Polyurethane Suspension Bushes

For many years now aftermarket vendors have been extolling the virtues of polyurethane suspension bushes. I have yet to be convinced. Several years back, at the request of a Healey owner we installed a set of these which he supplied. They came from a reputable supplier and were touted to be the best thing since sliced bread. Unfortunately I don’t have a photo of the bushes he supplied but they were red and looked to be of good quality.

I’m always hesitant to install customer supplied parts because if something goes wrong there is always some concern as to who is responsible for the problem and who will pay for the necessary corrective action.

After making my position clear I got the go ahead and installed these front suspension bushes in said Healey. Anyone who has worked on a Healey which has spent time in Eastern Canada is well aware of the difficulties that removing the lower inner suspension bushes can present and this car was no exception. As I recall this one was so bad that I ended up having to change all four pivot pins and washers after having to resort to using the great blue wrench to get out the original rubber bushes which, before my attack, were in great shape despite their age.

Polyurethane bushes replaced the anti roll bar bearings, the anti roll bar link bushes (which required that the links be replaced because the ends of the threads were badly damaged) the upper trunnion bushes and the afore mentioned inner lower bushes. No lubricant was supplied with this set but as per the instructions we applied silicone grease to the bushes.

In less that one week the owner was back complaining of creaking noises from the front suspension; however, despite the creaking noises, he swore that the handling of the car had improved immeasurably.

I took it all apart and re-applied the lubricant and reassembled everything. This fix lasted about 10 days before the creaking started up again and one of the threads broke off one of the new anti roll bar links. The car was back and I was on vacation. My chief mechanic of the era was a distinctly crusty individual who was somewhat disinclined to “humour” customers and, without discussing the options with the owner, fixed the problem and sent him on his way.

When I ran into him at the next club gathering the owner was to be heard extolling the virtues of the “genius” I had working in the shop while I was on vacation and how he, the mechanic, had managed to correct the creaking problem in his suspension. He took pains to point out that this was something that I wasn’t able to do, and how wonderful the handling was with the polyurethane bushes. When back at work on the following Monday I mentioned the praise to mechanic he said “Wait till he finds out that the bushes are rubber”.

I think this incident says a lot about polyurethane bushes in Healeys and MGs for that matter.

The problem with these bushes, as I see it, is that joints are not designed for polyurethane bushes and without a major reconfiguration are completely inappropriate for them.

Take for example the upper trunnion of a Healey or, for that matter an MGB which is virtually identical. The same issues apply to the lower inner joints. This is what a cross section of the joint looks like.

Cross Section of a Healey Upper Trunnion

The brilliance of the design is that there is no relative movement between the surfaces. All the movement is taken up by the compliance of the rubber bush. No movement means no maintenance, no squeaks and no wear. What more could you ask for? The bush design is far cleverer than it at first appears. When the joint is tightened up with the suspension in mid range there is just enough rubber in there that it remains within its elastic limit throughout the entire movement of the joint and the rubber is squeezed in so tightly that it keeps moisture out. It’s brilliant!!

Now let’s consider the concept of installing polyurethane bushes into the same place. This is a scan of the instructions received with a set of such bushings.

And this is what the “improved” joint looks like.

Unlike the original rubber bushes, polyurethane requires sliding contact somewhere. In the case of the ones used in this application the sliding is meant to take place between the sleeves and the polyurethane and between the shock arms and the polyurethane.

The first problem is that there is no sealing medium at the edge of the sliding joint at the shock arm with the result that any lubricant in there gets squeezed out and replaced by moisture.

The next problem is that the force required to hold the bushes into the tapered holes, which is a factor of the compressibility of the urethane, the excessive length of the urethane which gets compressed to tighten the bush into the cavity and the length of the sleeve is quite substantial. This force produces friction at the shock arm to urethane intersection. Decrease this force and the bushes will start rotating inside the upper trunnion because they are a loose fit.

Way back in 1962 Triumph redesigned the suspension used in their TR series of sportscars and used nylon bushes in the then new TR4A. Around the same time they also launched the Spitfire which used a similar system also incorporating nylon bushes. I know that nylon isn’t exactly the same as polyurethane, but it does have very similar properties. The joints in these cars were redesigned to incorporate the nylon and this redesign included fitting a complex “O” ring sealing system to keep moisture out and lubricant in. They worked fairly well, although they do wear out frequently and are often reassembled incorrectly during maintenance which does not improve matters.

