[Closed] Kid's bike geometry?

My daughter has an Orbea MX24. The MX20 looks like it has similar geometry and is 'quite' relaxed compared to some other bikes like Isla, Cube etc

https://www.orbea.com/gb-en/bicycles/mx-20-team-disc-19

I'd not go long, but I'd go low standover and slacker than most brand frames.

Short chainstays too. Most brands are waaaay to long.

To be honest, I'd look at spawns 20's or NS clash 20 and make the standover a little less and the front a little slacker.

I don't think kids would benefit from a longer reach as they are better off sitting more upright than forwards to encourage weight through the feet transition from sitting to standing.

Id wait till the 24/26 bike till you go all geometron.

I'd be careful of transferring "adult" geometries (designed for experienced adult riders) direct to kids frames. You need to think carefully and realistically about what sort of rider they are and what they want to do on the bike, but there's certainly room for tinkering!

Islabike geometry is hard to beat in my opinion, but that's not to say you couldn't tweak it a bit here and there. But starting with  the Beinn 20 geometry (Pro Series if you can) would be a good way to go. It's certainly not aimed at proper "trail" riding, so if that's the aim you maybe could slacken it a bit. But slacken it too much and you'll lose agility for slow speed riding and play - so you need to be honest about how it's going to be ridden.

The risk of going too long is that it makes it hard to get on top of for pumping - which is a key skill. Obviously they can do it, but adding too much length will complicate the learning I think. Of course if they've got another bike to play about on (maybe a mini BMX type) then that might enough. Adding reach may make the bike last longer of course, but the advantages of moving up to 24" or 26" wheels sort of stump that one. I see a lot of 20" bikes with really long backends - adds stability but makes it harder to pump (see above!) and get the front up. I think a tighter back end (a la Islablike) is better in most situations.

I've always aimed to set my kids bike up with the sort of riding position I'd want them to have as an adult - so I'd want to see a low front end.

Oh and make sure you've got room for some decent sized rubber!

Look also at what flow kids bikes are doing for adjustable reach/ adjustable 20/24 and 24/26.

Some compromises but there's a reasonable solution to be had.

Thanks chaps 🙂

I'm currently compiling a spreadsheet of geometry. This only started as a thought...earlier this morning!

Don't go long because their arms are shorter and torsos need to be more upright. BB as low as possible with short cranks (properly short). Don't make the seat angle too steep - but bear in mind you can use a layback post if need be. With a rigid fork I'd see if you can fit a 24" front wheel in but 20" at the back. I'd go with a short offset fork and pretty damned slack head angle - the smaller front wheel will easily go towards negative trail when it hits a bump.

My daughter's on the Beinn 20 Small and with bigger tyres (Little Joe 2.0) that's a pretty good rigid MTB for a five year old. Could do with some lower gears!

Have you seen those Chris Hoy kids bikes that look like the forks are the wrong way round?

I'm tempted to say if its worth doing it's worthy doing right....

As jonathon say's though you need to be honest (to yourself) about what it's for....

What's she riding now and likely to ride ?

What's the reason for the rigid fork ???

How tall  is she (are you making a 20" 29er as it were or a 20" 27.5) ???  and how long do you see it in use for?

What's great for say shooting down the canal towpath vs technical riding vs messing about?

Equally I guess is what rear cassette are you planning on the 20" wheel?  I'd guess a short cage Zee and 11-36?

Just using the 'standard' offerings we went 20" at age 5... couldn't tell you the height/inside leg anymore but he was just big enough .. (Pinnacle Ash) ... but 9 months later he was just big enough for a 24" .... HOWEVER he still had the room on the 20... it was really that the 20 was what it was and not worth spending more.

You aren't going to be stuck in the same place or not to the same extents.

Propain do a 20" carbon air fork that's surprisingly cheap.... MRP do one (as far as I can work out inherited from White Bros) that's eyewateringly expensive and stuck on the Lil Shredder.  I didn't check specifically but could ask (or you could .. if you post on his FB Si will answer if you don't have other contact info) what Henry (or 417/flyup) is using because it's obviously indestructible!  I'd assume it's the Propain one... as they sponsor him

The Orbea geo is IMHO pretty good for trail....  buy you could also base it on a Lil Shredder or Propain...

Personally I wouldn't try and be fancy with 2 wheel sizes... all the compromises get multiplied with kids.  I know mainstream companies are selling the one-geo-fits-all but that's IMHO more marketing than design...

We have a couple of those 20" forks. Ours are Brood (Spawn) branded and they are very good.
I've replaced a couple of damper assemblies but they are holding up well and super easy to pull apart and get running butter smooth.
Avoid the MRP fork. they seem to need too much love to keep them working..

Mainstream multiple wheelsize bike compromises are normally that the reach is too long for the smaller wheel, 2 short for the bigger wheel and BB height changes between the 2 sizes, and the seat stays still too long on the smaller size

With your creative design from the 16" bike, you could relatively easily adapt what I assume wont be a traditional design..
I napkinned some ideas for a 24>26 FS build but I don't have your skill set or access to fabrication.
20>24 might be a jump too far however.

I think 20" is probably the last size I would be happy making and letting my daughter ride with confidence that it isn't going to fall apart under her.

Cost will play a part so I think suspension forks are out (for now) and I can get a nice looking aluminium rigid one for £30.

I've done a speadsheet of 7 different bikes and there are many very similar dimensions.

Average HT = 69.1° (NS slackest @ 67°)

Average ST = 72.7°

Average Standover = 533mm

Average Reach = 312mm

I think the NS has the geometry I was thinking of but has a slightly higher standover so I can tweak that a bit.

Some info to work with for now though 🙂