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

Regular price: $399.00 Regular price:

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

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Sale price: $399.00 Regular price:

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San Francisco, 735 Market St. Options

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Model: 8629906

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Product Features

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.

Product Lifestyle Image Gallery

Image Gallery

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20 

Btwin Rockrider ST900 Mountain Bike 20

Product Customer Reviews

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Btwin Rockrider ST900 Mountain Bike 20
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Quality control needs to improve

I had this bike for my son for Christmas. It arrived quickly and was kept fully informed. What I am not happy about is that the back brake wasn't tight and I had to tighten the cable. I don't expect to have to do this on a brand new bike. Quality control should be better and I have read another review where someone had the same issue. I also ordered with a stand which i thought they could have fitted.


The only I would like to make is when we bought it I filled in my name which is not a problem ,but I am titled as a Ms. I hate Ms as I ama Mrs. I did comment to the assistant but it was not his fault. Please add a 3 letter word to your list. Thank you very much.

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It is a great bike and my son loves it

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Good value

Great spec for the price

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In Demand - Understandably!

Very good bike, high quality and to be fair the service provided by the store was also high quality. back to the bike, its a solid bike but not to heavy - by 6 year old is easily able to take it in and out the garage. highly recommend it.

Superb value

We bought this bike for our son, we had been looking to get a specialized Riprock, but £420 seems excessive for a bike for a 7 year old. The spec of this bike is not quite as good as the Riprock, it only has a disc on the front and the rear v brake and the brake levers are very cheap, although functional (I can see why Decathlon have chosen to save money here, however personally I would have been prepared to spend a bit more for slightly better components). However, what you get for your money with the rest of this bike is not available from any other competitor that I can find. The finish on the frame is excellent, the front disc brake and suspension forks make it ideal for an early introduction to off road riding early, and the weight is equivalent to any of the other more expensive bikes with suspension forks. Only note of caution to potential buyers is that suspension forks really add to the overall weight of a bike this size, so if it's just a bike for your child to ride on the road that you are looking for, choose a different bike without suspension. Most of the issues highlighted in negative reviews for this bike that I have read, appear to involve not setting the bike up properly for use. Our bike arrived pretty much set up for use, but if you have absolutely no confidence in checking a bike over before your child gets on it, you should probably buy a bike from a local bike shop instead.

Good quality. Excellent value for money

We chose the B'Twin Rockrider ST900 20" wheel mountain bike for our 5 year old for Christmas. He wanted a red bike. We chose this out of the only 5 red mountain bikes in this size/age category. Orbea, Vitus, Voodoo and Frog all offered a red 20" but nobody could get close to spec/price like B'Twin. Criteria was red frame, suspension fork and disc brakes. Only Decathlon could offer the combined spec criteria and undercut the others on price. Amazing. Delivery was covert and on time so hiding the pre-assembled and checked bicycle was easy as it came in a HUGE unmarked box. I must admit I wasn't expecting the bicycle to be assembled all I had to do was straighten handlebar and tighten bar stem bolts and attach both pedals (with a touch of bicycle grease). The bicycle had clearly been checked and set-up although out of the box the gears weren't perfect and needed adjustment as did brakes? Easy enough if you know how but many people won't be able to. I was also pleased to see a French product supplied with brakes installed on correct sides for UK riders. Shimano 6-speed gears with a single front chainring and guide for chain are excellent connected to a Sram Gripshift twist shifter and Hayes front disc brake and rear V-brake. Odd to see disc and V together but it obviously saves money and most people use front brake most. Would have been nice to see mounts for rear disc fitted but nothing there so can't be upgraded later. Aluminium frame is nicely welded and paint finish is good. Saddle is high quality and comfortable but plastic brake levers let the side down. However they are very chunky and easy to use. All in all very pleased with this bicycle choice and it really made his Christmas. He took to it immediately and was riding like he'd had it for years. Highly recommended.

Chain derailled twice after barely using it.

Chain derailled twice after barely using it.

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A bike

For my youngest, for Christmas (sshhh don't tell him). What a great bike, set up, great spec and it's an upgrade from his previous BTWIN bike, and this time it has gears...can't wait to let him loose on some trails.

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Terrible experience so far

Very disappointed with this bike. We got it for my son who goes to school with it. But in the past year we have almost spent what we spent to buy the actual bike, on parts replacement and servicing. And after a thorough check up in few days a new problem comes up. Shame!

Колелото е много добро

Колелото е много добро

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Благодарим Ви, за отделеното време да споделите обратната си връзка за нашия продукт!

Като спортна компания, която се стреми да направи спорта достъпен за всеки, нашата цел е винаги да предоставяме качествени и функционални продукти.

Спортни поздрави,

Екипът на Декатлон България

Спортни поздрави,

Екипът на Декатлон

Seřízení, dotažení

Po seřízení a dotažení upadla mému synovi šlapka za jízdy. Mohlo se mu něco stát. Tohle mě vážně vytočilo a této službě u vás už nevěřím.

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Kolo je super ,už se těším až se malá naučí pořádně jezdit

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Poměr cena výkon bomba, doporučuju

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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.