I have no doubt the polyurethane is a wonderful material for suspension joints BUT the joint must be designed appropriately and not cobbled up from something designed to operate entirely differently.

About Michael

Who is this guy? Born in New Zealand some time back. Went to Maori Hill Primary School then Kaikorai Valley High before joining the RNZAF and an Airman Cadet in 1968. Graduated 1972 with an NZCE in Aeronautical Engineering. Then embarked on the typical Kiwis "Big Trip Overseas". Got to see quite a few places, and spent a while in the U.K. "home" as it was refered to by many New Zealanders in those days, before travelling on to New York and then to Canada by bus!! This trip is presently on hold (has been for the last 34 years). Met my dear wife Judy not long after arriving in Ontario and we have been happily married since 1976. After travelling around New Zealand and the pacific in 1979 I started Precision Sportscar andfor the next 23 years grew the business and helped raise 2 boys Drew and Robin.
This entry was posted in BJ8 Restoration Information, Classic Rallying, New British Sports Car Parts. Bookmark the permalink.

24 Responses to Polyurethane Suspension Bushes

  1. John Close says:

    Good job, Mike – I had the same experience with my BN6. I now have rubber bushings and am happy. Your blogs are great! Prior to reading your blog on suspension upgrades – front and rear anti-roll bars, tube shocks – I had made those modifications. Your blog told me I had done the right thing! Nice to hear – doesn’t happen often enough. Thanks again – John Close

  2. Ed Kaler says:

    PERFECT, Mike!!

    Proves, beyond reasonable doubt, the K.I.S.S. Rule WORKS!!

    Ed

  3. Alan says:

    Hi Mike –

    From all the information I have gathered, the problems do not have to do with the upper trunion, but with the lower A-Arm.

    I have been told that the front and rear A-Arm chassis mounts are not on the same pivot line, and that’s really the main reason why the Polyurethane bushes don’t work that well when installed in the lower A-Arms. Another issue is the rubber bushes are designed to “give” by flexing the rubber material in the sockets (allowing the rubber outside surface to stay fixed in the socket, and forcing the pivot motion between the bolt and sleeve), whereas the polyurethane bushes, when used with grease and because of it’s inflexibility, causes the outside surface of the bush to rotate in the socket, which it should not do. This also overly stresses the bolt and sleeve. All suspension rotation should between the bolt and sleeve, not bush and socket.

    I have been told many times the best of both worlds can be had with Noltec bushings, which are softer like rubber, but made out of polyester similar to urethane. Not to plug them, but check it out:

    http://www.noltecsuspension.com/

    I love their stuff, and put them on my cars on every suspension rebuild.

    Alan

  4. Greg Lemon says:

    Micheal, intersting reading, and I value your opinion and experience on this, I did fit polyurethane upper trunnion bushings to my BN1 asw well as on the frame mountings to the anti-roll bar, I thought of the ARB bushings as a cheap alternative to the fitting of the thicker bar as fitted to the M and later cars (if I recall correctly).

    So far after a few years as far as I can tell no extra squeaking or premature wear.

    I will also say I didn’t feel an appreciable difference in handling. I did fit them to the upper (inner as I recall) front mounts on my TR4A years ago and felt it did make the handling sharper, purely be seat of the pants spirited street driving feel.

    I certainly see what you are saying, but does the relatively sheltered lives most of our cars lead lessen these concerns about wear somewhat. Fewer miles and less bad weather would seem to me to lessen the concerns about dirt/moisture infiltration and wear.

  5. Michael says:

    Greg, glad you enjoyed my comments and your point is well taken, however the thing that I’m trying to point out is that IMHO rubber is actually a really good material for the job so why not stick with it.

  6. Dave Russell says:

    Hi Michael,
    I really enjoy your thought provoking “blog”.
    To add to your comments on the poly suspension bushings:
    The following was written by Jim Hockert several years ago. I really believe that he is correct that the stock Healey front suspension lower mounts to the frame do not have the correct geometry/alignment to work with non-flexible bushings. I have quoted his comments below.

