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gw302 17th May 2020 10:04

*rear wishbone failure*
2 Attachment(s)
Hi all,

While driving through Cambridge the rear bottom wishbone snapped in half. Thankfully this was at low speeds (20 mph) and both I and the car were not damaged further, the AA also managed to successfully recover the car (the relay option is great).

I'm not sure why this happened yet. My guess is that it has fatigued and I was lucky it went when it did (there wasn't a collision prior to failure). It doesn't appear to be an obvious welding issue as it has broken through the tube and not along the weld line, which makes me wonder if the area needs to be strengthened. A very simplistic structural analysis (attached) confirms that this region is the area of highest stress on the wishbone under acceleration.

Has anyone else had any problems with this? The car is running the latest suspension setup from Marlin.

Grey V8 Pete 17th May 2020 11:11

Good job it happened at low speed! I would guess that the crack started along the edge of the weld in the heat affected zone. I would want to replace that flat gusset with a U-section piece, preferably wrapping it around the outer tube as far as possible on replacement components, as you need to suspect the other side of the car is also at risk of failure. So replacing both would be a good idea. I have experienced three failures over the years when accelerating from standstill. Two were half shafts (on a classic car) and one was a diff mount. There is a lot of stress on drive train and suspension components at this time. Peter.

bigrich 18th May 2020 19:10

Rear Suspension ?

Not seen that failure mode before on an exi. I'm running the original suspension with my own toe control mod but this lower wishbone is still standard. How many miles have you covered? 21k on mine and no signs yet.....I must inspect closely though next time I get her out.

Good it was low speed :)

Cheers Rich

gw302 19th May 2020 00:37

I'm definitely going to strengthen the lower wishbones. This is actually the second time one has snapped (in exactly the same place). The first was when I was rebuilding the car (engine swap) and I was pushing the bushes out by hand, levering the wishbones against the floor admittedly, but it snapped clean through. Not sure if I just have bad luck with them but I can't think how my setup could be causing this failure!

I also need to mod them while I'm at it: the on-board pick up points mean I have hardly any droop in the suspension at all before the wishbone collides with the bottom chassis rail. The maximum I can jack the car up is to about 110mm at the rear - the suspension travel is then all in compression. I made sure I altered the dampers to the correct length (so the wishbone doesn't actually collide with the frame).

Good to give it an inspection though.

Cheers, George

alackofspeed 16th November 2020 21:33

I had one fail on a track day. Crack propagated from the toe of the weld. When it failed undiscovered how thin the walls were and the tube was seamded. I remade using CDS and a heavier wall.

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