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Rec.Bicycles Frequently Asked Questions Posting Part 5/5
Section - 10.1 Suspension Stems

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 5/5
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by Brian Lee & Rick Brusuelas, 1994

ABSTRACT:  Discussion of the differences between suspension stems
and suspension forks, and a listing of the pros & cons of suspension
stems.
       
DESCRIPTION:  The suspension stem discussed here is the
Allsop-type, which employs a linkage parallelogram and a spring
mechanism to effect shock-absorption.  Two models on the market
using this mechanism are the Allsop Softride, and  a version
produced by J.P. Morgen, a machinist based in San Francisco.
There is also a version put out by J.D Components of Taiwan 
(advertised in Mountain Bike Action), however judging from 
illustrations, this unit does not employ the parallelogram design 
shared by Allsop & Morgen.  Other Taiwanese models may also exist.
The Girvin-type stem, which uses a simpler hinge and bumper, will
not be directly addressed here, although some of the comments
may also apply.

The Allsop-type suspension stem (suspension stem) works on a
different principal than a telescopic shock fork.  Instead of
only the front wheel moving to absorb shock, a stem allows the
entire front end of the bike to move with obstacles while the
rider's position does not change.

All suspension requires some form of "inertial backstop" to
operate.  A theoretical suspension (stem or fork) loaded with
zero mass will not function regardless of the size of obstacle
encountered.  This is because there is nothing to force the
compression of the spring mechanism.  It is essentially locked
out.

In a fork system, the weight of the bike & rider both provide the
inertial backstop.  In a stem system, the rider's weight on the
handlebars provides the backstop.  Because of this, the two
systems "ride" differently.

Since most of the weight comes from the pressure of the rider's
hands, the stem system encourages a more weight-forward style of
riding.  Or perhaps placing the stem on a frame with a shorter
top tube so the rider's weight is distributed more on the front
end.  (Shortening the front end has also been applied by frame
builders on frames intended for use with suspension forks.  Ex:
Bontrager.)

What does this mean to you and me?  It means the suspension stem
requires a certain amount of the rider's weight to be on it at
all times in order to remain completely active.  For the majority
of riding, it's just fine.  The only difference is in extremely
steep descents, where you are forced to keep the weight back in
order to keep from going over the bars.

In this situation, much less weight is on the bars to activate
the stem.  Further, if one were to encounter a largish rock on
such a descent, what does one do?  The instinctive thing is to
pull back a bit to unweight the front and help the front wheel
over.  This removes all the weight from the stem area, and you
are now riding a rigid bike again.

A fork system is also affected by weight shifts, but not quite to
the extent that a stem is affected, because of the weight of bike
& rider coming through the head tube to be distributed into the
fork.  Even if you were to remove your hands from the bars on a
gnarly descent and hang with butt brushing the rear wheel, you
are still applying weight to the bike through the pedals.

All this, of course, is theoretical and YMMV.  I, for one, am not
always able to react to obstacles coming at me and leave the
front end weighted.  When that happens, I'm very glad I have
suspension.

Now enough theory stuff, here's a summary of the advantages &
disadvantages of suspension stems:

PROs

1)  Lighter than a suspension fork.  This depends on the existing
stem/fork combination.  If the current stem and rigid fork are
heavy, then a suspension fork may be a better choice.  For
example, I've chosen the following items for comparison, as they
represent the lightest and heaviest of commercially available
stems & forks (weights for all stems are for conventional types -
non-Aheadset):

Litespeed Titanium          211g
Ritchey Force Directional   375g
Allsop Stem                 625g
Fat Chance Big One Inch     680g
Tange Big Fork             1176g
Manitou 3                  1360g
Lawwill Leader             1588g

So say you have a Litespeed stem and a Fat Chance fork.  The
combined weight would be 891 g.  Switching to an Allsop would
change the combined weight to 1305 g, while a Manitou 3 would
bring it to 1571 g.  The Allsop has a weight advantage of 266 g
(9.4 oz).

OTOH, if you have a Ritchey stem & Tange Big Fork, the original
weight would be 1551 g.  Allsop stem => 1801 g.  Manitou 3 => 1735
g.  In this case, keeping the boat anchor of a fork and switching
to the Allsop would be a weight penalty of 66 g (2.3 oz.).

Of course, YMMV depending on your original equipment.

