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Rockrider ST900 Mountain Bike 20" Kids 3'11" to 4'5"

Sale price: $200.00 Regular price: $399.00

20-inch bike with 6 speeds, front suspension, double disc brake

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Size: Select a size
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Sale price: $200.00 Regular price: $399.00

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Final Sale: No returns or exchanges

Free returns

Free pickup

San Francisco, 735 Market St. Options

Select an available color and size to see options.

Model: 8629906

Product Videos

Product Features

A photo of the Rockrider ST900 Mountain Bike 20" Kids 3'11" to 4'5" in use
A photo of the Rockrider ST900 Mountain Bike 20" Kids 3'11" to 4'5" in use

Rockrider ST900 Mountain Bike 20" Kids 3'11" to 4'5" | 6-8-year-old kids (4'1" - 4'7")

20-inch bike with 6 speeds, front suspension, double disc brake

Easy to use

6 speeds, quick-release saddle, quick-release front wheel

Bike safety

Double disc brakes provide powerful and easy braking.

Shock absorption

Suspension fork.

Lifetime warranty

B’Twin offers a lifetime warranty on the steel frame, stem, and handlebar.

Product Lifestyle Image Gallery

Image Gallery

Rockrider ST900 Mountain Bike 20 

Rockrider ST900 Mountain Bike 20 

Rockrider ST900 Mountain Bike 20 

Rockrider ST900 Mountain Bike 20 

Rockrider ST900 Mountain Bike 20

Questions & Answers

Look for product information, frequently asked questions, opinions...


How much does this bike weigh?


This bike weighs aproxamately 21lbs

By Thierry on Nov 22, 2023 | STAFF

Product Customer Reviews

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461 Reviews
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Rockrider ST900 Mountain Bike 20
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Happy little son at…

Grandson happy in every way on his Rock rider mountain bike...

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Nice bike. Effective braking,…

Nice bike. Effective braking, gear changes suitable for children.

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Worth the Money!


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Fed up for the moment

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Catastrophic spare parts supply

My son has not been able to use the bike for 15 months because Decathlon has failed to supply a crankset, let alone a new brake fitting.


Very good solid bike pretty

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Good but brakes too strong

Good product, somewhat expensive for a children's bike that is more expensive than some for adults. The front brake locks too much, easily blocking the front wheel, which can cause the child to fall.


Price/performance is simply brilliant.

Fragile bike

Bike that deteriorates quickly. I had to replace the brake levers and steering bearings after six months. This bike is poor value for money.

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Very nice, sturdy bike

Very proud 5 year old son. Very happy with his bike. Still need to practice with the gears, but other than that it's going well.

Very handy

Very handy

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Very light bike.. my…

Very light bike.. my son is happy. On the other hand, the brake levers are fragile

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High performance bike

It is a reliable brand. 3rd bike that we bought in 3 different sizes. I highly recommend it

the bike is perfect…

the bike is perfect overall except problem with the wheels and especially the rear wheel very low quality hub and rim completely veiled due to the new bike

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It was a Christmas present for my 6 year old. A very nice bike, I would buy it again and again. It costs a little more than the other models but is worth every cent. My son rides it in all weathers.

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Size Guide

Size Guide Visual

Additional Product Information


Frame (for bike): 100.0% Aluminium


Aluminum frame designed by B'Twin. The lowered frame geometry makes it easier to get on and off the bike.


6 speeds. Ultra-soft grip shifter for kids: Change gears using a grip shifter, which is easier and safer (kids don't have to let go of the handlebar to change gears). B'Twin chose this shifter as it is extra soft and specially sized for kids' hands.

Handlebar / stem / steering

Mid-rise stem and handlebar.Quick-release saddle. Comfortable, reinforced (plastic shell) saddle that provides a comfortable position. Fast, tool-free saddle adjustment. B'Twin selected this reinforced saddle for its tried-and-tested durability (should the bike fall over, for instance).


Very reliable double disc brakes. B'Twin chose to equip this bike with a double disc Brake system for its ease of use, easy maintenance, and greater braking power.


Lifetime warranty on manufacturing defects for all metal frames, rigid forks, stems, and handlebars.5-year warranty on all carbon frames and forks.2-year warranty on all other parts.Excludes damages arising from normal wear and tear, misuse, negligence, and unauthorized modifications.

Compatible accessories

KIDS BIKE STAND 100 (Model Code 2694189)20” & 24” KIDS BIKE STABILIZERS (Model Code 8550741)


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



Law label information

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Superior quality. Unbeatable prices.

For 40 years, Decathlon has delivered the best value in the retail sports industry by offering high-quality, sustainable and cost-effective products.

Decathlon makes the difference
  1. product-research

    Research and development

    delivers high-quality cost-effective products

  2. State-of-the-art production processes

    minimize manufacturing costs

  3. Optimized supply chain

    reduces retail prices

  4. laptop-checkout

    Minimal retail footprint

    lowers overhead and indirect costs


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