Cam Timing
Cam timing is the synchronization of a compound bow's top and bottom cams so both reach their draw stops at the same moment. When cams are out of time, the string tips slightly at full draw and imparts an off-axis impulse to the arrow at release — producing a vertical paper tear and inconsistent groups that rest adjustments cannot fix.Details
On a dual-cam bow, each cam is rotated by its own control cable. If the top cam rolls over (reaches its stop) before the bottom cam, the string tips slightly at full draw — the top nock end of the string moves forward while the bottom lags. When the archer releases, this string angle imparts a vertical impulse to the arrow at the nock, producing a nock-high tear in paper and erratic vertical grouping. The reverse — bottom cam leading — produces a nock-low tear. Horizontal tears from timing errors are less common but can occur when cable stretch or cam lean is involved.
Timing is checked at full draw. The standard method is to use draw-board reference marks or a bow press with an indicator to observe whether both cams reach their stops simultaneously. Many bow manufacturers mark the cams with timing dots or lines — when the bow is at full draw, these marks align with a string reference when timing is correct. A simpler field check is to observe both cams at full draw from the side: they should appear to roll over to their respective stops at the same time. If one cam clearly stops before the other, timing needs correction.
Correcting cam timing is done by adjusting the length of the control cables. Adding twists to the top control cable (which controls the bottom cam rotation) causes the bottom cam to advance — it reaches its stop sooner relative to the top. Removing twists has the opposite effect. Each twist changes cable length by approximately 1/16" to 1/8" depending on the cable diameter and material. It typically takes only one to three twists to bring timing back into spec, and re-checking at full draw after every adjustment is important before shooting again.
Cam timing should be verified any time a new string or cable set is installed, after significant string stretch during bow break-in, and whenever a paper-tuning tear persists in the vertical plane after nocking point and rest height have been confirmed. One practical note: some single-cam and binary-cam bows are marketed as timing-independent by design — but they still have timing-equivalent adjustments (draw stop timing on a single cam, for example) that serve the same function and should be checked the same way.
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Common questions
- When should I suspect cam timing rather than nocking point when I see a vertical paper tear?
- Check nocking point and rest height first — they are easier to adjust and account for most nock-high or nock-low tears. If those are confirmed correct and the tear persists, then check cam timing. A timing problem often produces a tear that is more pronounced at longer paper distances and does not respond predictably to small nocking point changes.
- Can I check cam timing without a draw board?
- Yes. Draw the bow fully and have a partner observe both cams from the side — or set up a camera on a tripod aimed at both cams. Look for whether the timing marks align and whether both cams appear to reach their stops simultaneously. It is less precise than a draw board but sufficient to detect obvious timing problems.
- How many twists does it take to fix cam timing?
- Usually one to three twists on the relevant control cable is enough for minor timing drift. Add a twist, re-check at full draw, and repeat. Avoid making large twist adjustments in one step — cable twist also shortens the cable slightly, which can shift draw length and draw weight.