Polyurethane or metalastic bushes?

[Heuer]

Nicely summarised Pete!

Having gradually moved from Polybushes to OEM I can attest that my car is no slower through corners than before but NVT has noticeably reduced (specifically by my wife) making the car more enjoyable to drive.

[PeterCrespin]

We did a tour of Europe in my S3 OTS but my wife moaned a lot about comfort and was very reluctant to get in the car again. Result? I bought a hard-as-nails D-type :-)

Now my Series 2 on OEM bushes has won her over and she has remarked how good the ride is and is a frequent passenger. Whether this is good or bad depends on the timing, but I have another D-type anyway.

That one is for sale incidentally if anyone wants to see it at Stoneleigh, or save me the trip. Reason for sale is I'm getting a third Lynx-clone D-type custom built by Zealia's Brian Wilkinson, to include live rear axle and some original D-type panels.

I'll post it in For Sale and Wanted

[Tony]

I do not know what my car has but I could not ask for a better ride. On the basis I cannot see any blue, red or orange around the suspension area, I presume I am on the normal rubber bushes.
What they will be like after 50,000 miles I do not know but I suspect I will not be around to worry.

[38E]

The Metalastic type of rubber bushing for pivoting applications has to be one of the most major advances in automotive suspension design, along with top and bottom ball joints for steering instead of king pins etc.

No moving parts to wear plus the rubber assists the suspension compliance (which the poly bushing does not).

One of the most common comments by people riding in my car for the first time is surprise at the supple ride it provides.

Nothing wrong with rubber bushes on a properly designed suspension. The original Lotus Elan had them too and would run rings, handling-wise, around an E type.

[christopher storey]

Metalastik when they were genuinely available were first class. Few of the bushes are now genuine metalastik ( unless you can obtain NOS from Jaguar dealers)but are modern replacements . The quality of rubber parts now is generally vastly inferior to that of 40 years ago , the hysteresis and other attributes often being radically different from the originals. Furthermore, rubber is highly susceptible to degradation from ultraviolet radiation , and this is why so many bushes show clear signs of crazing and cracking. Polybushes are not subject to this type of degradation, nor in general are they so subject to tearing under shear forces . The suspension on an E ( or indeed any IRS Jaguar of the 1961-1986 era ) is about as sophisticated as you can get, and it is this which accounts for the exceptional ride/handling compromise irrespective of the type of bushes that are used

[38E]

I must be misunderstanding the whole concept of poly bushes then. What shear forces are present that might cause a polyurethane bushing to tear? I thought they just rotated on the bolt or a metal sleeve or whatever. Isn't that how they work, and why they creak?

I'm not sure why there should be a problem obtaining decent bonded rubber bushes these days since just about every vehicle on the road uses them. The Metalastik company was of course taken over by a Swedish operation over 10 years ago so maybe the current offerings by the usuals for Jaguar replacements have a more oriental origin.

[christopher storey]

Clive : my understanding is that they do not work by rotation of, or around, the securing bolts. The whole point of flexible suspension bushes is to provide a degree of compliance by allowing the bush to flex in different directions and not be absolutely solid. Thus by way of example , the radius arm large bushes flex each time the lower rear wishbone rises or falls - a classic shear motion. The radius arm small bushes do the same and in addition both these sets of bushes provide a small measure of both lateral and fore and aft compliance, allowing the suspension to "lead" under braking and "trail" under acceleration without promoting rear wheel steering except to the most minute degree . The same , in principle, applies to the front suspension bushes, the rack mountings, etc

[PeterCrespin]

rubber is highly susceptible to degradation from ultraviolet radiation

Which is zero problem since UV doesn't reach the working rubber deep inside the forged steel suspension arm... Even the 'exposed' radius arm bushes are out of UV's way, unless you parked over a glass disco floor with UV uplighters I suppose.

