Suspension fork sag adjustment

One of the new repairs shortly coming to Bike Doctor is suspension sag adjustment. This is something that all mountain bike riders should do on a new mountain bike to account for their weight and riding style. It should also form part of an occasional maintenance routine.

Suspension on a mountain bike reduces rider fatigue and improves the bike wheels contact on rough terrain. When adjusting, your aim is to balance between a soft and a hard setup. Too soft will result in your bike wheels not travelling far enough in a dip and too hard will cause your bike to bounce off rough terrain.

The sag determines the amount that the mountain bike suspension compresses.

I’ve demonstrated the repair on a Voodoo Canzo bike which is a pretty high spec mountain bike.

Step 1: Measuring sag

If your bike doesn’t come with an O-ring, as the Voodoo Canzo does, then simply secure a zip tie around the leg. Then, slide it down the fork leg until it reaches the fork wiper seal.

Step 2: Sit on the bike

Next, you’ll need to get a friend to hold you in position or alternatively prop yourself up against a wall using your elbow.

Sit down on the bike in the normal riding position and be careful not to compress the fork too much as you get on. If you do then simply take your weight off the bike and reset the O-ring or zip tie.

Step 3: Aiming for 20-25%

Then, climb off the bike, whilst being cautious not to compress the fork further.

Measure the sag from the fork leg. You need to be aiming for 20% to 25% of the total travel available. The total travel available can be found in the bicycle’s handbook or, if you’ve lost that, then by looking it up online.

The Canzo features the Rockshox Ario R by Sram. This has 140mm of travel. The manufacturer recommends, 15-25% of travel is 28mm (20%).

Note that the 20% to 25% is a general recommendation that works for most. However, you may find yourself adjusting this depending on your riding style.

Step 4: Increasing or decreasing pressure

If your bike has air sprung forks then you decrease the air pressure to increase the sag. Alternatively, increase the air pressure by pumping more air into the fork to reduce the amount of sag.

Depending on your measurement, either increase or decrease the sag, remembering to aim for around 20% to 25% sag. You should make incremental changes and check them by re-setting the O-ring and climbing back on the bike.

Your mountain bike may use a coil-sprung system instead. In this case, adjustments are made using the dial on the fork crown that controls the spring. This has a limited range of adjustment. If you need to make further changes you’ll need a firmer or softer spring. However, the default spring should be fine for most situations. To increase the sag in a coil-sprung system simply turn anti-clockwise.

There is also one more factor to take into account. Something that is called bottoming out. This is when the suspension reaches the end of travel when you hit rough terrain. On the opposite end of the scale, is an effect called topping out. This is when the suspension reaches its full extension. Budget mountain bikes don’t have a system to control this and therefore the bike bobs up and down. On a budget mountain bike your only option therefore is to increase the spring rate if the fork is bottoming out too much. Note that some bottoming out isn’t a problem.

Higher end mountain bikes have a compression damping which can be increased if the fork is bottoming out too often.