount at their front end and a pair of mounts at the axle end. In race-type cars, the ladder bars are usually around 30 inches long, and often are only adjustable at the front end. Some "double adjustable" ladder bars have more adjustability in their lower sections and can be used to fine-tune the pinion angle, which is often 1 or 2 degrees "nose down" in a race car application.

The second type of ladder bars, which is much longer, was developed by the street rod aftermarket and is offered by numerous manufacturers today such as SO-CAL Speed Shop-whose Pete Chapouris, along with Jim "Jake" Jacobs, pioneered such parts some 30 years ago as Pete & Jake's Hot Rod Parts. Rather than being mounted parallel, these ladder bars are mounted at the axle end as far outboard as possible, tapering in toward their forward ends, where they are mounted around 8 inches apart, usually either side of the driveshaft at the transmission crossmember. This type of ladder bar has no adjustment at the forward end, and clevis pins at the rear that offer very little adjustment. During setup, the brackets that weld to the axlehousing should not be fully welded until the pinion angle has been set. The triangulated setup allows similar suspension movement to the early Ford arrangement.

Ladder bars are a simple, easy-to-maintain style of axle location that fit easily under most floorpans, though the second type offers a smoother ride owing to the greater length of the bars. The first type is designed relatively short, as they provide a harder hit when a race car leaves the start line. Either type will require some form of lateral link to prevent sideways movement of the axle, though the second type can be used with a transverse spring, eliminating the need for this.

Trailing Arms
More familiarly seen on '60-72 Chevy pickup trucks as stock equipment, trailing arms have been used in the past on kustoms to good effect. They essentially work the same as ladder bars, with a single front mount, but instead of bolting to a vertical bracket on the axlehousing, two vertical bolts pass through the trailing arm and the axle pad. The length of the bars will provide an exceptionally smooth ride if the Chevy truck arms are used, or aftermarket versions such as those offered by Classic Performance Products. The factory trailing arms also offer a great lower mount for airbags, as the original coil spring mounts on them too.

Four-Links
A four-link offers the most adjustability of any rear axle location system, probably best suited for drag cars but worth mentioning here in case you're building a car with a performance bent. It allows fine-tuning for improved 60-foot times on the dragstrip. We can't see too many R&C readers using a four-link in their cars, so we won't dwell too long on it, but it is a form of axle location, and as such should be mentioned.

Parallel Four-Bars
One of the most used systems in rodding today, and for good reason, the parallel four-bar takes up little, if any, interior room, works well, and is relatively maintenance-free. It's also inexpensive and fairly easy to install in most chassis.

As its name suggests, it comprises four bars, two per side, that run forward from the axle to brackets usually welded to the underside of the framerails. The upper and lower bars are parallel to each other when viewed from the side. They don't necessarily run parallel side to side; some kits feature angled urethane bushed joints at the end of the bars so the forward mounts are closer together than the mounts on the axle. This makes it easier to mount the forward brackets on most chassis and still mount the axle brackets reasonably far outboard. The system operates on a parallelogram principle, where the rearend always remains perpendicular to the ground, maintaining a constant pinion angle. With a Panhard rod providing lateral stability, the rearend is always in proper alignment.

Three-Links
Again, maybe slightly outside the usual R&C field, a three-link will probably apply more to Pro Touring cars and the like. Coupled with an adjustable Watts linkage, it allows more rearend articulation without bind, and is perfect if you take your cornering seriously.

Triangulated Four-Bars
A growing number of hot rodders and custom owners are coming to appreciate good handling, realizing that bad road manners are not some-thing they have to put up with today. The triangulated four-bar rear suspension goes a long way in helping your old car perform like a modern one. This type of axle location has been around on production cars since the mid-'60s (GM A-bodies, for instance), but it's only relatively recently that people have moved on from simply replacing parallel leaf springs, for instance, with a more modern version of the same thing, to replacing them with an improved system.

The triangulated four-bar uses a pair of lower bars in much the same way as a regular parallel four-bar, but the upper pair of bars are located on the axlehousing near the center, either side of the pumpkin, and are angled outward toward the chassis 'rails at approximately 45 degrees. Mounting the upper bars at an angle eliminates the need for any form of lateral locator. Each of the four bars is adjustable to perfectly align the axle and adjust the pinion angle. Art Morrison Enterprises uses this system in its Tri-Five and Max G chassis, where a low center of gravity and great handling are required, and Air Ride Technologies reports it is the company's most popular system.

Lateral Location
One thing we haven't mentioned yet is the various methods of lateral location; that is the ways to eliminate sideways movement of your rearend. The movement is limited anyway with a transverse leaf spring, and the triangulated four-bar eliminates sideways movement by its very design; but with the other forms of axle location discussed here, you will need some way to ensure your rearend remains central in the car. The most simple method is a Panhard rod, which is a bar running across the vehicle from the axle casing to the chassis. You could opt for a diagonal link if you have a four-link or four-bar rearend, or even a wishbone, though the latter is more of a race car option.

The Panhard rod shown on the Model A chassis on page 50 is bolted to welded brackets on the chassis and a bracket that bolts to the third member on the axle. The longer the Panhard rod, the smaller the arc it will travel in, and therefore limit sideways movement as the axle moves up or down. However, most hot rods have limited travel compared to OEM suspensions, and the movement is negligible.

Another option is to employ a Watts linkage, which is more complicated than the others but ensures your axle remains absolutely central at all times-something a Panhard rod can't do, as it has to move in an arc. You can see a Watts linkage in the Art Morrison three-link suspension photo on page 50. It consists of two bars, each attached to brackets on the chassis, their inner ends attached to a pivot on the rear of the differential. This pivot allows the axle to move up vertically, but not sideways.