REF: 8545706



This bike is designed for training and competing in cross-country marathon mountain bike races. Dig deep and take on the whole field! Its all-new full suspension carbon/aluminium 29" frame will boost your performance",while its 120mm suspension will help you get to the finish line quicker in marathon and long-distance events.


First use

Bedding in your brakes

The brake pads and discs on a new bike are new themselves and don't work to their full potential.Hard, continuous braking (on descents, for example) can cause the pads to overheat and "glaze".To prevent this from happening and to ensure that your brakes slow you down effectively, we recommend that you bed them in.You need to apply each brake firmly around ten times to bed them in properly. The bedding-in process involves getting up to a speed of 25 km/h and then applying just one of the brakes firmly to reduce your speed to 5 km/h, without locking the wheel.

How to convert your wheels to tubeless

To reduce the risk of your inner tube being punctured or pinched, you may want to consider making the wheels on your MTB "Tubeless". To do this, you will need to buy: - Puncture prevention liquid (tyre sealant): DECATHLON: TYRE SEALANT FOR TUBELESS READY TYRES AND RIMS (item 8550735)   STEPS TO TAKE:1. Remove the wheel2. Remove the tyre and inner tube from the wheel3. Apply rim tape (sealant tape) to the rim4. Insert a Schrader Tubeless valve through the rim hole (from inside the rim)5. Apply tyre sealant (puncture prevention) liquid6. Install your new Tubeless tyre7. Fully inflate the tyre until it presses firmly against the rim If you find it difficult to perform this operation, please visit your nearest Decathlon workshop and have this done for you.


Adjusting your fork

To properly adjust your Rockshox fork, see the instructions on the following link

Adjusting your rear suspension


Fitting your telescopic seat post



Getting the right gear to deal with problems

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

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).

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

Unfortunately, anyone can get a puncture. - The ground (thorns, brambles, glass, nails, etc.)- And how you use your bike (taking on obstacles like rocks, steps, kerbs and potholes)are the most common cause of perforation punctures (small holes) and pinching punctures (small cracks close together), respectively.However, if you're getting a lot of punctures, it might be because of:- Incorrect wheel assembly in our factory, which has resulted in the end of a spoke piercing the inner tube- A faulty tyre or inner tube (porous)- A poorly repaired initial puncture that has left a foreign body (thorn, glass, metal splinter, etc.) in the rim or tyre.It is therefore important to correctly diagnose the puncture and remove any foreign bodies before carrying out the repair.Not sure how to remove/fit a tyre, or how to repair or change an inner tube?1- The simplest thing is to not even touch your wheel. Instead, get yourself out of trouble using a puncture repair spray: Screw the end of the tube onto your punctured tyre's valve (valve pointing down) and press and hold to inject the foam. The tyre should return to its initial shape and feel hard again so you can get yourself back home 2- If you have taken out our 1 euro breakdown insurance, you can then get your wheel fixed properly at one of our Decathlon workshops 3- To repair or change the inner tube yourself, take a look at our video tutorials 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- 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.2- Have a full maintenance tune-up done at the workshop.

The rear gears are not shifting 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 a 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.

3. Bring your MTB to a Decathlon workshop to have the derailleur adjusted. 


4. To adjust your SRAM derailleur, use the SRAM DERAILLEUR ADJUSTMENT TOOL. This indispensable tool allows you to adjust the distance between the upper roller and the cassette.


5. 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.


Changing an internal drivetrain cable

The first thing to do when replacing a derailleur cable in a bike with carbon frames is to take the derailleur apart. All you will need to do this is an Allen key. The old cable will act as a guide when passing the liner through. The liner is a sort of temporary internal casing that makes it easier to push the cable correctly through the frame. Once you have removed the cable from the derailleur, you then need to thread the liner along it until it comes out of the frame, at the point where the cable is pushed in. Take care to slide the liner around the cable, without it PUSHING it. You can remove the derailleur cable when the liner is in place. The cable now has a clear path to follow inside the frame. It will be easier to insert the cable and to pull it out through the cable exit point. You can remove the liner once you have pushed the cable all the way through it.

You start pulling the brake lever and it touches the handlebars

The lever is spongy and/or touches the handlebars before you can brake:You need to bleed the brake system.Drop in at a DECATHLON workshop and get one of our technicians to do it.

The derailleur dropout is broken

A bent hanger causes the derailleur to be out of alignment and therefore unable to function correctly. Stand behind your bike and examine the alignment of the derailleur.If the hanger is twisted or broken, visit a Decathlon workshop to have it replaced, or use this video to help you change it yourself.


