Polybushes - Any Good?
Discussion
Hi All
I'm looking for some information on polybushes. I'm currently pulling the suspension of my MX5 apart, in readiness for a rebuild. This will include new bushes all aound.
I was planning on upgrading to polybushes, as a performance mod, and as they are easier to fit! However, speaking to a guy who works in the car industry as an engineer, he said I shouldn't do it.
His reasons were that in his experience normal rubber bushes were a better combination of control and damping. He also went on to say he didn't really see how polybushes could hope to work as well as rubber, given that the central metal sleeve is free to move like a bearing and as such you sacrifice a lot of the 'rotational damping' (for want of a better phrase) as it will try to return to its rest position.
I confess I have no knowledge on the matter, but hoped there might be those here who can explain it to me?
Cheers
I'm looking for some information on polybushes. I'm currently pulling the suspension of my MX5 apart, in readiness for a rebuild. This will include new bushes all aound.
I was planning on upgrading to polybushes, as a performance mod, and as they are easier to fit! However, speaking to a guy who works in the car industry as an engineer, he said I shouldn't do it.
His reasons were that in his experience normal rubber bushes were a better combination of control and damping. He also went on to say he didn't really see how polybushes could hope to work as well as rubber, given that the central metal sleeve is free to move like a bearing and as such you sacrifice a lot of the 'rotational damping' (for want of a better phrase) as it will try to return to its rest position.
I confess I have no knowledge on the matter, but hoped there might be those here who can explain it to me?
Cheers
They are better in some instances.
If the oe bush is just rubber like most arb bushes/track control arms etc are then 100% improvement you might get a bit more road noise but worth the trade off imo.
If it’s a encapsulated spherical bearing like on trailing arms, powerflex aren’t a good replacement. If you do change these the bush pin does move around causing unwanted steer.
They look like this underneath the rubber covers.
http://www.wychbearings.co.uk/gxsw-m08_spherical_p...
And use powerflex only. I’ve dismantled cars and you can always resell powerflex I just use the other makes as examples to sell powerflex.
If the oe bush is just rubber like most arb bushes/track control arms etc are then 100% improvement you might get a bit more road noise but worth the trade off imo.
If it’s a encapsulated spherical bearing like on trailing arms, powerflex aren’t a good replacement. If you do change these the bush pin does move around causing unwanted steer.
They look like this underneath the rubber covers.
http://www.wychbearings.co.uk/gxsw-m08_spherical_p...
And use powerflex only. I’ve dismantled cars and you can always resell powerflex I just use the other makes as examples to sell powerflex.
This forum is very generalised, you want specific info so i'd suggest trying elsewhere.
On some makes of car they are fine, on others they can creak, others they break.
http://www.mx5nutz.com/forum/index.php?showtopic=1...
On some makes of car they are fine, on others they can creak, others they break.
http://www.mx5nutz.com/forum/index.php?showtopic=1...
Lewis's Friend said:
Hi All
I'm looking for some information on polybushes. I'm currently pulling the suspension of my MX5 apart, in readiness for a rebuild. This will include new bushes all aound.
I was planning on upgrading to polybushes, as a performance mod, and as they are easier to fit! However, speaking to a guy who works in the car industry as an engineer, he said I shouldn't do it.
His reasons were that in his experience normal rubber bushes were a better combination of control and damping. He also went on to say he didn't really see how polybushes could hope to work as well as rubber, given that the central metal sleeve is free to move like a bearing and as such you sacrifice a lot of the 'rotational damping' (for want of a better phrase) as it will try to return to its rest position.
I confess I have no knowledge on the matter, but hoped there might be those here who can explain it to me?
Cheers
Your friend is 100% correct. Due to the fact that they behave like bearings, polybushes will have greater hysteresis. Large amounts of hysteresis in the vertical travel of the suspension will degrade the ride quality, the car can feel like it's never really settled. Also, the torsional stiffness of the rubber bushes makes up a proportion of your wheel rate, so removing may leave the car over damped.I'm looking for some information on polybushes. I'm currently pulling the suspension of my MX5 apart, in readiness for a rebuild. This will include new bushes all aound.
I was planning on upgrading to polybushes, as a performance mod, and as they are easier to fit! However, speaking to a guy who works in the car industry as an engineer, he said I shouldn't do it.
His reasons were that in his experience normal rubber bushes were a better combination of control and damping. He also went on to say he didn't really see how polybushes could hope to work as well as rubber, given that the central metal sleeve is free to move like a bearing and as such you sacrifice a lot of the 'rotational damping' (for want of a better phrase) as it will try to return to its rest position.
I confess I have no knowledge on the matter, but hoped there might be those here who can explain it to me?
