Solid height and coil bind, explained

A compression spring has a hard floor: the length where every coil is touching and it can travel no further. That is its solid height, and running into it — coil bind — turns the spring into a solid column. Getting the geometry right means the working stroke never gets there.

Solid height depends on the ends

Solid height is the wire stacked up, plus an allowance for how the ends are finished:

closed & ground: Ls = d·(Na + 2) · closed/squared: d·(Na + 3) · open/plain: d·(Na + 1)

For a 1 mm wire spring with 8 active coils, that is 10 mm solid ground, 11 mm squared, or 9 mm plain — the end treatment moves it a full coil either way. The spring pitch calculator runs all three end types and the pitch math below.

Travel, pitch and the gap

Travel to solid is simply what is left above the stack, and pitch spreads that travel evenly across the active coils:

travel to solid = L0 − Ls · p = d + (L0 − Ls)/Na · active-coil gap = p − d

A 40 mm free length over that 10 mm solid stack leaves 30 mm of travel, a 4.75 mm pitch and a 3.75 mm gap between turns. Know the pitch you want instead? The same relation reverses to L0 = Ls + Na·(p − d).

Why coil bind matters

The working stroke must stay clear of solid. At a 10 mm working deflection the example spring sits at 30 mm long with 20 mm of clearance to solid — comfortable. Drive a spring to coil bind and force climbs almost vertically, stress spikes, and the part it pushes against takes the shock. Rate, Wahl-corrected stress and buckling are the next checks, in the compression spring calculator; designing wire and coils from a target load is the spring design guide.

Common mistakes

• Using the wrong end allowance. Squared-and-ground vs plain ends shift solid height by a full coil of wire — size the stack for the ends you actually have.
• Designing the stroke to solid height. Leave clearance for tolerance, surge and accidental overtravel; the working point should never reach coil bind.
• Confusing pitch with the coil gap. Pitch is center-to-center (p); the open space between turns is p − d. Mixing them mis-sizes the free length.
• Treating the simplified stack as exact. Supplier drawings add grinding allowance, set and tolerance — confirm the final solid height against the spring drawing.

Frequently asked questions

What is the solid height of a compression spring?

The length when every coil is touching — the wire stack at full compression, where the spring can travel no further. It depends on wire diameter and the end treatment: closed & ground ends stack as Ls = d·(Na + 2), closed/squared as d·(Na + 3), open/plain as d·(Na + 1). A 1 mm wire, 8 active coil, closed-and-ground spring is 10 mm solid.

What is coil bind?

Coil bind is reaching solid height — all coils touching, no travel left. A spring driven to coil bind stops acting as a spring and becomes a solid column, spiking force and stress. The usable travel before it is travel to solid = free length − solid height: 30 mm for the 40 mm-free-length example.

How do you find spring pitch from free length?

Spread the available travel across the active coils: p = d + (L0 − Ls)/Na, the center-to-center distance between active coils. The active-coil gap — the open space between turns — is p − d. The 40 mm example gives a 4.75 mm pitch and a 3.75 mm gap.

How much clearance to solid should a spring have?

Enough that the working stroke never reaches coil bind, with margin for tolerance, surge and overtravel. The geometry tells you what is left — the 40 mm spring at 10 mm working deflection still has 20 mm of clearance — but the production margin itself comes from the application, fatigue and surge requirements, not a fixed rule.