On 4/12/2019 11:15 PM, Joy Beeson wrote:


I don't recall whether my handlebars came with the Raleigh Carleton or
the Fugi Grand Tourer -- we swapped components rather suddenly when a
mechanic overhauling the Raleigh found a defect in the brake bridge
that was above his pay grade. I don't think I ever knew whether the
handlebars and stems were included in the swap; we just left both
bikes with the mechanic.

It says "GB" in a circle in a recessed diamond on the stem.

Just above the point of the diamond, an adjusting barrel for the front
brake cable is set in the top of a hole through the stem.

I read somewhere that drilling a hole through the stem is a terrible,
horrible idea, and it makes sense that one should avoid weakening such
a vital component, but it's a tempting idea: It looks neater, it
makes the path of the cable slightly smoother, and it makes the cable
a more convenient place to hang my gloves.

So as long as the hole is there I might as well use it, but the
mechanic at the Trailhouse routes the cable around the stem, as if
there were no hole.

I'm going to have the bike overhauled pretty soon, and I'm thinking of
telling him to use the hole -- but is there some *other* reason why
routing a brale cable through the stem is a bad idea?

The hole causes what's called "stress concentration." That means when
there's a significant load on the stem, stresses will be higher in the
area right around the hole than they would have been without the hole.

Stress concentration is more of a problem for aluminum than for steel,
for reasons I won't go into here (unless asked). But it's not
necessarily a significant problem. Everyone accepts holes in aluminum
hubs (to attach spokes). I have holes drilled in my aluminum frame to
attach shift levers, bottle cages, etc. The problems arise only if the
concentrated stresses are higher than what the metal can resist for the
long term.

I gather from your posts that you're probably not a person who does lots
of high torque pedaling on sprints or tough climbs, where you're yanking
hard on the handlebars. Those yanks would put torque on the stem and
might generate some significant stress. But I doubt you'll see a problem.

More significant, perhaps, is that the hole is already there. If you
don't use the hole it doesn't improve the situation; it's still a stress
concentration. So leaving the cable out of the hole seems silly to me.

So I'd use it as intended. If you want to be super-diligent, check it
once a month to see if any tiny cracks are emanating from it. If you
want to be paranoid, replace the stem.


--
- Frank Krygowski