27.5" Mountain Bike - Red/Black


This Michelin tire has been designed and developed to best suit leisure mountain bike use. A comfortable ride, whatever level your skill is at.

This lightweight mountain bike offers greater efficiency and more control. It boasts ROCKSHOX 120mm fork with handlebar lockout, single chainring, and lighter 27.5" double-walled wheels.

Color: RED
Size: size guide


Product benefits

Cycling comfort

Cycling comfort

Enjoy the raised position, adjustable stem, 120mm suspension and hammock saddle

Directional control

Directional control

Control your trajectory: hydraulic fork and brakes, tyres and a wide handlebar.



Smooth riding: lighter wheels, single chainring and fork lockout.

Ease of use

Ease of use

Change speeds or lock your fork with a single motion on the handlebar.

Lifetime warranty

Lifetime warranty

Rockrider offers a lifetime warranty on the frame, handlebar and stem.

Technical information

Which is the right size?

Several sizes available:
S: between 1.50m and 1.64m
M: between 1.65 m and 1.74 m
L: between 1.75 m and 1.84 m
XL: between 1.85 m and 2.00 m

If you are between 2 sizes:
Choose the smaller size for easier steering (easier to handle) and comfort (straighter back).
Choose the larger size for easier pedalling with a more stretched-out position (position more suited to performance).


With a heavily sloped top tube and a wide range of sizes (from S: 1.50m to XL: 2m).

The ST 900 mountain bike frame is unisex.

The ST 900 mountain bike is available with the same frame in a women's version: fitted seat, narrower handlebar, shorter stem, fork with a more flexible spring, and brake levers that are closer together.

As a woman, if you choose the men's model, we recommend at least purchasing the Women's Sport 500 saddle which is fitted to the female form.


Our team of mountain bike enthusiasts created this ST 900 mountain bike at our B'Twin Village in Lille, France.

A comfortable, efficient mountain bike: 120 mm Rockshox fork to easily ride over obstacles, fork with handlebar lockout, single chainring and, 27.5" double-walled wheels that have been lightened for greater efficiency.
Women mountain bikers were involved at every stage of the design process.


For less weight and more efficient pedalling, the Rockrider ST 900 MTB frame is made from 6061 aluminium (only 1.6kg in size M). It is 500g lighter than the ST 540 frame, thanks to its double butted front triangle.

Our designers created this bike to be comfortable and easy to manoeuvre. Its slope has been designed to keep your back straighter. The lower bottom bracket helps keep you more stable during turns.


The 120 mm Rockshox 30 Silver Air hydraulic fork is 400g lighter than that on the ST 540 MTB. It is conical and offers even more stiffness and precision when steering.
Easily adjust the fork's bounce and stiffness.
Its quick Combo Lock lockout system, which you can control from the handlebar, helps prevent bobbing and loss of speed on faster sections.


This mountain bike has no rear suspension.

This is a "semi-rigid" mountain bike. Compared to a full suspension mountain bike (with rear suspension), the advantages are clear: lighter weight, less loss of energy on flatter terrain. If however your primary concern is comfort and manoeuvrability, the ST 900 is available in a full suspension version with suspension at the rear.


The single chainring is easy and efficient to use.
1 chainring and 11 speeds:
Accelerate on the flats and easily tackle the climbs. 1 single shifter on the handlebar: adapt your speed instantly like the pros as the terrain changes!

In detail:
-SRAM X1000 32-tooth single chainring
-Microshift M665 11-speed rear derailleur
-Microshift M8515 right shifter
-Microshift H113 11-cog 11 to 46 teeth cassette
-KMC X11 chain


The adjustable stem brings the ST 900 controls to your hands. The all-aluminium Rockrider stem is 70 mm (size S), 80 mm (size M/L), or 90 mm (size XL) and can be raised to 5 adjustment intervals: + 0mm, + 7.5mm, + 15mm, + 22.5mm, + 30mm.

In detail: - 720mm oversize Rockrider Sport aluminium handlebar
- Rockrider ergonomic grips for good grip and a comfortable feel.


Our teams designed and created an exclusive Rockrider Ergofit Pro saddle. Designed for all hip widths, its hammock shape is very comfortable for long, athletic rides. The Flex 27.2mm diameter aluminium seatpost also contributes to its increased level of comfort.

Quick and easy tool-free adjustment of your saddle height using the adjustment lever on the seat post clamp.


No matter what conditions you ride in (dry/wet), the Rockrider by Tektro 180 mm (front) and 160 mm (back) double hydraulic disc brakes will come through for you, even in the mud.
Hydraulic braking adds more power and progressive braking and requires less powerful use of the brake levers.
The aluminium brake levers are easy to grip, lightweight, and precise.


Lighter wheels: with sleeved double-walled rims and lighter tyres, these are 230g lighter than the wheels on the ST 540.
With their 28 spokes, they are lightweight, stiff, and durable.
The exclusive Rockrider All Conditions Light (60 TPI) 27.5"x2.2 tyres have side knobs designed for all terrain riding. A width of 2.2 means more grip on technical sections.
Tubeless Ready 23 mm rims: you will need rim tape, Schrader valves, and puncture sealant.

Platform pedals

The ST 900 comes with Rockrider Flat platform pedals with a wider pedalling surface and more pronounced dots for better grip on the pedal.


We added a quick release system for easily removing/reattaching the front wheel.
Our advice: a bike rack is an easy way to transport your mountain bike without dirtying your car.


Your ST 900 mountain bike is supplied with a bell, lights, wheel reflectors and pedals.

The frame is compatible with:
-front and rear mudguards
-bottle cage
-saddle bag
-frame-mounted child seat


Your bike's fittings (chain, derailleur, brakes, etc.) will adjust over time. Take advantage of a free servicing within 6 months after purchase. Our technicians will carry out a 13-point inspection of your bike. Just come to your nearest Decathlon workshop with your Decathlon card and/or receipt.


To optimise the durability of the components, we recommend a maximum load of 120 kg (rider, kit + bike).


Lifetime (frame, stem, handlebar). Availability of spare parts: 2 years.


S - 155-164cm : 12.85 kg
M - 165-174cm : 12.95 kg
L - 175-184cm : 13 kg
XL - 185-200cm : 13.1 kg
Weight without pedals.

Product concept & technology

Product Composition

Frame 100% Aluminium 6061


After washing, rub with a dry cloth to remove all traces of moisture. Store your mountain bike in a cool, dry place, away from corrosive products and salty air.

User Directions

Not suitable for XC, All Mountain, Enduro and BMX.


Our engineers always have real riders test all our products in real conditions. These tests round out our lab tests and let us validate all our products before they hit store shelves.


To have more informations about this product, consult the notice


27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black
27.5" Mountain Bike - Red/Black

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