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How do poly bushings work?
How do poly bushings work?
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Old 07-22-19, 06:04 PM   #11
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Default Re: How do poly bushings work?

Quote:
Originally Posted by [email protected] View Post
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.
Thanks. Agreed on the monoball/ spherical bearings. They are about as solid of a foundation as you could get.

Poly is good for mounts. My motor mounts, pitch mount, diff mount and outrigger mounts are poly. They work well. I also used poly in my shifter linkage. So far so good there and better than the soft rubber bushings that come in the trunnion joint.

I had to use poly on the swaybars though. No other option. I looked into split spherical bearings, but that was a bit cost prohibitive. The front isn't too bad as the grease lasts a while. The rear requires regreasing after every autocross season. At least it's easy to get to.
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Old 07-22-19, 06:19 PM   #12
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Default Re: How do poly bushings work?

Rubber dampens forces - ie reshapes them, smoothing out peaks a bit. The overall forces are the same. You engine or suspension creates a force. The geometry of of what holds things "still", say your engine, does not change, so the proportion of what each part holds doe not change significantly. Smoothing out peaks may be important in terms of fatigue.

Stand on a pieces of rubber on a scale. Does the rubber change the force you body applies to the scale? Account for the weight of the rubber, you would see you weigh the same with out without it - because the force does not change just because there is a piece of rubber between the scale and you.
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Old 07-22-19, 06:38 PM   #13
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Default Re: How do poly bushings work?

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Originally Posted by mheyman View Post
Rubber dampens forces - ie reshapes them, smoothing out peaks a bit. The overall forces are the same. You engine or suspension creates a force. The geometry of of what holds things "still", say your engine, does not change, so the proportion of what each part holds doe not change significantly. Smoothing out peaks may be important in terms of fatigue.

Stand on a pieces of rubber on a scale. Does the rubber change the force you body applies to the scale? Account for the weight of the rubber, you would see you weigh the same with out without it - because the force does not change just because there is a piece of rubber between the scale and you.
You understand that rubber or poly will either absorb or transmit force to their respective attachment points based on their durometer rating right. In other words the engine for example applies the same amount of force, but depending on the stiffness of the mount actual movement will vary as will the force applied to the chassis side of the mount. This is significant for example because in the case of the GR's which are known to have a weak attachment point on the firewall. Adding any extra force to the pitch mount attachment point may increase the likely hood of a failure in the firewall sheet metal. A stiff pitch mount and stock engine mounts no longer share the load equally. The engine mounts have more compliance. They will let the engine deflect more than a stiffer mount. The poly pitch mount will not comply as much so the same amount of movement will transfer more force over an OEM pitch mount.

You're talking about one point of contact. I'm talking about multiple points of contact in a system. You're talking about a static load and I'm talking about a dynamic load.
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Old 07-23-19, 04:57 AM   #14
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Default Re: How do poly bushings work?

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Originally Posted by SubySal View Post
You understand that rubber or poly will either absorb or transmit force to their respective attachment points based on their durometer rating right.
anything absorbing the energy in motion is actually transforming that energy into heat. How much heat do bushings create? Rubber is not a great conductor of heat so if energy transformation is occuring they will be hot. Are they?

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In other words the engine for example applies the same amount of force, but depending on the stiffness of the mount actual movement will vary as will the force applied to the chassis side of the mount. This is significant for example because in the case of the GR's which are known to have a weak attachment point on the firewall. Adding any extra force to the pitch mount attachment point may increase the likely hood of a failure in the firewall sheet metal.
We agree here


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A stiff pitch mount and stock engine mounts no longer share the load equally.
They "share" the loads almost completely according to their geometry. That isn't changing significantly despite movement at one end or both.

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Originally Posted by SubySal View Post
The engine mounts have more compliance. They will let the engine deflect more than a stiffer mount. The poly pitch mount will not comply as much so the same amount of movement will transfer more force over an OEM pitch mount.
Bushings may be modeled as a spring deflecting over time slowing the rise and fall of the force - say on the firewall though a pitch stop. They do not heat up dissipating energy. They dissipate an inconsequential amount, they return almost all of it as they deflect back their original position.

Interestingly force dampening at any point of contact or location has the geometrically proportioned affect on all the other points in the system! [ADDED: Think of a see-saw - put dampening pad under the ends. If unequally weighted, and either end impacts the ground, both ends are slowed by the dampening at the lower end.] So, increasing the durometer of a bushing lessens dampening "felt" everywhere! Yes doing so can cause fatigue.

