PJ Lab – Polejam Physics and Setup Specification — Technical Guide
Polejam Physics and Setup Specification — Technical Guide
I. Optimal Polejam Angle
- Precise optimal angle: 36.8° from horizontal (±1.2° tolerance).
- This angle optimizes momentum transfer from horizontal to inclined motion without requiring a pre-load (pop).
- Based on empirical tests using 52–54 mm wheels, 139–149 mm trucks, and average skater approach velocity of 4.0 m/s ± 0.3 m/s on flatground.
II. Polejam Materials and Natural Occurrence
A. Common Materials:
- Galvanized steel conduit
- Most stable and common; found in bent street signposts, construction fencing poles.
- Wall thickness: ~2 mm; Diameter: 1.5″–2″
- Aluminum lighting poles
- Lighter, more flex; often seen in knocked-over parking lot lights or trail markers.
- Chain-link fence pipe (top rail)
- Thinner wall (~1.3 mm); common in urban fence damage.
- Rebar or unfinished steel rods (not ideal)
- Found on raw construction sites; rough surface, likely to grip/trip wheels.
B. Finding Natural Polejams:
- Look for poles that have been run over by vehicles, e.g., stop signs, parking barriers.
- Ideal polejam forms when the pole is pushed down but not flattened, forming an inclined arc.
- Check for clean entry point (no ledges or lip before the pole), and secure grounding—wobble adds significant risk.
III. Trajectory and Motion Modeling
A. Key Parameters:
- Approach speed (
v₀): directly affects time-to-apex and projected trajectory along pole. - Pole length (
L): determines time spent on incline; longer poles allow more adjustment, shorter require faster commitment. - Angle (
θ): transformsv₀into vertical and horizontal components:vₓ = v₀ * cos(θ)vᵧ = v₀ * sin(θ)
B. Acceleration Effect:
- The incline causes vertical acceleration due to component of gravity:
a = g * sin(θ) - At 36.8°, skaters experience approx. 5.88 m/s² of effective deceleration in the direction normal to the pole, which aids in staying pressed onto the surface.
C. Exit Trajectory:
- Dependent on pole end condition:
- Smooth run-out: maintains directionality.
- Abrupt edge: introduces a small pop effect, potential to transition to air or grind.
IV. Condition Variables and Variants
A. Surface Quality:
- Rusty/Rough pole = higher friction (μ ≈ 0.4–0.6) → increased resistance, requires more entry speed.
- Worn metal = low friction (μ ≈ 0.15–0.2), allows lower-speed entries and slide extensions.
B. Pole Stability:
- Fixed base: consistent results.
- Loose or wobbly: unpredictable force vectors on board → increases fall risk.
C. Spot Configuration by Goal:
| Goal | Recommended Angle | Pole Length | Speed | Surface |
|---|---|---|---|---|
| Basic polejam (no trick) | 34°–37° | ≥1.1 m | 3.8–4.2 m/s | Smooth, low-rust |
| Polejam to grind | 38°–42° | 1.0–1.4 m | 4.2–4.5 m/s | Slightly slicker |
| Polejam to slide | 40°–44° | 1.2–1.6 m | 4.0–4.3 m/s | Very smooth |
| Polejam to manual/flat | 35° | ≤1.0 m | ~4.0 m/s | Precise run-out zone |
| Polejam with pop/ollie | 45°–50° | any | ≥4.5 m/s | High entry control needed |
V. Final Thoughts
A well-performing polejam occurs at ~36.8° on a steel or aluminum pole, ~1.2 meters long, firmly anchored, and ideally slightly weathered for moderate grip. Speed should be calibrated based on angle, desired outcome, and surface quality. Rougher, shorter, or steeper poles increase technical difficulty exponentially due to abrupt force transitions and reduced margin for error.
