Stiffness Balance

Ratio E×t_tension / E×t_compression. Target ≈ 1.0. Values 0.85–1.15 are considered well balanced. Tension dominant >1.15; compression dominant <0.85.

Neutral Axis

Distance from board base (ski side) to the neutral bending axis in mm. Layers below NA are in tension; above are in compression under positive bending.

Uniformity

Standard deviation / mean of the balance ratio along the board length, expressed as %. Lower is more consistent behaviour tip-to-tip.

EI (Bending Stiffness)

Flexural rigidity per unit width in GPa·mm³ × 10³. Midpoint: value at widest core section. Mean: average along board. Equiv: effective EI for a simply-supported deflection calculation.

Strength Balance

Ratio σ×t_tension / σ×t_compression. Indicates which side will fail first under bending. Target ≈ 1.0.

Strain Balance

Ratio of minimum failure strains (σ/E) on tension vs compression sides. The weakest-link layer on each side. Target ≈ 1.0.

Torsional Stiffness

GJ_equiv = L / ∫(1/GJ(x))dx, in ×10³ GPa·mm³/mm (per unit width — multiply by board width for absolute value). Torsional compliance adds in series, so soft tips dominate — directly analogous to Standard Hardness. ±45° layers are the dominant contributors; the core contributes little.

Midpoint Torsion

Torsional stiffness GJ at the board's thickest cross-section (widest core), before the tips thin it out. Compare to Torsional Stiffness: a large gap means the tips are torsionally soft relative to the centre — the board twists easily at the tips regardless of how stiff the body is.

Twist Drive

max(e) − min(e) where e(x) = y_NA(x) − T(x)/2 is the distance of the neutral axis from the geometric mid-plane. When e(x) varies along the board, each bending increment creates a small twist moment — the board steers itself without rider input. A uniform offset is harmless (the rider adapts once); only variation drives chatter. Thresholds: < 0.04 mm → clean, 0.04–0.08 mm → moderate, > 0.08 mm → chatter risk.

Chatter index

Measures how much the effective turn radius varies along the contact zone due to bending–torsion coupling. When the neutral axis is offset from the board's geometric mid-plane, bending loads also twist the board. This changes the effective edge angle θ_eff(x) and thus the local sidecut radius R_local(x) = R_sidecut × cos(θ_eff). Chatter index = (1/span) × ∫|R_local(x) − R̄_local| dx, converted to mm. Only variation of the NA offset along the board contributes — a uniform offset cancels out. Lower = more consistent edge contact. Requires the Bending & Carving section to be active.

E×t Values

Sum of modulus × thickness for all layers on each side of the neutral axis. Used to compute stiffness balance directly. Units: GPa·mm.

Flex Test

3-point bend simulation. Apply moment M at center; supports symmetrically placed. Deflection δ = M·L²/(12·EI·w·10³). Back-calc EI from measured δ for calibration.