How To Adjust Rear Suspension On Mountain Bike

If you have a full suspension mountain bike. setting the front and rear suspension to suit you and your riding conditions will make a difference to your off-road capabilities. Today we will take a look at how to set up and how to adjust rear suspension on mountain bike. 

To get the full advantage of your rear suspension, it has to be set up and adjusted correctly. To do this you will need to set up the sag and adjust the rebound damper.

Sag set up

What is sag on a mountain bike?

Suspension sag is a distance of how much the rear suspension compresses when the weight of the rider, including any gear carried, (hydration bag, helmet, spares etc.) is applied. Setting the correct sag allows the shock to work most efficiently. If you have a full sus bike, when you sit on it, you will feel it kind of sink underneath you. This is sag. 

As you learn how to adjust rear suspension on mountain bike, you will see that sag is the first thing to adjust.  Most manufacturers recommend around 25% as a starting point. This means that a quarter of the travel of your suspension unit should be taken up with the initial sinking down, but remember, this is a starting point.

Too much sag means you are absorbing too much travel before you hit your first obstacle. This may cause bottoming out. This means that when you hit some larger bumps, there is too little movement left in the suspension unit, so it can’t compress any more. Too little sag means that you’re not utilising the full travel. This means that as you hit bumps the resistance of the shock unit is too great and you won’t feel the suspension doing its job.

 To set up sag you will need:

  • A shock pump- This is a very small and thin pump. It has a button on the side that will release air a small amount at a time.  It also has a very accurate gauge that can go up to 300 PSI. This differs from a tyre pump because it delivers higher pressure and less volume. Note that your everyday pump will not work.
  • Ruler- This is required to take measurements of your suspension.
  • Cable tie- This is in case you don’t have an O Ring.
  • Piece of paper and pen or calculator- They will be essential to work out the correct sag.

You can set up sag with the help of another person or like in this example by using a wall or bench.  

Step 1: 

First set the rear suspension to the open or descent position.

Check to see if there is a thin rubber ring on the stanchion.  This is the “O” ring.

 If there isn’t one then put a zip tie around the stanchion and tighten it so that it is firm but can still slide up and down. 

Step 2:

While the helper holds the bike at the front or while you are leaning against the wall apply either one of the brakes and stand with both feet on the pedals.

When you activate the rear suspension unit, as the bike sags, the angles of the bike change and the wheel base, that is, the distance between the wheel, increases. By applying both brakes, neither of the wheels can move to allow the wheel base to increase so you will get an inaccurate reading.

Step 3:

Bounce up and down a few times to break the stiction and then sit down on the saddle.

Now, either you or your helper can move the “O” ring or zip tie to where the bottom of the stanchion is positioned on the shock body.

Step 4:

Get off the bike carefully so as not to move the “O” ring or zip tie.

Step 5:

Once off the bike, using your ruler, measure the distance in millimetres between the “O” ring and the bottom of the stanchion and write it down.

Most manufacturers recommend 20% – 30% aiming for 25% sag as your starting point. You can also check the correct sag for your rear suspension on the manufacturer’s website. If your suspension has markings on the shock body then it is easy to check if your sag is within the recommended guidelines.

If it doesn’t then you can calculate it. To do this you will need your current sag in mm and the stroke length of the suspension in mm. The stroke length is the overall amount of suspension.

Step 6:

To work out the percentage of your current sag.

First find the serial number or I.D. code This is usually marked on the main body of the shock.

Now go to the manufacturer’s website. 

An example of a Rockshox suspension unit: 44t84535530. The website is: Type your code here:

and the stroke length is shown here:

You will see that the stroke length for this particular example is 44mm.

An example of a FOX suspension unit: CG49. The website is:

Type your code here:

And the stroke length is shown here in inches:

You will see that the stroke length for this example is 2.5”. Convert this to mm and you have 63.5mm

Step 7:

Once you know the stroke length, write it down on the piece of paper.

Now,  use your current sag that you measured earlier. In our example the current sag is 33mm.

