Inputs
Example data table
| Setup | Motor rated | Distances | Rod | Span | Balance | Estimated limit |
|---|---|---|---|---|---|---|
| Compact grill | 10 kg @ 15 cm | Actual 16 cm | 10 mm mild steel | 55 cm | Slight | ~7–9 kg |
| Long spit roast | 20 lb @ 6 in | Actual 10 in | 3/8 in stainless | 28 in | Poor | ~8–11 lb |
| Supported rig | 15 kg @ 15 cm | Actual 15 cm | 12 mm mild steel | 70 cm | Balanced | ~10–13 kg |
Formula used
This calculator estimates a safe maximum load using two checks and then takes the smallest value.
- Motor torque capacity: T = Wrated · rref
- Allowable load at your distance: Wmotor = (T · η · kbal · ksup) / (ract · SFmotor)
- Section modulus (solid round): Z = π d³ / 32
- Allowable stress: σallow = σyield / SFrod
- Allowable moment: Mallow = σallow · Z
- Max bending moment: Mmax = W · L / 4 (center) or W · L / 8 (uniform)
- Rod limit: Wrod = (Mallow · factor) / L
Wlimit = min(Wmotor, Wrod, Wmanufacturer)
How to use this calculator
- Select your unit system and enter your motor’s rated load.
- Enter the reference distance that rating assumes.
- Measure motor-to-load center distance along the spit.
- Pick balance quality and your support setup.
- Enter rod diameter, material, and support span.
- Set safety factors, then press Calculate.
- Use the smallest limit for safer garden cooking.
- Download CSV or PDF to keep the results.
Motor rating is not the whole story
A rotisserie motor label usually assumes a balanced load at a stated offset from the spit axis. Treat it as a torque proxy, not a guarantee. If your motor is rated for 12 kg at 15 cm, the implied torque is 12×9.81×0.15 ≈ 17.7 N·m. Aging gearboxes, grease, and wind at an open garden station can reduce usable torque during long, slow cooks.
Distance and imbalance multiply the torque demand
Torque demand scales with the center-of-mass distance. If the distance increases from 15 cm to 30 cm, the required torque doubles and the safe load roughly halves. Imbalance adds dynamic peaks every rotation. The calculator applies a balance factor and a motor safety factor so you can model “slight wobble” versus “shifting load” and keep margin for start-up torque.
Spit diameter matters more than it looks
A solid round rod resists bending with section modulus Z = πd³/32, so strength rises with the cube of diameter. Moving from 10 mm to 12 mm increases bending capacity by (12/10)³ ≈ 1.73, a big gain for heavy roasts. Stainless is convenient for cleaning, but typical yield strength can be lower than mild steel; aluminum softens quickly with heat.
Support span and load distribution set the bending moment
The worst case is a concentrated load at mid-span, where maximum moment is W·L/4. A more uniform load lowers peak moment to W·L/8, effectively doubling the rod-based limit for the same span. Shortening the support span, adding an idle-end bearing, or using dual supports reduces bending and friction, improving stability and motor efficiency.
Use the smallest limit and verify in the real setup
Choose the minimum of motor limit, rod limit, and any manufacturer cap. Then do practical checks: the spit should spin freely by hand, the motor should start without stalling, and the load should not “walk” sideways. Re-tie the roast, center it, and re-run the calculator after adjustments.
FAQs
Should I rely only on the motor rating?
No. The label assumes a balanced load and a specific offset. Real setups lose torque to bearings, wind, and start-up inertia. Use the calculator, apply conservative safety factors, and always obey the manufacturer’s stated maximum.
What distance should I measure for the load center?
Measure from the motor drive point (where torque is applied) to the roast’s center of mass along the spit. If you are unsure, assume the center is farther out and re-balance the load after a test spin.
How do I choose a reasonable efficiency value?
For clean bearings and a healthy gearbox, 0.80–0.90 is common. If the spit feels stiff, the motor runs hot, or the supports are misaligned, use 0.70–0.80. Improving alignment often raises efficiency more than upsizing the motor.
When should I select “center load” instead of “uniform”?
Pick “center load” when the roast mass is concentrated in the middle or slides during rotation. Use “uniform” only when the load is spread evenly between supports and secured tightly.
Does cooking heat affect the spit limit?
Yes. Metals generally lose strength as temperature rises, especially near the firebox. Keep the rod as cool as practical with shields or spacing, avoid thin aluminum spits, and increase the rod safety factor for long cooks.
What if my result is higher than my manual cap?
Use the manual cap. The calculator cannot account for every gearbox, mounting, or warranty constraint, so the manufacturer limit is the hard ceiling.