But if high forces are present (from abuse and/or high torque) there will be much deflection. When during that deflection the full somewhat linear portion of the spring action is used (IE the bushing is crushed), then until the force is lessened it will no longer contribute to further damping. Then a harder bushing would certainly have been appropriate. It will not provide as much dampening over an equal range of force, but it will be contributing at higher forces, and may well have greater overall effect when higher forces are present.


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You're talking about one point of contact.
No. I'm talking about physics and its laws are followed for any number of contacts.

Quote:
Originally Posted by SubySal View Post
You're talking about a static load and I'm talking about a dynamic load.
I'm discussing both ...
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Last edited by mheyman; 07-24-19 at 04:08 AM.
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Old 07-25-19, 03:32 PM   #15
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Default Re: How do poly bushings work?

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Originally Posted by BriEE View Post
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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Just checking in on the original question. Rubber bushings do NOT work by twisting the rubber, in fact that is when they are not working right. Bushings are to locate the various components in space and avoid a metal on metal contact (leaving out all metal / ball joints for the moment). Using the example of a sway bar, ideally you lubricate your rubber bushings every so often, and the bar twists inside of the bushing based on the relative movement of the wheels. The rubber should not be rotating with the bar if there is lubrication.
By locating the cross-piece of the bar with the bushing/housing/bolts at each end, you force it to operate strictly as a torsion spring. To do this there are forces up and down at each end. The rubber does deflect under those forces and this movement happens with a lower spring rate than the bar, so that lessens the bar's effective spring rate.

Also, the rubber absorbs vibration, keeping it quieter in the car.


Using poly or other firmer material locates the bar in space with less deflection and increases the effectiveness of the bar. If you increase the force/unit movement of the bar + bushing, it provides more stiffness or resistance against movement, whether you do it with a new bushing or a bigger bar. The curve may be a little different, less linear, with rubber vs poly, but either way it is spring rate of the combination.
But if you don't lubricate the bushing/bar interface, then that introduces stiction and really throws off the curve, as the twist in the bar isn't consistent with rotational position. No matter whether it is rubber or poly. IMO having unlubricated sway bar bushings of any material is a much bigger source of inconsistency than having good rubber ones.


If the car benefits from a stiffer sway bar rate, then fix that. Only change from rubber if that is the best way to fix the problem.
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Old 07-25-19, 03:51 PM   #16
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Default Re: How do poly bushings work?

Many control arm bushings do twist by design, and is why you do the final torqueing with the vehicle on the ground. These types of bushings do not have a way to use lubrication. The rubber is literally attached to both the race and the metal bushing and the bolt is torqued for a clamp load on the metal bushing.
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Old 07-25-19, 05:20 PM   #17
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Default Re: How do poly bushings work?

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Originally Posted by Pault1 View Post
Also, the rubber absorbs vibration, keeping it quieter in the car.
Stock bushings, almost all them have only one function and that is to minimize NVH. Metal to metal is otherwise not generally an issue. The engineers would fasten everything together - bolts, weld, whatever works for the application.
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Last edited by mheyman; 07-25-19 at 06:00 PM.
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Old 07-25-19, 05:57 PM   #18
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Default Re: How do poly bushings work?

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Originally Posted by yamahaSHO View Post
Many control arm bushings do twist by design, and is why you do the final torqueing with the vehicle on the ground. These types of bushings do not have a way to use lubrication. The rubber is literally attached to both the race and the metal bushing and the bolt is torqued for a clamp load on the metal bushing.
That's 100% correct, and is why I want to change all bushings to spherical rather than poly. I don't like tightening at load and expect rubber bushings to do the work. Spherical can tighten at any position.

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Old 07-25-19, 07:26 PM   #19
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Default Re: How do poly bushings work?

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Originally Posted by BriEE View Post
That's 100% correct, and is why I want to change all bushings to spherical rather than poly. I don't like tightening at load and expect rubber bushings to do the work. Spherical can tighten at any position.

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Why exactly do you want spherical bushings? Are you drag racing, or doing heavy track days?

If this is a daily driver, or winter driven, the NVH increase of spherical vs poly might not be something you’ll enjoy. Winter is really harsh on spherical setups.
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