To a lesser extent, the stiffness of the stem
will depend on the material used (modulus of
elasticity, E). For the stems tested, the
only sample that was made of a significantly
different material was the ITM The Stem, which
was constructed out of a magnesium alloy.
Typically, magnesium alloys have a modulus that
is 30% less than aluminum alloys. In order
for the lower modulus materials to have
equivalent stiffness, they must have a larger
extension radius, or a thicker wall. If
neither of these design variables are changed,
the stem will lose stiffness. The ITM stem
did not significantly change its geometry, and
as a result had a lower stiffness than the rest
of the stems tested.
This previous stiffness equation will
adequately describe the performance in the
vertical test configuration.
Looking at
the measured stiffness and the predicted
stiffness we can see the following relationship:
There is a very good relationship; most notably,
the Forgie stands out as the stiffest. Of
course, it is the stiffest because it has the
largest extension tube radius by nearly 20%.
Note that the oversized stems and the alternate
material stem (magnesium) are omitted from the
following plot.
It should also be clear that bar bolt pattern
has no material affect on stiffness. For
example, it is a common belief that a four bolt
pattern will be stiffer than a one or two-bolt
pattern. The TTT Forgie lays this notion
to bed. The Forgie uses a two bolt
pattern, yet has the highest stiffness measured.
Of course, the high stiffness of the Forgie is a
function of the large diameter of the extension
tube.
The following table summarizes the stiffness of
all the stems in both the horizontal and
vertical direction.
The average stiffness
are accurately captured in the final
stiffness ratings as well:
In summary, stem stiffness can be reasonably
well predicted by the following variables:
-Length (shorter is stiffer)
-Extension tube diameter (larger is stiffer)
-Wall thickness (usually pretty constant, and
thicker is stiffer)
Mass
The manufacturing community has gravitated to
mass as a key selling point. This is
probably due to the fact that it is the most
tangible concept that consumers can understand
and even measure. There is foundation to
the argument and mass does affect ultimate
cycling performance, but usually only in the
very specific circumstance of climbing steep
hills. Mass is generally a second order
affect (nearly ten times less significant than
other performance variables) on overall cycling
performance; however, it does play a role and
was therefore, measured and compared.
The range of weights for all stems covered a
mere 90 grams, with the ITM The Stem coming in
at 120 grams and the Forgie at 208 grams.
It is interesting to note that most of the
lightweight stems try to save weight in the
hardware, mainly in the bolts. Slight mass
savings were had in the faceplates and the
extension tubes. The following graph shows
the ratings of all the samples.
Durability
Two basic designs were used for the stems
evaluated. One style consisted of separate
components welded together at the steer tube
clamp and handlebar clamp areas. The other
design combined the clamps and the extension
tube into a single continuous piece. This
single piece was generally made by either
machining (cutting/removing) the shape out of a
block of material, or forging (forming the
material under temperature and pressure) into
its final shape.
There were distinctly different failure modes
and durability performance characteristics for
these two construction techniques. The
welded stems failed sooner and at or near the
weld beads. This type of failure is to be
expected, since welding does not guarantee full
penetration/fusion of both components and also
because no geometrical reinforcement is provided
for when using constant thickness extension
tubes. On the other hand, the one piece
body stems allow for a thickening of the walls
at the juncture of the clamps and the extension
tube – this is a good thing and is represented
by the superior performance of this type of
stem.
The one piece stems also failed in similar
manners amongst themselves. The primary
cause of failure in the standard bar clamp size
stems was due to stress concentrations.
The concentrations observed included thickness
transition points and cutting tool/forming tool
marks. The best example of a failure at a
stress concentration point can be seen on the
failure surface of the ITM The Stem.
Interestingly, the oversized stems performed the
worst in the durability testing. This is
the opposite trend that has been observed with
oversized steer tube forks. In the case of
forks, the larger steer tube contributes to a
measurable improvement in their strength
(assuming they have been manufactured well).
The stems, on the other hand, failed quickly –
though it should be mentioned that the failure
mode was not extension tube failure but instead
hardware failure.
The TTT Zepp XL tested was identical to the
style recalled by Cannondale late in 2002. This
stem was not recalled by the manufacturer,
though, and is still available from at least one
national mail order company. The reason
for the Cannondale recall was that the thread
engagement length for the faceplate bolts was
insufficient. A general rule of thumb is
that the thread engagement length for a fastener
should be at least 1.5 times the diameter of the
bolt. Thus, for the M5 bolt used on the
Zepp XL, the engagement should be at least
7.5mm. The engagement on the stem tested
was only 5.9 mm. The M5 steer tube clamp
bolt had a measured 7.5mm engagement, and the
other non-oversized stems all included hardware
that abided by the 1.5 times the screw diameter
engagement rule of thumb.
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