Cheers
The bearing like behaviour of non-bonded polybushes also means that they are not particularly durable compared to a rubber bush, as there is wear between the metal inner tube and polyurethane.
In my opinion, it is not necessarily a good idea to remove compliance from a suspension system without a good idea of what this will do to the handling of the car. For example, stiffening all of the bushes may make the car unstable at the limit, which might reduce your enjoyment and make you slower.
JoeMk1 said:
Your friend is 100% correct. Due to the fact that they behave like bearings, polybushes will have greater hysteresis. Large amounts of hysteresis in the vertical travel of the suspension will degrade the ride quality, the car can feel like it's never really settled.
I'm struggling to see how using a bearing could *increase* the hysteresis. It would reduce it, surely.GreenV8S said:
I'm struggling to see how using a bearing could *increase* the hysteresis. It would reduce it, surely.
Rubber has a natural damping effect, reducing hysteresis, a bearing has no damping so would increase the frequency if anything. Most suspension systems have rubber bushings in all or most axis, remove the damping effect in any one direction can have all sorts of weird consequences.Damping will effect hysteresis, damping is not hysteresis.
Per-se an increase in damping normally results in an increase in hysteresis.
Hysteresis is the difference between input and output in its simplest form.
If we talk of a "step-change" on a suspension set up (likened to driving over a kerb) the input is a step, the actual movement of the suspended load will depend on the level of damping.
Too much damping & the suspended load doesn't smooth out the step, too little will have it over-shooting.
Per-se an increase in damping normally results in an increase in hysteresis.
Hysteresis is the difference between input and output in its simplest form.
If we talk of a "step-change" on a suspension set up (likened to driving over a kerb) the input is a step, the actual movement of the suspended load will depend on the level of damping.
Too much damping & the suspended load doesn't smooth out the step, too little will have it over-shooting.
Edited by E-bmw on Monday 4th December 15:22
E-bmw said:
Damping will reduce hysteresis, damping is not hysteresis.
In the context of damping occurring when a bush deflects, hysteresis is the effect that causes the damping and it's reasonable to refer to these as the same thing.If you think that reducing hysteresis will increase damping, I don't think that hysteresis means what you think it means.
GreenV8S said:
E-bmw said:
Damping will reduce hysteresis, damping is not hysteresis.
In the context of damping occurring when a bush deflects, hysteresis is the effect that causes the damping and it's reasonable to refer to these as the same thing.If you think that reducing hysteresis will increase damping, I don't think that hysteresis means what you think it means.
They are inexorably linked but damping is not hysteresis.
Damping is damping & it does affect hysteresis, they are not the same thing.
E-bmw said:
They are inexorably linked but damping is not hysteresis.
Agree with that and your clarification below. What confused me was the implication that hysteresis and damping were inversely related, which I don't think is correct and I don't think you are suggesting based on your clarification.GreenV8S said:
E-bmw said:
They are inexorably linked but damping is not hysteresis.
Agree with that and your clarification below. What confused me was the implication that hysteresis and damping were inversely related, which I don't think is correct and I don't think you are suggesting based on your clarification.GreenV8S said:
I'm struggling to see how using a bearing could *increase* the hysteresis. It would reduce it, surely.
A thought experiment for you. I have two simple suspension systems, one with bonded rubber bushes, the other with polybushes (essentially plain bearings if they're not bonded). Assume that the geometry is such that the joints all work purely in torsion, and I've also removed the spring and damper. I have them on a rig that holds the body in place and supports the wheel at it's ride height position.
The rubber bushes have a torsional stiffness, i.e. if i move the suspension up or down, they will resist the movement and provide a force that tries to return the wheel to it's original position. Some of the energy I expend moving the suspension is stored in the bushes.
On the other hand, my polybushes have friction (especially after a few months when the grease is no longer effective and the metal tubes have corroded) When I move my suspension up and down, I am losing some of the energy expended to this friction. (damping)
So lets say my rig moves the wheel up and down through the suspension's travel, and I measure the vertical force at the tyre's contact patch as I do it. As I move the wheel up, my rubber bushed suspension will have a steeper force/displacement curve due to the torsional stiffness of the bushes. However, when I reach the top of the travel and start to move the suspension down again, the friction in the polybushed suspension will hold the wheel up ever so slightly, reducing the force at the contact patch vs the rubber bushed suspension, where the bushes are pushing the wheel down.
Therefore, the vertical wheel Force/Displacement curve will have more hysteresis with polybushes than rubber bushes. The same phenomenon happens with ball joints due to their inherent friction, made worse by their high breakaway torque or 'stiction'. If you've ever handled a new ball joint and tried to move it you will know this, they take a lot of force to get moving, so when your suspension stops and travels the other way, you get the same hysteresis effect.
I hope that explains it well enough, I might have used too many big words that I don't understand
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