RD421

A rack end — also called the inner tie rod end or inner tie rod socket — is the threaded pivot assembly that screws into or onto the end of a rack-and-pinion steering rack and connects it to the outer tie rod end. It is the innermost joint in a rack-and-pinion steering linkage, directly attached to the moving rack bar, and it transmits the rack's linear motion to the adjustable tie rod assembly that steers each front wheel. Every rack-and-pinion vehicle has two rack ends, one per side.

Also called: inner tie rod end, inner tie rod, rack end socket, inner ball joint. Note: a rack end is NOT the same as a tie rod end (outer tie rod end). The rack end is the inner pivot that threads directly onto the steering rack and is protected inside the steering rack bellows boot; the tie rod end is the outer ball-jointed fitting at the wheel end that connects to the steering knuckle and is exposed to the elements. They are separate parts that fail independently and are replaced separately. In some markets and catalogues the rack end is also listed as just "inner tie rod" — the outer is then "outer tie rod end" — to distinguish the two.

How it works

Linear motion transfer from rack to wheel

The steering rack moves left and right in a straight line inside its housing. The rack end is threaded onto the end of this rack bar, so it moves with the rack. As the rack translates, the rack end pushes or pulls the outer tie rod (and with it the steering knuckle), swinging the front wheel through its steering angle. Without the rack end's articulating joint, the outer tie rod could not accommodate the angular change between the horizontal rack motion and the arc the tie rod follows as the suspension rises and falls.

Ball-and-socket articulation inside the bellows

The rack end contains an internal ball-and-socket joint — similar in principle to the outer tie rod end but larger in section because it is under higher force at this point in the linkage. This joint allows the rod to pivot in multiple planes as the suspension travels, preventing the steering from binding through bumps. The entire rack end assembly lives inside the steering rack bellows boot, which keeps road grit, water and salt away from the joint and the rack surface itself.

Toe adjustment via thread engagement

The outer tie rod end threads onto the rack end's threaded shaft with a jam nut that locks the assembly at the correct length. Turning the outer tie rod end in or out on this thread changes the effective length of the tie rod, adjusting front toe. During a wheel alignment, the technician unlocks this jam nut and turns the outer tie rod end to set toe, then re-tightens the jam nut. The rack end itself is not adjusted during routine alignment work.

Structure and construction

A rack end comprises a machined steel or sintered-iron socket housing threaded externally to engage the rack bar, with an internal spherical bearing that carries the rack end's ball stud. The ball stud's threaded shaft protrudes outward and accepts the outer tie rod end. Housing-to-rack attachment varies by design: staking (the housing is deformed mechanically against the rack after installation), a lock wire, a roll pin, or a separate jam nut — the correct method is vehicle-specific. The socket is sealed by the rack bellows boot, which collapses and extends with every steering input. On replacement, the bellows boot should also be inspected and replaced if cracked or torn. MOOG, TRW, 555 and CTR supply rack ends with forged housings and pre-greased bearing surfaces; MOOG Problem Solver units include an external boot kit.

Common failure symptoms

• Knocking or clunking over bumps from the steering rack area. A worn ball joint inside the rack end allows the internal stud to knock against the socket walls each time the suspension cycles or the steering is loaded. The noise comes from inside the rack housing area, distinct from the wheel-end clunk of a worn outer tie rod end.
• Excessive steering wheel play or loose steering. The worn joint absorbs steering input before it reaches the outer tie rod — the driver turns the wheel but the wheels lag behind. This type of play typically feels softer and more gradual than gearbox play because the joint has to travel through its worn range before transmitting force.
• Vague centre feel and on-centre wander. A rack end that is worn primarily in the straight-ahead position creates a dead zone exactly on centre — the most common position for motorway driving — causing the vehicle to wander without clear steering wheel feedback.
• Uneven front tire wear. A loose rack end allows the effective toe to shift under cornering and braking loads. The result is feathered or inner-edge tire wear. Because the rack end is inside the bellows and not easily visible, this wear pattern is sometimes the first observable sign of inner tie rod wear.
• Torn or collapsed bellows boot. If the bellows boot splits, road grit and water enter the rack housing and attack the rack end joint directly. A torn boot is not itself a rack end failure, but it accelerates wear so rapidly that the rack end typically fails within one to two seasons unless the boot is replaced immediately.

Common questions about Rack Ends