Chassis Tuning Calculators

Two essential chassis setup tools — lateral G from skidpad testing, and coil spring rate from physical measurements. Both work with real-world measurements you can take at the circuit or workshop.

Skidpad G-Force

Drive a circle of known radius at the limit of grip — time several laps and use the average. Works best on a smooth, level surface with a consistent radius. A 150ft (45.7m) radius is the Car & Driver standard. Measure the circle centre-to-centre of tyre contact patch.

Circle radius (metres)

Centre of tyre contact patch to centre of circle

Circle radius (feet)

Lap time (seconds)

Average of several laps at the limit of grip

Road car (standard): 0.7–0.85G Performance road car: 0.85–1.0G Track/sprint car: 1.0–1.4G Full race car: 1.4–2.0G+

G = (4π² × r) / (g × t²)
r = radius (m), t = lap time (s), g = 9.81 m/s²
Equivalent to: G = (r × 0.03948) / t² (constant = 4π²/g)

Coil Spring Rate

Calculates spring rate for steel coil springs using the standard torsional stiffness formula. Does not apply to variable-rate, rubber, or composite springs. All measurements in inches — use a digital calliper for accuracy. Spring rate does not change over the spring's life unless it is damaged or fatigued.

Number of active coils

Do not count end coils that rest on the perch — only count coils free to deflect

Wire diameter (inches)

Measure with a digital calliper — accuracy matters, value is raised to the 4th power

Mean coil diameter (inches)

Average diameter of the coils viewed from above — not the outside diameter

k = (G × d⁴) / (8 × n × D³)
G = 11.25 × 10⁶ psi (steel shear modulus)
d = wire diameter (in), D = mean coil diameter (in), n = active coils
Result in lb/in. 1 lb/in = 0.1751 N/mm = 0.01786 kg/mm

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