Not Geometron slack, but getting there. I thought I would share the build I’m doing for my soon to be 9 yr old who is pretty tall (139cm) and is getting close to outgrowing his Orbea MX24.
Most of us learnt to ride on old XC bikes (mine was a ’96 Kona Lava Dome) and I don’t think it helps learning to ride off road on what is effectively a road bike with knobbly tires. Surely you will develop faster if you ride something more suitable that gives you increased confidence from the start?

Anyway, after trying a 13″ Voodoo Nzumbi I decided he wasn’t far off the next wheel size. So I started with a Brand X HT-01 (27.5″) small frame from CRC.

Plan to run it with some 26″ wheels when I can find a reasonable cheap/light set (Crest or Arch), got some non TNT Barzo’s from PX, I’ve heard they work fine tubeless so will be lighter than TNT. (560g each).

Forks: found some Fox FIT 27.5″ 120mm on eBay, they have a bent rebound needle which I’ve straightened as best I could without it breaking, putting some new SKF seals and Gold 20wt oil in them. They had what looks like engine oil in and at 4*C in the garage it was like treacle… hope the Fox stuff isn’t that thick in the cold. Oil was fresh and they had SKF green wipers in so I have to assume they were looked after.

Headset: Going to run a 2.0* Works headset, this is where the slack part comes in, HA is 68* in stock form. I gave this a fair amount of thought as the top cup will go from ZS to EC so will raise his bars a bit, but I think the result should be ok.

Cranks: I have some 150mm cranks, going to fit a 30t O-O ringmaster

Drivetrain: Will probably rob this from one of my bikes which I need to cull from the fleet, 1×10 SLX/Zee mech with 11-36 cassette and M-785 brakes.

Bar and stem: 35mm Uno from eBay and some Marin kids/womens bars which have a reduced 19mm diameter grip area for little hands.

Seatpost: shimming an old 27.2 post down, found a San Marco Start saddle from PX for £8!

My next issue is the BB, using the square taper cranks from his current bike (68mm shell) for the BX (73mm shell), do I just use a BB with the same axle length or will I need to check the width of the stays to get the smallest Q factor?
Will update with pics when I can.

I wish you were my dad. Can i buy the mx24?

This sounds great! I totally agree with your thinking, slack bikes help build confidence and skills.

Can I buy it off you for my almost 7 year old once he’s too big for it?!!

Great stuff. Do you ride hilly places? The 30-36 lowest gear might be a bit tall- my 11yr old is lucky enough to have 30-46 on 11 speed, and definitely appreciates the low gear.

I did this for my son a while back. Small ht-01 with Stan’s 26inch crests. I’ve not got around to the slackset that it clearly needs yet.

Very interesting! I’d love to know how these ride – @Scienceofficer that looks great, and the short cranks should counter the lower BB with the smaller wheels, but what did your boy make of it, and did he need anything changed from the original build?

@howsyourdad1 The MX24 will be in use until next April, you’re welcome to PM me around that time!

@chiefgrooveguru Thanks, yes but I hope it’ll last a few years, the plan is to put all the bits onto a bigger frame when necessary and eventually 27.5″ wheels…

@TomB, not really, we live in the East Anglian mountains, so it should be enough range…

very nice Scienceofficer, what cranks are you running?

The postman brought me some goodies, managed to finish the lower leg service which it probably didn’t need (and I don’t need to spend £50 right now) but I wanted to make sure they were done. I can confirm the Fox Gold is indeed very thick in the cold, I wonder if I should have put some 5wt oil in that I already have? The forks feel pretty lethargic compared to the last 5 sets of RS forks I’ve owned.

Anyone a square taper BB guru that can answer my question from the first post?

using the square taper cranks from his current bike (68mm shell) for the BX (73mm shell), do I just use a BB with the same axle length or will I need to check the width of the stays to get the smallest Q factor?

but what did your boy make of it, and did he need anything changed from the original build?

He’s not that into bikes, because Hockey is his thing, but does ride with me when the weather is decent, and was fairly ‘unmoved’ until some of his friends went mental about how cool a bike he had.

It’s built from individual components to my spec so I didn’t change anything on the ‘build’.

what cranks are you running?

SR suntour 152mm cranks,