The bushes work as rubber in torsion, which is a superb corrosion-free way of arranging suspension pivots in the worst areas for corrosion damage - the undercarriage. Before they were invented it was bushes or and needle rollers, with lots of lubrication and mainetenance needed. I'm guessing the vast majority of cars from the last forty years have gone to the scrap yard still wearing the bushes they left the factory with.

The range of movement is not huge if you use a narrow cylinder of rubber like on the E-type wishbones, which is why you need to tighten at mid-laden. Since front wishbone polybushes don't act in torsion there's no issue with tightening at mid-laden AFAIK - they just rotate anyhow. The rubber for wishbone bushes is pretty low-tech and I have not heard of anybody's bushes failing very quickly if assembled correctly. Trelleborg are still around and making Metalastik stuff I think, which is not expensive, but my pattern rubber ones are fine anyhow.

The pressed-in shock absorber or radius arm bushes would flex but I think Chris was only referring to the fronts? Very little torsion on the shocks or radius arm bushes - they are mostly compression loadings and fail by the eye pulling to the edge of the bush when the high-load part of the rubber is eventually crushed.

[christopher storey]

Pete : the horizontal radius arm bushes are in shear,the small vertical ones in torsion ( which is of course in itself a shearing motion ). Incidentally, UV radiation is reflected off almost all surfaces so that even the underside of a car receives a fair amount unless the car is kept indoors

[Heuer]

............ the horizontal radius arm bushes are in shear,the small vertical ones in torsion ( which is of course in itself a shearing motion ).

And the reason the Polybushes squeak!

[PeterCrespin]

Pete : the horizontal radius arm bushes are in shear,the small vertical ones in torsion ( which is of course in itself a shearing motion ).

Not sure. Hopefully an engineer will give the definitive view but for now my take is as follows:

Shear is a sliding, er, shearing motion, with a large lateral component. The steering rack mounts are a perfect example. We can agree on those I'm sure.

Radius arms do not resist shear movement although they transmit fore and aft forces along the steel arm with respect to the bodywork - i.e the front mount 'shears off' the floor pan if corroded. That's fine as description of the radius arm action with respect to the bodywork, but the arm itself is in tension and compression along its beam axis. So are the bushes at each end which are de facto part of the arm IMHO. OK, I realise the large bushes are normally mounted with the gaps front and rear, so most of the tension and compression is taken by the blocks of rubber at the side so they are in shear - I'll buy that. As it happens, my replacement bushes are with the holes at the sides and rubber aligned front/rear and the blocks of rubber will be in compression/tension, not shear, but we'll move on.

I also accept that to the small extent that the dampers rotate as they compress/extend, there is a torsional component in those bushes as there is in the small vertical radius arm bushes as the suspension moves up and down. It's a geometry thing as you know.

But I'd suggest that the primary load on a spring/damper unit and a radius arm is along its main axis, slight rotational component notwithstanding. In that scenario, the primary load on the bushes must lie in the same direction. Therefore, the main operative parts of those shock and small radius arm bushes, and the parts which wear out, are when the rubber deteriorates along that line of to/fro forces.

Yes, in a circular donut of a bushing you could say that the pieces of rubber at the sides experience shear parallel to the direction of loading/unloading, but the main work is done by the rubber in line with the forces, which experience compression/tension. When you look at a collapsed shock bush you see the eye is always off-centre in the direction of load - i.e. the rubber is pumelled to death at the point between the sleeve and the damper body, not the sides.

Interesting discussion.

[christopher storey]

I think we may have a case here of a "rose by any other name" ! I agree absolutely that the axial forces on a bush - and indeed those forces at right angles to the bush - are of tension/compression . What I have in mind as shear forces are the rotational loads applied to the bushes. It may be that I am under a misapprehension, but I have always been under the impression that , to use an example, the upper front wishbone bushes have their outer periphery "anchored" to the bush housing , and their inner surface " anchored " to the fulcrum shafts . Thus as the wishbone moves up and down, is not the bush subject to a rotational shear force? Or does the fulcrum shaft move freely within the bush ? (and if so why did I have to burn my old bushes off in some cases!)

