The anatomy of the curve
At brace height, a compound bow already has some load on the string — typically 15 to 25 lb, depending on the limb stiffness and brace geometry. The archer is not drawing yet, but the system is already under tension. That is the left edge of the curve.
As the archer begins to draw, force climbs. The limbs are bending, storing energy, and the cams are rotating through the early part of their cycle. On most modern single-cam and binary-cam designs, this climb is fairly steep. By the time the archer reaches mid-draw, they are working against something close to peak weight — on a 70 lb bow, that means somewhere between 60 and 70 lb of pull.
Then the cams rotate through their let-off geometry and the force drops sharply. This is the signature feature of the compound bow. The cam profile is shaped specifically to create this drop — the geometry of the wheel redirects the force through a mechanical advantage that rises steeply over a short arc of rotation. The result is a draw force curve that crests near peak weight and then falls off a cliff.
At the bottom of that cliff is the valley. The valley is the region of the draw where the holding weight is at its minimum — where the archer is holding only 10 to 15 lb instead of 70. The valley exists because the cam has passed the peak of its let-off geometry but has not yet contacted the draw stop. It is a deliberate design feature, not an accident. The width of the valley in inches determines how forgiving the bow is about exact draw length — a wide valley means the archer can be a fraction of an inch short or long and still be in the low-weight zone. A narrow valley is much more demanding.
At the far end of the valley is the wall. The cam's draw stop — a physical pin or tab on the cam — contacts either the cable or the limb and halts rotation abruptly. The force curve at the wall is nearly vertical. The archer cannot draw further without dry-firing the bow or bending the limb stops. This is not a gradual resistance; it is a hard stop. An archer who draws into the wall and pulls into it feels an essentially rigid surface.
What the cam is actually doing
The draw force curve is not a property of the limbs. The limbs contribute the energy — their stiffness determines the peak weight. But the shape of the curve, including the let-off, the valley width, and the wall sharpness, is determined entirely by the cam profile.
A compound cam is an eccentric wheel. As it rotates during the draw, the radius at which the cable leaves the cam changes continuously. When that radius is small (early in the draw), the mechanical advantage is low and the archer pulls hard. When the cam rotates to the point where the cable is leaving at a large radius, mechanical advantage is high and the pulling force drops — this is the let-off zone. The draw stop pin then interrupts the rotation before the advantage can reverse.
This means the curve is entirely a function of where the cam is in its rotational cycle at any given draw length. Change the cable geometry and you change what rotational position the cam occupies at any given draw length. The let-off starts at a different draw length. The valley occupies a different position. The wall falls at a different draw length. Everything moves together, because everything is downstream of cam rotation.
The cam's starting position is not arbitrary
At brace height, before the archer has drawn an inch, the cam sits at a specific rotational angle. That angle is not a side effect of assembly. The manufacturer chose cable lengths precisely to place the cam at this position, because the draw force curve that results from drawing from this exact starting angle is the curve they designed, tested, and published.
The cam's eccentric profile is not uniform around its circumference. As it rotates during the draw, different radii engage, and the mechanical advantage changes continuously. The rising ramp, the let-off zone, the valley floor, and the wall each correspond to a specific arc of the cam's rotation. Those arcs were designed in sequence. The cam enters the rising ramp arc from the designed starting angle, works through the let-off arc, settles into the valley arc, and stops at the draw stop — all in the order and proportion the manufacturer intended.
When the cable length is wrong, the cam sits at a different rotational angle at brace. It does not enter the rising ramp arc from the designed position. It enters from somewhere else on the eccentric profile — a region the manufacturer did not intend to be the starting point. Every arc that follows is shifted accordingly. The ramp rate may be different. The let-off profile may feel different. The valley may be narrower or wider. The wall may be softer or sharper. These are not predictable, uniform deviations. They depend on exactly which portion of the cam's eccentric geometry has been displaced into the brace position, and every cam design handles that displacement differently.
The manufacturer's specified cable lengths are the precise inputs required to produce the curve the cam was designed to produce — nothing more, nothing less. Any deviation from those lengths means the archer is shooting a configuration the manufacturer never validated.
What cable twists actually change
A control cable is a physical length. Adding twists makes it shorter. Removing twists makes it longer. This is straightforward mechanics.
What is less intuitive is what a shorter cable does to the cam. Because the cable is shorter, it holds the cam in a retarded position at brace — earlier in its rotational cycle than the manufacturer's specified cable length places it. The cam has not yet rotated as far through its designed cycle before the draw begins. Every inch of draw from that point must advance the cam through more of its rotation than it was designed to cover at that draw position.
