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How do poly bushings work?
How do poly bushings work?
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Old 07-21-19, 11:32 AM   #1
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Default How do poly bushings work?

I'm still not really understanding fully how replacing OEM rubber bushings with poly works.

I understand that rubber bushings work by twisting the rubber to provide a counterforce to the moment of the suspension components such as rear trailing arm bushings.

But the poly bushings, they have a metal sleeve that rotates inside the 2 poly half pieces, so how does this provide counterforce to the moment?

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Old 07-21-19, 02:32 PM   #2
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Default Re: How do poly bushings work?

Generally not well. The sleeve is the pivot. The poly is filling the void. They tend to squeak and bind. Depending on the application there are OE pillow balls or SPL arms, which use pillow balls as well.

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Old 07-21-19, 08:10 PM   #3
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Default Re: How do poly bushings work?

It does it minimally, that's the whole point. Less absorption, better feedback to the driver, less comfort. Where as solid bushings provide no counter whatsoever providing the ultimate "raw" control, but the least possible comfort level.
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Old 07-22-19, 05:40 AM   #4
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Default Re: How do poly bushings work?

Quote:
Originally Posted by [email protected] View Post
Generally not well. The sleeve is the pivot. The poly is filling the void. They tend to squeak and bind.
Are you saying they don't work well for just street use? Or does one need to be careful when doing poly bushings for track use as well, due to the binding and such?
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Old 07-22-19, 06:03 AM   #5
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Default Re: How do poly bushings work?

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Originally Posted by raisingAnarchy View Post
Are you saying they don't work well for just street use? Or does one need to be careful when doing poly bushings for track use as well, due to the binding and such?


If you keep them lubed and clean, you’ll never have issues. I’ve had mine installed for 5 years now. I used to clean them every fall and spring when I would autox. They were always really clean with no cracks in the poly. I’ve since slacked on that to a yearly checkup. Still no issues. This includes being Canadian winter driven on heavily salted roads. Keep in mind, I didn’t go with cheap bushings either, Super Pro has been great.
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Old 07-22-19, 07:46 AM   #6
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Default Re: How do poly bushings work?

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Originally Posted by BriEE View Post
so how does this provide counterforce to the moment?
It doesn't... or at least not to any great extent. Forces transfer directly whereas the OEM rubber absorbs some of these forces.

So going to harder bushings means more consistent and improved suspension geometry under loading (bushing keep suspension pivot and mount concentric), increased feedback, as well as increased noise transfer to the chassis.


I would recommend starting slow and with the more common bushing upgrades to see how it suits you. It's not uncommon for people to go to far with this and have to reverse their work as they pass the threshold into irritatingly stiff, loud, vibrating, etc...
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Old 07-22-19, 08:26 AM   #7
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Default Re: How do poly bushings work?

^ ^ ^ ^ ^ ^ ^ ^ ^

I believe that in the street performance world (where actual performance hardly matters) there are people replacing various bushings who haven't fully or haven't well addressed the major factors in handling - basic mounts and suspension. If they had, they would would have gotten to that stopping point first!

"No one" wants to ride in my car but me and I haven't even addressed the myriad bushings I might.

ADDED: and I've recently had the importance of camber reinforced via experience. Not enough front camber can make a decent STI suspension useless.
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Old 07-22-19, 08:53 AM   #8
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Default Re: How do poly bushings work?

Quote:
Originally Posted by raisingAnarchy View Post
Are you saying they don't work well for just street use? Or does one need to be careful when doing poly bushings for track use as well, due to the binding and such?
I'm saying poly bushings work like poo regardless of how often you clean them on the street or the track. If you want proper racing suspension go monoball. Otherwise don't mess with the bushings.
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Old 07-22-19, 01:46 PM   #9
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Default Re: How do poly bushings work?

I agree that poly bushings are crap on anything that has to cycle. They work fine for mounts though. I've replaced every bushing and mount in my car. Well almost. The two small rubber bushings that secure the carrier bearing are still original.

All my suspension bushings that actually have to move are Group N. My mounts are a combination of poly and Group N. However both the Group N rubber and poly have nearly the same durometer rating. You want everything to bend at the same rate. I did them all at once too. I feel only replacing some bushings or mounts causes "hot spots". In other words mixing the bushings means the stiffer bushings will do more work. For example if you replace the pitch mount with poly and leave the motor mounts the pitch mount will still twist. However the stiffer bushings will transfer more of that twisting force to the firewall mount. This is because the soft rubber motor mounts have a higher compliance then the stiff poly pitch mount.

With suspension components you want to maintain as much consistency as possible. Changing some bushings will cause the remaining softer OEM bushing to flex more as they have to keep up with the higher force transfer of the stiffer bushings. This can lead to higher amounts of deflection in the suspension when it cycles. It may not cycle smoothly or consistently which could affect at the limit handling. This also pertains to stiff coilovers on an otherwise stock suspension system. Deflection is often the result. The bushings are the foundation of the suspension system. They keep everything in line. If they are overwhelmed it will degrade overall performance.

The real benefit of rubber bushings is not that they add spring rate. It's the fact that the bushing itself actually twists. This helps to make the suspension very smooth and consistent when it cycles. I don't care how often, how much or what kind of grease you use on poly bushings. You can't match that. Basically every time you hear a poly bushing squeak or creak it's binding. It hangs up then lets go. That inconsistency can cause problems at the limit under high forces.


I have a friend with a 2005 STI. He runs in autocross. He was getting terrible wheel hop in the rear. He's broken probably a dozen left rear halfshafts already. He was constantly taking apart the rear suspension and re-greasing the poly bushings. I told him I would install Group N lateral link and rear knuckle bushings and that should help. Afterward the wheel hop was gone. I don't think he's broken an axle since. I've actually broken two myself from wheel hop. I think I figured out the cause. My AST dampers were done. I've also heard recently that these dampers can get piston bind which would have the same affect as a bushing that binds. So proper maintenance of the serviceable components are important too. I went one step further and switched to KW Clubsports.

I'll also add that by sticking with rubber the car has been pretty tolerable on the street. A lot more than I expected. With street tires it's fairly tame. It's so tight though it can get twitchy on irregular road surfaces.

The best way to go for performance of course is full bearing or Heim joints. However that's essentially full race. They require more service and NVH would be very high. They would however keep everything in line with zero deflection. Also they would not in any way alter the spring rates by generating any resistance of their own.
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Old 07-22-19, 01:59 PM   #10
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Default Re: How do poly bushings work?

Well put SubySal!

I'll add this; I likemonoball suspension because I can do all the tuning where it should happen: shocks, springs and sway bars.

My 525bhp Porsche uses 100% monoball/spherical bearing suspension. The motor/trans mounts are poly.
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Last edited by [email protected]; 07-22-19 at 02:27 PM.
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