Product Colors and Sizes

  • Black Friday product
  • New Deal product
  • Cyber Monday product
  • Clearance product
  • Deal product
  • Best Seller product
  • Ecodesign product
  • New product
  • Staff Pick product
  • In The News product
  • Innovation product
  • product

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

Sale price: $249.00 Regular price: $399.00

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

Color: Select a color
Size: Select a size
Size Guide
Sale price: $249.00 Regular price: $399.00

Free shipping over $49

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

Btwin 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.

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

Skip to reviews
478 Reviews
Write a Review 
Btwin Rockrider ST900 Mountain Bike 20
Filtered by reviews Loading reviews…
Pleasant aesthetics, good ride,…

Pleasant aesthetics, good ride, solid, perfect for this age group

icon-circle-checkmark verified purchase

Rear wheel: the freewheel bearing is not mounted correctly, which makes it difficult for the wheel to rotate and does not roll straight. Even if it is cheap, the wheel should be freewheeling and of good quality. It should be covered by the warranty. Otherwise, I am satisfied with the bike.

icon-circle-checkmark verified purchase

Great product

icon-circle-checkmark verified purchase
Light and effective but with crippling flaws

At first glance this bike is perfect: even for 5 times more expensive there is nothing lighter on the market, and at this age that is decisive. The gears work pretty well and the front brake, although cable, does the job pretty well, it's hard to want better for this price. Two big problems mar this beautiful picture: the fork and the transmission. The fork is just perfectly useless, at this level of non-function you might as well put a rigid one, at least the bike would gain in lightness. At this price I suspect that there is no fork capable of cushioning a featherweight, but creating the illusion of one is not in line with the pragmatism of a Decathlon product. In terms of transmission, everything relies on the crankset protections which prevent derailment. Once this one is broken (which happens quickly since they are made of plastic and very exposed) the bike constantly derails no matter what you do to it. And as the part is not referenced it seems that changing the crankset is the only possibility, but we already know that it will only last a few outings. Too bad, it's nice to see a Decathlon product being on par with other much more (too) expensive offerings.

Very good bike

Only negative comment given the price, there should be a mudguard and a kickstand

icon-circle-checkmark verified purchase
Very light and…

Very light and robust bike.

icon-circle-checkmark verified purchase
Very comfortable bike

It goes very well

icon-circle-checkmark verified purchase

The bike is well made. My son is very happy.

icon-circle-checkmark verified purchase


icon-circle-checkmark verified purchase
Very good product

Very good product

icon-circle-checkmark verified purchase
aluminum bike

great product but still expensive in my opinion

icon-circle-checkmark verified purchase
This purchase was…

This purchase was made for my grandson's birthday. he was delighted.

icon-circle-checkmark verified purchase

I recommend this bike, the boy is very happy with it!

icon-circle-checkmark verified purchase

For my still "small" son, an excellent purchase. Adequate value for money

icon-circle-checkmark verified purchase

In two years without a single defect.

icon-circle-checkmark verified purchase
Skip to reviews

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 Inch & 24 Inch KIDS BIKE STABILIZERS (Model Code 8550741)


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



Law label information

Click here to see the product label


Size Guide Visual

Please Sign In to Post Your Question

In order to notify you about the answer it is required to Sign In.

Sign In