Rockrider ST 120 MTB Black

REF: 8530223



This 27.5" mountain bike is designed for getting started with MTB touring in dry weather, for rides up to 1.5 hours.

A high-performing mountain bike that's easy to handle! Enjoy the precision and light weight of the ST 120 MTB with its single chainring (1x9 speed) and mechanical disc brakes. Change speeds with ease!

First use

Inflating your tyres

The recommended average tyre pressure is 2.0 bar.

If you're looking for more comfort and grip AND/OR if you weigh under 70kg, you can reduce the pressure by 10% but without dropping below 1.8 bar (the minimum recommended pressure). On abrasive terrain, however, this will increase the risk of punctures.

You should check the pressure of your tyres regularly.


Adjusting the height of your handlebars

Your handlebar is set to the most raised position, which is the most comfortable position for riding.

For improved aerodynamics and more efficient pedalling, you can lower your handlebar so that your body leans forward and adopts a more racing-like position.

Follow the steps in this video to move the headset spacers below the stem and lower the handlebars on your bike.


Aligning your handlebar

Check out this video to learn how to align your handlebars.


Adjusting the stiffness of your fork

The BTWIN U-Fit fork has been designed to make adjusting it as easy as can be.
A genuine innovation: just turn the thumbwheel on the fork until your weight appears in the window.

This will make the spring harder or softer, in line with your weight.


Bedding in your brakes

When you first get your bike from the store, the brake pads and discs are brand new and therefore the initial braking performance is very low (weak). A sharp prolonged braking action (for example, during a descent) with brakes that have never before been used can cause the pads to overheat and to "freeze up".

To ensure that your brakes perform well, we recommend you bed them in. This takes about 10 braking actions per brake. For each braking action: apply a single brake to slow the bike down from about 25 km/h to 5 km/h, without fully blocking the wheel. Repeat with the other brake.

Adjusting the height of your saddle

Check out this video to learn how to adjust the height of your saddle.


Positioning your brake levers

Check out this video to learn how to orient your brake levers.


Changing your pedals

Check out this video to learn how to replace your pedals.



The right equipment in case of a hiccup

You need to have the right equipment with you when you're out and about on your mountain bike so that you can take care of a puncture, a broken chain or any other type of setback. Here's a selection of products that can get you out of a tight spot or two.

Have a problem?

The wheel is buckled

When you take your new bike out for a ride for the first time, a lot of parts can shift and fall into place.

You'll hear the spokes on your wheels creak because of the residual torsion in them.

Your bike is equipped with sturdy double-walled rims (on the inside of the wheel), so your wheels should be straight and not bent.

In the unlikely event a wheel is or becomes slightly bent or twisted, please bring your bike to any Decathlon workshop and they will make the adjustment free of charge, either:

1. As part of a free check-up in the first six months following the date of purchase.

2. As part of our €1 damage insurance plan (valid for two years).

Message for parents: Some technical manoeuvres by young riders (wheelies, skids, jumping a curb, etc.), if repeated too frequently, can also cause some play in the hub of the back wheel and cause the rims to twist.

If your child performs these tricks on a regular basis, speak to them and let them know the problems they can cause.

wheelies and buckling of the ROCKRIDER ST 100 mountain bike's wheels
You've got a puncture

Unfortunately, a flat tyre is something that can happen to any of us.

The terrain (thorns, brambles, glass, nails, etc.),

or the way the bike is used (jumping over obstacles such as rocks, steps, curbs, potholes, etc.),
are the most common causes, respectively, of flat tyres due to a puncture (small hole) or pinching (small grooves close together).

On the other hand, if a flat tyre is a frequent occurrence, this could be because:

- The bike was improperly assembled at the factory, leaving the tip of one of the spokes exposed, which is puncturing the inner tube.

- The tyre or the inner tube is faulty (porous)

- A previous puncture was improperly repaired and a foreign object (thorn, bit of glass, metal shard, etc.) got into the rim or the tyre.

