Ref: 8500763



This 27.5" mountain bike is designed for your MTB outings, all year long, lasting more than 3 hours Greater efficiency and control: 120 mm ROCKSHOX fork with lockout controls on the handlebar, single chainring, lightweight 27.5" double-walled wheels... Enjoy the light weight of this ST 900 mountain bike!

First use

Inflating your tyres

The recommended average tyre pressure is 1.8 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 1.6 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 in the highest position to help keep your back raised, which is the most comfortable position for riding.

For improved aerodynamics and more effective pedalling, you can lower your handlebars to lower your body. This will cause you to lean forward and adopt a racing 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
Adjusting the height of your saddle

Your ST 540 bike frame is fitted with a quick-release lever.

Stand next to your saddle. Tilt the quick-release lever to unlock it. Pull the saddle up or push it down to bring it level with your pelvis.

Tilt the lever again to lock it.

To make sure you get it right, take a look at our tutorial video.

Adjusting the tilt of your saddle
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: weigh yourself and then 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

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.

Positioning your brake levers
Changing your pedals
How to convert your wheels to tubeless

You can avoid the risk of the inner tubes on your mountain bike getting punctured or pinched by switching to TUBELESS.

To do this you'll need to buy the following:


- A pair of TUBELESS tyres:


- 21mm rim tape: 


- A pair of Schrader TUBELESS valves:


- Sealant:



STEPS TO FOLLOW:1. Take the wheel off.2. Remove the tyre and the inner tube from the wheel.3. Apply the tubeless rim tape.4. Insert a Schrader TUBELESS valve in the valve hole in the wheel.5. Apply the sealant.6. Fit your new TUBELESS tyre.7. You need to inflate the wheel quickly to ensure that the tyre sticks to the rim.If it seems too tricky, just visit your nearest DECATHLON workshop and ask them to do it for you.


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

When you take your new bike out for a ride for the first time, a lot of parts move around and fall into place.You'll hear the spokes on your wheels creak because of the residual torsion in them.Your bike is fitted with double-walled rims, which help make the wheels more solid. That means they should not buckle in any way at all.If they do, then call in at one of our DECATHLON workshops to have them repaired free of charge: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:Young riders like to do tricks on their bikes (wheeling, skids, jumping off kerbs, etc). If done too often, however, they can cause play in the rear hub and buckle the wheels. 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, 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 one or two things you can do to make your fork "softer":1. Adjust the stiffness: check the section "ADJUSTING THE STIFFNESS OF YOUR FORK"2. Do some basic maintenance: using a simple oil can. Squirt some oil on the pivots of the fork and move it around to work the oil into the blades.3. Have the bike fully serviced in a workshop.

The rear gears are not shifting properly

There could be several reasons why your gears (cogs) are not shifting correctly.1. Check the position of the housing ends.Housing ends that are out of position can obstruct the derailleur cable when it is being pulled and prevent the rear gears (cogs) from shifting smoothly. If a housing end is out of position, push it back into place.2. The cable and housings may have jammed up.If the smallest gears (cogs) do not move down, if the derailleur doesn't move even when the gear shifter is operated, then the cable and the housings have jammed up. They need to be replaced. Take your mountain bike to a DECATHLON workshop or replace them yourself with the help of this video.3. Take your mountain bike to a DECATHLON workshop to have the derailleur adjusted. 4. You should also check that the derailleur dropout is not twisted. If it is, check the section "YOUR DERAILLEUR DROPOUT IS BROKEN".   NB: If you regularly use your mountain bike in wet or muddy conditions or if you wash it a lot, then you will need to replace the housings and cables more often. We strongly advise you not to use a pressure hose to wash your mountain bike as it will force dirt into the housings. 

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.

Video tutorials

These videos show you how to perform your own repairs and replace parts.

- Fitting/removing a rear wheel                                            - Repairing a puncture

- Changing an inner tube                                                       - Adjusting the height of the handlebar

- Repairing a chain                                                                   - Replacing the derailleur hanger

Don't forget to check out our YouTube channel for even more video content.







Maintaining your mountain bike

Cleaning your mountain bike

To clean your bike, you'll need a bike cleaner, a sponge and a brush. First, hose your bike down. Next, use the sponge and brush to apply the cleaning products. Leave the cleaning products time to work, then rinse the bike. Do this from a reasonable distance so as not to damage the mechanical parts. Lastly, dry your 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
Repairing a puncture

To repair a puncture, you'll need: a pump, a bowl of water, a cloth and a puncture repair kit. Pump a little air into the inner tube. Locate the hole by submerging the tube in the water. Once you've found the hole, dry the inner tube with the cloth and use the sandpaper in the repair kit to sand the tube. This will enable the glue to stick better. Apply the glue generously to the hole, and wait for 3 to 5 min. Lastly, place the patch over the hole and press down for a few minutes. You can then put your inner tube back into your 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 calliper 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 calliper, spin the wheel and operate the brake lever. It is important to keep the brake lever engaged before screwing the calliper 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.

Compatible accessories

Mountain bike mudguard kit


mountain bike mudguard kit
500 Base stand

Fit the 500 CHAINSTAY STAND on your ROCKRIDER ST 540

500 Black bike bottle cage



Spare parts

Go further

Need to change some parts? You'll find all our spare parts for the ST 900 MOUNTAIN BIKE right here.

User guides

Instruction manual


Braking instructions


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. 


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