Arrow build for the Hoyt Carbon RX-8 Ultra (2024) — spine, weight, speed
The short answer. On a Hoyt Carbon RX-8 Ultra (2024) at 80 lb and a 29″ draw, the engine builds a 414-grain arrow on 250 spine flying about 321 fps, making 95 ft‑lb of kinetic energy, 0.59 slug‑fps of momentum, at 12.3% FOC. Every number below is this engine's output for this exact bow — not a chart, not a rule of thumb, and not copied from anybody's spec sheet.
The Hoyt Carbon RX-8 Ultra (2024) is rated at 338 fps IBO with 80% let-off. That IBO figure is measured with a 350‑grain arrow at 30 inches and 70 pounds — a test arrow, not a hunting arrow. The tables below show what the bow actually does with an arrow you would take into the woods, at every draw weight it offers.
The recommended starting build
- Spine: 250 — engine-picked for 80 lb at 29″ with a 100‑grain point
- Total arrow weight: 414 grains
- Estimated speed: ~321 fps
- Kinetic energy: 95 ft‑lb
- Momentum: 0.59 slug‑fps
- FOC: 12.3%
▸ Open this exact build in The Forge — free, and it loads with every component above already selected.
Every draw weight, side by side
Spine is not a property of the bow. It is set by draw weight, arrow length and point weight acting together, which is why the engine's pick changes down this column. If your bow is not maxed out, read the row that matches your actual poundage.
| Draw weight | Spine the engine picks | Total arrow weight (gr) | Speed (fps) | Kinetic energy (ft‑lb) | Momentum (slug‑fps) | FOC |
|---|---|---|---|---|---|---|
| 30 lb | 500 | 318 | 261 | 48 | 0.37 | 16.0% |
| 35 lb | 400 | 336 | 264 | 52 | 0.39 | 15.2% |
| 40 lb | 400 | 336 | 274 | 56 | 0.41 | 15.2% |
| 45 lb | 340 | 356 | 275 | 60 | 0.43 | 14.3% |
| 50 lb | 340 | 356 | 285 | 64 | 0.45 | 14.3% |
| 55 lb | 340 | 356 | 295 | 69 | 0.47 | 14.3% |
| 60 lb | 340 | 356 | 305 | 74 | 0.48 | 14.3% |
| 65 lb | 300 | 382 | 305 | 79 | 0.52 | 13.3% |
| 70 lb | 300 | 382 | 315 | 84 | 0.53 | 13.3% |
| 75 lb | 250 | 414 | 311 | 89 | 0.57 | 12.3% |
| 80 lb | 250 | 414 | 321 | 95 | 0.59 | 12.3% |
What one inch of draw length costs
| Draw length | Spine | Weight (gr) | Speed (fps) | Kinetic energy (ft‑lb) | FOC |
|---|---|---|---|---|---|
| 27″ | 250 | 414 | 301 | 83 | 12.3% |
| 28″ | 250 | 414 | 311 | 89 | 12.3% |
| 29″ | 250 | 414 | 321 | 95 | 12.3% |
| 30″ | 250 | 414 | 331 | 101 | 12.3% |
What this build is cleared for
We grade against five energy floors and we tell you which of them are Easton's numbers and which are ours. Three come from Easton's game-class chart, which Easton retired in 2012 and which we reproduce from archive captures on the kinetic energy chart by game class. Two are ours, labelled as ours.
| Game class | Floor (ft‑lb) | Whose number | This build at 95 ft‑lb |
|---|---|---|---|
| Small game | 15 | judged | clears by 80 |
| Deer / antelope | 25 | charted | clears by 70 |
| Elk / bear / boar | 42 | charted | clears by 53 |
| Heavy game | 50 | derived | clears by 45 |
| Big game | 65 | charted | clears by 30 |
Energy is a floor, not a plan. It tells you whether an arrow has enough to work with; it does not tell you where to put it. For that you want the target size — broadside vital dimensions for 61 species are on the vital zone size page, with the sourced shoulder heights they derive from.
