JamesMcD
JamesMcD New Reader
4/3/11 9:03 p.m.

Lots of people replace their rubber suspension bushings with polyurethane, but in places such as a lower control arm on most FWD cars, I can't understand how they can possibly work. The stock set-up is generally a steel tube that is clamped to the car and does not move. This tube is bonded inside of a cylinder of rubber which is in turn bonded to the control arm. So when the arm goes up and down, it is the flex/stretching of the rubber that allows the movement...

Now, if you throw this in the trash and replace it with a hard plastic sleeve of polyurethane, how the heck is this supposed to work? Is the chunk of poly rotating on the tube? Is the tube is being allowed to rotate on it's bolt? Neither of those possibilities sound like a good plan to me.

novaderrik
novaderrik HalfDork
4/3/11 9:18 p.m.

the poly moves on the metal tube instead of sticking to it like rubber. the result is a smoother movement with less friction. poly can squeak if you don't get it lubed up, which is why they sometimes have grease zerks in the outer shell and matching grooves in the bushing itself.

the ultimate setup is to go all steel, with only a thin layer of grease between the moving metal parts. but this results in faster wear and more noise and harshness being transmitted to the passenger compartment.

a good compromise between solid and poly is delrin, but it's more expensive.

Will
Will HalfDork
4/3/11 9:25 p.m.

Lots of people believe that poly is a poor choice for applications like control arms because it tends to bind. Given the choice, I'd take delrin or something similar.

procainestart
procainestart Dork
4/3/11 11:08 p.m.

Poly can wear out, so that the center hole through which the steel sleeve fits can ovalize over time.

Keith
Keith SuperDork
4/3/11 11:58 p.m.

It makes sense if you think of the poly bushings as bearings, not bushings.

curtis73
curtis73 Dork
4/4/11 3:06 a.m.

poly is good at squeaking and wearing out. That's why its a performance addition

Been there, done that... I keep it either rubber or go delrin/solid. The best suspension design is one that offers no resistance. Rubber twists and adds phantom spring rates (tiny, but can get in the way of tuning). The main reason for going poly is to prevent deflection which changes camber/caster during cornering, but at the cost of slightly increased NVH and maintenance.

Keith
Keith SuperDork
4/4/11 9:18 a.m.

There's enough spring in the rubber bushings that tightening them up with the wheels hanging will jack up the car due to the extra spring rate. All Miatas will do this to some extent, but the 2006+ cars can experience a huge ride height change - it's over an inch. I'd say that the bushing spring rate is more than tiny in some cases.

92CelicaHalfTrac
92CelicaHalfTrac SuperDork
4/4/11 9:32 a.m.

Solution:

GregW
GregW New Reader
4/4/11 9:33 a.m.

I suggest, if you want a solid connection, use prelubed "oilite" bronze bearings. Be aware these will wear due to dust and debris grinding away the bronze but the clunking will let you know when they need replacing. They are much better than steel on steel bearing.

These should be available in any size at your local industrial bearing supply store. Granger might carry them as well.

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