What does a trailing arm do, and why do they wear out?
Author: SuperPro
What does a trailing arm do? It locates the axle or hub in the fore and aft plane, holding it in place against the shove of acceleration and the drag of braking, and on a live axle it sets pinion angle. Its bushes carry those loads directly, which is why they wear out first.
What does a trailing arm do that a coil spring or a shock absorber cannot? It stops the axle moving backwards and forwards. The rest of your rear suspension controls vertical movement or damps it, while this one runs along the length of the vehicle and pins the axle or hub carrier to the chassis. We manufacture polyurethane bushes, kits and complete trailing arms for a wide slice of the Australian fleet.
The arm itself usually outlives the vehicle. What gives up is the bush at each end, for a reason worth understanding before you spend money.
Rear Adjustable Upper Trailing Arm Kit to suit Toyota Prado 150 Series (non KDSS vehicles only), FJ Cruiser & GWM Tank 300
The job it actually does
Longitudinal location
Picture the rear axle of an 80 Series or 105 Series LandCruiser. Coils hold it up, a panhard rod stops it sliding sideways, and shocks damp the movement. None of that stops the axle being pushed forward under power or dragged backward under brakes. That is the trailing arm's work, done by triangulating the axle to the chassis through a pivot at each end.
Pinion angle on a live axle
Because the arms bolt to the axle housing, they also control its rotation. The angle of the diff pinion relative to the tailshaft is set almost entirely by where those arms hold the housing. Get it wrong, through lift height or worn bushes, and the universal joints run at an angle they were never designed for.
Fore and aft compliance on an independent rear end
On a Falcon BA through to FGX, a Territory, and many Japanese and European independent rear ends, the arm does the same job for a single hub carrier rather than a whole axle. It holds the hub against braking load and keeps rear toe stable while the suspension moves. Once that bush softens, the rear of the car starts steering itself on bumpy corners.
Why the bushes take such a hammering
Most suspension bushes only allow a pivoting movement while a spring does the heavy lifting. A trailing arm bush is different, because driving and braking loads pass straight through it on the way into the chassis. Accelerate hard and it is squashed one way, brake hard and it is squashed the other. Do that thousands of times a week with a couple of tonnes behind it, and bonded rubber cannot hold its shape forever.
Australian conditions accelerate the process. Heat cycles the rubber, UV and ozone attack the exposed faces, and corrugated dirt roads hammer it constantly. Add a loaded touring 4WD, a camper on the towbar or a set of 33s, and the load rises beyond what the original design brief assumed.
What worn bushes feel like from the driver's seat
The first symptom is almost always a clunk on takeoff and again on the brakes, because the axle is now free to move a few millimetres before the bush catches it. Vibration through the floor at highway speed follows as pinion angle drifts. On an independent rear end you get a loose feeling under brakes and uneven rear tyre wear.
Symptoms, causes and what to check
|
What you notice |
What is likely happening |
What to check |
|
Clunk on takeoff and on brakes |
Bush has lost its bond, so the axle moves before it loads up |
Rock the vehicle in gear and watch the arm ends |
|
Vibration through the floor above 80km/h |
Pinion angle has drifted as the bushes collapsed |
Bush condition first, tailshaft universal joints second |
|
Loose rear end under brakes |
Fore and aft compliance in an independent rear arm |
Lever the bush and watch for movement in the void |
|
Uneven rear tyre wear |
Rear toe moving under load |
Rear alignment reading, then bush condition |
|
Bush cracked or sitting off centre |
Rubber has torn away from its shell |
Remove the wheel and inspect both ends |
The driveway check that tells you most is the lever test. With the vehicle safely supported, put a long bar against the arm near the bush and load it fore and aft. Any visible shift of the sleeve inside its shell means the bush has stopped doing its job.
The repair we would recommend
On most vehicles the arm is still straight and sound, and only the bushes have failed. That is the repair we engineer for. Our polyurethane resists oil, heat, ozone and UV far better than bonded rubber, it does not tear or delaminate, and it holds the axle where the geometry says it should be instead of deflecting under load.
With more than 7,000 parts engineered and tested at our Brisbane facility, we can usually supply the wearing item on its own, backed by a lifetime warranty. Where an arm is bent, cracked or badly corroded, we supply complete arms.
Frequently asked questions
Looking for more information? Below we answer the most commonly asked questions
What does a trailing arm do?
What is a trailing arm also called?
Why do trailing arm bushes wear out so quickly?
Can you drive with a bad trailing arm?
Do trailing arms affect wheel alignment?
How long do trailing arm bushes last?
Can you replace just the bushes instead of the whole arm?
Are trailing arms the same as control arms?
If the clunk on takeoff has started, the cheapest time to fix it is now, before pinion angle takes your tailshaft joints and rear tyres with it. Have a look through our polyurethane bush kits and complete trailing arms, and talk to us about the right option.
