A modern compound bow runs three strings: the main string the arrow nocks onto, and two control cables that connect the cams to the opposite limbs. The convention of replacing all three at once is common, and for a full refresh it is sensible. But it is not a requirement, and it is not always the right call. Think of it the way you would rear tires on a sports car: replacing all four at once is the thorough option, but if the rears are worn and the fronts have plenty of life left, replacing only what needs replacing is entirely sound. The logic is the same here.
The main string and the control cables lead very different lives. The main string takes direct contact from the nock on every shot, runs through a peep, anchors a D-loop, and passes over the cam grooves. It is the highest-wear component in the system. The control cables run in the cable slide, contact the cam tracks, and otherwise stay relatively protected. They stretch and wear, but typically on a longer timeline than the main string.
Replacing only the main string when it needs it — while leaving cables that are still in good condition — is not cutting corners. It is matching replacement to actual wear. The condition that makes it acceptable, however, is not negotiable: the cables must be verified in spec before the bow goes back into service.
Why cable spec verification is non-negotiable
Cable length is not cosmetic. It directly controls draw length, draw weight, cam timing, and cam lean. A cable that has stretched beyond its original spec will throw off all four of those parameters — subtly if the stretch is minor, significantly if it is not. A new main string installed on top of out-of-spec cables will bring the bow back to neither the original tune nor a predictable new tune. It will simply be wrong in ways that are tedious to diagnose.
The problem is that cables can appear fine visually while being out of spec dimensionally. Serving wear is visible. Strand damage is visible. Elongation is not — at least not without measurement. This is why verification requires numbers, not just a look.
How to verify the cables are in spec
There are three practical checkpoints:
Axle-to-axle measurement. With the bow at rest, measure ATA. Compare it to the manufacturer's published specification for your exact model. ATA is sensitive to cable length — if the cables have stretched, ATA will typically be longer than spec. A bow that measures correctly at ATA is a reasonable indicator that cable lengths are in the right range, though it is not the only check.
Cam timing marks. Many cam systems have timing marks or reference lines that should align at full draw or at rest. If the cams are still indexed correctly relative to each other and relative to the limbs at rest, the cables are holding their relationship. If the timing marks are off, the cables have moved — either through unequal stretch or damage — and need investigation before you proceed.
Draw length and draw weight. If your draw length has crept longer than your set modules or draw stops should allow, or if draw weight has dropped noticeably without a limb adjustment, cable stretch is a likely cause. These are secondary indicators, but consistent with a cable that has moved out of spec.
If all three checkpoints are within acceptable range and the cables show no serving separation or strand damage, replacing only the main string is a sound decision. If any checkpoint is off, replace the cable that is causing the deviation — or all cables if you cannot isolate which one.
Cable serving condition
Verify the cable serving visually before making the final call. Run your fingers along the cable serving, paying attention to the cable slide contact area and the cam track contact areas — both see mechanical wear. Serving that is worn to the point of exposing core strands in those areas changes the effective cable diameter at that location, which in turn changes how the cable seats in the cam track. This affects timing. A cable with serving that is worn but not yet dimensionally affecting the system can continue in service; one where strand exposure is changing the seating geometry needs re-serving or replacement before the main string goes on.
After the string is installed
After installing a new main string, re-verify the checkpoints. A new string that comes in at the correct length will typically shift ATA slightly as it settles, and timing should remain correct if the cables held their spec. Check timing marks at full draw. If they are off, the new string's length needs adjustment — twisting in or out as appropriate — before the bow is tuned.
The expectation with a main-string-only replacement is that the bow should return close to its previous tune once the string settles, because the fundamental geometry controlled by the cables has not changed. If it does not — if the bow requires significant nocking point or center shot adjustment to return to its previous tune — something has moved in the cable geometry and needs a closer look.