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Demystifying MTB Suspension: Sag

Demystifying MTB Suspension: Sag

Writer: Ryan Kershek- Owner Fluid Focus, LLC in San Marcos CA. Previously, a factory suspension tech at WP for Rockstar Husqvarna SX/MX

We’ve all been there, the excitement of a new bike or dusting off your old bike after life got in the way. At some point, you’ll have to add air pressure to your fork and shock, but chances are you never took any measurements to see the correct air pressure or spring preload. In today’s world, mountain bikes cost just as much or more than a motorcycle, but most people don’t take the time to set them up properly. The bicycle suspension manufacturers give you recommended starting points for fork and shock air pressure, but often those are wrong. Your local bike shop will add some air and send you on your way. Let’s look at what sag is and why it’s essential.

There are two types of sags: Static and Dynamic. Static sag is the amount your fork and shock compress with you just sitting on the bike. This allows your wheels to be able to “fall” into holes as you ride over them. Dynamic sag is the average position your fork and shock are in while riding on a trail. We’ll be referring to static sag to keep things simple for the remainder of this article. You should always reference how much sag your frame manufacturer recommends running because this is what they feel works best for their design.

Generally, the shock sag zone is 25% on xc bikes, 25-30% for trail bikes, and 30% for enduro and downhill. Forks should be running around 20-30%. These percentages are based on the stroke of your shock and the travel of your fork. One thing to note is that sag is a starting point, and the numbers aren’t set in stone. This gives you a good starting point and is a reference to revert to when you start to tinker with your setup and it goes awry.

Setting sag is the first step in setting up your suspension. Correct air pressure or spring preload will significantly contribute to a supple and compliant ride. Too much air pressure or stiff coil springs will result in harshness and the bike not tracking over the terrain. The suspension then deflects instead of absorbing. Too little air pressure or a soft coil spring will sit the bike deep in the travel mimicking harshness. When your fork and shock sit too deep into the travel, it takes much more force to break through the rest of the stroke. Ideally, throughout your ride, you want the fork and shock to sit in the “sag zone”. This will allow your suspension to move freely and track the terrain. Think of a trophy truck flying through a set of woops. The truck’s cab doesn’t move because the suspension articulates and tracks the ground. This is also how you want your bike to work.

Follow these simple steps to set up sag. First, you must know your shock’s stroke and your fork’s travel in millimeters. Most of the time, these numbers are printed on the components, or you can look up most serial numbers on the manufacturer’s website. You will also need a shock pump explicitly designed for suspension and something to measure with. For example, let’s use an enduro bike with a 65mm shock stroke and fork travel of 170mm. We want 30% shock sag and 20% fork sag. 30% of 65mm is 19.5mm (65x.3=19.5), 20% of 170mm is 34mm (170x.2=34).

  1. Open compression and rebound all the way. This will allow the fork and shock to move freely.
  2. On a flat surface, have someone hold you up or use a wall if you don’t have a helping hand. Put your feet on the pedals and take your hands off the brakes.
  3. Jump up and down to cycle the suspension a few times with your hands off the brakes. Once you and the bike stop, gently sit down on the seat with your feet on the pedals.
  4. Move the sag O-ring to the fork and shock the wiper seal. On a coil shock, push the bump-stop up the shaft until it meets the seal head. Then, gently step off the bike without compressing the suspension.
  5. The suspension will extend without your body weight. Your sag is the distance between the sag O-ring or bump-stop and the wiper. If that measurement is less than 19.5mm and 34mm remove air or preload. More than 19.5mm and 34mm add air or preload. If you have a coil shock, the ideal amount of preload is 1mm.

After you’ve accomplished this, the last thing to do is set your adjusters. For starters, using the manufacturer’s recommended settings is fine IF you go 2-3 steps softer(compression) and faster(rebound) than they recommend for the psi or spring rate you use.

If you have learned nothing this far, remember that if you have a coil shock, using a spring that gives you no more than 2mm (ideally 1mm) of preload to achieve your desired sag is essential. Please understand the worst thing you can do is compensate with more air pressure to gain support and hold up, and never run less air pressure with tons of volume spacers to get a softer ride. Using a digital shock pump is recommended because they are the most accurate, but if you have a cheap shock pump and you have never put in the effort, that is far worse!

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