Tuning this bow: Hoyt HBX cam spacers
Cam lean on this Hoyt is corrected by moving spacers on the HBX cam. It is a press job. Hoyt's 2026 XTS system replaced this workflow with a no-press wedge; this bow uses the earlier spacer method.
The full tear-to-cause doctrine, and the cam-lean correction system for all 196 bows in our catalogue that have one, is on the bow tuning page.
Bow specifications
- IBO speed: 338 fps
- Let-off: 80%
- Axle-to-axle: 33.5″
- Mass weight: 4.2 lb
- Draw weight range: 30–80 lb
- Draw length range: 26.5–31″
Common questions
What arrow spine do I need for a Hoyt Carbon RX-8 Ultra (2024)?
At 80 lb and a 29-inch draw with a 100-grain point, the engine picks 250 spine for a 414-grain finished arrow. Drop to 30 lb and the pick moves to 500 spine at 318 grains. Spine is not a property of the bow — it is set by draw weight, arrow length and point weight together, which is why the table above shows every draw weight this bow offers rather than one number.
How fast does a Hoyt Carbon RX-8 Ultra (2024) shoot a real hunting arrow?
About 321 fps with the 414-grain build above at 80 lb and 29 inches. The bow's advertised 338 fps IBO is measured with a 350-grain arrow at 30 inches and 70 pounds, which almost nobody shoots. The gap between 338 and 321 is not the manufacturer lying — it is the difference between the IBO test arrow and a huntable one.
Is this arrow heavy enough for elk?
Yes on energy: 95 ft-lb against the 42 ft-lb elk, black bear and boar floor — a margin of 53 ft-lb. That 42 is Easton's charted figure, not ours. It also clears the 25 ft-lb deer and antelope floor. Energy is only half the question though: penetration on elk is driven by momentum and FOC as much as by kinetic energy, and this build sits at 0.59 slug-fps and 12.3% FOC.
What game does this Hoyt Carbon RX-8 Ultra (2024) build clear?
At 95 ft-lb it clears small game, deer / antelope, elk / bear / boar, heavy game, big game. It clears every floor we publish. Two of those floors are Easton's charted numbers (25 and 42 ft-lb), one is Easton's top band (65), one is ours placed between charted anchors (50), and one is ours with thin evidence that we label as such (15). We publish which is which rather than presenting all five as industry standard.
How do I fix a left or right tear on a Hoyt Carbon RX-8 Ultra (2024)?
Correct it at the cam, not at the rest. This bow uses Hoyt HBX cam spacers, so cam lean is adjustable on the bow itself. Chasing a horizontal tear with rest windage moves the arrow's launch point without fixing what is throwing it, which is why the tune looks fine on paper and falls apart when you screw on a fixed-blade broadhead.
Where do these numbers come from?
The same physics engine that runs The Forge, with no hand-tuning and no chart lookup. Launch speed is IBO-corrected for your actual draw weight, draw length and arrow mass. Kinetic energy and momentum use the Easton method. The spine pick comes from a model validated against the published manufacturer charts — 98% agreement with Easton, 85% with Gold Tip. Bow specs (338 fps IBO, 80% let-off) are the manufacturer's published figures. When you chronograph your real bow, your measured speed replaces the estimate everywhere.
Method, and what we are not claiming
Launch speed is the bow's published IBO corrected for your actual draw weight, draw length and finished arrow mass. Kinetic energy and momentum are the Easton method. The spine pick comes from a coefficient model published in full on the spine what-if page, including its measured agreement with each manufacturer's chart.
What we are not claiming: that this is the arrow you should shoot. It is the arrow this engine builds from a sensible hunting default on this bow, so that the numbers on this page are reproducible rather than editorial. Your draw length, your point weight and your broadhead all move every figure here, and the permalink above opens the build so you can move them.
Built by archnerds, for archnerds — The Forge by Sparrow Expeditions.