The picture above shows typical stress cracking due to the lack of post weld heat treatment. The tube below the crack has peeled outwards due to the stress induced as the weld material shrinks on cooling. Stop looking for answers in the design or use of the component and ask a metallurgist for an educated opinion. Look here to see how I fixed mine
http://www.bleazey.co.uk/M3W.html#chassis I have also stress relieved the other, uncracked, side to avoid future problems. All good so far, after 4,000 miles.
PS. Not the picture which is obviously the result of high speed application of a lamp post or some such unrelated disaster.
