Reliability of Steerer Materials

On 9 Aug 2004 13:05:38 -0700, (Chalo) wrote:



It's not a precision quantity in question; it's big. Tandems have
been known to generate braking forces that flex the fork back enough
for the front tire to scuff the downtube, *without* permanently
bending the fork.

I'm surprised to hear that this flex occurs in the steerer and not
mainly or exclusively in the blades.

As I said before, I have bent many fork steerers
myself, and CrMo steel does not yield before first flexing deeply.

Does this bind the head bearings in any way.

You must be exceedingly lightweight if you have not perceived steer
tube flex in your own bike.

I'm on the slightly light side for a normal guy of average height, or
the average size for a bike racer.

Even then you must be noticing something,
but attributing it to some other cause.


Maybe. I have a bike with a carbon steerer tube that I don't like as
much, but there are a lot of other things about it that differ from my
bike with an aluminum steerer.

JT

John Forrest Tomlinson wrote:

Lou Holtman wrote in message ...
John Forrest Tomlinson wrote:

On Mon, 09 Aug 2004 14:07:10 +0200, Lou Holtman wrote:

Believe me the steerer tube flexes.
You can easily think of a more
precise method yourself to measure this.
Use your imagination.

Have you used a precise measuring system?

I don't have to I can calculate how much it flexes.

Interesting. Not persuasive, but I guess we'll have to trust you.


If you think you can see it by eyeballing it as you suggested it's
likely that is is _your_ imagination that is producing the effect.


You can use a steady reference and a little help from a second person
who pushes the fork against a wall for instance.

Have you ever done this? Have you ever measured this effect?


Fix your frame and fork rigidly (BB shell across the jaws of a bench
vice will work) so that the steerer is horizontal. Add some weight to
the fork ends and measure how much they bend back. You will see that
there will be considerable movement at the weights typical of a hard
pull on the brake, and if you remove your stem you will readily see the
steerer tube bending. Headset bearings are designed to accommodate this
misalignment because of the cup and cone design, which is very
appropriate for that application.

Braking very hard produces a backwards force on the forks roughly
equivalent to your weight, plus the weight of your bike.

John Forrest Tomlinson wrote:

On 9 Aug 2004 13:05:38 -0700, (Chalo) wrote:

It's not a precision quantity in question; it's big. Tandems have
been known to generate braking forces that flex the fork back enough
for the front tire to scuff the downtube, *without* permanently
bending the fork.

I'm surprised to hear that this flex occurs in the steerer and not
mainly or exclusively in the blades.

There are two blades, often longer in the relevant axis than the
steerer. They do flex, but the steerer flexes more by virtue of its
location immediately adjacent to the head bearing (which serves as a
fulcrum), its limited size, and the fact that there's just one to take
the load.

As I said before, I have bent many fork steerers
myself, and CrMo steel does not yield before first flexing deeply.

Does this bind the head bearings in any way.

Headsets perform some ball-and-socket type motion in addition to
swiveling on axis. Jobst Brandt has repeatedly pointed out that
Shimano headsets have a spherical bearing interface between the
cartridge and the frame-mounted cup, in addition to the ball bearing
elements within the cartridge (and that this is a good thing for their
longevity).

Generally speaking, though, steerer flex is hard on head bearings.

Even then you must be noticing something,
but attributing it to some other cause.


Maybe. I have a bike with a carbon steerer tube that I don't like as
much, but there are a lot of other things about it that differ from my
bike with an aluminum steerer.

Carbon steerers vary somewhat in their bending stiffness, but to my
knowledge they never approach the effective stiffness of steel--
although in principle they possibly could. The carbon steerer on your
bike may be no stiffer than the aluminum one on your other bike, and
might be even less stiff.

Many riders happily tolerate lots of flexibility in their bike frames
and components. As long as it's not symptomatic of a dangerously
underbuilt bike, it's really just a question of taste. My personal
experience is that a stiffer bike maneuvers more predictably and turns
pedal effort into speed more effectively than a flexible one. Rider
weight and riding style no doubt have a huge part in this.

Chalo Colina

The True Temper Alpha Q is a different all-carbon fork in that the
carbon fiber is of the high modulus type and the majority of the fibers
are unwoven, running from the dropouts all the way to the end of the
steerer. The blades are then wrapped in a layer of woven carbon and
the steerer has a mid wall wrap of fiber ina helical pattern to resist
torsion. Because of this use of high modulus fiber and orientation, the
stiffness of the Alpha is considerably greater than steel. Note, some
carbon fiber can have elastic modulus twice that of steel. With use of
low void processes, the finished product can have a modulus 20-50%
greater than steel.


--
Weisse Luft

On Wed, 11 Aug 2004 04:49:52 +1000, Weisse Luft
wrote:


The True Temper Alpha Q is a different all-carbon fork in that the
carbon fiber is of the high modulus type and the majority of the fibers
are unwoven, running from the dropouts all the way to the end of the
steerer. The blades are then wrapped in a layer of woven carbon and
the steerer has a mid wall wrap of fiber ina helical pattern to resist
torsion. Because of this use of high modulus fiber and orientation, the
stiffness of the Alpha is considerably greater than steel. Note, some
carbon fiber can have elastic modulus twice that of steel. With use of
low void processes, the finished product can have a modulus 20-50%
greater than steel.

I thought the steerer was cured first, then bonded (co-molded?) in a
rooting process? In other words, that the fibers don't run from tip to
top, but the steerer tube ends inside the fork crown?