What are binding and side loading?
Binding is when the joint or the linkage sticks or goes stiff as it moves. Side loading is a sideways force on a part that is made to take force only along its length. Sideways load bends rods, wears seals and makes a drive work harder against mechanical binding. A push-pull rod has the same weakness: the rod is meant to carry push and pull, so a bend or twist added by a poorly aligned joint is a load it was not sized for (Q1-02, Q1-03). The principles below come from careful push-pull installations in general; for your own machine, follow its manual.
How do you set up the geometry?
Think of the rod as a two-force member: it carries only axial load when the force acts through the two pin centres and nothing else holds it sideways.
- Pivot at both ends. A rod end or ball joint at each end lets the rod align itself as the mechanism moves. Rigid mounts on a mechanism whose geometry changes put side load into the rod by design.
- Keep the load in line. The force must lie along the axis of the rod. Sight along a straight-edge at mid-travel: any visible angle between the rod and the load direction creates lateral force.
- Support a long rod. For long travel with moderate to heavy loads, add a guide alongside to take any off-axis force. The rod itself should not also serve as the guide.
How do you check the ends move freely?
Rotate the rod between your fingers. A self-aligning rod end must show a slight movement you can see or feel, and this freedom has to be checked at both extreme positions of the travel, not only at the middle. A rod end that cannot articulate freely will bind, make the linkage stiff, or pass bending load into the tube. Check that the ball sits roughly centred in its housing at the neutral position, then move the linkage through its full travel. Binding at any point means a misaligned bracket, a wrongly sized rod end or a rod of the wrong length, and it must be corrected.
How far can a rod end tilt?
Only as far as its limit. The maximum deflection is reached when the outside of the rod end's head touches the side of the fork, clevis or fitting it is bolted into, and running past the angular rating loads the edge of the ball abnormally. Once the housing or adjacent hardware makes contact the joint has used up its articulation, and further movement can bind or redirect force into the shank and mounting hardware (rod-end maker IBT makes the same point). See K03-Q1-09 and K02-Q1-05 for how the usable angle is worked out.
How tight should the pin be?
Tight enough that the ball does not turn on the bolt, not so tight that it binds. The inner race must be clamped securely to the attaching fitting; otherwise the bearing action may not work and the rod end rotates around the bolt instead. In most installations the bolt is preloaded to clamp the ball, but excessive force expands the ball and binds it in the race. If the ball is not clamped, motion takes place on the bore, so the bolt, the bore or both then need suitable surfaces.
How much thread must stay in?
Follow the maker's rule. As a general starting point, Q1-04 shows where the figure comes from: about 0.7 times the thread diameter when both parts are the same material, about 1.4 times if the softer part has half the strength, and 1 diameter as a common starting figure in steel (K05-Q1-04, K04-Q1-23).
Many tube ends have an inspection hole: when the shank is properly engaged, the hole is blocked and a wire cannot pass through. If you can push a wire through, the shank is not in far enough.
How do you lock the length, and stop the linkage going too far?
- Lock the length. A jam nut tightened against the end fitting locks the thread, but it never replaces correct thread engagement: the nut prevents rotation but does not make up for too little thread. Safety wire is another method on some installations. See Q1-05 for how jam nuts work.
- Stops. A push-pull linkage with a bell crank (a lever that turns the motion through an angle) must have its travel limited to prevent jamming at an extreme or being rotated past centre, with positive stops at both ends of the travel.
What should you check when fitting a link to a vehicle?
A suspension link moves through a long, three-dimensional path, so check the whole path, not only the static position. With the vehicle maker's procedure for unloading the springs, move the suspension from full droop to full bump and the steering from lock to lock, and watch for: the housing or hardware touching the bracket, the link touching a tyre, brake line or chassis tube, and the bolt axis sitting at the wrong angle to the link's swing. The rod end's tilt limit must not become the travel limit: bump stops and droop limits should act first. In a symmetrical layout, left and right links start from the same centre-to-centre length (Q1-04); the final setting comes from the axle position in the vehicle's set-up data. Use spacers to set the rod end square in its bracket and keep the housing clear, and the radius rod buyer's guide for the link forms. Bolt grade and torque come from the vehicle maker, not from this article.