Re: tech: physics and materials

On 8/21/2024 11:05 AM, AMuzi wrote:

On 8/21/2024 9:07 AM, John B. wrote:

On Wed, 21 Aug 2024 07:55:51 -0500, AMuzi <am@yellowjersey.org> wrote:
>

Our customer wants higher handlebars.  We sold him a new
bike and as usual I asked for dimensions from post to
handlebar on centers and handlebar to ground from his old
bike. That's a tall riding position:
>
http://www.yellowjersey.org/daily.html
>
Since the new model has a carbon steerer, I swapped in an
aluminum column carbon blade fork of same dimensions.
>
I reasoned that modern aluminum bars are farther from the
stem clamp (stress riser) to the bottom, where climbers pull
forcefully, than from the top bearing to the stem.
Handlebars are thinner than columns with cold formed curves
of various radii while columns are a simple cylinder.
Failures in handlebars are rare now (usually precipitated by
crash damage, corrosion or both) as are threadless column
failures (I have never seen one) so I can't reasonably
assess frequency. This was intuitive not calculated.
>
The new wrinkle is that after 50 miles he says another 50mm
would be better. Yikes! That's a lot, and the local shop
refused to add an extender:
>
https://www.yellowjersey.org/UPSTEM.JPG
>
They also refused to run longer gear cables and brake line
if he installed the extender himself.
>
I was OK with the bike as delivered but I'm not so sure
about more height. Comments?

>
If I was in business and a similar question came up I believe I would
contact the manufacturer or U.S. Distributor.

 I did, naturally.
Carbon steerers warn about maximum stem height over top bearing (and reasonably so IMHO) but there aren't firm limits for metal columns.

That's a very tall setup, all right. I think the bike is severely undersized for the rider, and a much bigger frame would be the real solution. I'd be very wary of going any higher - not that I have your experience viewing failures.
This phrasing confused me: "modern aluminum bars are farther from the stem clamp (stress riser) to the bottom, where climbers pull forcefully, than from the top bearing to the stem."
But loads applied in bicycling are largely unknown and vary tremendously with the individual, which makes any attempt at calculations impossible. Will this guy really be pulling on the bars while doing steep climbs?
Vaguely related: Our Bike Friday New World Tourists
https://www.flickr.com/photos/16972296@N08/7410976626/in/dateposted-public/    have extremely tall gooseneck stems, made of steel. Mine flexes disconcertingly when I pull on the bars while climbing. I'm told it's also tough on the headset.
--
- Frank Krygowski