Suspension 101: Part I - Sag

Tomy screwing a shock pump onto the fork of a Santa Cruz Hightower

Paying close attention to how your suspension is set up isn’t just for professional riders; a well set up bike can give you both extra confidence over rough sections of trail and shave seconds off your times. But with modern suspension components having 4-way adjustable dampers, tokens to change the rate of progression on your fork or shock, and occasionally even multiple air chambers, setting up a new fork or shock can be a daunting prospect. 

The aim of this setup guide is not to help you squeeze the final tenth out of your MTB suspension, that will come in a later article, but rather to give a simple, step by step process to follow which should give you a decent starting point to begin your suspension setup journey. 

Before we start, let's take a closer look at how a modern suspension system works.

Air cap on a Rockshox Super Deluxe rear shock

The vast majority of modern suspension forks and shocks are air sprung, meaning they use pressurised air to hold up the weight of the rider, rather than the coiled steel spring that you find on most modern cars. By increasing the air pressure in your fork or shock you are increasing the spring rate of the suspension which makes it feel firmer; reducing the air pressure reduces the spring rate. The amount that the suspension compresses under rider weight is called sag. 

As the suspension compresses the volume of the air spring reduces, increasing the pressure, giving the feeling of the fork or shock getting firmer as it gets deeper into its travel. This is known as progression, or ramp up. Volume spacers, or tokens, do just that; they reduce the volume of the air spring. Starting with a smaller overall volume means the air pressure increases faster as it goes through its travel, giving more ramp up. 

A sealed damper unit provides Compression and Rebound adjustment, but we’ll save that for a later date. 

Focusing on getting a good spring rate is the first, and most important step in unlocking the performance of your suspension, so arm yourself with a shock pump and let’s get started!

Rockshox sag gradients on Super Deluxe rear shock

Most modern full suspension mountain bikes are designed to ride best with around 30% sag. Rockshox shocks give clear markings on the stanchion showing 20%, 30% and 40% sag. Fox units don’t have markings so in order to find 30% sag we first need to find the stroke of the shock, which is specific to your model of bike and can be found in your owner's manual or on Google. Once we have the stroke length of the shock, find 30% by multiplying the overall stroke length by 0.3 and rounding to the closest millimetre. For example, a bike with a stroke length of 62.5mm will need to be set up with 19mm of sag in order to achieve 30%. 

We’ll begin by setting sag on the rear shock. 

Sag window on Santa Cruz Hightower

  1. Find a level piece of ground and drop your saddle to its lowest position. 
  2. Set your compression adjustments, if you have them, to fully open (or all the way to the negative).
  3. Slide the o-ring on the shock stanchion down to rest against the seal
  4. Standing over the bike, sit down slowly and smoothly until all of your weight is going through the saddle. Bringing your feet slightly off the floor can help with this.
  5. Stand back up and climb off the bike.
  6. The position of the o-ring gives you the position of your seated sag. 
  7. If you’re using a Fox shock, or just want to be millimetre perfect, measure the distance from the seal to the o-ring. This is your current sag figure. If the number you have measured is less than your desired 30% sag figure, reduce the air pressure in the shock. If the measured number is greater than the desired sag figure, increase the air pressure.
  8. Repeat this process until you are sitting at 30% sag. This normally takes 2 or 3 attempts so don’t get disheartened if you don’t get there straight away.
Tomy checking rear sag on Santa Cruz Hightower

This suspension set up is built around repeatability, being able to get the same set up time after time. A big part of that repeatability comes from getting consistent measurements, it’s for this reason that when setting the rear shock sag we didn’t stand on the bike, as this can give different measurements. Each time you stand up on the bike, the amount of weight distributed between your hands and feet can be slightly different, and this would skew our readings making it much harder to get an accurate setting for rear sag. By sitting on the saddle, something you can do the same each time, the process becomes very repeatable, meaning we can make meaningful changes quite quickly. 

With your shock sitting at 30% sag, let’s shift our focus to the fork. 

Air cap removed on Fox 36 fork

Every fork comes with manufacturer’s recommended settings, and while these aren’t always perfect, they give a decent place to start. So to set up for fork:

  1. Look at the chart either printed on your fork leg, or in the fork’s user manual and inflate your fork to the recommended pressure for your weight. 
  2. Set your compression adjustments, if you have them, to fully open (or all the way to the negative).
  3. Slide the o-ring on both the fork and shock so it rests against the seal.
  4. Ride your bike around on flat ground and bounce up and down on the bike in your riding position. The aim here is to have both the fork and shock using similar amounts of travel.
  5. Smoothly stop the bike and climb off, take a look at the front and rear o-rings. If the fork has used proportionately less travel than the shock, reduce the air pressure in your fork. If the fork has used proportionately more travel than the shock, increase the pressure in your fork. 
  6. Repeat this process until you’re happy that both fork and shock are compressing similar amounts. 
  7. Go ride, sliding the o-rings against the seals before each new trail and keeping an eye on the balance between the fork and shock travel. 


Tomy trying to bottom out the Santa Cruz Hightower

Once you have these baseline pressures, write them down and experiment. How does it feel with your fork inflated with an extra 5psi? Having your baseline written down should give you the confidence to try new set ups knowing you can easily change back to something you know works. The most important thing to try to develop a feel for is balance; are the fork and shock compressing the same amount? Is one end firmer than the other? If you have a bike set to 30% rear sag with a balanced set up front to rear, that gives a great baseline set of pressure to build on and make adjustments to fine tune your suspension performance. 

This is in no way the definitive way to set up your suspension, nor is it the only way. There are many different methods to arrive at the same outcome, but this is how we set our own bikes up and how we get riders feeling comfortable on our demo bikes. 

Shock pump connected to Rockshox Super Deluxe rear shock

To take your suspension setup to the next level, consider a Suspension Setup day with TVB Tech. Using Motion Instruments Data Telemetry equipment, and expert knowledge, we can help you get more out of your bike, increasing your confidence on the bike. Choose from a basic setup day, all the way up to 1-1 guided uplift day using out telemetry after each run to make meaningful changes to your suspension. 

In the next instalment of our Suspension 101 Series we’ll look at volume spacers, A.K.A tokens, and how they can be used to change the feel of your suspension. Check out Part II HERE.

Fox 36 volume spacers being installed

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