    Regards,
    Dave Russell

    “Suspension Bushings

    * Subject: Suspension Bushings
    * Date: Mon, 03 Sep 2001 13:49:04 -0500

    Bob:

    I believe that changing out the OEM type suspension bushings for a harder material is not beneficial to the car or driver in the case of big Healeys.

    The reason is that the design and manufacture of the front suspension causes the bushes to bind as the suspension travels up and down. That is the reason that you hear squeaks, clunks and thumps when poly bushes are installed. They bind up, then release with a ‘pop’. That is also why the ride is so harsh with poly bushes.

    The castor on the front suspension is built into the chassis by having the horizontal centerline of the rear suspension bracket one-half inch closer to the top of the chassis than the horizontal centerline of the front suspension bracket. This provides two degrees of positive castor for the
    suspension. Unfortunately, in the manufacturing process, the brackets are installed at a 90 degree vertical angle to the chassis instead of 88 degrees. In addition, the shock tower is 90 degrees vertical to the chassis instead of 88 degrees. Thus, both the upper and lower A-arms do not travel in plane with the castor angle causing the suspension to bind slightly as it works.The OEM rubber bushes absorb the binding and
    actually snug up somewhat in a tight turn.

    The best solution, absent blueprinting the suspension and changing the angles of the brackets, is to replace the rubber bushes with OEM type bushes as a regular maintenance item.

    Best regards.
    Jim Hockert
    BJ8 Rallye
    Dallas, TX”

  7. Dick Matson says:

    Thanks all. Great stuff. On the 2004 front end rebuild of my owm Bj8 I finally did decide on rubber bushing replacements all around. All are rubber except the upper trunion which is an offset poly bushing. Gained was the ability to adjust camber. Making the adjustments took some fiddling but in the case of this car it was a big plus. The car tracks really nice on the road now. No down side to report here so far, and no squeeks. I would do it again.

  8. Steve Gerow says:

    My experiences are slightly different–

    I must have been mis-describing the bushings I bought from Cape. They’re plastic but soft and flexible like the OEM rubber.

    They are one piece (similar to your drawings but without the center seam) and you remove the center tube, lube the outside with the white assembly lube, then hit them with a sledge hammer in order to drive them into the hole in the a-arm, where they definitely do not rotate. As far as I can tell the only rotation is between the center tube and the bolt. I lubed them with multi-purpose chassis grease, including on the faces adjacent to the chassis mounts. Tightening up the castellated nut all the way only produces slight friction. I don’t believe the center tube is stationary–I think it rotates around the fulcrum bolt.

    Before the installation of the springs, moving the suspension up and down produced a very slight friction at the joints, but no binding or mis-alignment was observed.

    I also installed Cape’s bronze offset upper trunnion bushings, but feel the offset part is not important.
    The steering seems crisper with the rigid upper trunnion. I did some extra work there machining lubrication grooves in the trunnion as well as fitting a grease fitting and fitting the bushings to the trunnion itself. Cape recommends anti-sieze (Copper-something) as a lubricant. It all works fine in our non-corrosive, dry, pampered So Calif environment.

    I also installed Cape’s roller-bearing thrust washers, which seem like a pretty good investment.

    Having said that, I don’t recommend these bushings as they provide only one degree of offset, and, in my case, required a fair amount of machining to make them fit.

    The combination of a hard (nylatron) upper trunnion bushing with a new upper bolt might be a pretty cost-effective way to get sharper steering.

  9. Steve Gerow says:

    Mike,
    Looking at your “rubber” drawing–shouldn’t there be a very slight gap between the rubber and the shock arm, where the center tube protrudes slightly?

  10. Michael says:

    Hi Steve,
    No there is no gap between the rubber and the shock arms when things are correctly installed, if there was the bushes would slide out of the trunnion until they stopped against the arm but this is a far from ideal condition. The rubber bush is actually a bit large for the hole in the trunnion so that when the through bolt is tightened the rubber is squeezed back a fair bit and then starts to push up through the gap between the shock arms and the trunnion as the spacer tubes come together and prohibit further tightening. It is real tight in there.
    Another interesting thing about the rubber bushes is that there are some poor quality ones around which have a split spacer tube rather than the original seamless tubing. The split tube can distort easily when the through bolt is tightened allowing the bolt to over stress the shock arms by bending them in.

