I think the geometry makes some angle change at the top of the stanchions unavoidable - see this, which shows the principle, but is not necessarily a realistic representation of the actual change in angle on today's GS design.
I would say there are three ways in which the crimps could come under pressure.
The first is longitudinal. However, I find it hard to believe that the simple tromboning of the telescopic parts would be able to exert sufficient longitudinal pushing or pulling to put pressure on the crimps.
The second is where hard braking would cause the lower slider to try to rotate around the ball joint on the suspension arm which carries the damper, and this movement would be transmitted to the top of the stanchion, and opposed by the joint links in the top yoke. In this case the pressure on the joint would be exerted perpendicular to the direction of the stanchion, and I would think the joint link is fairly well designed to resist this. It also puts pressure on the crimped section, but not, IMO, in a direction which could easily distort and loosen it, though it is definitely a possibility.
A similar rotation effect, possibly of higher magnitude than braking, could be caused by an impact high up on the front of the wheel, but I think this is more in the realms of a crash impact than normal use, in which case I think all bets are off anyway.
This leaves the third form of motion where the angle of the stanchion with respect to the yoke changes due to the geometry of the suspension changing as the suspension arm moves up and down. The ball joint attachment to the forks moves through an arc which has the effect of pushing and pulling on the forks which slightly changes the angle at the point where they attach to the yoke.
The pressure from this angular movement at the top of the stanchion tends to tilt the inner part of the joint link with respect to the outer, and there is a very limited amount of movement available. If this movement is fully used up in extreme suspension deflections, then it could lock the top mount, meaning that any further change of angle can only be accommodated by flexing in the stanchion/slider assembly. In my opinion this sort of angular flexing is the most likely sort of stress to do damage to the crimps by rocking the joint back and forth and gradually deforming the top of the stanchion.
Fred
I agree with almost all of that and it all seems to make sense - the only bit I am struggling with is being able to quantify any angle change of the slider w.r.t. the bush, that bush doesn’t see to have much compliance at all. I have stared at that animation long and hard but I have yet to be convinced that on a real bike there is any significant change at all - I guess I’ll have to bite the bullet and do some measurements on my bike
The resonant thing I found quite interesting, one could imagine a horizontal ‘thump’ on front wheel sending a shock wave up the tubes and done repeatedly I found imagine it stressing the crimped section. I have seen damage to surface mount capacitors on printed circuit boards suffering fractures as a result of a shock wave passing along the board due to break out or HALT testing.
