Trailing arm, Radius arm or Control arm: What is it actually called?

Author: SuperPro  

Control arm is the umbrella term for any suspension link that locates a wheel. A trailing arm is one type, defined by running along the length of the vehicle. A radius arm does the same job on a live axle front end, and a wishbone runs across the car. 

Are trailing arms the same as control arms, or is your workshop using two names for one part? Both, depending on who is talking. Naming in Australia mixes engineering terms, workshop shorthand and 4WD slang, so one arm can arrive with three labels on three invoices. We catalogue over 7,000 parts, including polyurethane bush kits and complete trailing arms, so working this out is a daily job. 

Here is the plain English version, organised by what each arm does.

 
Rear Trailing Arm
Trailing arm
Radius Arm
Radius arm

Control arm is the umbrella, and also a specific part 

Used broadly, control arm means any rigid link connecting the chassis or subframe to a wheel hub or axle and controlling how it moves. By that definition a trailing arm is a control arm, and so is a wishbone. 

Used narrowly, which is how most workshops and catalogues use it, a control arm is the lateral arm that holds the hub in from the side and sets camber, like the triangular arm at the bottom of a strut front end. Our guide to what a control arm does covers it.

Lower and upper control arm 

Lower control arm sits below the hub centre, usually carrying a ball joint and taking the bulk of cornering load. Upper control arm sits above it and shapes camber gain through travel. A strut front end has a lower arm only, while double wishbones run both. 

The arms that run along the vehicle

Trailing arm 

The chassis pivot sits ahead of the wheel or axle it locates, so the arm trails behind its mounting point. Its job is fore and aft location, holding the axle or hub against acceleration and braking. On a live axle rear end, such as an 80 Series or 105 Series LandCruiser, a Patrol or a Wrangler, the arms also control rotation of the housing, which sets pinion angle. On an independent rear end, including Falcon BA to FGX and the Territory, the arm holds one hub carrier and keeps rear toe stable under load. 

Radius arm 

Same plane, same job, different end of the vehicle. Radius arm is the term for the fore and aft arm on a live axle front end, where coil sprung 4WDs live. Because it resists rotation of the axle housing it sets caster, which is why lift kits include castor correction. 

Leading arm 

A leading arm is a trailing arm running the other way: its pivot sits behind the wheel, so it leads forward to the axle. The job is identical, and the difference shows in how it is loaded, in compression rather than tension under brakes. 

The arms and rods that run across the vehicle 

Wishbone and A-arm 

Two names for one shape: a triangular arm with two widely spaced inner pivots and a single outer joint. A-arm is the American term and wishbone the British one, and both describe a control arm in the narrow sense. The wide spacing lets one arm resist load in two planes. 

Track bar and panhard rod 

Not an arm, but it belongs in the family because people ask about it in the same breath. A panhard rod, or track bar, runs across the vehicle from the chassis on one side to the axle on the other. Its only job is stopping a live axle moving sideways, which is why a coil sprung axle needs one as well as locating arms. See what a panhard rod does. 

Tie rod, which is not a suspension arm 

A tie rod connects the steering rack or centre link to the steering arm on the hub, so it belongs to the steering system. The giveaway is its adjustable length, because toe is set with it. 

Suspension arm names, translated

Name 

Also called 

Which way it runs 

What it controls 

Where you find it 

Control arm 

Suspension arm 

Across the vehicle 

Camber, lateral location 

Independent suspension 

Lower control arm 

Lower wishbone, LCA 

Across, below the hub 

Camber, location, ball joint 

Strut front ends 

Upper control arm 

Upper wishbone, UCA 

Across, above the hub 

Camber and caster in travel 

Dual cab utes 

Trailing arm 

Rear control arm, rear link 

Along, pivot ahead of wheel 

Fore and aft location, pinion angle, toe 

4WD and Falcon rears 

Radius arm 

Caster arm 

Along, axle ahead of pivot 

Fore and aft location, caster 

Coil sprung 4WD fronts 

Leading arm 

Forward radius arm 

Along, pivot behind wheel 

Fore and aft location, in compression 

Some live axle fronts 

Wishbone 

A-arm, control arm 

Across the vehicle 

Lateral and fore and aft location 

Wishbone suspension 

Panhard rod 

Track bar 

Across, chassis to axle 

Sideways location of a live axle 

Coil sprung 4WDs 

Tie rod 

Track rod 

Across, rack to steering arm 

Toe and steering angle 

Steering system 

Why the naming matters when you order parts 

Order the wrong name and you get the wrong part, especially on a corner with both a lateral arm and a longitudinal one. Describe the arm by where it mounts and which way it runs, then confirm the catalogue name. We make polyurethane bushes and kits across this family, so on most vehicles the wearing part can be replaced without buying a complete arm. Our polyurethane resists oil, heat, ozone and UV far better than bonded rubber, and holds geometry under load instead of deflecting. Everything is tested in Brisbane, with a lifetime warranty.

Frequently asked questions

Looking for more information? Below we answer the most commonly asked questions