  11. Steve Gerow says:

    Sorry to flog this one, but wanted to get it all straight in my mind, which this discussion as been great for.

    Based on the above & my own observations, some generalizations which may or may not be correct–for comment:

    1) Hard bushes are mostly not acceptable except for racers.

    2) There are aftermarket bushes which are referred to as “poly” which are as soft as the rubber ones.

    3) These soft bushes function similarly to the rubber ones and therefore do not improve handling.

    4) Since they’re as soft as the rubber ones, their only advantage comes from being oil- and grease-proof.

    5) This isn’t much of an advantage on a collector car which isn’t going to get that oily. The OEM ones are likely to last for years.

    6) Center-tube rotation around a fulcrum bolt isn’t that desirable, but also isn’t likely to be a problem on a collector car.

    7) Lastly, those of us (me included) who paid through the nose for sets of the soft “poly” bushes have likely wasted our money.

    That’s my summary.

  12. Michael says:

    Yep Steve, I would say that you have summarized things pretty well.

  13. John Kilmartin says:

    I would disagree with Dave Russell and Jim Hockert on observations of the factory installation of the suspension wishbone mount brackets. The brackets DO lie back 2 degrees from the chassis rails to give the required castor angle and this can be demonstrated by placing a 5/8″ shaft through the front mount, pass through the shock tower and then through the rear mount. Yes the shock tower IS welded 90 degrees vertical to the chassis,but the shocker mount plate on top does not weld parallel to the tower top (in side view) , the front vertical edge is wider than the rear, allowing the correct 2 degree lie back of the shock absorber. Both the upper arm and lower suspension arm are in the same plane.
    The two 5/8″ shafts are a great ready means to check the correctness of your front end geometry. They should be exactly parallel in both plan and side view, and sit at a 2 degree to the main rails.Use of a spirit level is handy for confirming parallelness.
    Have enjoyed reading your blog too, Michael.

  14. Ian B says:

    Hi Michael,

    A good overview on this !

    The key difference between rubber and poly is that the poly is a liquid. Rubber can compress, but the polyurethane will not – it flows.

    So your statement “force required …compressibility of the urethane… is quite substantial” comes as now surprise !

    I totally agree with you that the joint needs to be designed for poly and the most of the replacement parts fittings are totally unsuitable.

    I have a friend who’s sole business is damper rebuilding and he sees so much wear as well. Typical is a shackle bushes in 4x4s with leaf springs. The bushes last forever, but the salt/sand wears away at the housings and fittings.

    Cheers
    Ian

  15. Krish Smith says:

    I have been using urethane bushes for many years without any problems of noise or harshness in competition and fast road uses. Rubber bushes do not allow the crush tube to freely rotate as it is bonded to the rubber. The purpose of the rubber or the polyurethane is to hold the suspension joint in position, relative to where the crush tube is bolted into. The bush should be able to rotate freely around the crush tube if one wishes to get accurate suspension movements, which will give accurate camber changes, better steering, less tyre wear etc.
    If the polyurethane bush has internal cross hatching against the crush tube and is lubricated with correct lubricant it can make a significant difference.
    The best one I have seen is from AUTOBUSH who can be found at autobush.com. I have used their product, it fits very tightly in the housing, has a seal at each end to keep their lubricant in place, the suspension geometry works very well after installation. Also they supply stainles steel crush tubes so there is no corrosion and no deterioration caused by oil or grease on the rubber. Other companies probably offer similar products but I have only used this one.

  16. Steve Begg says:

    Very interesting discussion. One factor that has been dealt with indirectly but I think is essentially the main difference between rubber and urethane bushings. Rubber will flex in torsion,torsional shear is the correct term I think.This feature allows the rubber bushed suspension joint all of the positive attributes mentioned in the original post.
    Urethane does not possess this ability, and so has to rotate somewhere within the joint.
    The percieved negative with rubber joints is loss of accurate placement of suspension components under load.
    Depending on the OEM specification, IE: the amount and durometer of rubber used in the joint, this can be true.Adding to this issue is aged or worn out bushings.
    Enter urethane bushings, harder with less deflection, problem solved?
    Not necessarily. Suspension components, especially trailing or control arms, see loads from more directions than their primary direction of movement.A trailing arm moves up and down, but also has to handle high side loads. A front control arm moves up and down but also recieves high loads for and aft.Rubber suspension joints handle these loads without complaint, because they can flex in any direction, INCLUDING, rotation. Internal torsional shear. When urethane bushings are subjected to these same secondary loads, no matter how stiff they still have to flex slightly. However they still need to rotate within the joint, wether it is at the housing or shaft. The problem is that under load the bushing is no longer round, it is no longer free to rotate, so it binds. Then when the load is high enough it is forced free.This is why the joints creak and groan, why they need to be lubricated and to a certain extent why they seem to improve handling, a binding suspension is a stiff suspension.
    The best solution, as someone from Texas has already mentioned replace bushings as regular maintainence, especially for street use. The reason urethane bushings are so attractive is that they are dirt cheap to manufacture. Urethane is simply poured into molds and removed after curing. Rubber parts require expensive injection or compression molds and special equipment to get the rubber into the molds. So to improve a rubber suspension joint for performance use one would design the bushing with less rubber or a higher durometer rubber (stiffer). This would increase the stiffness of the joint, actually adding to the overall spring rate, and improve the location accuracy of suspension components. This would also be expensive, so most race cars use heim joints or rose joints, which handle suspension loads without binding and provide zero deflection. They don’t hadle dirt and moisture , so are no good for extended street use.As with most performance improvements its all about picking the best compromise.
    There is a place for urethane bushings or mounts and that is in static applications like engine mounts, sway bar end link bushings etc., anything that doesn’t require rotation and deflection.There is a reason OEM manufacturers have not jumped on the bandwagon , especially as this would save money.
    Check out a Ferrari,Porsche or Astin Martin and see how many urethane suspension bushings they use.
    A long winded opinion, late in what is a perhaps a dead post, hope it is helpful.

  17. Tore Lie, Norway says:

    I agree with everything positive that has been said about rubber bushings, but I find the real problem now is the lousy quality of the bushings that are available. The inner A-arm bushings on my MGB were completely shot after only two years, and that is in the cold Norwegian climate.
    This season I have used blue Autobush polyurethane bushes in the entire front suspension. They work well, but I think I will change back to rubber in the upper trunnions to try to reduce vibrations. Of course changing these bushings is a piece of cake compared to the lower ones.
    If anyone can point me in the direction of QUALITY rubber bushings, I will change back to rubber in the A-arms as well.

  18. Michael says:

    Tore, I would strongly recommend that you use the readily available MGBGTV8 lower inner bushes on your MGB. The work very well and last for ever. I have used them extensively in the front suspension of AHX12.

  19. Polyurethane bushings is specially design to deal with internal stress and other force and different obstacle on the quality of your ride.This will help you for smooth and bump free ride.

  20. Warwick Jenness says:

    Hi Mike
    Thanks for the interesting and informative blog. I had started to hunt for suitable polyurethane bushes for the rear radius arms on my E Type as the rubber ones I fitted 24 months ago have cracked. I am now perpared to replace the rubber bushes with a good quality rubberbush again and this time ensure that they are fitted and tightened into place with the load of the car on them. (I didn’t check this last time) Can you recommend a source of good quality bushes? I am after the small bushes that connect to the rear wishbone.
    Many thanks
    Warwick (Auckland, NZ)

    • Michael says:

      Hi Warwick,
      Very sorry but I’m not up on current sources of parts for Jaguars. With so many Jaguars on the rod with that suspension system I cannot imagine that there is not a good source of bushes.
      I would start by looking for original equipment Metalastic bushes.
      It has been my experience that these really do not deteriorate on the shelf so even if you find some 30 year NOS old originals they will almost certainly be ideal for your car.

  21. Angus says:

    Hello Michael
    I live in Perth, Western Australia and have owned my 1980 MGB for about 30 years. I wish to rebuild my suspension front and rear and was considering poly bushes (Superpro) but now have second thoughts. I think that I will go for front V8 rubber bushes where available. However, can you advise on where I should source the remaining bushes needed given that the reputation of currently available rubber bushes is not so great. Any help will be appreciated. Many thanks.

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