I am not understanding the physics behind a "too short" damper
causing a trailing arm pin to fracture like that, as in Spider's picture...
The saloon shocks look to be 7/16" shorter, or about 11mm in French.
If the suspension bottoms out, then surely the stress points would be
the 3/8" diameter bottom shock mount pin and the top rubber mount
going through the wheel arch?
In the normal course of events, yes, there is a load on the trailing-arm
shaft basically because the shaft is used as part of the "geared" linkage
to get the spring horizontal. In a standard system, there will be a rear
to front loading on this shaft assembly. The extremes of angular movement
will depend on the open/closed length of the damper.
Why would a reduction in angular movement, caused by a "too-short"
damper cause the pivot pin to fail?
Looking at Spider's pictures, I am thinking that either the shaft was machined
with too sharp a corner between the 1/2" diameter and the large diameter.
This sharp corner has introduced a stress crack which has caused the
failure.
Or:
The assembly was put together with some of those shims that they supply
in some kits. These are intended to make allowances for wear on the thrust
faces - A "too-short" shaft is supplied and then shimmed out to give the correct
amount of end-float between the main thrust washers.
If the nut works itself loose, these razor sharp shims will cut their own
sharp corner in the shaft, leading to the same problem as above. The
thread on Spider's shaft certainly looks distressed, so to me, this
looks like a contributory factor.
Maybe the nut was just done up too tight in the first place and the damage
was done on assembly? (Because of poor quality of the components).
The threaded section appears to have some kind of double-set in it...
They make these shafts out of cheese these days and often the hardening
is virtually non-existent.
Ian