Amongst other things, if the bushes were no fixed in this way, there would be no need to tightn up only in a mid-laden position, and as David has said 2 posts ago , there would by implication be no squeaking

As you say, it is an interesting topic , ( and bearing in mind just how wonderful an E type feels with new bushes of whatever sort, an important one ) and maybe there is someone who can give a definitive answer

[PeterCrespin]

A rose by any other name AND a case of apples and oranges......

There's no question the front bushings operate primarily in torsion - and that is a form of shear force, leading to premature failure if the deflection exceeds the designed degree of rotation. There are vehicles which use only the torsional rubber spring instead of metal springs. A nice benefit of this is that the rubber also provides slight damping, not merely springing.

Poly front bushes don't work this way. The bush is merely a cushion and waterproof rubbing surface around a bolt or sleeve. Hence Chris's comment on your earlier statement that poly bushes are better because they are less susceptible to shear degradation. I suppose strictly-speaking not being under shear at all means they are less susceptible - i.e. not at all susceptible - but in truth they're a completely different case and can't be compared to rubber in torsion bushes on that parameter.

Meanwhile, I too was on a different tack, discussing your radius arm example, not the poly front initial query. Specifically, I was talking about the small radius arm and shock absorber bushings. For those, the angular displacement (and therefore torsional loads) are far smaller than the front bushings. The reciprocating lateral loads predominate, involving tension/compression of the bush rather than shear.

There is a caveat to all this, in that for true race applications or the highest loads, even the firmest poly is too sloppy and solid pivots are required - bushes or bearings as necessary. Hence the IRS wishbone fulrcums are not rubber bushes and race cars often have totally play-free suspensoion systems with nothing to absorb vibration and enhance ride compliance except for the tyre itself and the spring/damper units. Even those often have solid rose joints rather than rubber eye bushes. There are E-type IRS designs which use rose-jointed radius arms with zero ability to absord length changes like the satandrad ones. The only way these avoid being torn to pieces are by pivoting the front end in line with the inner fulcrums, at which point the geometry works with out any fore-aft arc of deflection as the wheel moves up and down - in a mirror image of the fully-triangulated front wishbones.

But then a full race car is not an animal with which to cross continents or preserve one's dental work...

[mgcjag]

Hi Guys.....Just rebuilding the front suspension & using the Black Poly bushes as supplied by Barrats......During assembly I used soapy water as recomended in the fitting instructions (other polybush manufactuters recomend a variaty of special greases for their bushes).....& used coppa slip anti siez on the inner metal sleave......Then tightened the castle nuts which grips the sleave tight to the shaft (didnt read the manual re tightening under load)...Although tight the poly still rotates in its housing...my question is should neither the metal sleave rotate on the shaft or the poly/rubber rotate in the housing but the only movement for the wishbones be provided by the give in the poly/rubber.....Thanks Steve

[PeterCrespin]

but the only movement for the wishbones be provided by the give in the poly/rubber.....Thanks Steve

Polybushes are a totally different technology Steve. Rubber in torsion bushes have no moving surfaces relative to each other. The rubber is bonded to the inner sleeve and is a tight press fit into the outer suspension arm. The only 'movement' is flexure of the rubber itself.

With poly bushes, the poly only has enough 'give' to slightly reduce vibration transmission between the suspension arm and the chassis - depending on grade chosen. The rotation occurs where two surfaces rub against each other - poly and metal. Hence the frequent squeaks after any lubricant has been washed, rubbed or squeezed out.

Having the bush rotating in its housing doesn't sound right, but who knows what the designer had in mind? The inside of a wishbone eye is not polished enough to be a long -lasting bearing surface against soft material. I guess it would wear the poly slack in no time if there was relative movement. I would assume the larger surface area of the outer diameter means the bush should stay put in the wishbone and rotate around the inner sleeve which is polished and of far lower surface area (i.e. friction) than the outer diameter.

