Individual Trailing Arms with Fitting Position Front Axle, Rear Axle

Individual trailing arms are supplied singly rather than in pairs, for the side and position required.

Buying one is straightforward on some vehicles and a poor idea on others, and the difference comes down to whether the two arms have to match each other.

Why Two Trailing Arms Must Match in Length

On a solid rear axle the two trailing arms work against the same housing, which makes their effective length relative to one another more important than either length on its own.

Effective length means more than the arm itself, since it includes the bushes at each end, and a collapsed bush adds play that a fresh one does not have. A new arm with new bushes on one side working against an original arm with perished bushes on the other means the axle is being held firmly at one end and loosely at the other.

Under acceleration and braking the loose side moves and the firm side does not, so the axle rotates slightly rather than staying square to the vehicle.

That is a different situation to an independent rear end, where each arm locates its own hub and the two sides are not mechanically tied together. On those, a single arm replacement affects one corner only.

Establishing which arrangement your vehicle uses is the first thing to do, because it decides whether one arm is a sensible purchase or half a repair.

Thrust Angle and Why Mismatched Trailing Arms Make a Vehicle Crab

Worth understanding, since it explains a complaint people struggle to get diagnosed.

Thrust angle is the angle between the vehicle's geometric centreline and the thrust line, which is the direction the rear axle is actually pointing. When the two are parallel the angle reads zero and the rear wheels push the car straight ahead. When the axle sits skewed, the rear wheels aim somewhere else entirely and everything in front has to respond to that.

A vehicle with a thrust angle problem tracks slightly sideways down the road, described as crabbing or dog tracking, or as the vehicle feeling like it is going straight while the steering wheel sits off centre. The steering wheel is off centre because the front wheels are being steered to compensate for a rear that is aiming elsewhere.

Trailing arms of mismatched effective length are a common cause on a solid axle. One side holding the housing further forward than the other puts the axle out of square, and no front end alignment will correct it because the fault is at the back.

If a wheel alignment report shows a thrust angle outside specification, the trailing arms and their bushes are the first place to look. Worn or damaged bushings, or a bent arm, are a recognised cause of a steering pull on any vehicle using trailing arm suspension.

Why the Rear Gets Aligned Before the Front

Worth understanding, because it explains why a front end alignment alone will not fix this.

An aligner establishes the thrust line first and then sets front toe relative to it, rather than to the body of the vehicle. The rear axle decides what straight ahead actually means, in the same way a rudder decides where a boat points, and everything at the front is adjusted to follow it.

Skip that step and the front can be set perfectly to specification while the vehicle still crabs, because the reference it was set against was wrong to begin with.

That is also why repeat alignments happen so often. A vehicle that keeps coming back with the steering wheel off centre and tyres wearing unevenly, despite correct front readings, usually has a rear that was never measured.

A front end alignment measures and adjusts the front axle only. A thrust alignment adds the rear axle measurement and is what any vehicle with a solid rear axle actually needs. A four wheel alignment goes further again and is required where the rear is independent and adjustable.

If your vehicle has trailing arms and you are only being offered a front end alignment, ask for a thrust or four wheel alignment instead.

When to Have the Thrust Angle Checked

• After any collision, however minor it looked at the time
• After a hard knock to the suspension, including running up a gutter
• After replacing any rear suspension component, including a single trailing arm
• When the steering wheel sits off centre while the vehicle tracks straight
• When the vehicle feels loose or unsettled at speed
• When rear tyres are wearing unevenly with no other explanation
• When a previous alignment has not held or the same complaint keeps returning
• As part of a periodic geometry check every couple of years

Have the alignment done with the vehicle carrying its normal load rather than empty. A ute that spends its life with tools in the back, or a tourer loaded for a trip, sits differently to the same vehicle stripped out, and the figures should reflect how it is actually driven.

When a Single Trailing Arm Is the Right Purchase

Several situations genuinely call for a single arm rather than a pair.

Impact or damage on one side is the clearest, since a bent or cracked arm is a single sided fault by definition.

Corrosion affecting one side more heavily than the other is the second, which happens regularly where one arm has sat in more water and mud than its opposite number.

An independent rear suspension is the third case, where each semi trailing arm locates its own hub and the two are not working against a shared housing.

