What is an Ohlins MTB Spring Rate?
An Ohlins MTB spring rate refers to the stiffness of the coil spring used in an Ohlins rear shock absorber for mountain bikes. The spring rate dictates how much force is required to compress the spring by a certain amount. Choosing the correct spring rate is crucial for optimizing your mountain bike's suspension performance, ensuring it provides the right balance of support, control, and comfort for your riding style and terrain.
This calculator is designed for riders using Ohlins rear shocks, but the principles apply to most coil-sprung mountain bike suspensions. Riders who are unsure about their current setup, experiencing issues like excessive bottom-outs or a feeling of being "pogo-sticked," or those who have recently changed their bike, weight, or riding style will benefit most from using this calculator.
A common misunderstanding is that spring rate is solely determined by rider weight. While rider weight is a primary factor, the bike's suspension design (leverage ratio), shock stroke, and desired sag percentage significantly influence the ideal spring rate. Using a calculator that accounts for these variables provides a much more accurate starting point than simple weight-based charts.
Ohlins MTB Spring Rate Formula and Explanation
The calculation for an optimal MTB spring rate involves understanding how rider and bike weight, combined with the bike's suspension kinematics and shock specifics, translate into the force needed to achieve the desired sag. The core formula is derived from Hooke's Law (F=kx) and considers the leverage ratio of the bike's suspension linkage.
The primary calculation for the required spring rate in Newtons per millimeter (N/mm) is:
Spring Rate (N/mm) = [ (Rider Weight + Bike Weight) * 9.81 * Leverage Ratio ] / (Shock Stroke * Sag Percentage)
Where:
- Rider Weight: Your total weight including gear (helmet, hydration pack, tools, etc.), measured in kilograms (kg).
- Bike Weight: The total weight of your bicycle, measured in kilograms (kg).
- 9.81: The acceleration due to gravity (m/s²), converted to N/kg.
- Leverage Ratio (LR): The ratio of rear wheel travel to shock travel at a specific point in the suspension stroke. This is a critical factor determined by your bike's specific suspension design.
- Shock Stroke: The total length the shock shaft can travel, measured in millimeters (mm).
- Sag Percentage: The percentage of the total shock stroke compressed under the rider's weight when seated, typically expressed as a decimal (e.g., 30% = 0.30).
We also calculate intermediate values to better understand the forces involved:
- Total Force (N): The combined weight of rider and bike translated into force.
- Target Sag (mm): The absolute amount of travel the shock should compress based on stroke and desired sag percentage.
- Required Force at Sag (N): The force exerted by the spring at the target sag depth. This value should ideally match the Total Force * Leverage Ratio.
Variables Table
| Variable |
Meaning |
Unit |
Typical Range |
| Rider Weight |
Total weight of rider with gear |
kg |
60 – 120+ |
| Bike Weight |
Total weight of the bicycle |
kg |
12 – 20+ |
| Sag Percentage |
Desired suspension compression under load |
% |
25 – 35 |
| Shock Stroke |
Total travel of the rear shock |
mm |
50 – 75 |
| Leverage Ratio (LR) |
Bike's suspension linkage ratio |
Unitless |
1.8 – 3.5 |
| Spring Rate (N/mm) |
Stiffness of the coil spring |
N/mm |
250 – 700+ |
| Spring Rate (lb/in) |
Alternative common measurement for spring stiffness |
lb/in |
140 – 400+ |
Practical Examples
Understanding the calculation with real-world scenarios helps in choosing the right spring.
Example 1: Aggressive Trail Rider
- Rider Weight: 85 kg (with gear)
- Bike Weight: 16 kg
- Target Sag: 33%
- Shock Stroke: 65 mm
- Leverage Ratio (LR): 2.85
Calculation:
Total Force = (85 kg + 16 kg) * 9.81 = 101.01 kg * 9.81 = 991.00 N
Target Sag = 65 mm * 0.33 = 21.45 mm
Required Force at Sag = 991.00 N * 2.85 = 2824.35 N
Spring Rate (N/mm) = 2824.35 N / 21.45 mm = 131.67 N/mm
Spring Rate (lb/in) = 131.67 N/mm * 0.1751 ≈ 23.05 lb/in
Result: The rider would likely need a spring rate around 300-325 lb/in (approx. 52-57 N/mm) considering the conversion. Ohlins often uses different spring designations, but this provides a starting point. A common recommendation might be around a 300 lb/in spring.
