A-frame ball joint How to replace it
Ed Evans explains why it’s important to keep a check on the A-frame ball joint, why it’s there, and how to replace it
THE suspension A-frame fitted to Discovery 1, Classic Range Rovers and Defenders (including early Ninety and One Tens) helps to locate the rear axle position in relation to the chassis. It controls the chassis’ tendency to move sideways in relation to the rear axle when off-road and when cornering on-road, especially when the vehicle is heavily loaded.
It’s a simple mechanism formed of two arms connected by bushes to brackets bolted at each side of the chassis. The two arms run back and are bolted together (forming the ‘A’ shape) through the fulcrum bracket which houses the ball joint which, in turn, connects to a bracket on the axle casing.
The ball joint provides a positive fixing, while allowing for axle tilt in relation to the chassis when cornering and on undulating
terrain. The A-frame arms pivot on their forward mounting bushes to allow the axle to rise and fall vertically in relation to the chassis, with the ball joint accommodating the change in angle between the arms and the axle.
Fore and aft movement between the chassis and rear axle is also controlled by the trailing arms that locate the axle to the chassis. If the bushes on the trailing arms are worn, they allow the axle to marginally move out of alignment when acceleration torque is applied. This causes a slight rearwheel-steering effect which isn’t noticed by the driver until the clutch is pressed for a gear change. At that point, the torque is suddenly disconnected, allowing the axle to straighten, and the driver feels the steering twitch.
A very worn A-frame arm mounting bush can have a similar effect but, unlike the trailing arm bushes, they’re relatively unstressed and aren’t an ongoing problem. It’s the ball joint that takes most of the load in the A-frame and, because it’s mounted centrally, a worn ball joint doesn’t produce any steering effects. But it does knock, especially on undulating ground, and sometimes at the point of applying brakes or acceleration. MOT inspectors go for the ball joint. They don’t like looseness in the joint, nor do they like splits in the rubber gaitor that protects the joint from external grit and water.
The ball joint on the Ninety shown here was fine, but the gaitor was split, and that would have meant an MOT fail. Whatever the issue, fitting a new joint assembly complete with fulcrum bracket (except on vehicles with a self-leveller) and new gaitor is always the answer. Here’s how it’s done.