On Thursday, February 1, 2018 at 10:24:16 AM UTC-8, Joerg wrote:
Yesterday on the MTB I had to look downwards between my legs to see what
made a rattling noise on the bike, saw some brush tangled in the rear
and hit both brakes quite hard. That's when I noticed how much the upper
diagonal strut in a Horst link bends when applying a lot of brake force.
The center of it bows down several tenths of an inch and also outward a
little. It's a pretty beefy strut:

http://www.analogconsultants.com/ng/bike/Muddy4.JPG

Similar on other bikes:

https://ep1.pinkbike.org/p5pb9586988/p5pb9586988.jpg

One can see such bowing also on aircraft wings which as built and tested
to high stress standards. They make the spars out of stuff such as 7178
which I assume bike mfgs don't:

https://www.popularmechanics.com/fli...-stress-tests/

Has anyone else with a Horst link bike taken a look while applying the
rear brake hard? Can this fatigue the strut to the point where it
eventally breaks? Should I shore that up with maybe an L- or U-profile
strapped around it?

I am asking because I use my MTB for transportation a lot and ride about
2000mi a year on it, hard, not just the occasional weekend loop. It'll
see hundreds of such strut load cycles per ride.

Yes, everything breaks after enough fatigue cycles, particularly aluminum -- which has no fatigue threshold. Even small amplitude fatigue cycles will affect aluminum. It will break. You will died. Mountain lions will eat your corpse. Or it will break and you'll buy a new one, which is what most people do. If this is a problem, buy a steel hard-tail.

-- Jay Beattie.