If you are trying to dial in a coil shock, this mountain bike spring calculator gives you a practical starting point in seconds. Enter your system weight, travel, and target sag, then get a spring-rate recommendation in lb/in and N/mm.
Rear Shock Coil Spring Calculator
Tip: Start with this number, then fine-tune based on actual sag, terrain, and riding style.
How this mountain bike spring calculator works
The goal is to estimate a coil spring rate that puts your bike in the right sag window. Sag is how much the suspension compresses under your normal riding load (you + gear + bike). When sag is correct, your bike balances traction, support, comfort, and usable travel.
The core equation used here is:
Spring Rate (lb/in) = Rear Load (lb) ÷ (Sag Fraction × Rear Wheel Travel in inches)
Then we convert to N/mm for riders and shops using metric spring specs.
Input guide (what each field means)
Rider, gear, and bike weight
Use realistic numbers from your normal trail setup. If you ride with a full pack and water, include it. The more honest you are here, the closer the first spring choice will be.
Rear weight bias
This estimates how much total system weight is carried by the rear wheel at sag. A centered trail stance often lands around 58–62%, while very steep or seated climbing positions can shift this.
Rear wheel travel and target sag
Most trail bikes run around 25–32% sag. Enduro riders may choose a touch deeper sag for traction, while pumpy bike-park setups may run less for support.
Shock stroke
Stroke helps compute leverage ratio and sag at the shock. This is useful for sanity-checking whether your frame is in a typical leverage range.
What to do with the result
- Use the recommended spring as a starting point.
- If your exact value is unavailable, choose the nearest stock spring (25 or 50 lb/in increments).
- Re-check sag in full riding kit on level ground.
- Then tune damping (especially rebound) after spring rate is correct.
Example setup
Suppose your total system weight is 222 lb, rear bias is 60%, rear travel is 150 mm, and target sag is 30%.
- Rear load = 222 × 0.60 = 133.2 lb
- Travel = 150 mm = 5.91 in
- Sag displacement at wheel = 5.91 × 0.30 = 1.77 in
- Spring rate = 133.2 ÷ 1.77 = 75.3 lb/in wheel-equivalent
Converted to a shock spring recommendation through real geometry assumptions, this calculator gives a practical spring-rate target and nearest commercially available options.
Common mistakes riders make
- Using body weight only and forgetting pack, shoes, and helmet.
- Running too much preload to “fake” the correct spring rate.
- Ignoring rear weight bias and riding position.
- Changing rebound/compression before spring rate is sorted.
- Judging setup from parking-lot bounces instead of trail feedback.
Quick post-install checklist
- Set preload to manufacturer minimum starting point (often 1–2 turns).
- Measure actual sag in full kit.
- If sag is too deep, go one spring step stiffer.
- If sag is too shallow, go one step softer.
- Reset rebound after every spring-rate change.
Final note
No calculator can replace on-trail tuning, but a strong first estimate saves time, money, and frustration. Use this tool to get close, then refine for your terrain, speed, and riding style.