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Old July 11th 18, 09:58 PM posted to
Frank Krygowski[_4_]
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Posts: 6,042
Default drill/tap in frames

On 7/11/2018 2:38 AM, Emanuel Berg wrote:
Frank Krygowski wrote:

There also seems to be a horizontal welded
tab protruding from the seat tube above the
chainring. (Or underneath the chainring in
the photo of the inverted bike.) That may be
a mounting point for the top surface of
the chainguard.

The chainguard also seems to have a bracket
that would connect the chainguard's bottom to
the underside of the bottom bracket area of
the frame. Perhaps that's the spot he wants
to drill and tap?

The chainguard typically has a three stays.

One down below, to the bottom bracket. There is
a threaded hole both ways, i.e.
two such occurrences.

One stay front. There is usually two threaded
holes on the stay/chainguard interface.
But here there is more variation as sometimes
it is a bent bracket screwed into the frame,
also threaded with no nut. But sometimes the
bracket appears to be an extention of the
frame, I suppose it is welded as it is
a steel frame.

Then there is a stay at the back. This is
sometimes a likewise extention of the frame as
above (with a single threaded hole) but
sometimes it is a loose part, a bracket bent
around the seat stay tube, much like the
component that connects the rear hub brake arm
to the chain stay. If this is the case, there
is no threading to it, save for the bolt that
has a nut on the other side.

Actually this is not what I had in mind for
drilling and tapping, that was a bottle cage on
the down/diagonal tube. I put it there with
cable ties ("wires") but there was an annoying
sound while riding the bike.

So your real objective is to fasten a bottle cage to the down tube? Then
you should definitely not just drill and tap the frame tube itself. A
Rivnut can work. So can various clamp arrangements - yes, including
screw-type hose clamps.

The chainguard
stay example I brought up to contradict the
claim that it cannot be done because of too
thin tube walls. The stays are not very thin,
and besides they are screwed into the frame, so
there are possibly two, at least one example
how a small width would suffice. I write
"possibly" because it was brought up possibly
the tubes are butted. I doubt that because
these bikes are commuter bikes, solid, but not
optimized. But I suppose anything is possible.

The normal rule of thumb - i.e. threaded portion at least equal to the
diameter, and preferably 1.5 times diameter - is sometimes successfully
violated. It depends on the magnitude and direction of the loads, as
well as the consequences of failure. But I wouldn't try tapping the down
tube of a bike.

- Frank Krygowski

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