Aerodynamics and bike fit for speed

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Some practical wisdom on endurance sports nutrition from the book is “The woman Triathlete“, reprinted with permission by Human Kinetics.

“How fast you finish the cycling portion of a race depends on the
power you’re able to produce during the ride. Ultimately, power output
depends on just two variables: force and speed. Very simply, it depends
on how hard you push and how fast you pedal. The three forces you need
to overcome to move forward are air resistance, rolling resistance,
and, on climbs, gravity. Because gravity and rolling resistance depend
on weight, most cyclists try to minimize weight. This is most easily
achieved by using a lighter bike and componentry, but these come at a
high cost. Rolling resistance also depends on the road surface, as well
as the make, thickness, and pressure of your tires. The biggest
resistive force, however, is air resistance, which is dependent on your
speed and frontal surface area. At 20 miles per hour on a flat road
(gravity is zero), rolling resistance makes up less than 25 percent of
the total resistance, while air resistance makes up more than 75
percent. The most effective way to reduce air resistance is to draft
behind (or even next to) another rider. For a triathlete without the
option to draft (drafting is not permitted in most amateur triathlon
racing
), reducing frontal area has the greatest effect on performance.
Aerodynamic equipment–such as bike frames with tear-shaped tubes,
deep-dish wheels and discs, narrow water bottles, tight skin suits, and
streamlined helmets–can reduce some of the frontal area. However, a
rider’s body is by far the biggest obstacle. Bike fit for a triathlete
is therefore optimized with biomechanical fit and aerodynamic
positioning; many triathletes even choose to ride a less comfortable
setup in favor of better aerodynamics. Keep in mind, though, that a
comfortable setup that incorporates aerodynamics will usually result in
increased power output. Because road cyclists are allowed to draft,
they tend to place greater importance on biomechanical fit, comfort,
and handling of the bike than triathletes do, but triathletes would be
well served in finding a comfortable setup.

It is relatively
easy to adjust a traditional bike fit to a more aerodynamic fit. The
most cost-effective investment is a set of aerobars. Better yet, using
an ergo-stem along with your aerobars will allow you to more completely
adjust the position of your handlebars. A second seat post and saddle
combination will allow you to quickly move back and forth between a
road position and a time trial position with just one bike frame.
Because a traditional road bike fit often results in better (i.e.,
easier) handling of the bike, it is useful to be able to switch back
and forth between setups. You can convert your bike to match your
workout–aerodynamic position for solo efforts and time trials or a
traditional bike fit for group rides and hilly routes. Before you
adjust your bike fit to a more aerodynamic position, measure (and mark
with tape) how your bike is set up. It is always a good idea to have
the option of going back to a position that already works for you. Once
you have the necessary measurements, move your saddle forward one or
two centimeters. Because this reduces the distance from your saddle to
the bottom bracket, you may also need to move the saddle up (usually
about half the distance that you moved it forward). Now check your
reach by leaning forward into the aerobars. The front of your shoulders
should be aligned vertically with the back of your elbows. This
position allows you to rely on the skeletal rather than muscular
support of your upper arms for the weight of the upper body. Your
comfort and flexibility should determine the height of the handlebars
relative to the saddle. For example, if your hamstrings feel tight,
your handlebars need to be moved higher. Most likely, your cleat
position and your saddle tilt can remain in the same position as they
were in before.

No matter how aerodynamic you want to be,
injury prevention and comfort should be your main concerns with regard
to fit. Your knee rotates through many cycles on a ride–in just one
hour of racing at 90 revolutions per minute, you are completing 5,400
rotations per leg! If your bike is not properly fit to your
biomechanics, you will be at high risk for injury. Also, if you are
uncomfortable on the bike, you may become distracted by repetitive
twinges instead of being able to focus on your effort. Because a proper
bike fit is critical, you should be fit at a reputable triathlon or
cycling shop, by a certified fit specialist, or by a coach or physical
therapist who has experience in bike fit. A proper bike fit should
always include setting up your cleats (on the bottom of your shoes) in
the proper position: If your knee is restricted to the wrong range
through each pedal cycle, you’re almost guaranteed injury. Athletes
looking to be very competitive in triathlon should consider being fit
by a professional fit specialist who will take into account every
aspect of their biomechanics when adjusting their position. Look for
someone who specializes in triathlon-specific fitting, and expect to
pay $50 to $100 for the service (and anywhere from $200 to $1,000 for
services that include power output measurement or wind tunnel testing).

Even with a good bike fit, you may find that you are
uncomfortable on your saddle at times. If you experience this, consider
the following:

  • Never wear anything under your cycling shorts. The shorts are
    designed so that there are no seams in sensitive areas. Wearing
    undergarments adds those seams back between you and your saddle. Also,
    make sure you buy women’s shorts to ensure a proper fit.
  • Wash your shorts after each ride to avoid infections.
  • Use a chamois cream or ointment to prevent saddle sores and
    chafing. Apply it to both your body and the shorts for maximum
    protection.
  • Use a women-specific saddle. They are designed to support the wider sit bones of a woman’s body and provide increased comfort.”