Still planning out my front suspension rebuild the question of rubber vs poly bushings comes up. Who has any experience with the differences?
Some will say that poly makes the suspension less refined. While it may very well be true in absolute terms from āas mintā new to poly, changing ours from tired rubber to poly was certainly an improvement also in refinement. Now I need to do my XK and will definitely go with poly.
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John, when I restored my E types, I purchased black bushings, the softest available from Polybush in the UK. I have never had a problem. Just don`t get the red stiff ones they are too firm and need lubrication. Also did my steering rack mounts in polyurethane. Still fine for decades.
Allen
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Good feed back thanks!
I tried both on the 2+2, I preferred the rubber ones.
We put those Urethane bushings on a carās front suspension, and the squeaking would drive you crazy. Never again, and yes, we used the ālubeā they supplied. It did firm up the ride though.
I have zero on-car experience with poly bushes although Iāve done some study of several suppliers and am currently bench testing rubber vs poly bushes and mounts to understand the differences. Iāve also done a lot of computer simulations to understand and explore the behaviour and interactions of the various front suspension elements. Less so at the rear as I donāt have such a detailed model.
My first question would be - what do you want to achieve from the proposed change? Durability, performance, refinementā¦?
Well possible squeaking would be a concern. I donāt see pushing my car to its limit , those days are over for me. Iām more interested in cruising. Not being knowledgeable about poly bushings I just wanted to be informed. Sounds like they are more advantageous for those that want to push their car rather than cruise.
If properly installed the only movement of the bushing is the twisting of the rubber (or poly). The outer āraceā is pressed and locked into the arms and the inner āraceā locked on the shaft by compression the outside nuts. No metal on metal friction to cause a squeak.
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Sorry no outer race just locked by compressing. Trying to recall from when I did mine a few years backā¦ā¦
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Mark, Iām sure they can vary from mfg to mfg and even type to type but my experience with the polybushes, on my Porsches, is that they come with grease and are expected to rotate. But maybe on the Etype they simply deform. That said, if they arenāt a perfect fit, maybe they rotate anyway and cause the squeaking?
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Harvey,
They are meant to deform/ twist which is why youāre supposed to tighten the shaft nuts when the vehicle is on the ground, at normal ride height. The idea is to avoid or minimize the amount they are twisted at rest in order to extend their life.
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Doesnāt that suggest the PU bush has similar stiffness to the original rubber part? In which case, what is the value of using them? I always thought the underlying purpose was that a PU part can be made higher stiffness than rubber.
The last few posts illustrate the confusion that seems to exist with regard to āpoly bushesā. I use the generic term here, rather than a particular manufacturerās product name. I have seen it stated on this forum that (a) poly bushes are designed like their rubber forebears so that the center steel sleeve does not rotate on the fulcrum shaft, and (b) poly bushes are different from their rubber forebears in that the center steel sleeve is designed to rotate on the fulcrum shaft, and requires lubrication to prevent it squeaking.
The only way I can rationalise these diametrically opposing views is by assuming that (a) someone doesnāt know what they are talking about, or (b) there are different kinds of āpoly bushingsā, and they operate quite differently. Based on a post I read here fairly recently in one of @CliveR 's threads, I am inclined to believe (b). In that thread, it was stated that the Polybush (manufacturerās name) black bushing product is designed to behave like the rubber bush - ie not rotate, but that other, stiffer, poly bushes may not be. It would really help the original poster here, and many of the rest of us if we could finally clarify this issue, as we may well be talking apples and oranges. I installed Polybush black bushings on the front suspension of my Series 2, and in 13 years and 30-40K miles havenāt noticed any squeaks FWIW.
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Clive,
By no means Iām an expert on this subject but I believe I understand the basics of an e type front suspension. With that I would compare the bushing to a torsion bar. Once you tighten them in a position their torsional rigidity will want to bring the a-arms back to the original āat restā position whether they were extended or retracted. Their durometer value would impact the amount of force required to compress or extend the arms and the ride comfort since the vehicles load is directly(?) transferred thru them.
This is my un-educated interpretation
Yes, that is also my understanding. To be specific, it applies to the condition where there is no relative slip between the bushing and the fixed parts (housing or spindle) so all the motion takes place by twisting the flexible material.
Youāre correct, the stiffness of that flexible material determines the force required to deflect the connected suspension link and is thus a part of the total suspension stiffness (resistance to vertical motion of the wheel relative to the body). The contribution of the bushings is typically not a large part of the total - perhaps 10% or less? It will vary from one car type to another but itās going to be lower than due to springs and tyres. That will apply equally to rubber or PU bushings.
Also significant is the inherent damping of the bushings - that plays a part in the overall ride comfort / quality feel / road noise equation because itās typically felt in situations where the road input may be too small to operate the dampers. I have not driven back to back but have seen data suggesting that PU has lower inherent damping than natural rubber - that could show as a loose, ājigglyā ride feel in certain situations. Maybe not always apparent as there will be interaction with the tyres.
It still leaves me wondering what we gain from PU bushings, if they are soft enough not to rotate and squeak.
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Yes, Steve H [MGCJAG] has gone to great lengths to clarify the situation relative to Polybush parts. I donāt disagree with him, nor do I have a particular position on your options - I may be person (a) while holding view (b)! Iām afraid Iām not the person to give the clarity you seek - Iām still trying to understand the motivation to use PU bushes on an otherwise unmodified E-type.
I make that distinction because I see the appeal on a ātunerā version of a mainstream street car, where an eager youth will make drastic spring and damper mods in search of reduced ride height. The resulting reduction in wheel travel could reduce the slip/squeak risk of the PU bush, but the E-type owner retaining full travel may not be as fortunate.
I spent a couple of hours this morning reading through a long thread on this very subject, I think from 2017 / 8. Iād prefer to add something new than repeat the previous stuff, so Iām working on a PU rear subframe mount. I believe it has the potential to give helpful geometry control and no squeaks.
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@Marco @davidxk I am on the same page with you both that the original rubber bushings were meant to grip on their ID and OD and deflect in the rubber portion. I also agree with David that some of the aftermarket products may achieve this state of affairs but based on reports of sqeeking, some donāt. Iād like to be able to give the OP some better advice on which may be which but I have almost zero experience, except to say that I would expect the harder ones (the more racey or high performance ones) probably fall over the line into slipping and squeeking.
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With less experience of this application than yours, I agree with your comments.
Hi all,
Just from experience, for what is worth: In my MG, Mini (racing) and XJ-SC I have poly bushes, no squeals at all. My brotherās racing E is poly (blue, out of memory). Mine also seemed to be blue, metal collared ones.
I can only confirm that durability and resistance to oil seems better than rubber. Their compressibility (force you need to compress the thing, rather than twist it) is higher than rubber, giving stiffer lateral control.
When my XK bushes are due (they are) they will be poly.
Best,
Ll.