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MG MGF Technical - Cracked/broken wishbones

Hi, its been a while since I posted here but had a shock this week 2 days after the car was serviced/MOTed. This could be a heads-up for others with a similar setup. Car is a 2000 registered 1.8MPi with about 45000 miles on it and I had B&Gs poly-bushes fitted 2 years ago.

I had to have both front wishbones replaced this week after one end of one snapped right off (drivers side). Fortunately I was only going slowly over a speed-hump near my office at the time. The other was also cracking. Brown & Gammons (who took pictures) said they had never seen this type of damage before but probably due to the poly-bushes making this area _too_ stiff. Replacement parts were also hard to get hold of.

Pictures of the damage should be viewable at http://www.kjkapur.demon.co.uk

Anyone else with a similar setup and who has to negotiate lots of speed bumps might be advised to check this - as you can see it went in a difficult to see area (cracking starting on the top near the bush).

Cheers,

Graham.
Graham Berridge

Graham what is really good to have in cases like this are detailed macro photos of the fracture surface. The photos you provided a link to are not really detailed enough to get a good indication of what went on, but from the 1st photo in the link certain shiny? surfaces might indicate the crack started at the bottom of the wishbone. As you say the wishbone failed over a low speed speed bump I would expect the failure had started far earlier and may be caused by stress corrosion cracking. Have you always owned the car, it may be the car had been badly kerbed or similar and started a crack in the past and had been propagating during your ownership.
David Billington

That was some nasty pictures ! It seems that the bush has gone solid into the frame-mounting and have been unavailable to move at all. Correct me , but shouldnīt the metal parts on the poly-bushes be stainless steel just to prevent this from happening ?
Would be interesting to know from dissmantling if it is the inner "tubing" to the bolt on the polybush or the sidewalls that has seized solid.
My wild guess is that if the arm could twist on the poly material only you would have had a very stiff suspension, but the arm would still cope with it. Here it seems that the whole thing has gone totally solid and the only movements has been flexing of the arm.
Luckily it didnīt let go on the motorway...
What are the proceedings with a thing like this in UK when you had a MoT just a few days before ? / Carl.
Carl Blom

what carl says.

Rubber bushes flex, polly bushes do not.

The effect of a rubber bush happens by locking the inner and outer surface to the local surface and as the suspension moves the rubber twists

The effect of a poly bush is that it stays solid and acts as a bearing on a stainless steel insert, doing up the bolts grips the insert solid, but the bush is free to rotate around it.

The loads thru the bushes are all cornering and breaking loads. The suspension loads all go thru the top arm.

This does look suspiciously like the bushes were prevented from twisting
Will Munns

With you on the idea of stiff _bearing_.

Graham, from what make where these bushes. It isn't Powerflex, cause they were blue coloured.

Anyway, I wonder why there was no movement possible.
May be the inner stainless steel pipe was to short, or just rust blocked them ?
Dieter

With both sides fsiling together, it is apparent that there was either a problem with the parts or the fitting of them.
The parts need to go to B&G for an explanation why together with the Bill for fixing it.
We would await the outcome with interest.
Geoff F.
G. Farthing

Brown & Gammon had the car back to fix and showed me the broken part (and were the ones who took the pictures) - about 30% of the broken surface was shiny so the rest must have been going slowly. The bushes are B&G ones, not Powerflex - they did not refit with the new wishbones!

I've had the car since it was 1 year old: no bad impacts. However the speed humps (7 in all) on my office site are quite extreme: supposedly 20mph limit but 10 is an upper limit for the humps so a year of these ....

Graham.
Graham Berridge

actually, it's usual for the shiney bit to be the old break, new breaks look dull like glasspaper and very old breaks look rusty.

BTW, when we had to replace the pans due to rust, they came next day.
Will Munns

This thread was discussed between 28/04/2007 and 06/05/2007

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