Step 8:

The calculation: 

Let’s divide that by the stroke length:

It equals 0.639mm

Finally, let’s multiply this by 100 to give us a percentage. The sag on the bike is 69% which is above the recommendation.

To reach the desired percentage between 25% and 30% some air needs to be added. Adding or releasing 10PSI at a time is advised.

Step 9:

Now it’s time to recheck the sag. If there’s still too much sag then keep adding some more air. If there’s not enough sag then release some of the air using the button on the shock pump. Repeat the process until you reach the recommended percentage

Rebound damping

What is rebound damping?

Rebound is the suspension returning to its original position. Damping controls the rate at which the suspension returns. The amount of rebound depends on the type of terrain. A series of small bumps need the suspension to rebound quickly between them If the rebound is too slow then the suspension might not return quickly enough to be able to take the next hit.  While going over bumps the suspension will keep compressing the suspension without time for it to return, squashing the unit more and more and not taking advantage of the full range of travel.  

Rebound that is too fast creates the effect of a recoiling spring and can make the bike leave the ground and the rider can lose control. Terrain that includes big bumps and/or drop offs needs a slow return. the harder the landing the more your suspension will compress.  A fast rebound from full or nearly full compression will push the bike quickly to its original position and could cause the rider to lose control.  You will need to experiment with the type of environment in which you are riding.

Rebound damping is normally controlled by a small lever on the top of the shock unit.  Each click alters the speed of the rebound. To increase the rebound, turn the lever anticlockwise and to decrease it turn it clockwise. Turn it a few clicks at a time and then check the rebound. The aim is to have the rebound returning fast enough to recover after a hit but not so fast that the bike leaves the ground and loses traction.

Adjusting rebound. Method 1

If you have a Rockshox suspension unit then there will be pictures of a hare and a tortoise. Fox shocks have a “+” or a “-“. Move the lever towards the hare or the “+”. This is the fastest rebound. 

Step 1:

First, make sure that your sag is set up correctly.

Step 2:

The aim to adjust the lever so that the rebound speed is the fastest possible without bouncing up and down. Stand to the side of the bike with both your hands on the saddle but not gripping. 

Step 3:

Push down hard and let the bike spring up. If the bike bounces off the floor, then the suspension rebound is too fast. 

Step 4:

Move the lever towards the “-” and try again. Keep adjusting it so that it doesn’t bounce. 

Now you have reached critical damping and you will probably find that this works for most of your riding. 

Adjusting rebound. Method 2.

Step 1:

Find a high kerb. Set the adjuster for the fastest rebound. This means that when you come off a bump the suspension will compress, then rebound past the sag, compress again, rebound past the sag and continue to do so 2 or 3 times, giving a trampoline effect. The aim is to dial the rebound so that the rebound speed is the fastest possible without trampolining. 

Step 2:

Now, seated on the bike and without pedalling, drop off the curb. See how much bouncing goes on.  Dial the lever about half way.

Step 3:

Ride off the curb again.  How many times do you bounce? If you don’t bounce at all move the lever 2 notches towards the fastest rebound.  If you are still bouncing, move the lever 2 clicks towards the slowest rebound. When you have found the point with the fastest rebound and no bouncing you have reached critical damping. You will probably find that this works for most of your riding. 

Final note

The critical damping is set at the slowest speed without bouncing, and shock manufacturers recommend only adjusting three clicks towards the fastest rebound to ensure that you get the best range of suspension from the shock.

As each cyclist, bike and trail conditions are different, adjustment of sag and rear damping is an individual preference. This tutorial is a starting point. Now it’s your turn to play with suspension to find  the perfect suspension sweet spot.

Overview of how to adjust rear suspension on mountain bike

As each cyclist, bike and trail conditions are different, adjustment of sag and rear damping is an individual preference. This tutorial is a starting point on how to adjust rear suspension on mountain bike. Now it’s your turn to play with the bounce to find the perfect suspension sweet spot. Next week we will explain how to adjust front suspension on mountain bike.

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