Trek Suspension Calculator

Your expert starting point for dialing in your Trek mountain bike’s rear suspension.

Suspension Settings Calculator

Recommended Air Pressure

205 PSI

Sag in Millimeters

15.4 mm

Rebound Setting

5 Clicks

Formula Explanation: Pressure is estimated based on rider weight, with an adjustment for your desired sag percentage. Rebound is a suggested starting point based on pressure (higher pressure needs slower rebound). These are starting points for your first ride.

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What is a Trek Suspension Calculator?

A suspension calculator for Trek bikes is a specialized tool designed to provide a highly accurate starting point for setting up your mountain bike’s rear air shock. Unlike generic calculators, it considers factors and terminology relevant to Trek’s unique suspension designs, like RE:aktiv and Thru Shaft. It translates your weight and preferences into a baseline air pressure (in PSI) and a rebound setting (in clicks).

This calculator is essential for any Trek owner because proper suspension setup is the single most important factor for bike performance. An incorrect setup can lead to poor traction, harshness over bumps, or excessive bottom-outs. While Trek’s official calculator is excellent, this tool provides an alternative model and explains the “why” behind the numbers, empowering you to make your own adjustments on the trail.

The Trek Suspension Formula and Explanation

While Trek’s exact formulas are proprietary and bike-specific, we can use a widely accepted physics-based model to get very close. The core principle is that the air spring must support the rider’s weight at a specific sag percentage.

Primary Formula (Air Pressure):

Recommended PSI ≈ (Rider Weight in lbs) * (Pressure Factor based on Sag)

Secondary Formula (Sag Measurement):

Sag (mm) = Shock Stroke (mm) * (Desired Sag % / 100)

Suspension Setup Variables

Variable	Meaning	Unit	Typical Range
Rider Weight	Total weight of the rider with all gear.	lbs or kg	100 – 300 lbs / 45 – 135 kg
Shock Stroke	The travel of the rear shock shaft itself.	mm	40mm – 75mm
Sag	The percentage of shock travel used under rider weight.	%	20% (Firm) – 35% (Plush)
Air Pressure	The pressure inside the shock’s main air chamber.	PSI	100 – 300 PSI
Rebound	The speed the shock returns after compression.	Clicks from closed	0 (Fast) – 20 (Slow)

Practical Examples

Example 1: Trail Rider on a Trek Fuel EX

A rider weighs 190 lbs and has a Trek Fuel EX with a 57.5mm stroke shock. They want a balanced setup and aim for 28% sag.

• Inputs: Weight: 190 lbs, Stroke: 57.5mm, Sag: 28%
• Calculated Results:
  • Recommended PSI: ~195 PSI
  • Sag in mm: 16.1 mm
  • Rebound: ~4-5 clicks from closed

Example 2: Enduro Rider on a Trek Slash

A more aggressive rider weighs 210 lbs with gear and has a Trek Slash with a 62.5mm stroke shock. They prefer a slightly plusher ride for descents and aim for 30% sag.

• Inputs: Weight: 210 lbs, Stroke: 62.5mm, Sag: 30%
• Calculated Results:
  • Recommended PSI: ~205 PSI
  • Sag in mm: 18.8 mm
  • Rebound: ~3-4 clicks from closed

How to Use This Trek Suspension Calculator

1. Enter Your Weight: Put on all your riding gear (helmet, shoes, pack with water) and weigh yourself. Enter this number and select either ‘lbs’ or ‘kg’.
2. Find Your Shock Stroke: Check the side of your rear shock or look up your specific Trek model and year on their website to find the shock stroke in millimeters. Enter this value.
3. Choose Your Sag: Decide on your desired sag. For general trail riding, 25-30% is a great starting point. Enter this percentage.
4. Calculate: Click the “Calculate” button.
5. Interpret the Results: The calculator will provide a starting air pressure, the physical sag measurement to check for, and a starting rebound setting. Use a shock pump to inflate your shock to the recommended PSI.
6. Verify Sag: With your gear on, carefully sit on the bike and let the suspension settle. Slide the rubber O-ring on the shock up to the seal. Carefully get off the bike and measure the distance the O-ring moved. Adjust air pressure up or down in 5-10 PSI increments until you reach the target ‘Sag in Millimeters’.

Key Factors That Affect Trek Suspension Settings

• Riding Style: Aggressive riders who hit jumps and drops need more pressure and slower rebound than smoother, cross-country focused riders.
• Terrain Type: Rough, chunky, high-speed terrain may require slightly lower pressure and faster rebound to improve tracking compared to smooth, flowy trails.
• Volume Spacers: Adding volume spacers inside the air can makes the suspension more progressive (harder to bottom out), which allows you to run slightly lower initial pressure for better small bump sensitivity.
• Temperature: Air pressure can change with ambient temperature. A setting dialed in at 50°F will be higher at 90°F. Always check pressure before a ride.
• RE:aktiv and Thru Shaft Shocks: These Trek-specific technologies are designed to provide a firm pedaling platform while still reacting quickly to bumps. They can sometimes feel different from other shocks and may require slight adjustments from a standard setup.
• Leverage Rate: Some Trek bikes have a “Flip Chip” that changes the leverage rate. A more progressive setting will naturally resist bottom-out more, potentially allowing for lower PSI.

Frequently Asked Questions (FAQ)

1. Why is my calculated PSI different from the official Trek calculator?

This calculator uses a generalized physics model. Trek’s official tool uses a massive database of test data specific to each model and year. Both are excellent starting points. Trust the physical sag measurement above all else.

2. How do I adjust rebound damping?

The rebound knob is almost always red. Turn it fully clockwise until it stops (this is the slowest setting, or “fully closed”). Then, turn it counter-clockwise by the number of clicks recommended.

3. What if my suspension feels too harsh?

This means you have too much air pressure or too much compression damping. First, try reducing air pressure by 5-10 PSI. If that doesn’t help, ensure any compression levers (usually blue) are in the ‘Open’ position.

4. What if I’m bottoming out too easily?

You need more support. First, add 10-15 PSI of air pressure. If you still bottom out but the initial part of the travel feels harsh, you are a perfect candidate for adding a volume spacer.

5. What does “clicks from closed” mean for rebound?

It’s the standard way to measure rebound. “0 clicks from closed” is the slowest setting. “5 clicks from closed” means you turn the knob fully clockwise, then back it off five clicks counter-clockwise.

6. Should I set sag sitting or standing?

Always set sag in your “attack position” – standing on the pedals, with your weight centered, arms and legs bent, and looking down the trail. Having a friend help stabilize the bike is very useful.

7. Does this calculator work for the front fork?

No, this is specifically for the rear shock. Forks have different sag requirements (typically 15-20%) and different air spring characteristics.

8. How often should I check my suspension pressure?

It’s good practice to check it before every ride, or at least once a week. Air can slowly leak over time, and temperature changes will affect the pressure.