(Fatigue) Crack Rear Lower A-Arm

That is a great post.  Thank you for sharing.   I am glad it was a discovery prior to full separation!

It is the first example of rear a-arm fatigue that I have seen. Anecdotally, I have heard of cracks caused by the bushings, located at the lower control arm/hub carrier, freezing up and the control arm forced to accept the flex.  This looks like the bushing connecting the a-arm to the frame was stiff and the fighting the shock absorber's rebound.  Or perhaps someone tried to jack the a-arm from underneath or road debris struck it.  The later seem unlikely w/o collateral dent damage.

Do you have a hypothesis as to the cause?

I guess it is a combination of things.

Looking at the crack, location and growth pattern it has to be a fatigue crack which started at the top side of the tube and not a crack due to a single moment of overstressing. I would therefor consider it unlikely that is was damage from jacking although this could have initiated the crack. I would rule out road debris as a cause for this.

For a fatigue crack to grow which can be at a very slow pace, there has to be a cyclic (tensional) force. I guess it is the horizontal component of the shock force exerted on the flange during the movement of the suspension. This force provides a bending moment (Force x height mounting flange hole) and thus tension almost exactly where the crack started, which is on the top side of the tube.

I would also assume that the weld of the flange might have decreased the skin thickness there locally , thus increasing the tension in the material.

It could there for very well be that other A frames, where the weld has been made slightly different, will not exhibit this cracking.

And lastly, the location of the shock mounting on the lower A -arm is not ideal as it  as it is not in line with the wheel axle thus introduces a torsional force on the  A frame as well.

Maybe others have some ideas as well?

I see a grease nipple in the lower shaft, I wonder if it has been rebuilt and at some time in it's history had the common shaft bind in the hub, which would have put extra stress on the a-arm.

Hydrogen embrittlement from welding in the presence of moisture. This is a guess since i would think we would see more of this on other Panteras.

Joep, your a-arm is the second cracked one I've seen in 40+ years of messing around with the car, which includes wrecks. So I suspect its not a common defect. And that one was also a lower rear, which is the most highly loaded of the a-arms on a Pantera. So if I was especially worried about a-arm cracking, my attention would be upon lower rear a-arms. Finally, their saving grace is while being quite light, Pantera a-arms are mild steel that easily gas or arc welds. So maybe some owners that find this problem simply fix them.

Hi BW,

Thanks for your reply, good to hear that it is not a common problem. I think it is probably due to the weld why the A arm had this fatigue crack. I dont have shaft bind now but indeed it could have done so in the past wrt joules's comment. I own the car now for a about a year, so do not have much on the history other than what I have found out so far...My RH lower A arm is good although it shows signs of jacking on the a arm .

In the mean time, I have made a repair of the arm by (mig)welding and adding some extra (sheet)material = thickness, thus lowering the tension levels. Will monitor it over the coming months but for the short term it should be ok.