The chain is broken

Have you broken your chain? You've got 2 options: 1- Repair it yourself with the help of this video. You're going to need a chain tool and a quick link.  2- Visit one of our Decathlon workshops to get our technicians to repair it.



Maintaining your mountain bike

Cleaning your mountain bike

To clean your bike properly, you'll need some bike cleaner, a sponge and a brush. Spray the bike with a hose before applying the cleaning products. Use the sponge and brush to apply the products. Allow the product to act before rinsing the bike. Rinse it from a distance to avoid damaging mechanical parts. Finally, dry the bike with a soft, clean, dry cloth.


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. Then apply the degreaser to the whole of the drive train and scrub the cassette, chain and derailleur jockey wheels with the brush. Leave to act for a few minutes before rinsing the degreaser off from a suitable distance. Allow the drive train to dry before applying the lubricant. Shake the lubricant and apply to the inside of the chain, where it will work best. 


Changing a gear cable

Is your gear cable dirty, rusty or damaged, or are your gears hard to shift or shifting randomly?  Then your gear cable needs to be changed! To do this, you'll need some pliers, a screwdriver, a cable cutter, an Allen key and a new set of cables and housing. First of all, you'll need to remove the cable from its housing. Start by removing the plug from the shifter. Shift the gears all the way down, then cut the cable so you can pull it all the way out of the housing. Next, hold the housing against your bike to work out how long it needs to be. Cut it and place the end caps on. You can now install the housing. Pass the cable through the shifter and feed it through the housing. Next, screw the plug back in and fix the cable to the derailleur. Adjust the drivetrain by tightening the cable if the gears are struggling to shift up and slackening it if they are struggling to shift down. Once you're done, cut the cable and use the pliers to crimp the end cap onto it.


Removing a rear wheel

Removing and installing a wheel if your bike is equipped with a derailleur cage plate lock can be a tricky operation. First, put the chain on the smallest cog, then unlock the derailleur cage plate. To remove the wheel, simply open the quick release without changing its adjustment. To put the wheel back on, insert the wheel, place the chain on the smallest cog, tighten the quick release, and lock the derailleur cage plate to keep the chain from jumping!


Repairing a puncture

You need the following to repair a puncture: a pump, a basin of water, a cloth and an inner tube kit. Put a little pressure on the inner tube. To find the puncture, submerge the inner tube in the basin of water. Once you've spotted it, dry the inner tube with a cloth and scrape the area around it with the sandpaper provided in the kit. Scraping the inner tube will allow the glue to adhere better. Apply the glue generously around the hole and wait for three to five minutes. Place the patch over the hole and hold it there for a few minutes. You can then fit your inner tube back into the tyre.  


Changing hydraulic disc brake pads

You will need the following: new brake pads, a flat screwdriver and a 3mm Allen key. Remove the wheel first of all. Then use the screwdriver to move the pads apart and put the pistons back into their original position. Take out the pin holding the pads in place and remove them from their housings. Position the counter spring between the new pads. Insert the new pads in the brake caliper and then put the pin back in place to complete the process. Put the wheel back on and check to make sure that the brake pads are properly in place and do not rub against the disc.


Adjusting a hydraulic disc brake calliper

You will need a 5mm Allen key. Unscrew the brake caliper, spin the wheel and operate the brake lever. It is important to keep the brake lever engaged before screwing the caliper back in place.  Check to see that the disc and the pads are still in contact. 


Changing the derailleur hanger

You will need a new derailleur dropout that fits your bike (check the "SPARE PARTS" section) and a 5mm Allen key. Take off the wheel and the damaged derailleur dropout. Position the new dropout on the frame.  Remove the old dropout part from the derailleur before fitting the new one with the Allen key. Fit the derailleur on to the new dropout and check the gears.


Changing a chain

You will need the following: a chain tool, a new chain and a quick-release link. Put the chain on the cog and on the smallest chainring.  Remove the quick-release link using a 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. The chain should pass close to the top pulley of the upper jockey wheel on the derailleur. Once you've determined the length of the chain, use the chain tool to cut it as required. Then fit the quick-release link on the chain and turn the pedal to position the quick-release lever on the top part of the chain. Press it with your hand to check that it is on the right position.



Need to replace a part on your bike? You'll find all our spare parts for the XC 900 S mountain bike right here.


User guides


Instruction manual


Braking instructions


Suspension instructions

Drive train

Drive train instructions


Wheels user instructions


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.