Most archers think the string is working hardest at full draw. That's when it looks taut, when you can feel the resistance, when the whole system seems most energized. It makes intuitive sense.
It's also backwards.
The string on a compound bow is under its highest tension when the bow is just sitting at brace — resting against your bow hand, doing nothing. The moment you start drawing, string tension starts dropping. By the time you reach full draw, the string is holding almost nothing at all. The cables have taken everything.
The actual numbers
At full draw on a 70 lb compound with 80% let-off:
- String at full draw: ~10 lb
- Each cable at full draw: ~209 lb
The cables are doing almost everything at full draw. The string is nearly along for the ride.
Load by draw weight — at full draw
Numbers assume 80% let-off and two control cables. Scaled from a measured 70 lb reference.
| Peak draw weight | Each cable at full draw | Main string at full draw |
|---|---|---|
| 40 lb | ~119 lb | ~6 lb |
| 50 lb | ~149 lb | ~7 lb |
| 60 lb | ~179 lb | ~8 lb |
| 65 lb | ~194 lb | ~9 lb |
| 70 lb | ~209 lb | ~10 lb |
| 75 lb | ~224 lb | ~11 lb |
| 80 lb | ~239 lb | ~11 lb |
Why the geometry does this
The cam is not a wheel. It's an eccentric — an asymmetric rotating lever with the string groove at one radius and the cable grooves at another. As the cam turns, those radii change their relationship to the draw axis, and that's what moves the tension around.
At brace, the string is wrapping around the largest part of the cam profile. The effective lever arm on the string side is long, which sounds like an advantage — but the cable grooves are sitting at a short radius at this moment, so the cables are doing almost nothing. The string is holding the whole system together more or less by itself.
Start drawing and the cam rotates. The string begins peeling off the narrow part of the profile. The cable grooves rotate into a longer lever arm position. You're not adding force — you're transferring it. Every degree of cam rotation that shortens the string's mechanical advantage lengthens the cables'. By the time you're at full draw, the cam has rotated far enough that the cable grooves have maximum leverage and the string groove is at minimum. The string is barely contributing. The cables are doing 209 lb of work each because the cam is giving them the leverage to do it.
This is why let-off exists. At full draw, the cam has rotated past the peak — the cable lever arm is so long, and the limb has so little travel left, that holding the draw requires almost nothing. The cam literally gives the cables so much mechanical advantage that the system becomes self-locking. You're not holding 70 lb. You're holding whatever the cam geometry left over after it moved almost all of that tension into the cables.
What this means for your string
The string's worst moment is not full draw. It's every time the bow returns to brace after a shot.
The shot cycle, from the string's perspective, goes like this: tension high at rest → tension drops to almost nothing at full draw → arrow leaves → string slams back to full brace tension as the bow recovers. That return event — the snap from low tension back to brace — is the load cycle the string experiences on every shot.
The repetitive load event is that snap back to brace — thousands of times across thousands of shots.
The dry fire
On a normal shot with a 400-grain arrow, roughly 82 ft-lb of stored energy leaves the system with the arrow. The bow absorbs the rest — about 18 ft-lb — through limb oscillation, cam movement, and whatever damping is built into the riser. That's the residual the system is designed to handle. It does it on every shot and it's fine.
On a dry fire, there's no arrow. All 100 ft-lb stays in the system. What changes is not just the amount of energy — it's where it goes and how fast it arrives.
A dry fire converts what would have been a controlled cam rotation into a sudden, uncontrolled one. With no arrow to resist the string, the cams and limbs accelerate almost freely until the main string catches at brace and the system stops dead. That snap — 100 ft-lb absorbed in roughly 5 milliseconds — delivers an impulse load of over 600 lb to the string, cams, and limbs simultaneously.
That's usually not enough to break the string outright. The string body has a safety factor that covers it. But 600 lb is more than enough to bend cams, crack limbs, or cause catastrophic failure elsewhere in the system. And at the peep — where the string is forced to open around a plastic insert under that same impulse — the string is significantly weaker than in open air. That's where dry fire failures show up when the string does go.
Safety factors
A 24-strand 452X string has a break strength around 1,680 lb. The safety factor on string tension alone is enormous.
Strings are not failing because they're close to their tension limit. They're not close. The safety factor is enormous. What actually kills a string is time and repetition: wear at the serving, separation where the strands separate at the loop, and creep that changes the bow's tune over hundreds of shots. The loads are not the problem. The miles are.
Cable safety factors at full draw are tighter — a 24-strand cable against 220 lb peak load has a safety factor around 7. Still comfortable, and still not the failure mode you should be worried about.
The thing that stays with you
At full draw on a 70 lb bow, the string is holding about 10 lb. The cables are holding 209 lb each. The component that looks like it's doing the work is the one that's nearly along for the ride.
The string holds more than you think. And less at the moment you think it holds the most. The geometry set that up the day the bow was designed, and every shot you've taken since has followed exactly that pattern. The inversion isn't an accident. It's how compound bows work.
Published 2026-09-14 · Axial Bowstrings