A trailing arm suspension is a type of independent or semi-independent suspension where each wheel is connected to the chassis by a single, longitudinally mounted arm that pivots at its forward end. The arm extends backward (trails) to the wheel hub, hence the name. This design is most commonly found on the rear axles of front-wheel-drive cars, small trailers, and some off-road vehicles.
Background and Common Uses
The trailing arm design has been used in automobiles for over a century. It became particularly popular in the 1960s and 1970s as a compact, cost-effective way to provide independent rear suspension on front-wheel-drive economy cars. Notable examples include the Volkswagen Beetle (which used swing axles, a related design), the Citroën 2CV, and the original Mini. Today, you’ll find trailing arms on many small trailers, the rear axles of some SUVs and pickup trucks (like the Honda Ridgeline), and as part of the suspension on many modern electric bicycles and cargo bikes.
How a Trailing Arm Works
In a trailing arm suspension, each wheel hub is rigidly attached to the rear end of an arm that extends forward to a pivot point on the chassis. The pivot axis is perpendicular to the vehicle’s centerline, so the arm swings up and down in a vertical arc. When the wheel hits a bump, the arm rotates upward, compressing a spring (coil or leaf) and a shock absorber mounted between the arm and the body. The key characteristic is that the wheel’s path is a fixed arc, which means the wheelbase changes slightly as the suspension compresses and extends. This can cause a subtle steering effect if the arms are not perfectly parallel or if the pivot points are not aligned.
Types of Trailing Arm Suspensions
There are several variations of the trailing arm design, each with distinct characteristics:
- Full Trailing Arm: The pivot point is directly ahead of the wheel center, and the arm is the sole structural link. This is common on small trailers and some vintage cars.
- Semi-Trailing Arm: The pivot axis is angled relative to the vehicle’s centerline, so the arm swings both up/down and slightly inward/outward. This reduces camber change and improves handling, making it popular on many European sports sedans like the BMW 3 Series (E30 and later) and the Mercedes-Benz W123.
- Multi-Link Trailing Arm: A trailing arm is combined with additional links (e.g., lateral links, toe links) to provide more precise control of wheel motion. This is common on modern vehicles like the Ford Focus and Mazda 3.
Pros and Cons
The trailing arm suspension offers several advantages that explain its longevity:
- Compact and simple: It takes up minimal space, leaving more room for cargo or a fuel tank.
- Cost-effective: Fewer parts and simpler manufacturing make it cheaper than multi-link designs.
- Durable: The robust arm design can handle significant loads, especially in trailers.
However, it also has notable drawbacks:
- Limited tuning: The fixed geometry makes it hard to adjust camber or toe, limiting performance tuning.
- Bump steer: The arc motion can cause the wheel to steer slightly when hitting bumps, especially on full trailing arms.
- Poor ride quality on rough roads: The lack of lateral compliance can transmit harshness into the cabin.
Trailing Arm vs. Other Suspension Types
To understand where trailing arms fit, it helps to compare them to other common designs:
- MacPherson Strut: This uses a strut that combines a shock absorber and coil spring, with a lower control arm. It’s lighter and offers more steering precision, so it’s used on the front of most modern cars. Trailing arms are almost never used on the front because they don’t allow steering.
- Multi-Link: Uses multiple arms to control wheel motion in all directions, offering the best ride and handling but at higher cost and complexity. Trailing arms are simpler but less capable.
- Live Axle: A solid beam connects both rear wheels, so they move together. It’s durable and cheap but has poor ride quality. A trailing arm setup can be independent, which is a major advantage over a live axle.
Common Problems and Maintenance
Trailing arm suspensions are generally reliable, but they do have specific failure points:
- Bushing wear: The pivot bushings (usually rubber or polyurethane) wear out over time, causing clunking noises and vague handling. Replacement is straightforward but requires pressing out old bushings.
- Corrosion: On trailers and vehicles in salt-belt regions, the arms themselves can rust, especially at weld points. This can lead to structural failure.
- Misalignment: If a trailing arm is bent (e.g., from a curb impact), it can cause uneven tire wear and pulling. A visual inspection and alignment check are recommended after any significant impact.
Maintenance typically involves periodic inspection of bushings and mounts, and greasing any grease fittings if present. On trailers, check the spindle bearings at the same time.
How to Choose a Trailing Arm Suspension
If you’re considering a vehicle with a trailing arm suspension, or upgrading an existing one, consider these factors:
- Vehicle type: For a small trailer or a lightweight rear axle, a full trailing arm is fine. For a performance car, a semi-trailing arm or multi-link is better.
- Load capacity: Check the arm’s rating and the bushing’s durability. Heavy loads can accelerate bushing wear.
- Adjustability: If you need camber or toe adjustment, look for aftermarket arms with adjustable mounts.
- Compatibility: Ensure the arm’s length and pivot location match your vehicle’s specifications. A mismatch can cause poor geometry.
Safety Considerations
Safety is paramount with any suspension component. A failing trailing arm can cause a wheel to detach, leading to a loss of control. Always inspect for cracks, excessive rust, or worn bushings. If you hear clunking from the rear, have it checked immediately. When replacing bushings, use a press and never heat the arm, as that can weaken the metal. For trailers, ensure the safety chains and breakaway brake (if equipped) are functional, and check the arm’s pivot bolts for correct torque.
Frequently Asked Questions
Is a trailing arm suspension independent?
Yes, in most cases it is independent, meaning each wheel moves independently of the other. However, some trailers use a solid axle with trailing arms, which is not independent. The term “trailing arm” refers to the arm itself, not the overall axle type.
What’s the difference between a trailing arm and a control arm?
A control arm is a general term for any arm that connects the wheel hub to the chassis. A trailing arm is a specific type of control arm that is mounted longitudinally (front-to-back) and pivots at the forward end. Other control arms include lateral arms (side-to-side) and diagonal arms.
Can you lift a vehicle with trailing arms?
Yes, many off-road vehicles use trailing arms because they allow for long wheel travel. Lifting a vehicle with trailing arms typically involves installing longer arms or relocation brackets to correct geometry. However, excessive lift can cause driveline vibration and poor handling, so it’s best to consult a specialist.
Why do some cars have trailing arms on the rear instead of multi-link?
Cost and packaging. Trailing arms are cheaper to manufacture and take up less space, which is why they’re common on economy cars and compact SUVs. Multi-link systems offer better ride and handling but are more expensive and complex, so they’re reserved for premium vehicles.
Do all trailers use trailing arm suspension?
No, many trailers use leaf springs with a solid axle. However, torsion axle trailers (which use a rubber cord inside a tube) are a type of trailing arm design, and many boat trailers and small utility trailers use independent trailing arms for better ground clearance and smoother towing.
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