TECHNOLOGY STORIES BIKE INFO, GEOMETRY ... - Vintage Trek

Developed for the Trek Professional mountain bike
team, the basic concept of Pro Geometry is a bike that
better handles the higher speeds of Pro racers. There
are several key features used in Pro geometry to accomplish
these goals.
Position
The rider compartment is slightly more upright.
A road rider needs to be bent into an aerodynamic
position because wind resistance is a major source of
fatigue on the road. A road racer’s average speed is in
the 20mph range and higher. Mountain bikes usually
only go this fast downhill, so you don’t need to be
bent over as much. Another argument for an upright
position is balance. In humans, the body’s balance
mechanisms are mostly in the head. Your sense of
body position is anchored by your vision. If your
eyes are parallel with the horizon, your balance is
improved. With a more upright position, it’s easier to
keep your head oriented for optimal balance. Better
balance is critical to handling technical terrain or
carving a fast turn on singletrack.
Long front/center
Pro Geometry uses a long front/center. Front/center is
the distance from the bottom bracket to the axle of the
front wheel. The location of the front wheel is important,
since it’s the first part of the bike to meet obstacles in
the trail. The front axle is also the pivot point of the bike
when a rider takes a flyer over the handlebars.
With a longer front/center, the front wheel is pushed
further ahead of you. When you find yourself moving
back on your bike, it’s usually in response to your body
wanting to flip over the front axle. This happens on
steep downhills, and also any time the bike is moving at
high speed in rough terrain.
When the Trek engineers moved the front axle forward,
it added resistance to over-the-bars flight. With
this added stability, you’re more relaxed at speed, and
since you’re more in the saddle than behind it, you’re in
a better position to keep the power on the pedals.
Steering
With a long front/center, a bike needs a longer top
tube. To correctly place your hands when riding a bike
with a long top tube, you must use a shorter stem. The
shorter stem used with Pro Geometry puts your hands
closer to the steering axis so steering can be done with
your arms instead of a sweeping sideways movement of
your shoulders. Your hands can move faster than your
shoulders, so technical steering is precise at high speed.
Pro geometry is designed around today’s longer forks.
Thanks to the long front/center, Pro geometry places
slightly less weight on the front wheel. Due to a combination
of steering angle, trail, weight distribution, and
a slightly longer wheelbase, a bike with Pro geometry
likes to be steered by angulation, an advanced skill
that allows a rider to stay balanced over the bike’s tire
contact patches through a turn.
The technique is much like a downhill skier's position,
where the torso remains upright while the lower body
is angled for steering. This position keeps the center of
gravity over the skis for maximum edge hold. And if the
skis should slip, the skier can extend to control them.
As a high speed cycling maneuver, this angled position
makes controlling the bike in a corner much easier. If
the tires slide, extending your body keeps your center of
mass on top of the tire contact patches.
2002 Trek Technical Manual
Pro Geometry
Handling
Instead of making a bike that steers quickly so you
can adjust your line in a turn, this bike has additional
directional stability that lets you pick a line early and
hold it. It has a touch of understeer, so if you go into
a corner a little too hot, just lean it in a bit more with
a touch of rear brake, and go. Instead of skittering
around and washing the front tire, the additional lean
puts more edge knobs onto the ground, and a Pro
Geometry bike really carves. Coupled with a lightweight
frame, Pro Geometry makes a bike quick from
edge to edge, so it handles tight turns really well. And
the longer wheelbase works like a giant slalom ski so
high speed fire road riding is way fun.
Doesn't the short stem make the bike climb poorly?
Common sense tells us that a longer front center
places less weight on the front wheel. Intuition tells
us that with less weight on the front wheel, the bike
might not climb well. But geometry charts only tell
part of the story, and a Pro Geometry bike actually
climbs very well. Here’s two reasons why: with a shorter
stem, your shoulders stay more over the centerline
of the bike, even when turning. When your center of
gravity stays over the frame centerline, the bike stays
in better balance. With Pro Geometry, it’s even easier
to hold your line on steep, slow speed climbs. Secondly,
when climbing hard in first gear any bike will respond
to the pressure of pedaling. Imagine if the headset
were placed in the middle of the bike, right below the
saddle. The bike would hinge in the middle, between
contact patches of the tires. With every pedal stroke
the rear wheel would turn away from the pedaling
force. As a result, the front wheel would turn toward
the pedal side, and the bike would swim like a salmon
heading upstream. But the further ahead you move
the pivot (headset), and the closer to your hands, the
straighter the bike will climb. With the shorter stem
used in Pro geometry, you stay over the bike, and
the bike tracks straighter, making it climb very well
indeed.
Fitting Pro Geometry frames
Pro Geometry bikes (OCLV hardtails, Alpha SLR,
ZR9000, Fuel, and STP) are designed to put you in
a similar position to our other performance mountain
bikes. The only difference in position is that the larger
sizes of Pro Geometry use taller head tubes than we
offered in the past. With taller head tubes and 25mm
of spacers it may be necessary to move some spacers
to the top of the stem if you prefer a more deeply
bent-over fit.
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