Rockrider ST900 20" upgrade

[Ludo]

So I got that bike for my son a couple of month ago, great bike at that price
https://www.decathlon.com/products/kids-bike-20-st900?variant=39409795104830

but suffers from the same issues as most of the kids bike:
-rotoshift, but he doesnt have the strength to rotate it (5yo), especially when loading the pedals
-gearing is for kids to mess around in driveway and paved roads, not on the trail

I had him testing mine and my wife bike thumb shifters, and that seems to work for him, so I'll upgrade to shimano TX 30 I think, it seems many people do that. For the chainring, it is currently a 34T I am thinking of changing to 32 or 30 so he can actually use more than the top two sprockets when riding on a trail. I am kind of tempted to move to 30T with shorter crank to gain some ground clearance too (135-140 instead of 145mm).

Anyone's done that and has some feedback?

[Vipassana]

We have a 20" Prevelo Alpha 3 for my daughter that we got as a scratch-and-dent sale earlier this year.  Here is a link to the bike/spec.

https://www.competitivecyclist.com/prevelo-bikes-alpha-three-20in-complete-bike-kids-pve0004

We have found the gearing range to be quite good for her needs on and off road. It has a 32T front ring and 11-34T cassette. It also has trigger shifters that she has no issues activating and both the trigger shifters and brake levels are designed for children's hands. Very well thought out.

The site says 110mm crank, but that seems a bit short to me; I would need to check when I get home.  They do fit well and are very compact, whatever they are.  145mm seems irresponsibly long for a bike designed for a 5-8 year old (the bike you linked too), I don't know what in the world they were thinking there.

Unrelated to your question, but for anyone thinking of going with a Prevelo, the Alpha series is very nice, but limited on tire clearance, which limits it's use to smoother trails (which is fine for my daughter's use).  If you are looking for a better MTB, the Zulu series is the way to go. But I'm that price range, there are great options from several brands.  It should be noted that the Alpha is also decently light which is VERY important for children.

About to build a Chiner carbon coaster bike for the 18month old...

[Ludo]

Thanks for thorough answer! You have me doubting on crank length, I’ll check that when I get home!

[Ludo]

So it is 145mm, and square hole crankset. Off to AliExpress to try and find some 30/32T with 120/130mm cranks (10%of his height according to this http://www.bikesmithdesign.com/Short_Cranks/cranks4kids.html) Crankset is one piece so no choice but replacing cranks too…

[Ludo]

Not much luck so far, I didn’t find cranks with square holes in 120-130 so I will have the replace the spindle too. Found a goodie crankset that fits the bill, I will have to buy some tools to be able to make the change :/

[Ludo]

So I ended installé à Shimano tx30 shifter, so much easier for him compare to revoshift, install was straight forward.
I also got a crankset but not yet installed:
https://a.aliexpress.com/_mMoyBwo

Some mixed reviews but didn’t find a whole lot of alternatives…

[Ludo]

So I got a replacement crankset for this bike, however it didn’t come with the screw for the chainring, I know it’s M5, unsure of length, is it somewhat of a standard?
Just found this amazing item on AliExpress. Check it out! $32.19  22％ Off | GOLDIX Children's Bicycle Crank 110/127/140/150/160mm Ultralight MTB Bicycle Crankset Black 28T 30T 32T 34T 36T 7/8/9/10 Speed
https://a.aliexpress.com/_mNABfJm

[Ludo]

Ended up doing quite a few changes:
as stated above, replaced crankset for easier climbing, new pedals (283g saving, much improved crank bearings), elitaone carbon seat and seat post (348g), uni stem (9g saving should have checked that first...) and 2 Schwalbe little joe tires (202g saving each). still waiting for a carbon handle bar. Total weight saved so far, 1,044g, and probably some more with the bar.
It's been fun working with my sons on changing all parts, teaching him about mechanicals and swearing in the confine of the garage!

[Vipassana]

Subs like fun. Maybe I should look into some upgrades for my daughter's bike... I'm afraid to open that can of worms. 

[Ludo]

I know what you mean, asking around for 20” carbon wheels, a must have as it’s not necessary lol

The fact is that the weight of the bike vs the weight of the kids is so important that going downhill on bumpy ground, the bike is in charge, not the kid. But short of going full carbon, I am not sure anything will make a big difference. If my son keeps on growing the passion for mtb, his 24 will be full carbon…

[Ludo]

short video of the weekend ride, fun to work on the bike and video editing with him, sharing and learning! you can see and exemple of the bike taking over in one of the drops... I havent been able to find a good angle to show how steep the drops actually are: