Rubber suspension upgrade

[RichardMc]

The Jowetteer (vol66/12) arrived today, so obviously it was mandatory lunch time reading.

On P28 John Airey has a technical note about changing from rubber to poly bushes, Hi John! I just wanted to add a word of caution. When doing research and planning for my SC Jupiter build I too considered getting some poly bushed made. I made a few enquiries and eventually spoke to someone at powerflex, the general advice was don't.

The reasoning was that the rubber bush is gripped on the outer faces by the cone shape of the suspension arm, compliance comes from the rubber twisting internally. With a poly bush this doesn't happen (the twist) and the outer faces become a bearing surface that 'slips' against the suspension arm. This then wears the face of the mounting and eventually damages the arm. I dug a bit deeper and found that even with rubber bushes the suspension needs to be in its mid point or natural resting position before the assembly is tightened so that the rubber isn't 'pre loaded' in any way. If for example you build the front suspension on a Javelin with the engine out, putting the engine back will have twisted the rubber bushes as the front drops lower.
I must admit I didn't test the theory, it could be complete rubbish, but I stuck with rubber bushes.

[Forumadmin]

I agree with Richard https://jowett.net/forum/viewtopic.php?p=13623#p13623 and I did the same when racing and asked the question about poly bushes.
The only place I have replaced rubber is with nylon fitted in the front torsion bars.
http://jowett.net/forum/viewtopic.php?f ... ion#p17654
https://jowett.net/forum/viewtopic.php?p=20209#p20209
https://jowett.net/forum/viewtopic.php?p=35987#p35987


Read about the making and insertion here.
https://jowett.net/forum/viewtopic.php?p=23020#p23020

I will also add that setting up the suspension correctly with new rubber all round transforms the car's handling. See how I did it in that same post further down. Note this is for the Jup .
Javelin here.
https://jowett.net/forum/viewtopic.php? ... ion#p22575
https://jowett.net/forum/viewtopic.php?p=43842#p43842
https://jowett.net/forum/viewtopic.php?p=41998#p41998
Note in the last topic advice on changing from metal to rubber.

And a post by David on the subject here who has had poly on his for now 14 years.
https://jowett.net/forum/viewtopic.php? ... ion#p14861

[johnairey]

Hi Richard,
Thanks for passing on the advice from Powerflex on the function difference between the poly and rubber bushing. Looks like care is needs to be taken with the overloading the King Pin bearings with lubrication to prevent the contamination of the rubber bushing for a longer life.
John

[David Morris]

Hi All,

I have only used poly bushes at the bottom of the swivel pins, where I found the standard ones very soon went 'squidgy' and failed. Perhaps I am too eager with the grease gun? The poly ones have been in there for years and still look fine. I see that Keith has kindly found my original post from 2013, which quotes the part number and source.

I guess using them elsewhere might well cause problems?

All the best,

David

[David Morris]

Hi John,

My latest issue of the Jowetteer arrived late last night and I have just read your article. Very interesting and I like the colour you have chosen for the parts! Good move to incorporate a ball thrust bearing, as the thrust washers I have taken off are usually deeply scored. Let me know if you need a third hand to get the mounting bolts in, under the wings. They are a pig to locate.

All the best,

David

[Forumadmin]

It maybe the grade of rubber used in the ones which went squidgy. I have some conical bushes which are soft and some which are hard, so it maybe the rubber started off more squidgy than it should have been. There is probably a specification that is the correct compromise between torsional compliance and compressive compliance as well as oil contaminant resistance.
I usually coat the rubber with silicone grease before assembly, but I cannot remember where I picked up that tip but have been doing it for thirty years. I think it helps keep the oil off and prevents oxidisation of the rubber. This also might provide a lubricant when the rubber tries to twist and I have not noticed any wear as a result on the sockets. This might put additional load on the shock absorber as the spring effect of the rubber is nullified.
https://forums.mercedesclub.org.uk/inde ... hes.72966/
I also have some rubber grease by Castrol that I think I have had for forty years!

There may also be a way to remove the wear on poly bushes/sockets using a suitable lubricant (perhaps calcium grease).
Using teflon rather than poly?? might be tried. I use teflon plugs in my piston gudgeon pin after I had a circlip break on the Pirelli Marathon.
More research required.

[David Morris]

Hi All,

I am no chemist, but I did work within a technical group within the MoD that included some excellent chemists. They explained to be that many rubber and plastic products contain chemical stabilisers within the mix to ensure reasonably long lives for the products. Think of foam plastic and how this can go all 'gooey' and degenerate to almost a powder over time? Javelin and Jupiter steering wheels also break down with age and I have had one that 'leaked' a sort of goo all over the driver's seat.

The difference between the life of rubber bonded products, like the bushes we are discussing here, might be down to the shelf life they have experienced already on the shelf or have left before they become 'squidgy', due to using up the life of their in-built stabilisers. They could have been on the shelf as 'new old stock' for several years? Just a thought, but the difference in performance on the road that Keith has noticed with new bushes might be due to the in-built age of the bushes?

I suspect that poly bushes might last longer than rubber ones, due to their inherent chemical stability? I am sure there are other members who could advise on the chemistry, as I am out of my comfort zone here!

All the best,

David

[Forumadmin]

The difference in perfomance was due to correct suspension setup and taught unworn rubber. The torsion bar rubbers are seldom changed except on a total rebuild. Ask a hundred Javelin owners and I doubt if 5 will reply they have.

Most modern cars fail MOT on suspension rubber after 5 years, mainly due to the severe pounding they get from speed bumps.
Remember my Jup has endured numerous rallies and journeys on VERY rough roads, including Welsh forest stages, Latvia and the stony piste of the Sahara.

[Keith Clements]

http://jowett.org/media/98marahi.WMV
Put on the website in 1998 so not at modern fidelity!
Turn up the sound.

[PJGD]

I had a preliminary look about 5 years ago at what might be involved in adopting a poly bush suspension, and at the time I was not able to locate a poly bush that could be a straight swap with the Metalastik rubber bush. Possibly the main issue being that the suspension arm must rotate on the static bush and this requires that the receiving bore in the suspension arm be profiled to match the bush [or vice versa] and that the surface finish be up to an appropriate standard as specified by the bush supplier. For our rubber bush, the surface finish is not critical.

I reported on this at the time and I was able to do a durometer test on a new Metalastik bush which gave a value of 71 Shore hardness - see photo.

Rubber Bush Durometer Test_156.jpg

Note on Front Suspension.pdf