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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 Guide
Sale price: $200.00 Regular price: $399.00

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Final Sale: No returns or exchanges

See our return rules for heavy items

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

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

Product Customer Reviews

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409 Reviews
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Rockrider ST900 Mountain Bike 20
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Good value for money

Good value for money

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Bikes not possible…

Bikes not possible to buy in store (even at btwin village) so order and delivery in store Upon receipt I ask this one was adjusted and checked I was told yes First use on my son's birthday and the surprise nothing have to adjust correctly and worst of all not tighten the pedal which causes the loss of it during the first ride with the added bonus of a fall for my son Thanks to the guy who told me yes everything is ok In conclusion good bike and perfect for the practice just remember to have it checked in front of you on site in the store

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Excellent product

They set it up in the store. The gear shift, the brakes and everything works great. My son is delighted with his bike.

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Just front tire which...

Just deflating front tire

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Great bike for my 5years old kid


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Very good wheel.

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Bicycle wheel 20

Mountain biking very well equipped and of good quality. Child 5 years 114cm, handles well. Effective with side wheels. Very satisfied

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Very good

Excellent and light bikes for my 7 year old son who is very happy with the performance of the bike and rides it every day for free time.

good product

in general a good product, perhaps it could have cost a little less.

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The bike is very…

The bike is very beautiful and light Impeccable braking Top!

Brake and derailleur not...

Brake and derailleur not properly adjusted. Good bike.

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Disappointed with the…

Disappointed with the assembly 1st right pedal output not tight (assembly made in store) and a bowl for my son in the woods fortunately there was the helmet… 2nd left pedal output And now with correct use the rear brake which blocks the bike high-end in its category not necessarily interesting

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Very disappointed

Did I get the wrong batch? Problem with braking, shifting, the gear selector pointer, and the icing on the cake: the pedal falling off during a hike!

Just an adaptation time .... otherwise it's a great bike

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Size Guide

Size Guide Visual

Additional Product Information


Bike frame: 100.0% Aluminum


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



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.

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