Rockrider ST 120 MTB Black

REF: 8602020



An MTB that's efficient and easy to ride! 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 average recommended air pressure is 1.8 bar.For greater comfort and grip, or if you weigh less than 70 kg, you can reduce the air pressure by 10%, but in any event do not go below 1.6 bar (minimum recommended pressure). Lower air pressure also increases the risk of a puncture when riding on rough terrain.Don't forget to regularly check the air pressure of your tyres.


Adjusting the handlebar height

Your handlebar is factory set at its highest position (upright riding position, which is the most comfortable).

For a more aerodynamic position and greater pedalling efficiency, you can lower your handlebar to stretch out your riding position (a more sport-oriented position with the back more bent forward).

To do this, simply follow the steps described in this video to remove the spacers under the bike stem.


Straightening your handlebar


Adjusting the stiffness of your fork

The B'Twin U-Fit fork is designed to make adjustments as easy as possible. Simply turn the dial until you see your weight indicated on the back of the fork. Turning the dial increases or decreases the compression of the spring to adjust it to your weight. That's innovation!


"Breaking 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 avoid this issue (which can render the brakes completely unusable), we recommend first "breaking in" your brakes. 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 your seat height


Rotating your brake levers


Replacing your pedals



Having the right gear in case of a mishap

Be sure to have all the right gear when you set out on your MTB rides in the event you need to fix a flat, a broken chain, or another mishap. Here is a selection of products that can help you get out of a number of technical misadventures.

Have a problem?

A wheel is bent

The first time you ride your brand new bike, it's normal that many of its components will move about slightly and settle into their proper places. This is the case of your wheel spokes which you might hear make a "cracking" sound which is naturally due to the residual torsion built up during assembly. 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 the free tune-up available within 6 months of the date of purchase of the bike. 2- Covered by the €1 extended repair warranty, if purchased (good for 2 years). A note 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 you notice that your child is doing these types of bike tricks frequency, it's a good idea to make them aware of this issue.

truing the wheels on the ST100 Rockrider MTB
Repairing a flat

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.How to remove/install a tyre, and repair/replace an inner tube.1- The simplest is to leave the wheel alone and temporarily fix your flat using puncture repair tyre sealant: 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 EUR 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 MTB Tyre Puncture
Your fork is too stiff

There are a few things you can do to "soften" your fork:1- Check the stiffness level: see the section ADJUSTING THE STIFFNESS OF YOUR FORK2- Perform a basic maintenance operation: 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 a full maintenance tune-up done at the workshop.


Rear gears don't shift properly

There could be several reasons the chain isn't shifting properly between cogs on the rear cassette:1- Check the position of the cable housing end caps (tips).Incorrectly positioned cable housing caps can hinder the pulling action on the derailleur cable and cause the shifting of the gears to be off. If either cable end cap is incorrectly positioned, put it back in its place.2- The cables or the housing could be locked up.If the chain isn't shifting down to the smallest cogs, and the rear derailleur doesn't move when activating the shifters, then the cable and/or the cable housing is almost certainly locked up. In this case, the cable and housing need to be replaced. Your local Decathlon workshop can assist you with this, or you can replace them yourself with the help of our video tutorial.3- Take your MTB to a Decathlon workshop to have the derailleur adjusted. 4- Also check that the derailleur hanger is not bent. If it is, refer to the "YOUR DERAILLEUR HANGER IS BENT" section   PS: 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. 


The brake lever touches the handlebar too easily before braking starts

Your brake lever is too easy to squeeze and/or touches the handlebar before braking action begins:  Unscrew the barrel and the locknut until the cable is sufficiently tight and the brake lever is firm but not too hard. Once you've got the tension right, tighten the locknut firmly against the brake lever to secure the setting. Please visit a Decathlon workshop to have this done by one of our technicians.

Your chain has 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.


Caring for your mtb

Cleaning your mtb

To clean your bike properly, you'll need some bike cleaner, a sponge, and a brush. Rinse off the bike with a hose (not at high pressure), and then 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 your drivetrain

To clean and lubricate your drivetrain, you'll need a degreasing spray, a suitable brush, and bike lubricant. The first step is to protect your brake disc with a clean cloth to prevent any grease or lubricant getting on 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 drivetrain 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.


Replacing the drivetrain cable

Is your drivetrain cable dirty, rusty, damaged, or do you find changing speeds to be difficult or unreliable?  If so, you need to replace your drivetrain 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. Start by removing the shifter cover, and placing the indicator on the highest gear; then cut the cable to be able to remove the entire cable, followed by the housing. Next, measure out how much cable housing you need against your bike, cut it, and put caps on either extremity. Then install it on the bike. 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 drivetrain, 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


Repairing a flat tyre

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 in 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 now install the repaired inner tube into your tyre and onto your bike! 


Replacing the brake pads on mechanical disc brakes


Adjusting the mechanical disc calliper


Replacing the chain

To do this, you will need: a chain tool, a new chain, and a quick-link. Place the chain on the largest cog and the smallest chainring.  First, remove the existing quick-link using the quick-release tool, then remove the chain from the bike. Place the new chain on the largest cog and the smallest chainring in order to more easily define the optimal chain 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

Mtb mudguard kit

Install the MTB Mudguard Set onto your Rockrider ST540

500 Base stand

Install the 500 Base Bike stand on your Rockrider ST540

500 Bike bottle cage black

Install the 500 Bike Bottle Cage (Black) on your Rockrider ST540


Spare parts

Learn more

Need to replace a part on your bike? You'll find all our spare parts for your ST 120 MTB here.

User guides


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


Our commitments

ROCKRIDER guarantees the frame, handlebar and stem of your mountain bike for life (in normal conditions of use).

The guarantee for other parts is two years.

As soon as your child starts using the bike, some components will move slightly. It's for that reason that we offer a free check of your mountain bike six months after purchase.