Suspension Sag Calculator

Enter three measurements taken at the rear axle in millimetres: fully topped-out length, bike-settled length (bike weight only), and rider-on length (rider aboard in full gear, hands on bars). The calculator returns static sag, race sag, and a spring-rate diagnosis.

Static sag, race sag, and the spring diagnosis

Race sag (rider sag) is the primary tuning target: the amount the rear suspension compresses with the rider aboard in riding position, measured at the rear axle. Suspension-tuning references (Race Tech, Pro Circuit, MXA) consistently use race sag as the baseline because it captures the weight the spring actually supports in use.

Static sag (free sag) is the amount the suspension compresses under the bike's weight alone. Once race sag is set by preload adjustment, static sag is a spring-rate indicator. If static sag is outside the 25-40 mm range with race sag correct, the spring rate does not match the rider weight: preload cannot fix it, and the spring must change.

The spring diagnosis in this calculator applies the 25/40 mm static-sag thresholds: below 25 mm means the spring is too soft (excess preload was needed to hit race sag, consuming free sag); above 40 mm means the spring is too stiff (rider load alone can't compress it enough). These thresholds are consistent across suspension-tuning references for rear shocks.

How to take the three measurements

All measurements are taken vertically from the rear axle center to a fixed reference point on the subframe or fender directly above it. Use a tape measure or sag scale. Measure twice and average if readings differ.

Measurement 1 (topped-out): fully extend the rear suspension so the wheel hangs free, or hold the rear of the bike up until the suspension is at full extension. Record the axle-to-reference distance.

Measurement 2 (bike settled): set the bike on the ground on a flat surface with no rider. Let the suspension settle; gently push down and release, then gently lift and release, and take the midpoint. Record the distance.

Measurement 3 (rider on): rider sits in normal riding position, feet on pegs, hands on bars, in full riding gear. An assistant holds the bike upright. Same gentle-push-and-release, gentle-lift-and-release protocol. Record the distance.

Discipline target bands

Race sag targets from suspension-tuning references: motocross rear 100-105 mm, off-road/enduro rear 105-110 mm. Fork race sag targets are model-dependent; 30-35 mm is a common published starting point for motocross. Sport-bike and street targets vary by manufacturer; consult the service manual.

Static sag target: 25-40 mm with race sag set to the discipline target. This range is consistent across Race Tech, Pro Circuit, and MXA published setup guides. If static sag is in range, the spring rate suits the rider weight. If out of range, use the Spring Rate Calculator to find the correct rate.

Sag examples computed at build time

All values computed by the same tested Go functions the calculator calls. Static sag and race sag are derived from the three axle measurements; diagnosis applies the 25/40 mm static-sag thresholds.

FAQ

How do I set race sag?

Race sag is set by adjusting the preload collar on the rear shock (or adding/removing preload spacers on some designs). Turning the collar clockwise typically increases preload, which raises the ride height and reduces race sag. Adjust, bounce the bike, take the measurement, and repeat until race sag reaches the target for your discipline. A C-spanner (shock spanner wrench) and mallet are the standard tools; some bikes require the shock to be removed to access the preload collar.

What is free sag (static sag)?

Free sag, also called static sag, is the amount the suspension sags under the bike's weight alone, with no rider. It is not a direct tuning input: you do not set it by itself. It is a diagnostic: once race sag is correct for your discipline, static sag tells you whether the spring rate matches the rider. A static sag of 25-40 mm with correct race sag means the spring is right. Outside that range, the spring rate is wrong for the rider's weight.

My sag numbers are right but the bike still feels off. What is wrong?

Sag sets the static spring preload; it does not adjust damping. If sag is correct and the bike still feels harsh, loose, or unbalanced, the problem is most likely compression or rebound damping. Harsh bump absorption with correct sag points to excessive compression damping or a bent/stiff fork leg. Vague or wallowing feel with correct sag points to insufficient rebound damping. These require separate clicker adjustments or a suspension revalve, not a spring change.

Can I use this calculator for the front fork?

The formulas are identical: static sag = topped-out minus bike-settled; race sag = topped-out minus rider-on. Measure from the bottom of the lower fork leg to the bottom of the upper tube (or a fixed reference on the fork guard), not the axle. Fork race sag targets are model-dependent and shorter than rear targets; 30-35 mm is a common published starting point. The spring diagnosis thresholds (25/40 mm static sag) are designed for the rear; fork static sag targets differ by model, so verify against your service manual.

Sources

Race sag measurement method (axle to fixed reference, bounce protocol): standard procedure in Race Tech's Motorcycle Suspension Bible (Bremmers), Pro Circuit setup guides, and MXA suspension setup articles.

Race sag targets: motocross 100-105 mm, off-road/enduro 105-110 mm, from published suspension-setup references for each discipline. Individual models may differ; verify against manufacturer or suspension-tuner recommendations.

Static sag thresholds (25-40 mm) and spring-rate diagnosis: consistent across Race Tech, Pro Circuit, and MXA published references. Implemented in formulas.SpringDiagnosis.