It's important to properly diagnose the flat and make sure the tyre and wheel rim are free from any foreign objects before doing the repair.

You don't know how to put a tyre on or take one off, how to repair or change an inner tube?

1. The simplest thing to do is not to touch your wheel and use a can of puncture repair foam as a temporary solution:

Screw the end of the sealant tube onto the valve of the flat tyre (valve should be in the lower position) and squeeze and hold to inject the foam into the inner tube. The tyre should become full and firm once more, allowing you to get back home safely.

2- If you've purchased our €1 breakdown insurance, you can have your tyre repaired at any Decathlon workshop.

3- To repair or replace the inner tube yourself, please refer to one of the video tutorials which you'll find below.

ROCKRIDER ST 100 mountain bike puncture
Your fork is too hard

There are a couple of things you can do to "soften" your fork:

1. Adjust the stiffness: check the section "ADJUSTING THE STIFFNESS OF YOUR FORK"

2. Do some basic maintenance: you can regularly care for your fork with a small oil can. Simply put a few drops of oil on the fork pivots and work the fork to help the oil penetrate into the blades.

3. Have the bike fully serviced in a workshop.


The rear gears are not shifting properly

There are several things you can do to ensure your rear gears shift properly:

1. Check the position of the housing ends:

1a. Housing ends that are out of position can obstruct the derailleur cable when it is being pulled and prevent the gears from shifting smoothly. If a housing end is out of position, push it back into place.

1b. If the derailleur is not moving even after you have engaged and disengaged the grip shifter, it probably means that the cable and its housings have seized up. You will need to have the cable and housings replaced.

You can choose to replace them yourself by watching the video under "CHANGING A GEAR CABLE" or bring your mountain bike to a Decathlon workshop.

2. Check that the derailleur hanger isn't bent. A bent hanger causes the derailleur to be out of alignment and therefore unable to function correctly. Stand behind your bike and check the alignment of the derailleur.
If the dropout is twisted or broken, take your bike to a DECATHLON workshop to have it replaced.

REMEMBER: You can have all the settings on your bike checked for free in the first six months following the date of purchase.

NB: If you regularly ride your MTB in wet or muddy conditions, or if you regularly wash your bike with lots of water, then your cables and cable housing will need to be replaced more frequently. Washing your bike using a high pressure hose is highly discouraged as it can push dirt and dust into the cable housing.

You start pulling the brake lever and it touches the handlebars

The lever is spongy and/or touches the handlebars before you can brake:

Loosen the thumbwheel and the locknut to adjust the cable tension so that the brake lever is firm but not too hard. Once you've got the setting right, tighten the locknut against the brake lever.

Drop in at a DECATHLON workshop and get one of our technicians to do it.

The chain is broken

Did your chain break? You have two options:

1- You can repair it yourself using our video tutorial. You'll need a quick-link and a chain tool. 

2- Visit a Decathlon workshop to have the operation done by one of our technicians.


Maintaining your mountain bike

Cleaning your mountain bike

To clean your bike properly, you'll need some bike cleaner, a sponge, and a brush.

Rinse off the bike with a hose and apply the bike cleaner.

Next, use the sponge and brush to apply the cleaner effectively.

It's essential to give the bike cleaner some time to act before rinsing the bike off.

When hosing the bike off, make sure you do so from a few steps away so as not to damage the mechanical components.

Lastly, dry your bike using a cloth that is dry, clean and soft.


Cleaning and lubricating a drivetrain

To clean and lubricate your drive train, you will need a degreaser spray, a soft bristled brush and a lubricant.

The first step involves covering the brake disc with a clean cloth to prevent any lubricant from getting on to it.

Next, spray the degreasing agent onto the entire drivetrain, scrub the cassette, the chain, and the rollers on the derailleur with the brush.

Let it sit for a few minutes before rinsing with a hose from a moderate distance.

Wait a little while to let the drive train dry before moving on to the greasing step.

