Front Wishbone Failure due to Polybushes?

Here is a picture of my failed wishbone.

And a close up of the fracture surface

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The fracture surface shows that a crack has propagated from both sides of the arm over time (the original failure surface has gone rusty) and this must be due to cyclic bending stresses on the arm.

Give yourself a point if you've just gone " wait a minute, there shouldn't be any significant bending stress on a wishbone. Correct, as both ends of the wishbone are effectively pin jointed, it should only see a compressive / tensile load. If you try to apply a bending moment the arm is supposed to move.

What I think has happened is that the poly bushes I used are slightly oversize and a very tight fit in the subframe. When I tightened the through bolt it clamped the bush in the subframe and stopped it rotating. This means that every time I hit a bump it put a huge bending load on the wishbone and eventually broke it.

Just to be clear, the wishbone is supposed to be free to rotate and the clamping bolt is supposed to clamp on the sleeve through the middle of the bush and the poly bit is supposed to rotate on the sleeve - this is why they give you grease with the bushes.

For info it took about 18 months but only about 10K miles to break.

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Front Wishbone Failure due to Polybushes?