How to Set Fork and Rear Shock Sag on a Mountain Bike
What sag is, how much to run on your fork and rear shock for each discipline, how to measure it step by step, and what suspension pressure depends on.
Quick answer: sag is how much the suspension compresses under your weight alone. On the rear shock, aim for 27% for XC, 30% for Trail, 32% for Enduro and 35% for DH. On the fork, the usual range is 15% to 25% of travel, depending on what the manufacturer specifies. The pressure needed to get there depends on your weight, your fork or shock and your bike: MTBPro calculates it for you.
Sag is the first and most important suspension setting: if it's off, you can't set rebound and compression properly. The percentages in this guide are the same ones MTBPro uses.
What sag is
It's how much the suspension compresses when you get on the bike in riding position, without bouncing or braking. It's measured as a percentage:
- Fork: millimeters of compression ÷ fork travel × 100.
- Rear shock: millimeters the shaft compresses ÷ shock stroke × 100. Note: this is the shock's stroke (for example, 55 mm on a 210×55 shock), not the rear wheel travel.
With too little sag, the suspension feels harsh, bounces and loses traction. With too much, it rides low, the bike's geometry changes and you bottom out easily.
How much sag to run
Rear shock
| Discipline | Sag | Why |
|---|---|---|
| XC | 27% | Firmer, more pedaling support |
| Trail | 30% | Balance between support and traction |
| Enduro | 32% | More traction and sensitivity on descents, without going too soft |
| DH | 35% | Maximum sensitivity and traction; watch out for bottoming out if your pressure is low |
In millimeters, based on your shock's stroke:
| Stroke | XC (27%) | Trail (30%) | Enduro (32%) | DH (35%) |
|---|---|---|---|---|
| 45 mm | 12.2 mm | 13.5 mm | 14.4 mm | 15.8 mm |
| 50 mm | 13.5 mm | 15 mm | 16 mm | 17.5 mm |
| 55 mm | 14.9 mm | 16.5 mm | 17.6 mm | 19.3 mm |
| 57.5 mm | 15.5 mm | 17.3 mm | 18.4 mm | 20.1 mm |
| 60 mm | 16.2 mm | 18 mm | 19.2 mm | 21 mm |
| 62.5 mm | 16.9 mm | 18.8 mm | 20 mm | 21.9 mm |
| 65 mm | 17.6 mm | 19.5 mm | 20.8 mm | 22.8 mm |
With a coil shock, aim for 25–30% sag with no more than 1–2 turns of preload. If you need more preload to get there, the spring is too soft: swap it for a stiffer one instead of cranking the preload.
Fork
Forks usually run less sag than the rear shock, typically 15% to 25% of travel. Each manufacturer specifies its own: some XC or dirt jump forks call for 10–15% and others go up to 25–30%. When MTBPro has the official chart for your fork, it shows the manufacturer's recommended sag and the pressure at each end of that range.
In millimeters, based on travel:
| Travel | 15% | 20% | 25% |
|---|---|---|---|
| 100 mm | 15 mm | 20 mm | 25 mm |
| 120 mm | 18 mm | 24 mm | 30 mm |
| 130 mm | 19.5 mm | 26 mm | 32.5 mm |
| 140 mm | 21 mm | 28 mm | 35 mm |
| 150 mm | 22.5 mm | 30 mm | 37.5 mm |
| 160 mm | 24 mm | 32 mm | 40 mm |
| 170 mm | 25.5 mm | 34 mm | 42.5 mm |
| 180 mm | 27 mm | 36 mm | 45 mm |
How to measure sag step by step
You'll need a zip tie or the O-ring that comes on the fork stanchion and shock shaft, plus a ruler or caliper. It's easier with someone holding the bike.
- Inflate to the starting pressure (MTBPro calculates it for you; see below for what it depends on). Do it gradually: on the fork, start at 20 psi, compress the fork 5–10 times and repeat until you reach the final pressure; on the shock, add air in 15–20 psi steps and compress it several times after each one. This equalizes the positive and negative chambers so the reading is accurate.