For greater effectiveness, shake the can of bike lubricant and spray on the inner side of the chain.


Changing a gear cable

Is your drive train cable dirty, rusty, damaged, or do you find changing speeds to be difficult or unreliable? 

If so, you need to replace your drive train cable. To do so you'll need a pair of pliers, a screwdriver, a wirecutter, an Allen key, and a new set of derailleur cables and cable housing.

First, remove the cable from its housing. Remove the hood from the gear shifter, move the indicator to the last gear and cut the cable. This will allow you to remove all of the cable and then the housing.

Cut the housing to size and fit the end caps on the end of the cable, which is now ready to fit.

Install the cable on the shifter and pull it through the housing. Once done, put the cover back on the shifter and attach the cable to the derailleur. To adjust the drive train, tighten the cable if you have a hard time shifting to higher gears, and loosen it if you have a hard time shifting to lower gears.

Once adjusted, cut the cable and put a cap on the extremity using a pair of pliers.


Removing a rear wheel

Check out this video to learn how to remove your rear wheel.


Repairing a puncture

To repair a flat tyre, you will need: a bicycle pump, a basin with water, a dry cloth, and an inner tube repair kit.

Partially inflate the inner tube. Submerge the inner tube into the water to locate the puncture by the source of the bubbles.

Once you have located the puncture hole, dry off the inner tube with the cloth and rub the spot to be repaired using the sandpaper included in the inner tube repair kit.

This prepares the inner tube for glueing and ensures that the patch sticks to the inner tube as securely as possible.

Generously apply glue to the inner tube around the hole and wait 3 to 5 minutes.

Then apply the patch to the hole and hold it in place for a few minutes.

You can then fit your inner tube back into the tyre.


Replacing the brake pads on mechanical disc brakes

Check out this video to learn how to replace your brake pads.


Adjusting the mechanical disc calliper

Check out this video to learn how to adjust the brake calliper.


Changing a chain

To do this, you will need: a chain tool, a new chain, and a quick-link.

Put the chain on the cog and on the smallest chainring. 

Remove the quick-release link using the quick-release chain tool and then remove the chain from the drive train.

Put the new chain on the last of the cogs and on the smallest chainring. This makes it easier to determine its optimal length.

Pull the chain down, pass it under the upper pulley of the derailleur.

Once you've defined the correct length, use the chain tool to cut the chain.

Lastly, install the quick-link onto the chain, turn the pedal so as to position the quick-link on the upper part of the chain, and use your hand to apply pressure and ensure it is firmly locked in place.


Compatible accessories

Mountain bike mudguard kit


mountain bike mudguard kit
Kickstand 300 central

Fit the Kickstand 300 Central on your Rockrider ST 120

MTB Rockrider ST 120 - Black/Blue
Kickstand 500 central

Fit the Kickstand 500 Central on your Rockrider ST 120

Kickstand 500 Central
500 Black bike bottle cage

Fit the Mountain Bike Bottle Cage 500 - Black on your Rockrider ST 120

Pannier rack 500 onesecondclip

Fit the Pannier Rack 500 OneSecondClip on your Rockrider ST 120

MTB Rockrider ST 120 - Black/Blue
Child seat 100 frame

Fit the Child Seat 100 Frame on your Rockrider ST 120

MTB Rockrider ST 120 - Black/Blue

User guides

MTB Rockrider ST 120 - Black/Blue
MTB Rockrider ST 120 - Black/Blue

Need instructions for your ST 120 mountain bike? 

Women's MTB Rockrider ST 120 - Blue

Haven't been able to fix your problem or find what you need?

Please contact one of our technicians who will be happy to help

Rockrider ST 120 MTB Black

Our commitments

ROCKRIDER provides a lifetime warranty on the frame, handlebar and stem of your mountain bike (under normal conditions of use).

All other parts have a two-year warranty.

As soon as you start using the bike, some components will move slightly. This is why we offer a free tune-up of your mountain bike within six months of purchase.