Tech Talk
Mountain Bike Geometry
The modern off-road bicycle is a simple device that provides
recreation, exercise and fuel-efficient transportation. It consists basically of
two wheels connected to a seat, pedals, gearing system, and a frame which
incorporates a steering mechanism. However, all frames are not created equal-not
only are there various materials used in construction and various sizes (length
and height) available, the "geometry" varies from one mountain bike to
the next. Frame geometry is the term used to denote the length of the tubes
involved and the angles at which they intersect. It is an extremely important
factor in a bicycle's overall function as it affects how the bike feels, steers,
handles, balances, and performs.
The most common shape for bicycle frames is a
parallelogram with a diagonal line added. Effectively, the result is two large
triangles, which provides sufficient strength to minimize flexing of the frame
under the stress of the rider's weight, rough terrain, and strong pedaling
action. Lateral flex that occurs during pedaling is "lost energy",
lost in the sense that part of the energy the rider imparts to the pedals does
not go to turning the rear wheel, thus reducing the efficiency of his/her
effort. Over the past few decades there have been numerous frame designs that
have deviated from the traditional double triangle design-many have failed
miserably simply because of flex.
Most bicycle manufacturers give a few critical
measurements in their catalogs to describe the geometry of their bicycles. The
common measurements are head tube angle, top tube length, chainstay length, and
wheelbase. Early mountain bikes were based on the geometry of old one-speed
cruisers, designed only to go downhill. The result was a bike with a very long
wheelbase and shallow angles of the head tube and seat tube-it only liked to go
straight ahead. Over the past decade a bicycle frame more suited to all-around
use (not just downhill) has evolved, providing a middle ground between stability
and responsiveness, somewhere between "sluggish" and
"twitchy".
Head tube angle relates directly to how a bike steers. The
closer the head tube is to perpendicular to the ground (larger angle, called
"steeper"), the more agile the bike, the more quickly the bike
responds to steering input. As you lessen the head tube angle (more like the old
cruisers), stability and braking response at high speed improves.
Top tube length directly affects rider comfort-too long or too short for an
individual rider translates to an undesirable riding position (for example,
being uncomfortably stretched out to reach the handlebars or finding that your
knees hit the handlebars as you bring your weight forward on climbs). If angles
are held constant, increased top tube length parallels increase wheelbase, thus
affecting handling (discussed below).
Chainstay length has been steadily decreasing over the
past decade for most off-road bicycles. When you get out of the saddle and stand
on the pedals for a steep climb, traction dramatically improves as the
chainstays shorten (it brings the rear wheel forward, more underneath the rider,
increasing the proportion of weight over the spinning tire). However, longer
chainstays contribute to a more stable, comfortable ride, especially on a
downhill.
The longer the wheelbase on a bicycle, the better it tracks straight ahead.
Image a bike with a 10-foot long frame, turning it sharply would be impossible
(such a bike existed around the turn of the century)! Shorter wheelbases create
more responsive, lively feeling steering.
In addition to the factors described above, a
consideration of a bike's geometry may also note the height of the top tube
(better crotch clearance and greater range of possible saddle height), bottom
bracket height (better ground clearance), fork offset (greater offset gives
results similar to decreasing the head tube angle), and stem length and height
(primarily affects how the cyclist sits when riding, secondarily affects
steering response).
The proper geometry for any individual cyclist depends
upon his or her riding style. Novice or purely recreational riders typically
prefer bikes which allow them to sit relatively upright. This often means a
bicycle that's a little too tall, with high handlebars and a short top tube.
This "sitting on a park bench" position feels pretty comfy, but limits
pedaling power output and precision of balance (especially on hills and
corners). More experienced, aggressive riders, seeking to achieve their full
riding potential, are typically willing to give up some comfort in order to get
the most out of their bicycle. With this differentiation in bike geometry being
intimately related to the seriousness/expertise of the cyclist, generally
speaking, the more expensive the bicycle, the better handling, more
energy-efficient and uncomfortable the riding position it provides. Often when
choosing a bicycle, it comes down to the emphases most critical for each
individual--stability or quickness, downhilling or climbing, and handling or
comfort.
