Technical Panda 4x4 rear shock replacement

[Herts Hillhopper]

My 3-year old Panda 4x4 went back to the FIAT main dealer (NE England) on Friday for its annual routine service. They informed me that both rear shock absorbers had defective top bushes, and recommended replacement at a cost of £224 (not sure whether that was parts only or included labour)....

I'm sure you've already looked, but Shop4Parts list the rear shocks at £30 each... I still am amazed at the hourly rate Fiat charge!

Although the rubber 'lip' may have peeled back a bit, is the rest still bonded (ie, can it be solid along the metal shaft at all?). So long as that bits still stuck, prob OK for a while (and the MOT tester seemed happy)

My 4x4 Panda also came from a NE Dealer (Richard Hardie, Newcastle-ish). I have always reckoned I had one of the very last UK-registered 4x4 Pandas with a diesel engine. I bought it as a pre-reg from them on 4 August 2018 with just 4 miles on the clock, and they reg'd it on 31 May '18. But, seeing this post, maybe not?

 

[Panda IIs]

On a rubber moulding that tapers down to a thin section, there is always a tendency for the lip to curl away from the steel sleeve, as in those photos, but strength of the bond lies in the main block of rubber, not in a small lip with only thin air behind it.

When I was involved in machinery management (not vehicle suspension, I hasten to add, but similar bonded rubber bushes are very widely used in industry), we checked the positions of inner and outer sleeves (most, but not all, are designed to be concentric) and how easily the components could be levered away from the rest position – radial movement (across the diameter), axial movement (side to side) and in some cases rotational movement about the axis.


Those tests were reckoned to reveal any defects in the metal-to-rubber bond, and the condition of the little thin rubber lip was regarded as cosmetic.

But perhaps we were doing it all wrong in those days.

Agreed; my point was the Dealer inspection was very "sharp-eyed".

N.B. the taper to a thin edge section is primarily to avoid the stress raiser of a sharp corner where the rubber meets the metal and therefore significantly reduce crack initiation and subsequent growth, it also provides easier separation from the mould and environmental protection for the adhesive bond.

The design is essentially fail safe - you will get metal to metal "clunking" to let you know things have gone seriously wrong.

 

[Jabe]

N.B. the taper to a thin edge section is primarily to avoid the stress raiser of a sharp corner where the rubber meets the metal and therefore significantly reduce crack initiation and subsequent growth, it also provides easier separation from the mould and environmental protection for the adhesive bond.

You may be right, although logic suggests to me that:

• a convex form, like that where the rubber core meets the outer sleeve, would minimise stress on the rubber;

• by contrast, the thin tapering rubber lip (where the "skirt" makes contact with the inner sleeve) would be rather susceptible to cracking or tearing;

• the tapering design might have nothing at all to do with stress control, and exist simply to suit the moulding process.

However, I can happily admit that my own experience relates mainly to metals rather than to rubber, that my engineering degree was awarded a long time ago (1969), and that it is 20 years since I was last investigating stress cracking on a professional consultancy basis. For those reasons, I will certainly defer to anybody who has up to date knowledge and qualifications relating to stress control in shock absorber bushes.

 

[Jabe]

But, seeing this post, maybe not?

"A clever piece of deduction, H." "Elementary, my dear Jabe".
Or to put it another way, I think you and I might have twins.

 

[Panda IIs]

You may be right, although logic suggests to me that:

• a convex form, like that where the rubber core meets the outer sleeve, would minimise stress on the rubber;

• by contrast, the thin tapering rubber lip (where the "skirt" makes contact with the inner sleeve) would be rather susceptible to cracking or tearing;

• the tapering design might have nothing at all to do with stress control, and exist simply to suit the moulding process.

However, I can happily admit that my own experience relates mainly to metals rather than to rubber, that my engineering degree was awarded a long time ago (1969), and that it is 20 years since I was last investigating stress cracking on a professional consultancy basis. For those reasons, I will certainly defer to anybody who has up to date knowledge and qualifications relating to stress control in shock absorber bushes.

Your experience probably remains relevant for metals as does mine for non-metallics the issue is that there are important differences in behaviours.

Convex shapes in the rubber can lead to ozone cracking where there is any preloading or stress (think of inflated tyres). The rubber (or elastomer) can be compounded to provide some protection from this usually with the inclusion of waxes which migrate to the surface; I think you might be able to see this in the photo of a new bush in post #11 of this thread? There can be permanent tension in the outer layers of a convex shape, this tension will induce a tendency to ozone cracking, once these initiate a fatigue crack may then propagate.

I have some published reference material that (if I can find) I will post that describes a study of bond durability performed at a respected test lab. This describes the benefit of the thin, tapering layer to the edge of the inner bush.

Its all very niche I know but if you step back and think about it many failures in cars are due to the non-metallic bits - belts, bushes, plastics etc...

 

[WitleyPanda]

If they are the same length etc they probably make no appreciable difference. You "can" get new rubber bushes or even poly bushes, but the shocks internals won't be much better so might as well change them.

Where “can” you get new rubber or poly bushes please?