Know Your Springs - Suspension Secrets

Here is a view of the torsion bars on a Chevy 2500 HD with a BDS suspension lift. One end of the torsion bar is mounted to the chassis (fixed), while the other is mounted to the control arm (movable).

Here is a view of the torsion bars on a Chevy 2500 HD with a BDS suspension lift. One end

Torsion Bars
Torsion bars are perhaps the least common springs in use today, but are still used in a number of current General Motors vehicles, especially the heavy-duty line of pickup trucks. Other 4x4 vehicles to use torsion bars over the years are Toyota IFS (pre-Tacoma) trucks, '98-to-current Ford Rangers, Dodge pickups, and Hummer H3s, to name a few.

The torsion bar, or torsion spring, is a long metal bar that is attached to a fixed mount on the chassis on one end and attached to the suspension-typically, a lever that is mounted perpendicular to the direction the torsion bar is mounted (a control arm, for example)-on the other end. As the wheel moves up and down, the torsion bar twists on its axis, resisting the suspension movement and returning it to center. The strength, spring rate, and load-carrying capacity of torsion bars are determined by the thickness and length of the bar.

This is a CAD drawing of a Chrysler axle on the rear of a Ram 1500. This is the older leaf-sprung suspension.

This is a CAD drawing of a Chrysler axle on the rear of a Ram 1500. This is the older leaf

Perhaps the simplest form of all springs, torsion bars are known for their durability and ease of replacement and maintenance. Small changes in ride height are achievable through "twisting" or "cranking" of the bars, which effectively moves ride height to a different part of the suspension travel by adjusting the torsion bar preload through the adjuster nut. However, this will limit downtravel, or droop, diminishing ride quality. Additionally, it can contribute to other negative effects if it goes beyond the generally accepted "safe" range of 11/2 to two inches (depending on the vehicle, of course).

Other drawbacks to torsion bars are a lack of a progressive- or variable-rate option, giving a torsion bar less flexibility when loaded or going over harsh terrain, along with limited wheel travel.

Pros: Durable, simple, easy to make minor adjustments in ride height.

Cons: Spring rate not progressive, limited wheel travel, few aftermarket options.

This is a CAD drawing of the same Chrysler axle with the new coil-sprung suspension. Despite being more complex, the coil-sprung suspension saves Chrysler 40 pounds of weight and provides vastly improved ride and handling, while losing very little in capability on the '09-and-later Ram 1500.

This is a CAD drawing of the same Chrysler axle with the new coil-sprung suspension. Despi

Coil Spring
Because of the excellent ride characteristics, low noise, and ease of manufacturing, compression coil springs are the most common type of automotive spring in use today. A compression coil spring is a metal wire that has been wound into the shape of a helix to resist compression. A coil spring's rate is determined by the thickness of the wire and pitch of the winding.

Coil springs can either be linear- or variable-rate, and you can easily spot the difference between them. A variable-rate coil spring is wound with varying pitch and spacing, and is designed to be softer first and firmer later. Variable-rate coil springs are used when a firmer spring rate is desired to handle more weight, decrease sway, and improve handling without sacrificing ride quality. A linear-rate coil has equally-spaced windings and will maintain the same rate throughout its compression travel.

Additional benefits of the coil spring include compact size, ability to precisely tune the suspension, as well as excellent articulation and wheel travel. Coils also have no internal friction and weigh much less than leaf springs.

While the benefits are many, these benefits do come at a cost. Because coils don't locate the axle, they require links to work. Those links, while adding expense and complexity to a suspension's design, are what makes a coil-sprung suspension so good at resisting axlewrap. Coil springs are also limited to how tall they can get. The taller the coil, the more they are prone to bowing or deflection, which can diminish their effectiveness over time and cause interference issues. Additionally, coil springs, when repeatedly loaded to the max, can weaken or sag prematurely and are not as durable as leaf springs in heavy-duty applications.

Pros: Quiet, better ride, easier to tune, lighter weight, eliminate axlewrap, capable of big articulation.

Cons: Need links, can sag over time or if overloaded, more susceptible to sway, expensive to retrofit, limited lift heights, not as robust as leaf springs.

What About Coilovers?
Once thought of as purely for race applications, coilovers have gained a huge amount of popularity over the last several years. Even more compact to mount than coil springs, a coilover includes the damper (shock) and coil spring in the same unit, and because the coil spring is mounted to the shock and not the car, greater lift heights are achievable without coil deflection. Coilovers also mount with hemispherical bearings, which essentially allow the coilover to move on its mounts, preventing bind under articulation.

Coilovers further distance themselves from other spring setups by their incredible range of adjustability and their ability to run a two-stage spring setup. Changing spring rates is as easy as pulling the unit and swapping to a different coil. Threaded bodies allow preload and ride height to also be easily changed, all while the coilover is still mounted on the vehicle.

The biggest drawback to a coilover setup is the expense of the unit itself, as well as the fabrication work that may be required to fit one on a vehicle, as most coilovers are not direct replacement units.