Often as not these posts are stimulated by questions sent in by my students. In this case part of one question included this:
“Remember those 65% cam letoff modules? I didn’t notice any difference so I put the original 75% ones back on after a few months. Since using my trainer with two tensions, I see the higher tension seems to go off easier. In addition, I’ve read the higher tension lets you hold on target better. I have noticed even with my stabilizer with different weights I don’t get as steady as I’d like or should be.”
Many coaches not raised in the world of compound bows are a little baffled by the concept of letoff. Simple stated, the letoff is the percent of the peak draw weight of a compound bow that you lose getting to the holding weight. So, if you have a 50# peak draw weight compound, if it is designed with 50% letoff, you are holding 25 of those 50 pounds at full draw. If you have a 60% letoff bow, then you are holding 40% or 20#. If you have a 75% letoff bow you are holding 25% or 12.5 lbs.
So, why not 100% letoff, it sure would be easier holding?
In the early days of compound bows (Hint: the 1970’s) compound bows had 35-40% letoff at best. The archers choosing these bows were shooting 40 and 45 pound recurves, holding 40 and 45 pounds at full draw, so knocking off a third or more of that was quite a deal. Shortly thereafter letoff reached 50%, then 65%. When I got started in archery the Compound-Fingers archers were often shooting 50% letoff bows and the release shooters were shooting mostly 65% letoff bows. The difference between the two groups is understandable if you grasp that the fingers do not leave the bow string in a finger loose, the bow string pushes them out of the way on its path toward the bow. If there is very little tension on a bowstring at full draw, where the loose occurs, then there is little force to move the fingers out of the way, which means the string will move much more than we want it to in response to the force exerted on the fingers by the string (action-reaction). This makes for inconsistent wobbly releases.
Bow manufacturers have raised letoffs up to 75%, even 80% but these are not used much for target applications. They are mostly used by bowhunters who may have to wait at full draw for a deer to present itself, for example.
Target archers still need a bit more string tension for the reasons implied in the question and more. As more and more compound archers switched to using release aids, which make our releases so much cleaner, we tended to give back some of that full draw bowstring tension (high letoff = lower full draw string tension), trading it for comfort. The holding weight at full draw which creates the string tension is a force we exert on the bow that (a) makes the bow easier to hold up (as the draw arm is pulling up, somewhat, as well as back), (b) makes the bow easier to hold steady, and (c) gives a reasonably clean release.
In my student’s case, his bow has replaceable modules that attach to the cams that change the letoff from 65% to 75%. (Letoff is an element of design that varies slightly with draw weight and draw length and since those are adjustable, these numbers are approximate.) He is saying that with the 65% letoff modules installed (giving a slightly greater holding weight/bowstring tension at full draw), that the release goes off more crisply and that he seems to be able to hold steadier. He is quite right.
Basically there has to be a happy spot in the middle of the letoff range, somewhere where the amount of resistance at full draw is not taxing yet the tension on the string is enough to facilitate a stable hold and release. For target Compound-Release archers this happy middle ground is currently around 65% letoff. As with all things of this type, this is not a dictum, it is just an indicator that the farther you get away from that number the less easy things get. As the letoff goes down (toward, say 50%) the holding weight goes up and so fatigue becomes a factor on long shooting days. If you are in very good shooting shape, this may actually be desirable. As the letoff goes up, the ability to hold steady goes down a little, but if you are rock solid steady, that may be an acceptable tradeoff. So some archers favor higher and some lower letoffs than what most archers do.
If you hear compound archers arguing over “what amount of letoff is best” realize that the discussion is probably pointless as what is best for one person may be quite different for another. It also depends on the application: bowhunting, target, field, 3-D. But compound archers, shooting more complicated mechanisms (bows), have more of an equipment focus than do recurve and traditional archers. Arguing over “what <fill in the blank> is the best …” seems to be a way to talk about their sport and stay engaged. I never pay those discussions/arguments much attention.