| Most people in archery can’t expand on the virtues of good nock
travel because they don’t understand it, let alone know how to measure
it. Nock travel has always been an issue in bow design but it was never
referred to in those terms until recently.
Most archers accepted the importance of correct tiller starting with
early bow designs like the Recurve and Long Bow. Good tiller was recognized
as the correct balance or bend in each limb to get good cast from the bow.
If the top limb was too stiff the arrow nocking point would travel up and
inversely if the lower limb was too stiff it would travel down. This was
compensated for by experimentation with the location of the nocking point
to get good arrow flight, but arrow cast was compromised. Some custom bows
were tillered to the individual shooting the bow. Most Traditional Bows
were made with 3/8" to 3/4" greater dimension for the top limb when the
distance was measured at the top and bottom of the riser perpendicular
to the bowstring. An example would be to measure a 3/4" larger dimension
at the top fade out of the riser for someone shooting with a low grip while
someone shooting with a straight wrist may require almost even tiller.
The introduction of the compound bow caused new dynamics to become part
of the picture. With the early four (4) wheelers there was more concern
with tuning the cables so the eccentrics would turn over in unison rather
than any thought directed at nock travel. Because the power cables anchored
at the riser, eccentric balance was very critical and grip pressure had
a direct influence on limb and eccentric balance. One of the first attempts
to overcome this problem was the introduction of pylons extending from
the riser to anchor the power cables. This made the system less critical
but did not eliminate the problem.
Jennings made popular the two (2) wheel compound bow with the power
cables anchored at the opposite axles. This eliminated most of the concern
dealing with circular eccentrics and limb balance for only a short time.
Higher energy storing cams began showing up and tuning problems started
to surface for unknown reasons. It soon became recognized by a few in archery
that the timing of the cams was much more critical than timing circular
eccentrics. Most cams were set up by checking the cable gap at the bowstring
groove before the bow was drawn. This was adequate with eccentrics but
wasn’t good enough for checking cam timing. The most accepted and best
way to check cam timing at the time was to observe the bow while at full
draw and to adjust cables to put the cams in sync at that point. If this
procedure was followed most arrow tuning situations could be dealt with.
As technology continued to move forward in the quest for more speed,
tuning problems related to poor arrow flight continued to plague the average
archer. Fast Flight Bowstrings and Power Cables gave more speed but had
an inherit problem that became known as creep. Not only was there a problem
maintaining cam balance but the bowstring would continue to creep, changing
the draw length and stored energy characteristics of the bow. The bowstring
would creep enough in some cases to cause damage to the cam and string.
The problems with Fast Flight created more awareness of the importance
of cam tuning because almost everyone experienced it.
Easton Aluminum created a high-speed video for the study of arrow flight
about this time. This video demonstrated some erratic flight from bows
shot by top archers. It demonstrated that even bad arrow flight could result
in good scores if you happen to be one of the top archers in the country.
The video did not attempt to explain the reasons for the bad arrow flight
but it did create questions.
The popularity of draw stops seemed to create even more tuning problems.
There wasn’t much effort to explain the reason behind these problems. PSE
did come out with a video that stated tuning the top cam to start moving
first when the bow was let down would be the best way to tune your bow
if the cams couldn’t be kept in perfect sync. This became common practice
when tuning Two Cam Bows.
You may ask what does this have to do with nock travel, so now a little
bit of back ground on nock travel and modern archery.
Darton Archery created a new focus in the archery industry on nock travel
with the introduction of the C/P/S in 1996. Prior to the C/P/S there was
no design focus on nock travel, you tuned your bow to do the best it could
under the circumstances. In 1997 Darton ran an ad showing an arrow being
shot from a bow equipped with the C/P/S cam system and compared it to the
typical Single Cam of the day.
Facts about nock travel;
You may ask what problems? If you can tune a bow to get good arrow flight
what problems are left for the archer?
