J.Michael Manning
The easiest solution would be to try to duplicate your existing setup.
But, if you wish perform at your best...then you need a time trial position.I
do not use a mathematical formula, but instead I consider the geometric
configuration. You are right on in assuming that the reach to the
pedals is crucial (distance to the BB). With a laser and digital level,
I can perfectly duplicate that from one bike to the next. There is
another method by which you can closely approximate that. The idea is
to draw a triangle that connects 3 points: the center of the BB, the
sweet spot of the saddle (the widest point), and the intersecting point
created by drawing a connecting line that runs horizontal from point 1
and one that runs vertical from point 2. You have now created a right
triangle with the hypotenuse being the line drawn from the BB to the
sweet spot. You can now duplicate that triangle on any bike regardless
of the seat tube angle differences. Fitting a TT bike presents a
different situation though, simply because
you may choose to change the shape or size of that triangle. The key
thing to note here is that the length of the hypotenuse should
not change. A simple method for ensuring that is to cut a piece of
string that matches that length, and while maintaining the BB as your
pivot point, rotate the string forward or backward to the desired
saddle fore/ aft
setting. Now that you have established a way to re-create the correct
saddle height, you must determine exactly where to shift the saddle.
Unless, you are concerned with UCI regulations regarding TT bikes
(which are simply meant to prevent an unfair aerodynamic advantage) try
moving as far forward as you can tolerate. Consider that an aero
advantage can be easily negated if you cannot maintain some degree of
comfort. Longer TT events are going to be much more challenging in this
forward position. Being perched out on the tip of the
saddle may be more aerodynamic, but try not to slide the saddle so far
out that you are unable to occasionally slide back for comfort's sake,
or for the occasional thrust of additional power to climb a short hill.
On a hilly course, or in a race where there are
repeated surges of speed or attacks that require bursts of energy, it
is best to remain in a more rearward position in order to enable the
more powerful quads to take over. In a TT, you generally want to smooth
out the pedal stoke for a more
even distribution of power over the distance of the race. That does
mean sliding the saddle forward and engaging the hamstring muscles and
hips more effectively. However, if you are too far forward, you will
lose some of the leverage that is
provided by keeping the lever system of the lower leg/knee/upper leg
behind the pedals. Think of it in terms of using a lever (the femur)
with a rock (the knee) as a fulcrum to lift a heavy object. Another way
you might be able to better visualize this comes from your experience
in the gym. When you are squating, you are weakest when the knee is
more bent. As you stand and the angle at the knee opens up, you
generate more power. The same is true in cycling. The greatest amount
of power can be generated near the bottom of the pedal stroke. The
longer you are able to keep the knee in position the longer you are
able to generate power and the momentum that will carry through the
backstroke. The shorter stroke that comes from being too far forward
shortens the time during which you are able to generate power. The last
thing to consider is having the correct reach so that the thoracic
region is able to relax. This enhances breathing and prevents back
pain. One of the keys I look for is the angle created by the upper arm.
The arm and shoulder angle should be at 90 degrees and the line drawn
from the center of the shoulder down through the center of the elbow
should ideally imitate the angle of the steertube and fork. This makes
the steering mechanism an extension of the body so that it feels
relaxed and natural. If you have aero bars that provide for fore/aft
adjustment, it becomes easy to set the aero handles in a position that
allows for this position. Otherwise, you may have to change the stem to
achieve the correct reach. You also must consider the height
difference between the top of the bars and the saddle. A deeper drop
makes you more aerodynamic with a flatter back. But if the difference
is too great, you will find your thighs banging against your ribs. Take
a look at your flexibility and try to achieve balance between all of
these important variables.Mike ManningCycleOne LLC> Date: Thu, 22 Feb 2007 10:28:53 -0800> From: sugahara@mac.com> To: obra@list.obra.org> Subject: [OBRA Chat] Bike Measurement Transfer Question> > Hey folks,> > A question for the list:> > I had a really good bike fit this year on my road bike. I want to transfer those measurements to a TT bike. > My road bike has a STA of 74 degrees (52.5 ETT) and the TT bike has a STA of 76 degrees (50.5 ETT).> > I know the more aggressive TT angle engages more the hamstrings and glutes- for tri-geeks- to save the legs for running. As I don't do tri's, would it be more advantageous to set up the bike as close as I can to my road bike setup? If that's the case, is it correct that I need to move the saddle back and drop the saddle a bit to maintain the same distance to the BB? (.5 cm drop for every cm movement back?)> > Would the correct equation for the required change in distance be the following?> (52.5*cos(74)) - (50.5*cos(76))= difference (2.26 cm)> > Any suggestions, pointers would be appreciated.> > Thanks!> > Kenji> _______________________________________________> OBRA mailing list> obra@list.obra.org> http://list.obra.org/mailman/listinfo/obra> Unsubscribe: obra-unsubscribe@list.obra.org
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