Thanks guys for the input.
I have repaired it in a fashion that increases the plate from 3mm to 6mm hopefully this will banish this gremlin.[ although if I make this area strong enough it only moves the stress elsewhere ?]
So I need to find out why it happened ?
a) so will it happen again ?
b) can I stop it ?
c) is tilting a contributory factor ?
Looking into the side of the joint these are the forces/loads attempting to push the joint into the ground.
Now looking at my construction
1)the 3mm right angle is of infinite length compared to the 3mm plate
2) the M12 bolt and rod end bearing is also infinite length compared to the 3mm plate
This is the reason it broke where it did and not across the bolt hole , which theoretically was weaker due to the M12 hole in it.
Other forces are also at play
On the left the 2 wheels at the rear are on level ground and so the frame is horizontal and so the front wheel is vertical [ unless turning ] .
On the right the left hand wheel has encountered a bump causing the frame to tilt upwards , this tilt is transmitted to the front through the pivot and attempts to twist the front end to follow the rear.
Of course the 3mm part of the plate feels these forces the most a tearing action across it.
gee it has a hard life
So to tilting ?
Now tilting involves the elastomers allowing me to go off centre , now my gut feeling is that as long as both rear wheels stay planted on the ground the left example above tilting has no part to play in the failure.
Unless tilting causes one of the wheels to lift off the ground then we are at the right example above and so tilting could cause tearing forces across the 3mm plate.
To the best of my knowledge I have never had the Tilt#6 on 2 wheels.
So am I right ?
Where are the forces going now ?
What will happen tomorrow ?