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Old July 12th 18, 04:59 PM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default drill/tap in frames

On 7/12/2018 11:39 AM, Jeff Liebermann wrote:
On Thu, 12 Jul 2018 00:22:57 -0700, John B. Slocomb
wrote:

On Wed, 11 Jul 2018 20:16:33 -0700, Jeff Liebermann
wrote:
Nope. I'm not trying to measure if the tubing is strong enough.
Assuming identical lengths of tubing, I wanted to see if the presence
of a Rivnut significantly changed the tension required to bend or
break the tubing when compared to the identical tubing that did not
have a Rivnut inserted. If I'm able to pull hard enough, I should be
able to eventually break both tubes. If they break at the same
tension, then I'll declare the Rivnuts are safe to use. If there's a
substantial difference in tension, then I'll declare the Rivnuts
weaken the frame.


I'd think it obvious that any hole drilled laterally into a tube would
have an effect on the bending strength of the tube. The question
wouldn't be whether the rivnut changed the strength of the tube but
whether the tube was strong enough with the rivnut installed in the
tube.


Again, I expect some reinforcing effect from the Rivnut's clamping
action. For an analogy: Did steel frames fail in significant numbers
when water bottle bosses were brazed on and tapped? I doubt it. While a
crimped-on Rivnut wouldn't add as much strength (as a guess) I think it
may add enough to get the strength back up to that of the un-drilled tube.

Also, I'm pretty sure a bike down tube sees little if any bending
stress, except perhaps in a crash. The stresses of concern are torsional.


Agreed.

Reading between the lines, what others are apparently suggesting is
that even with a hole drilled into the frame, the tubing is still
sufficiently strong to consider the bicycle rideable. In other words,
if the drilling a hole and installing a Rivnut decreased the bending
strength by 10%, I would agree that the hole and Rivnut don't pose a
risk. However, if it decreased the strength by 50%, I would consider
it a hazardous modification. I'm not too sure what to do about
numbers in between or even if the 10% is realistic.

To complicate matters, there's the problem of the notch required by
Rivnuts to prevent rotation. That's a stress riser by anyone's
definition and will probably be the start of any break during testing.
I'll try to position it where it will do the least damage.


The notch is small enough to be enveloped in the crimped portion of the
Rivnut. I doubt that it's effective as a stress riser. Really, I doubt
that it feels any significant stress, other than compression from the
clamping or crimping action.

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- Frank Krygowski
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