You're not going to get any squeaks probably, with your Copa-Slip treatment and you don't need to worry about tightening under load (mid-laden) since you have rotating, not twisting, bush material. You've chosen to go non-standard so I hope it works out for you. Nobody who has splashed out on 'go faster' or 'hi-tech' poly is likely to say they wasted their money, but I honestly doubt you'll get much improvement for your money except bragging rights. <shrug>

If you never actually drove the car on rubber bushings before restoring it, so you know what the standard car feels like before modifying it, then 'double shrug '

[mgcjag]

Hi All......Just spoke to the Tech guys at Polybush, Wrexham. who supply the Black type bushes to Barrats (which are the same as there Blue bush).....They have designed the front Wishbone fulcrum bushes to work in the same way as the original rubber ...That is that the inner sleeve is bonded to the poly & when fitted to the shaft will be tightened so as not to move.....the outer poly face of the bush to be a tight fit in its housing with the only movement being flex of the poly.....I mentioned that after fitting my bushes they would still rotate (with force) in the housing this was explained as the soap used to install had not dried out & this would be the same with rubber.......They do realise that poly & rubber do not perform the same but have produced a bush for good road use to as near as possible OEM standards but with a much greater life span & not degraded by oil etc ...as per the squeeking no one has come back to them with any problems & I agreed that if mine squeek they can investigate.......Please note that there are other manufacturers of polybushes whos bushes work in a totaly different way & use lublicants to install.......Weather they are better than OEM is not for me to say as ive only driven on my worn out old rubber....Steve

[Dave K]

Steve,

You have done the right thing in talking to the designers not these two shade tree mechanics trying to outdo each other with a load of supposed technical BS.

Dave

[Tony]

Dave,

You nearly had me there till I spotted the smile at the end of comment. Nice one

[38E]

Hi All......Just spoke to the Tech guys at Polybush, Wrexham. who supply the Black type bushes to Barrats ....

I mentioned that after fitting my bushes they would still rotate (with force) in the housing this was explained as the soap used to install had not dried out & this would be the same with rubber...........Steve

That's a bunch of BS right there because the outer diameter of the original Metalastik bushes are bonded to steel sleeves. And the steel sleeve is a tight press fit in the wishbone so there's no way they can rotate.

What is absolutely amazing is how marketing people are able to convince some folks that their plastic bushes are somehow superior to the original Jaguar engineered needle and taper roller bearings for the rear suspension fulcrums.

[PeterCrespin]

That's a bunch of BS right there because the outer diameter of the original Metalastik bushes are bonded to steel sleeves. And the steel sleeve is a tight press fit in the wishbone so there's no way they can rotate

What is absolutely amazing is how marketing people are able to convince some folks that their plastic bushes are somehow superior to the original Jaguar engineered needle and taper roller bearings for the rear suspension fulcrums.

Yes and No Clive, or should I say no and yes... The E-type rubber bushes don't have an outer steel sleeve - not in the shock absorbers or front suspension (including drop links) at any rate. The rubber is squeezed into the wishbone which acts as the outer shield and is held by rubber-to-metal friction, not bonding. The inner sleeve is bonded sure enough.

Steve's supplier is fine about the bonding to the steel sleeve and IMHO that's better than a sliding fit, which is the type of polysush I've come across before. So that part is good to know. It's also true that soapy residues can take a while to dry out etc., so tearing off down the road on newly-installed tyres on chrome rims, for example, can rip a valve out if you're not careful adn there's still soap around the beads.

But I must say that if a bit of soap film on the outside of their polybush (most of which must surely be wiped off during pressing into the wishbone) is enough to let it rotate in the wishbone with a little force, then the fit is nothing like the OEM rubber. There is no way on God's green earth that an OEM rubber outer bush will rotate in that wishbone after correct installation. In fact we know that if not tightened up at mid-laden they tend to shred rather than slide around. It takes a good heave in a big vice to install them, so if polys rotate....? Hmm.