Example 2: Lighter Enduro Rider
- Rider Weight: 68 kg (with gear)
- Bike Weight: 15 kg
- Target Sag: 30%
- Shock Stroke: 60 mm
- Leverage Ratio (LR): 2.60
Calculation:
Total Force = (68 kg + 15 kg) * 9.81 = 83 kg * 9.81 = 814.23 N
Target Sag = 60 mm * 0.30 = 18 mm
Required Force at Sag = 814.23 N * 2.60 = 2117.00 N
Spring Rate (N/mm) = 2117.00 N / 18 mm = 117.61 N/mm
Spring Rate (lb/in) = 117.61 N/mm * 0.1751 ≈ 20.59 lb/in
Result: This rider would likely need a spring rate around 200-225 lb/in (approx. 35-39 N/mm). A common recommendation might be around a 200 lb/in spring.
Note: Ohlins springs are often specified in N/mm, while many aftermarket springs (and calculators) use lb/in. The conversion factor is approximately 1 lb/in = 0.1751 N/mm.
How to Use This Ohlins MTB Spring Rate Calculator
Using the Ohlins MTB Spring Rate Calculator is straightforward:
- Enter Rider Weight: Accurately weigh yourself with all the gear you typically wear while riding (helmet, pack, shoes, etc.).
- Enter Bike Weight: Weigh your bike if possible. If not, use an estimated weight based on its model and components.
- Set Target Sag Percentage: For most trail and enduro riding, 25-35% is a good starting point. Aggressive riders or those on downhill bikes might prefer slightly lower sag (e.g., 25-30%), while freeride or softer applications might use higher sag (30-35%).
- Input Shock Stroke: Find the total travel length of your Ohlins rear shock. This is usually stamped on the shock body or can be found in your shock's specifications.
- Select Shaft Diameter: Choose the correct shaft diameter for your Ohlins shock. Common sizes are 9.5mm, but verify for your specific model.
- Determine Leverage Ratio (LR): This is crucial. Check your bike manufacturer's website, your bike's manual, or use a dedicated mountain bike leverage ratio calculator (internal link placeholder). It's often a value between 2.0 and 3.5.
- Click "Calculate Spring Rate": The calculator will display your recommended spring rate in both N/mm and lb/in, along with the calculated target sag depth and the required force.
- Interpret Results: Compare the calculated rate to available Ohlins springs or aftermarket options. Small adjustments (e.g., +/- 25 lb/in or 5 N/mm) might be needed based on personal preference.
- Reset: Use the "Reset" button to clear all fields and start over.
- Copy Results: Use the "Copy Results" button to easily save or share your calculated values.
Always consult your Ohlins manual or a qualified suspension technician if you are unsure about any settings.
Key Factors That Affect Your MTB Spring Rate Choice
Several factors beyond basic weight influence your ideal spring rate. Fine-tuning these can significantly impact suspension performance:
- Riding Style: Aggressive riders who hit jumps, drops, and compressions hard may benefit from a slightly firmer spring (or lower sag) to prevent bottom-outs. Smoother riders might prefer a slightly softer spring for better initial sensitivity.
- Terrain: Riding primarily on rough, technical terrain might require a different spring setup than smooth flow trails. A stiffer spring can offer more support through rough sections, while a softer one might absorb small bumps better.
- Bike's Suspension Design (Linkage): Different suspension designs have vastly different leverage ratios. A high-LR bike amplifies the forces acting on the shock, requiring a proportionally stiffer spring compared to a low-LR bike for the same rider weight and sag. This is why the LR is a critical input.
- Shock Stroke Length: For a given amount of wheel travel, a shock with a shorter stroke will have a higher effective leverage ratio, demanding a stiffer spring. Conversely, a longer stroke shock requires a softer spring to achieve the same sag.
- Air vs. Coil Shock: While this calculator is for coil springs, it's important to note that air shocks have a progressive air spring curve, meaning their effective spring rate increases throughout the travel. Coil springs offer a more linear rate, making precise spring rate selection paramount.
- Personal Preference: Ultimately, suspension tuning is subjective. Some riders prefer a firmer, more supportive feel, while others prioritize plushness and small-bump sensitivity. The calculated rate is a starting point; fine-tuning by +/- one spring rate increment is common.
- Riding Environment: Downhill riders often use lower sag (firmer springs) for stability at speed, while cross-country riders might opt for slightly higher sag (softer springs) for climbing traction and efficiency.
- Tire Pressure and Volume: While not directly part of the spring rate calculation, tire pressure and the volume of the tire can affect how the suspension feels. Lower tire pressures can mimic a slightly softer suspension feel.
Related Tools and Resources
Explore these related tools and resources for a deeper understanding of mountain bike suspension tuning:
