Polyurethane or metalastic bushes?

[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.

[38E]

Hmmm. Maybe I remembered the front bushes incorrectly then. It's a long time since renewed them (they're still good, BTW). In any case, it would be very poor engineering practice to design a poly bush to actually wear on a part like an E-Type wishbone.

Most Metalastic bushes of that type do have a bonded outer steel sleeve though.

I can distinctly remember having to thread a hacksaw blade through some of the burned out bushes to cut out the seized in outer sleeve. Radius arms maybe.

[PeterCrespin]

Yes the radius arms, and most bushes in modern cars too, but not the E-type front wishbones, drop links (F+R anti-roll bars) or shock absorbers. In fact my Front Konis dont have the eye bushes bonded to the innder sleeve OR shcock eye, they are just two cones fitted over the same sleeve from opposite sides and clamped by the picture frame up top or the large washer and wishbone at the bottom.

[Larry Wade]

Hi guys,

I've somewhat different perspective than most of you. I live in a really hot place. I've driven for hours above 100 mph with the outside temperature over 120 F (I drank six liters of water and never had to pee that day).

In desert climates rubber doesn't last very long. This is especially true in high ozone locals (like Los Angeles).

I do believe there are real longevity advantages to poly in hot climates.

As to the performance merits.....I think it probably depends on the application. If I'm looking for a material to damp high frequency vibrations metalastic is a great choice. On the other hand if you are transering force between two structural elements (for instance between the swaybars and the monocoque) then squishiness (an important engineering parameter!) would add lean or slop without improving the ride.

So it seems to me that the optimal solution is a mixture of the two materials. For the IRS mounts, the radius arms, etc. a metalastic bushing is probably the best material. For some of the other fittings I think that poly would be the best choice. To me the best material depends on the specific fitting and it's function.

As to lubrication.....soap sounds dopey to me. Waterproof synthetic a high concentration of teflon (PTFE) or molybdenum disulfide are the appropriate choice to deal with the chirping that occurs.

My own car has many polyurethane bushings and some that are metalastic. I'm happy with the mix. I've had no squeaks in six years and ~35 K miles and have not had to relube....since I properly greased and installed them (my first attempt was dry...and not very satisfying).

If you are really worried about squeaks the solution worked out by Mike Frank would be a good choice:
http://www.coolcatcorp.com/faqs/PolyBushGrease.html
Then if you have a squeak you can just give it a squirt!

Personally I think the most important thing is to go drive your car and have some fun.

Cheers,
Larry

[Larry Wade]

Hey guys,

I wonder if we aren't all wrong. The earlier discussions assumed that the properties were uniform and well defined. That's just not true.

Note that Polyurethane products are available in a wide stiffness range. The polyurethane that Mike Frank (Coolcat) uses has a durometer of 85. That is a moderately stiff rubber...but still has plenty of flex....a good bit softer than a bushing marked Metalastik that I'm about to install.

By the same token I did some searches for metalastic (actually metalastik). That seems to be an originally Dunlop trademarked term for a (patented?) metal/rubber bonding or vulcanization process. These days the material of that name is made by Trelleborg. In their engineering literature they state that they use a wide variety of rubber compounds and formulations. 'Each compound is carefully formulated to obtain the best performance fro specific properties. The compound chosen depends upon the most important properties for the application's requirement. Strength and fatigue requirements, operating temperature,' etc., etc. The material range was really wide and included both synthetic and natural rubbers!

The point is that there are polyurethanes and there are metalastiks. In a given formulation one can be stiffer and in another formulation the relative characteristics could be reversed.

The good news is that I like the ride and handling of my car. That's about all that I can meaningfully report.

My recommendation for those who are buying new bushings is to consider how soft a ride you like, how hard you drive, what temperatures you will drive in (rubber properties tend to vary much more than polyurethane properties do), and then go for it. The aftermarket rubber stuff is softest. The softest polyurethane and metalastik fittings are probably pretty similar in character (unless it's really cold or really hot.. then the metalastik will likely be harder or softer respectively). If you want to autocross or track the car and don't drive hundreds of miles day after day you might go for a really hard polyurethane.

Just put them in, whatever they are, and drive!

Larry

PS you can always change them out.