A two cam compound in perfect tune will have level and straight nock travel
if drawn from the true center of the bowstring, with the arrow rest at
the vertical center of the bow. This is assuming the cams are the same
size, the geometry is the same for both cams and the limbs are of the correct
The more the arrow is positioned above center, the more upward slope there
will be to the path of the nocking point as the bow is drawn. In the late
70’s Darton introduced their patented True Center Shot Design. This design
altered the geometry of the riser to align the bowstring with the lateral
center of the grip and put the arrow parallel with the direction of the
stabilizer. Darton also put the grip below the vertical center, aligning
the arrow rest closer to the true center of the bow. This created a bow
that was extremely easy to tune and shoot. The Darton SL50 became one of
the most successful bow models sold for almost 10 years.
If one cam stopped rotating because of contact with its stop before the
second cam hit its draw stop there would be an abrupt change in nock travel
and serious tuning problems would develop. If the top cam hits the draw
stop first, the let out of the lower cam will continue causing the nocking
point to rise abruptly. When the bowstring was released the arrow would
come down hard on the arrow rest and create a condition that was almost
impossible to tune. The reverse condition created a less critical situation
and good arrow flight could be obtained in most cases. Because of these
problems the Single Cam became popular. With one Power Cam and one draw
stop there was no problem keeping things in sync. Nock travel became a
function of the second let-out groove on the lower cam. Any problems that
were caused by up and down nock travel or slope could be compensated for
by how the bow was tuned.
The C/P/S (Controlled Power System) was introduced to address problems
the Single Cam bow left for the archer to deal with.
One popular term you hear regularly is, “how large a window do you have
for error”. Early in this discussion it was noted that top archers could
shoot good scores, even when their bow doesn’t shoot a perfect arrow. You
will also note the same individual doesn’t always win. This points out
that even top archers have a problem shooting perfect all the time. Straight
and level nock travel creates a larger window of tolerance for good arrow
flight for all archers. This results in improved scores, better hits and
more enjoyable shooting when compared to shooting a bow with anything less.
Another advantage is better retained energy down range because of smoother
arrow flight and less dissipation of energy from arrow vibration.
The C/P/S design makes it easy to achieve straight and level nock travel
by using a non-concentric two (2) groove upper Control Wheel that is attached
to and controlled by the lower cam. This allows the bowstring let-out groove
of the upper Control Wheel to be nearly symmetrical with the lower Cam
for even tiller and to achieve more stored energy early in the draw cycle.
This puts less demand on a steep and sudden letdown near the end of the
draw cycle to get good stored energy. The take-up groove on the upper Control
Wheel permits complete control of the rotation of the top let-out of the
bowstring to maintain straight and level nock travel. The take-up groove
of the upper Control Wheel is coupled by the Control Cable to the let-out
control of the lower Cam so that one wheel can’t rotate without the other.
The let-out control of the lower cam is incorporated in the draw length
module so you have the same control of nock travel no matter how you adjust
the module for draw length.
Another unique feature of the C/P/S system is its ability to maintain
similar draw characteristics and stored energy efficiencies through all
draw length adjustments. The tuning marks introduced by Darton and used
on the C/P/S show the correct location to set your bow up for optimum straight
and level nock travel. This check is independent of optimum stored energy.
Optimum stored energy is achieved by following the chart included on the
Technical Bulletin supplied with each bow detailing the draw length adjustments.
On the upper Control Wheel there is a secondary adjustment provided for
the bowstring that is used to make draw length adjustments as small as
1/8" and to aid in optimizing stored energy. The shortest draw length adjustment
will be achieved by adjusting the draw length module to the #1 position
and anchoring the bowstring at the (A) position. This optimized level of
stored energy can be maintained by making each adjustment accordingly,
i.e., 2B, 3C, 4D, 5E. Each adjustment of the module is 1/4" (C/P/S Extreme)
and each adjustment of the bowstring is 1/8". By going + or - one (1) adjustment
for each optimized position you have the ability to adjust the draw length
as little as 1/8" and not adversely effect the stored energy efficiency.
If all of this sounds too complicated just adjust the bow so the tune lines
are close to the same for both cables and shoot it, you won’t affect the
efficiency of the system and you will still have a bow that is easier to
tune than anything else out there.
by Rex Darlington
President DARTON ARCHERY