I’ve noticed my broad heads have been slowly drifting left over the last few months. Tried moving my rest right and broadheads are still left of field points.
The chart below has always worked for me. However, pragmatism is the number one rule in tuning...if moving something in one direction doesn't fix the problem, try moving it the opposite direction (regardless of what anyone says is the "correct" direction).
I appreciate it may be the string stretching, though current string has been on for two years, but wouldn’t this make my arrows stiffer relative to draw weight meaning broad heads hit right of field points?
String/cable stretch can affect a bow in various ways. Stretching of the shooting string causes draw weight to increase. Stretching of the buss and/or control cable causes draw weight to decrease. It's possible that string/cable stretch could affect the horizontal tune of your bow, but it wouldn't be my first guess as to the root cause of your broadheads hitting left.
Also, the old "broadhead left = arrow is too stiff, broadhead right = arrow is too weak" (for a right-handed shooter) advice doesn't apply to a modern compound bow shot with a mechanical release. Today's compounds are cut well past center with the rest mounted in line with the string path, so the arrow doesn't have to bend around the riser during the shot (like it does on most recurves/longbows). And a mechanical release eliminates the left/right effects of shooting with fingers. Arrow spine can affect how a bow tunes, but the effect doesn't necessarily manifest as broadhead left if too stiff/broadhead right if too weak. Modern compounds can typically tolerate a fairly wide range of arrow spines.
Could it be cam timing? If so, any advice on how to check without a draw board? I live in the remote Northern Territory of Australia so don’t have access to a shop.
Cam timing primarily affects vertical point of impact. If your broadheads are vertically aligned with field points, I wouldn't suspect a cam timing issue. It never hurts to check though.
Without a draw board, you could try taking a slow motion video or having someone else watch as you draw to see if the top and bottom draw stops are hitting at the same time. A draw board is fairly cheap and easy to build; however, you would (likely) need a bow press to actually make an adjustment if you were to find that the timing is off. APA bows and Mathews' V3X/Phase4 are notable exceptions that can be timed without a press.
Check for vane contact. Put a tattletale substance (e.g., lip stick, chalk, spray foot powder) on the edges of the vanes then look for marks on the rest, cables, etc. after shooting.
Confirm the trend with multiple arrows to make sure that what you're seeing is not just an artifact of one particular broadhead or shaft.
Put the arrows on a spinner to visually check for misalignment.
Try moving the rest the opposite way.
Try adjusting cam lean/lateral position per the chart above. This will likely require a bow press.
Try bareshaft tuning. If you can get bareshafts to hit alongside fletched shafts (both tipped with field points), broadheads (on fletched shafts) should follow suit. If they don't and a tiny rest adjustment can't get them there, you may not have enough fletching to adequately steer/stabilize your broadhead.
