F-Body Europe

Often I can read about Polyurethane bushings they "bind".
What does this mean? A look in the dictionary brought a lot of meanings but not one that sounded applicable to PU bushes.


Can somebody give me clarification?

Gunnar

Roy:
Do you have poly bushings on your car?

Ben

Suspension/bushing bind:

Bind occurs when the the pivot points of the suspension cannot move as intended. As it relates to third- and forth generation F-body car, bind is when the pivot points do not move smoothly under load (stiction, or static friction) or when the suspension moves outside its (or one or more of its components) design parameters (in this case, range of motion restricted by the lack of compliance in the polyurethane bushings)

Under hard cornering, bind can create dangerous handling conditions.

Polyurethane bushings:

Polyurethane (or 'poly' or 'plastic') bushings are popular, as they noticeably 'tighten up' the suspension, and the aftermarket arms look cool, but most purchasers to not understand how they work, or the problems they create. This is an old issue, but the recent popularity of poly-equipped aftermarket control arms has drowned out the proven drawbacks of this material, including stiction, binding, squeaking, harshness, and need for regular greasing.

Poly is used in place of the rubber portion of the factory bushings. Unlike the rubber bushings, they are not bonded to the inner and outer sleeve, and movement of the control arm will cause the internal surfaces of the sleeves and bushing to slide to accommodate the shearing forces. The two main issues:

Stiction (Static Friction): Under light loads, they may squeak as the arm moves. Regular greasing reduces the squeaking, but under high loads, especially cornering on uneven pavement, the "sticky" properties of polyurethane causes the plastic to "grab" the steel liner, adding harshness to the ride, as the steel sleeves can't slide smoothly over the poly bushing surfaces. At worst, when the suspension then cannot move as designed, the handling of the car will be affected and this can create unexpected and dangerous oversteer.

Binding: To complicate matters, the trailing arm type rear suspension in the late model f-body cars has bushings that flex or 'deflect' as a normal and required part of their function. As the solid 'live' axle moves to comply with an uneven surface, it (and the LCA mounting points) rotate in relation to the chassis. To illustrate:

Normal axle movement
(missing the picture here)

1. The factory rubber bushings in the lower control arms accommodate this rotation/twist by design;
2. Poly bushings force the lower control arms to twist.
3. Tubular or boxed control arms - with poly bushings - force the LCA mounting brackets on the chassis and axle to bend;
4. Now that the car relies on twisting metal for suspension, it may at times bind, as the axle cannot move smoothly comply with bumps. The resulting oversteer condition can result in loss of control on high speed corners - not what you want in a high-power rear-drive car.
5. Subframe connectors (SFCs), added to brace the chassis, reinforce the forward LCA mounting points, but the twisting force is still there on the arms and rear mounts; poly bushings may seen to ride firmer after the addition of SFCs due to the more solid mounting of the LCAs.
6. Even for lower speed handling, the poly bushings increase the rear roll stiffness - something you should be tuning with the spring rates and sway bars.
7. Greasing the poly bushings does not help this issue.