The fourth is replacing an arm on a vehicle where the other side was already done at some point, bringing both back to the same condition rather than creating a mismatch.

Where the fault is age related wear on a solid axle, both arms are the sensible purchase.

Vehicles Where a Single Arm Is Not Available

Worth checking before searching, because not every layout allows it.

A twist beam rear suspension joins the two trailing arms with a crossbeam, and the whole thing is a single welded assembly. There is no individual arm to buy, and the repair is either the complete assembly or the bushes within it.

Some multi-link rear ends integrate the trailing arm with other links in a way that makes the assembly the serviceable unit rather than the individual arm.

Where the arm carries the hub, as on many semi trailing arm designs, it is a considerably larger component and is supplied as one piece with the bushes and sometimes the hub carrier included.

Checking your vehicle's layout before ordering saves looking for a part number that does not exist.

Handed and Unhanded Trailing Arms

This one catches people out in both directions equally.

On plenty of solid axle vehicles the two rear trailing arms are the same part, with no left or right to get wrong. That makes ordering simple and means one part number covers either side.

On others they are handed, with different shapes to clear the exhaust, the fuel tank, the diff housing or a chassis rail on one side only. Those look near identical in a photograph and will not fit the opposite side.

Semi trailing arms are almost always handed, since the arm carries the hub and the geometry is mirrored.

Check the listing rather than assuming either way, since both arrangements are common and the arms often look similar enough to order the wrong one confidently.

Checking the Other Arm Before Ordering

A few minutes with the vehicle supported settles whether you need one or two.

Lever each arm against its mounts and watch for movement at the bush rather than flex in the arm. Compare the two sides directly, since the comparison tells you far more than either result alone.

Look at the bolt holes at each mounting point. Elongation on one side and round holes on the other is a clear single sided indicator, while elongation at both means age.

Check the chassis brackets as well as the arms. A cracked bracket is a separate repair and a new arm bolted into one achieves nothing.

Measure the axle position in each wheel arch. If one side sits further forward than the other, the arms are already mismatched and both need attention regardless of how the bushes look.

What Else to Check While the Trailing Arm Is Off

Access is the expensive part of this job, so it is worth using it properly while the arm is out.

The chassis bracket is the first thing to look at. Arms bolt into brackets that crack on older and hard worked vehicles, and that crack is visible with the arm removed and almost invisible with it fitted.

The panhard rod and its mounts come next on a solid axle, since the rod handles sideways location while the arms handle fore and aft. A vehicle that has moved the axle out of position usually has something wrong at both.

Rear brake lines and the handbrake cable frequently route along or near the trailing arm, and both are worth inspecting for chafing where they have been rubbing against a moving component.

On an independent rear end, the arm coming off gives clear access to the hub bearing, the rear brake components and the other links, all of which are difficult to reach otherwise.

Ask for the whole corner to be looked at while the access is free. It costs nothing at that point and it avoids paying for the same disassembly twice.

Replacing One Trailing Arm Without Skewing the Axle

Support the vehicle and the axle before releasing either end, since the axle moves once an arm is free and it carries more weight than it appears to.

Where the vehicle uses two arms on a solid axle, work on one at a time so the other keeps the axle located while you work.

Expect seized fasteners at both ends on anything with age or salt behind it. Penetrating fluid and time beat force, and heat is frequently needed on the larger bolts.

Do the final tightening with the wheels on the ground and the vehicle sitting at its normal height. The rubber takes a set from wherever it is clamped, so anything torqued while the axle hangs will fight the suspension for the rest of its life.

Have the alignment checked afterwards, paying attention to the thrust angle reading rather than just the front figures. When only one arm has been replaced, that reading is what confirms the axle is square.

Suspension has been Fulcrum's business since 1976, through 16 Service Centres and more than 200 authorised workshops across the country. If the alignment has shown a thrust angle problem, having it measured properly before ordering is what tells you whether one arm or two is the answer.

Find Individual Trailing Arms for Your Vehicle

A replacement arm is a good time to fit new trailing arm bushes, and you'll find control and radius arms in our full suspension arms range.

Select your vehicle to see the single trailing arms listed for it, and check whether the arm is handed and whether bushes are included. Everything carries a warranty and a fitment guarantee, with workshops around the country standing behind it.