The cam reaches its let-off geometry later in the draw stroke — at a longer draw length than the original spec. It reaches its draw stop later too. The result is a longer draw length, but not in the way a module change produces a longer draw length. A module change relocates the physical stop point. A cable twist relocates every point on the curve simultaneously.
The valley compresses. The angular distance between the let-off point and the draw stop is fixed in the cam design — it does not change. But the translation from cam rotation to linear draw length is not constant; it depends on where in the cam's rotational cycle that distance is being traversed. When the cam is retarded out of its designed starting position, that fixed angular distance maps to a shorter linear distance at the nock. A bow that had a 1.5-inch valley at factory spec might have a three-quarter-inch valley after adding a meaningful number of twists to the control cables. The archer may not immediately notice. The bow still draws. The wall is still there. But the comfortable window of full draw has tightened significantly.
Why archers reach for the cables anyway
Draw length adjustment by cable twist is genuinely tempting. It requires a bow press, but no replacement parts and no ordering wait. A few extra twists in the control cable and the draw length increases by a quarter inch or more. For an archer who is a half-inch short and does not own the right module, it seems like the obvious answer.
The problem is that draw length is not the only thing that changes. Brace height may shift if the main string length is not adjusted to compensate. The valley shrinks. And the overall energy storage of the bow changes because the cam is no longer traveling through the arc it was designed to travel through during the power stroke.
The archer who adds twists to lengthen their draw length by a quarter inch has also, without intending to, tightened the valley and changed where on the limb's deflection curve the shot breaks. None of these changes announce themselves. The bow still shoots. The paper may still tear clean. But the bow is no longer operating at the geometry it was built around, and small deviations from that geometry compound over time as additional changes are layered on top.
The right tool for draw length: the module
Draw length modules exist specifically to change draw length without disturbing cable geometry. A module change physically repositions the draw stop — the pin or tab that the cam contacts at full draw. Moving the stop changes where the cam's rotation halts, which changes draw length, but the cable lengths remain exactly at spec. The let-off geometry is unaffected. The valley width is unaffected. Cam timing is unaffected.
This is the correct approach. If the bow needs to be a quarter inch shorter, the module is replaced with the next shorter module. The cable lengths are confirmed against the manufacturer's spec sheet for that module. If they have drifted from the target — from previous cable twist adjustments or from string stretch — they are corrected first. The result is a bow at the correct draw length with the correct draw force curve for that draw length.
Module availability is occasionally cited as the reason archers reach for cable twists instead — the right module is not in stock, the bow is older, the manufacturer no longer lists it. These are real constraints. But they are reasons to order the part and wait, or to contact the manufacturer about alternatives, not reasons to introduce a geometric distortion into the system that the archer will be chasing indefinitely.
The broader principle: the bow is a geometric system
Every number the manufacturer publishes — brace height, axle-to-axle length, draw length, draw weight — is achieved at a specific set of cable and string lengths, at a specific number of twists, with specific modules installed. Those numbers are not suggestions. They are the output of a cam geometry that was designed to deliver a specific draw force curve at those conditions.
When cable lengths change, the curve changes. When string lengths change, the curve changes. When modules are swapped without updating cable lengths to match, the curve changes. The bow does not know what the archer intended. It produces the curve that its geometry dictates, and the geometry is determined by every physical dimension in the system simultaneously.
An archer who has been adjusting draw length with cable twists for several seasons may have a bow that has never, in their ownership, had the draw force curve it was designed to produce. The valley may have been compressed since the first month. The cam timing may have been off since the first tuning session. The bow may be shooting adequately despite all of this, but it is not shooting what it was built to shoot.
Restoring a bow to spec
If a bow has been modified by cable twists and the original geometry is unknown, the path back to spec starts with the manufacturer's documentation. The bow's model and year determine the target brace height, axle-to-axle length, and the cable and string lengths (in inches and twist count) for each draw length module. Most manufacturers publish these in the owner's manual or on a spec card included with the bow. Some publish them on their website by serial number.
With the target numbers in hand: install the correct module for the desired draw length, set cable and string lengths to the published targets, confirm brace height, confirm axle-to-axle. That sequence, in that order, restores the bow to the geometry the draw force curve was designed around. Any additional tuning — paper tuning, walk-back, cam timing on two-cam bows — is done after the geometry is correct, not instead of it.
A bow that has been returned to factory geometry will often feel noticeably different in the hand. The valley may feel wider. The wall may feel more distinct. The draw cycle may feel smoother or more aggressive depending on which direction the previous modifications had pushed it. These changes are not the bow being different — they are the bow being correct.
Published 2026-07-29 · Axial Bowstrings