- Open the compression (levers in open or descend mode) so it doesn't hold the suspension up.
- Gear up as you'll ride: helmet, pack, water and shoes. It all counts.
- Get on the bike leaning against a wall or a friend, without bouncing. For the rear shock, sit in pedaling position; for the fork, stand with level pedals in descending position.
- Push the O-ring down against the seal and get off the bike carefully, without compressing the suspension further.
- Measure the distance between the seal and the O-ring, and divide it by the fork travel or shock stroke.
- Adjust and repeat: too little sag, let some air out; too much, add air. Repeat until you hit your target sag.
What rear shock pressure depends on
Fox (Float DPS, Float X, X2), RockShox (Deluxe, Super Deluxe, Vivid, SIDluxe) and Marzocchi (Bomber Air) don't publish a weight-to-pressure chart, because pressure depends on each bike's frame (its leverage ratio). What they say is to start with as many psi as you weigh in pounds and fine-tune by measuring sag. Other manufacturers, such as DVO or Cane Creek, do publish a chart.
- Your geared-up weight, without the bike. The suspension only supports what sits on top of it: you, your pack, water and the rest of your gear.
- Your target sag. Less sag means more pressure: for the same weight, XC runs more pressure than DH.
- E-bike: a bit more pressure.
- Your shock's maximum: for example, 350 psi on the Fox Float DPS and Float X. Never go over it.
MTBPro takes all of this into account (your shock's rule or chart, your weight, gear, discipline and whether it's an e-bike) and gives you the starting pressure and the manufacturer's maximum.
What fork pressure depends on
Every fork is different: pressure depends on the brand, model, travel and sometimes the model year. That's why there's no single chart. MTBPro has the official charts for many Fox, RockShox, Marzocchi, SR Suntour, DVO, Öhlins, Manitou and other forks, and gives you the manufacturer's pressure for your weight. When there's no official chart, it estimates it with a generic formula for air forks and tells you so.
The sticker on many forks' stanchions also has a rough chart. Either way, pressure is only the starting point: what matters is the sag you measure.
After sag: rebound
Once sag is set, the next step is rebound (how fast the suspension returns). As a general rule, the more pressure you run, the slower the rebound needs to be. Many official charts list rebound clicks for each weight, counted from closed, and MTBPro shows them when the manufacturer publishes them.
The values in this guide are a starting point. Always follow the maximum pressure and instructions from your fork, shock and frame manufacturers.
Frequently asked questions
What is sag on a mountain bike?
It's how much the suspension compresses under your weight alone, without bouncing or braking. It's expressed as a percentage of fork travel or rear shock stroke. For example, a rear shock with 60 mm of stroke that compresses 18 mm has 30% sag.
How much sag should I run on my rear shock?
MTBPro uses 27% for XC, 30% for Trail, 32% for Enduro and 35% for DH. With a coil shock, between 25% and 30%.
How much sag should I run on my fork?
Usually between 15% and 25% of travel. Each manufacturer specifies its own, ranging from 10–15% on some XC or dirt jump forks to 25–30% on others. If MTBPro has the official chart for your fork, it shows you the manufacturer's range.
What rear shock pressure should I start with?
Fox, RockShox and Marzocchi say to start with as many psi as you weigh in pounds (geared up) and fine-tune by measuring sag, because pressure depends on the frame. MTBPro calculates the starting pressure for your weight, your shock and your riding discipline.
Should I count bike weight when setting suspension pressure?
No. The suspension only supports what sits on top of it, meaning your body weight plus your pack, water and the rest of your gear. That's why MTBPro asks for gear weight without the bike.
Does an e-bike need more suspension pressure?
Yes, a little more. MTBPro adds 5% to fork and rear shock pressure on an e-bike, unless the manufacturer has a dedicated e-bike chart.