On 5/26/2019 8:55 PM, Jeff Liebermann wrote:
On Mon, 27 May 2019 07:24:56 +0700, John B.
wrote:

On Sun, 26 May 2019 15:23:41 -0700, Jeff Liebermann
wrote:
More on Rivnuts:
http://www.pardo.net/bike/pic/fail-001/FAIL-178.html
Frame failed at water bottle boss. This is an aluminum frame
with a crimp-in "Riv-Nut"-style threaded boss. The frame is drilled
and the boss installed; the hole slightly weakens the frame and
the crimping creates some local stresses. A typical alternative
is to weld in a boss. In some cases, the weakening and stress-riser
effects of welding and of the boss section may weaken the frame
more than drilling a hole and using a Riv-Nut.

Yup, that's one. If I had my camera I could post you a photo of a bike
that must be 15 years old now with rivnuts in the down and seat tubes.
No failure yet.

The problems start when you drill a hole for Rivnuts on the bottom of
the top tube. All the tubes on a bicycle are both in tension and
compression depending on weight distribution. However, the down tube
sees the most tension which is why it's often a larger diameter than
the others.

If a Rivnut hole were drilled on the top half of the down tube, the
hole would be mostly in compression. As long as there are no sharp
burrs around the hole, the Rivnut should be strong enough to resist
compression. If there were any failure here, it would be due to
buckling, which is unlikely.

However, a hole drilled on the bottom of the down tube sees more
tension, which tends to create or open cracks along the circumference.
The strength of the Rivnut does nothing because there is no pressure
on the Rivnut when the down tube is in tension. For strength, the
Rivnut might as well be absent. Once a tear or crack starts, it's
just a matter of time before the down tube fails at the hole.
Fortunately, most water bottle mounts are attached on the inside of
the main frame triangle, which is the part that is mostly in
compression.

What holes in frame tubes under tension do is decrease or remove the
safety factor designed into the various tubes. The safety factor
varies with class of service. If low weight is paramount, the safety
factor will be quite low. If safety and structural integrity is
paramount, such as in children's bicycles, the safety factor will be
very high. When holes are drilled in such structures, the resultant
structures are usually strong enough to support themselves and the
rider, but with a lower safety factor. A hard landing or repetitive
impacts might exceed this lower safety factor and cause a failure.
These will survive if treated gently:
http://bicyclepatents.com/a-bad-case-of-drillium/1293/

But never mind the rivnuts the damned front fork broke, and the rims,
and what appears to be the bottom bracket.

They just don't make them like they used to.

For every advancement in bicycle material science, finite element
modeling, assembly methods, and testing, there is an equal and
opposite tendency to build bicycles as close to the point of failure
as possible. The design goal is for everything to fail
simultaneously. Otherwise, the surviving components will be deemed
"over-built" with an excessive safety margin.



None of that's wrong but the torsional cyclic forces on a
downtube are significant whether the anomaly is top or bottom.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971