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B'TWIN Rockrider ST 900 Kids Alloy Mountain Bike - 20"

until £219.99 -%


This price includes a recycling fee for electrical equipment and batteries DecathlonDeeeTaxMentionAfterPrice / Since

From To

* Sales from to Until end of stock
* From and limited stock
£219.99 -%
This price includes a recycling fee for electrical equipment and batteries DecathlonDeeeTaxMentionAfterPrice / Since

* From To

* Sales from to Until end of stock
* From and limited stock
Check availability in store
Designed for mountain bikers aged 6 to 9 years old (120–135 cm tall), the B’Twin Rockrider ST900 has 6 speeds which, when combined with its front suspension fork, make it easy for younger riders to overcome rough and hilly terrain. Featuring a grip shifter for easy gear changing, it’s a bike that makes mountain biking safe, simple and fun.
  • Lightweight aluminium frame
  • Ergonomic saddle
  • 20-inch wheels
Not Available Now

We currently have limited capacity to build bikes in our warehouse. Due to this, we are limiting the number of bikes we sell per day. If you are unable to process your order at checkout, please return the following day to make your purchase.


Product benefits

Ease of use ALU frame and chainset for less weight.
Grip shifter for the 6 speeds.
Brake control Front cable disc brakes: powerful braking in all conditions.
Efficiency Suspension fork. Skinwall tyres for less weight, with side knobs.
Cycling comfort Sport saddle combined with an aluminium seat post for added comfort.

Technical Information

Frame designed for mountain biking

Our engineers have designed a new aluminium frame. Durable, lightweight and responsive: designed for tackling any obstacles when out mountain biking.

Controlled braking

Effective braking thanks to the cable-activated disc brake at the front and a V-brake at the rear.
The disc brake presses on the wheel axle to produce powerful braking under any conditions (dry, wet, muddy, etc.).
The V-brake is composed of 2 levers equipped with pads that exercise more or less pressure on the rim to slow or stop the rotation of the wheels.

Front suspension to cushion unpredictable terrain

Take on new challenging terrain (roots, stones, gravel, etc.) with ease thanks to the travel of the front suspension fork.

Changing gears (6 speeds)

This bike is equipped with a grip shifter to change between the 6 speeds: an easier, safer system because the child keeps their hands on the handlebars.
This makes changing gears to match the terrain simple and easy!

Why have we created a brake lever specifically for children?

Because children have smaller hands than adults, it's not always easy for them to reach the brake lever. This is why our engineers designed a brake lever that is closer to the handlebars and thus easier to hold.

A reinforced, ergonomic saddle

A saddle with a sporty design, designed with care to adapt to the morphology of children. Combined with an aluminium seat post (sturdy and flexible) for even greater comfort.

Riding position

We equipped this mountain bike with a short aluminium stem (60 mm handlebar) and an oversized 520 mm handle for more controlled steering. This gives you greater accuracy and more responsiveness in the turns.

Aluminium chainset

The new aluminium chainset has a 140 mm crank that is designed for a child's leg length. This lightweight chainset allows you to start pedalling more easily.
The double chain guard prevents the chain from derailing.

20" wheels and MTB SkinWall tyres

The tyres are designed for mountain biking. The side knobs provide good grip on corners. The tyres use SkinWall technology. This process makes it possible to reduce their weight, provide greater flexibility, therefore increasing their performance, grip and comfort.
The recommended tyre pressure is indicated on the side of the tyre (2 to 3.5 bar).
20-inch 1.7"-2.2" wide inner tube with Schrader valve.

Bike accessories

This MTB is equipped with a bell, lighting, wheel reflectors and platform pedals.
The frame is also compatible with other B'Twin accessories:
- front and rear mudguards
- stand
- bottle cage
- bag

Dimensions of the bike ready to use

Length: 140cm.
Height: 80cm.
Width: 54cm.

What if I order online?

For smoother shipping, our bikes come with their pedals removed and the handlebar unscrewed (instructions and tools are supplied)


11.7 kg

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Composition / advice

100.00% Aluminium
Stock advice Store the bike in a clean, dry place. Clean and grease the chain regularly. Our advice for before each ride: inflate the tyres to the pressure indicated on the sidewall.
Restricted use : Wear a cycling helmet.

Our commitments

Guarantee : for life (frame, stem, handlebars, rigid fork). Find out more about the bike guarantee.

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Rockrider ST 900 Kids Alloy Mountain Bike - 20
Maintenance and repair
Maintenance and repair
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