How To: Set Up Suspension Sag - Step By Step!

In this mountain bike How To, we are going over suspension sag; What is suspension sag, why is it important, what is the right amount of sag, and how to calculate it. Let's do it!

What Is Suspension Sag?

Suspension sag is the amount of suspension travel used when you are sitting on your bike in a natural riding position. This means that when you get on your bike and have your weight distributed like you would when riding, the suspension should be compressed some amount. The reason that we want sag is so the wheels are able to conform to the contour and shape of the ground while riding over rough terrain. Think about when you ride over a whole in the ground, when you have the correct amount of sag, your wheel will actually match the contour of that whole. Keeping your tires on the ground is what improves traction and gives you bike control on the trail.

With too much sag, meaning the suspension is compressed too much into its travel when weighted, the suspension may bottom out too often. The bikes balance and geometry might also not be in the most optimal position, making it harder to corner, jump, etc. With not enough sag, you won't be getting maximum suspension performance and the bike will struggle to find traction. 

How Much Sag Is The Right Amount?

It's important to set up your suspension with the correct amount of sag, that way the bike is balanced front to rear and the suspension performs at its best. Every bike uses a different suspension design and leverage ratio and will perform best with a different amount of sag. Each rider also has different preferences to how they want their bike to feel and handle and so these are just recommendations.

• For the rear shock - 20% - 30% sag is recommended. For the most part, this can be used across the board for trail bikes and downhill bikes. Again, keep in mind you may want more or less sag depending on your bike, weight, and riding preferences.
• For the front fork- 10% sag is recommended. 

Also note that here we are only discussing downhill bikes and trail bikes. Full suspension cross country bikes also need to have some sag, but in that case, some might say in that case pedaling efficiency is more important than suspension performance. For that reason, we won't talk much about XC bikes. 

For this How To, we are going to be setting the sag on a 2020 Santa Cruz Hightower with a Fox DPX2 rear shock, and we are aiming for approximately 30% sag. Some bike manufacturers will give you sag recommendations on their website for each specific bike. The process for setting sag for both the fork and the shock is the same, so follow the same steps when setting sag on your fork once we are done here.

What Tools Do We Need?

Before we start, let's go over what tools we are going to need to set the appropriate sag. Most enduro bikes now use air suspension front and rear, and changing the spring rate is as simple as adding or removing air pressure from the shock. If you are using a coil shock or coil fork, you will need to change the physical coil spring and swap it for a different spring rate to change the sag.

• Shock Pump: Digital shock pumps are great for getting repeatable pressure measurements.
• Measuring Device: Park Tool Calipers make getting accurate sag measurements nice and easy but a metal tape measurer will also work just fine.
• Allen Keys: If you are setting the sag on a bike with a coil shock, you will need to remove the shock from the bike to change the spring.

How To Calculate The Percent Sag

Because there are different shock stroke lengths for different bikes and every rider weighs a different amount, there is no set spring rate to achieve the correct amount of sag. Setting the sag on your bike will be one of the first steps to setting up your mountain bike suspension for the first time. 

% Sag = (Stroke Used) / (Total Stroke Length) * (100 %) 

• The Stroke Used is the amount of suspension stroke used while you are on your bike in a static riding position. 
• The Total Stroke Length is the length of the shock shaft that is available to be compressed. A lot of bikes use different stroke lengths. 
• The Santa Cruz Hightower here uses a 210mm x 52.5mm shock, meaning the eye to eye length of the shock is 210mm and the stroke length is 52.5mm.

Now Let's Set The Sag

Now that we know how to calculate sag, let's get the correct sag set for this Santa Cruz Hightower. We know that this Fox DPX2 shock uses a 52.5mm stroke. Using the equation above, to achieve 30% sag, we are looking for approximately 16 mm of Stroke Used. This means that as you are standing on the pedals in a neutral riding position, the shock should be compressed about 16mm to achieve 30% sage.

• To start, I aired the shock up to 160psi and equalized the positive and negative air chambers. This is for the most part a guess and check method. I decided to start with 160 psi, measure and calculate the sag, and then make adjustments from there towards 30% sag.

• Stand on the pedals and find a neutral riding position. Next, reach down and slide the o-ring all the way up against the dust wiper. Then gently hop off the bike without compressing the rear shock any more. As you take your weight off the bike, that o-ring will mark how much stroke you have used.

• With your calipers or metal ruler, measure the Stroke Used from the dust seal to the o-ring. With 160psi, I measured 22mm of sag. Now let’s calculate the % Sag at 160psi.

(22 mm) / (52.5mm) * 100% = 41.90% Sag

Since 42% sag is going to be way too soft, let’s add some air pressure to the shock and see if we can get closer to 30%. Next we are going to try 180psi and repeat the same process to see where that puts us.

• Again, stand on the pedals and find a neutral riding position. Reach down and slide the o-ring all the way up against the dust wiper. Gently get off the bike without compressing the shock further. With the metal ruler, at 180psi we measured 18mm of Stroke Used. Now we can calculate the % Sag at 180psi.

(18 mm) / (52.5mm) * 100% = 34.28% Sag

We are getting closer! Time to add a bit more air pressure and go through the same steps. Next, we are going to pump up the shock to 188psi, equalize the positive and negative air chambers, and stand on the pedals in a neutral riding position.

• Slide the o-ring up against the dust wiper, gently hop off the bike, and measure the stroke used. With 188psi, we are measuring 16mm of stroke used, or sag!

(16 mm) / (52.5mm) * 100% = 30.47% Sag

If you are using a coil shock, changing the spring rate to get a specific sag means changing the physical spring. DO NOT compensate for having too much sag by tightening the preload on the spring. Fox SLS springs come in 25lb increments to give you more adjustability. 

Follow the same steps for setting the sag for the front fork. Feel free to call us at the shop if you have any questions setting up your suspension sag.