2)  Does not affect frame geometry.  A suspension fork
retrofitted to a frame, *not* designed for suspension, raises the
front end - sometimes as much as 1".  This reduces the effective
head angle and slackens the steering, slowing it down.  This is
especially true for smaller sized frames which, with their
shorter wheel base, are affected to a greater degree by the
raising of the head tube.  A suspension stem provides suspension
while preserving the handling of the bike.

3)  Torsionally rigid fork.  Telescopic forks all have a certain
amount of flex to them,  and the sliders are able to move up &
down independently.  This aspect of front  suspension forks has
spawned a new line of suspension enhancing products:  stiffer 
fork braces, and bigger, heavier suspension hubs.  All to stiffen
up the fork.  This is  one reason suspension stems are favored by
some riders who ride lots of tight, twisting single track.

4)  No stiction.  Stiction, or static friction, is friction that
exists as the fork sliders rub  against the stanchion tubes. 
This friction is an extra force that must be overcome  for a fork
system to activate.  Not a problem on large hits.  But more of a
problem  on small- and medium-size impacts.  Because the stem has
none, the stem responds  better to small, high-frequency bumps
(washboard) than many air-oil forks.

5)  More boing for the buck.  The Allsop stem provides up to 3"
of stiction-free travel,  at a cost of about $250, depending
where you go.  The majority of forks in this  price range only
offer 1" - 2" of travel, and are often heavy, flexy, and fraught
with  stiction.  The fork could be stiffened, but at the
additional cost of a stiffer fork  brace or perhaps a suspension
hub and a rebuilt wheel (e.g. fork brace - $90; hub -  $80;
rebuild - $100.  Plus the original $350 for the fork.  YMMV).

6)  Better "feel".  The stem allows you to have a rigid fork,
which transmits more "information" back to the rider.  This is a
benefit when riding through creeks where you cannot see where
your wheel is.

7)  Less exposed to the environment.  The stem is higher, more
out of the way than suspension forks.  Thus you can ride through
creeks and mud without having to worry about your fork seals, or
about contaminating the innards of the fork.  Even if mud
splashes on a suspension stem, the pivots are less sensitive to
grit than sliders and stanchion tubes.

8)  Ease of maintenance.  There are no seals to replace or
service, no oil to replace, no air pressure to adjust, and no
bumpers to wear out.  An occasional lube of the pivots is all
that is needed.  An extension of this is the ease of initial set
up.  For best results, you have to set suspension (fork or stem)
to react according to your weight and riding style.  With air/oil
forks you may have to change oil, adjust pressure or change
damping settings (if the fork has them).  With bumper forks you
may need to swap out bumper stacks and mix-n-match bumpers until
you get what works for you.  With the stem, the only adjustment
is to increase or decrease the spring tension with an allen
wrench.


CONs

1)  No damping.  This is one of the main complaints from
proponents of suspension forks.  The suspension stem will give
way to absorb shock, but the return is not controlled and cannot
be adjusted.  JP Morgen currently makes a suspension stem which
employs oil-damping, but Allsop does not.

2)  Requires adjustment to riding style.  As mentioned above, the
stem requires weight to be applied to it to function.  This is
also one of the complaints applied to the Softride rear
suspension beam.  The flip-side to this, according to riders of
the Beam, is once the adjustment is made to "plant your butt on
the saddle" the ride is extremely comfortable and affords
excellent control by sticking the rear tire to the ground.

3)  Stem "clunks" on rebound.  The feeling is about the same as
suspension bottoming out, except it happens on the rebound.  This
is not as much a problem on the Allsop as on the Morgen stem,
which uses a hard plastic top-out bumper.  This is a subjective
complaint, as some riders claim not to notice it.

4)  Stem not torsionally rigid.  Another trade off.  The stem is
not proof to twisting forces and may be noticeable in hard,
out-of-the-saddle efforts.  Allsop has redesigned the top beam of
their aluminum stem for 1994 to address this problem.  Instead
of the aluminum "dog bone" structure for the top linkage member,
they've substituted a machined aluminum beam, reminiscent of a
cantilever bridge.

SUMMARY:  In my opinion, a suspension stem is an excellent choice
if one is retrofitting an existing bike, which has not been
designed around a suspension fork.  A suspension stem is also a
very good choice if one's primary riding is twisty singletrack,
where you need the sharp, precise steering of a rigid fork. 
There are undoubtedly situations for which a stem may not be
ideal, but stems should not be dismissed as a viable form of
suspension.  The best thing to do is to try both types of
suspension if you can, and see what you like better.

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Previous Document: 10 Off-Road
Next Document: 10.2 MTB FAQ available

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