What Vance said seems to make sense, even if the crack is between the hub
carrier and the spring pan.
Take the situation where the trailing arm has compressed the spring and the
shock has finally halted upward movement (of the arm and wheel)
Everything then starts moving back downward again. The spring is compressed
and is pushing down on the middle of the trailing arm with a lot of force.
(as it always is) The shock is way out there at the far tip of the
trailing arm and is going to be exerting a force upward at that point.
(resisting the downward motion during rebound)
So the portion of the trailing arm between the spring and the shock mounting
point is going to be stressed in such a way that the bottom edge of the
trailing arm is going to be in tension - and so would crack just like is
shown in your picture.
During rebound - the stronger the spring constant and/or more amount of
shock damping - the more stress the trailing arm is going to see.
Just like Vance says.
Hitting the upper bump stop wouldn't make a crack like the one you have
there.
Even if you removed the shock completely and hit the upper bump stop very
hard. It is going to transmit force down around the hub carrier and if
anything - will start a crack in the opposite direction.
If you hit the lower bump stop really hard, you could start the crack like
you have though.
(which is the same thing as running out of travel on a tube shock conversion
- a sudden, drastic increase in the upward force out there at the tip where
the shock mount is.)
Playing devil's advocate - I suppose if you had a really stiff spring and a
really weak shock - you could start that crack during compression. It'd
have to happen by hitting a pretty hard bump and jolting the suspension
though - (made worse if the hit happens with the suspension already
compressed.)
For my two cents - the chipped arm causing a stress riser was the real
problem. Stress risers really have a dramatic effect on things.
If it is a know weak point - then my question is - why not reinforce it even
on the new arm you install?
--
Scott Tilton
(Signature below blatantly stolen from Bud Rolofson:)
70TR6 CC5#### (Good 6)
63TR4 CT157## (The Wreck-Almost Parts)
64TR4 CT25### (The Project)
97F-150 L TRUCKO (Triumph Support Vehicle)
1989 Honda Prelude Si 4WS (economic commuter Vehicle)
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