Kids bike sizing help

Both of my rugrats are in need of new bikes as they’re outgrowing / grown the old ones but I’m not really sure what sizes to start looking for

before going to the shops to try for size, I want to have a good idea of what I’m looking at first

rugrat 1

140cm with 64cm inseam 11 years old in November

wants something geared, was thinking something with 24” wheels but a little worried he’ll grow out of it quickly, my small frame 27.5 bikes absolutely drown him

rugrat 2

127cm with 59cm inseam 9 in October

doesn’t care what she’s riding

any help / suggestions as to what sizes I should be looking for as a starting point would be greatly appreciated

I tend to look at the frog bikes website for a start, as the model names are the nominal inseam measurement for them, so it’s really easy.

https://www.frogbikes.com/bike-range/mtb.html

Rugrat 1 would suit a frog 69, for example, which they say covers 63-75 cm inseam. So he’s nicely at the bottom of its range – room to grow. Now we’ve learned that’s a 26” wheel, you can look at other brands smaller 26” bikes. My gut feel was 26” btw, 24” seems too small for an 11 year old.

26″ for the older, you can get 27.5s that’ll fit but there is an argument the smaller wheel aids skill development. Also keep an eye on crank length. Should be around 145mm for that height, you’ll probably have to swap those though as most kids bikes are fitted with cranks that are too long.

24″ for the younger but be prepared to swap cockpit and cranks lengths to get the fit right.

Both should be good with proper gearing.

Similar suggestion to above, have a look at some company’s websites for an idea of recommended sizing.

Vitus Nucleus 26 Youth Hardtail Mountain Bike

Personal experience is Vitus’ size guidance is fairly spot on. Both mini-a11ys progressed from 20″-wheeled Islabikes to Vitus Nucleus 24s, then on to the 26″-wheeled version linked above when they reached 140cm. No idea what either’s inseam was at the time.

Youngest (9) is now 153cm and still comfortable on a Nucleus 26, complete with a 105mm dropper post. Eldest moved on from the Nucleus 26 when she turned 11yrs and 156cm, now riding a small adult 650b HT.

I tend to look at the frog bikes website for a start, as the model names are the nominal inseam measurement for them, so it’s really easy

i don’t know that, quite handy to know, thank you for the insight

@nixie thank you for that also, seems I was looking at the wrong sizes for both

27.5 xs should work for 140cm. My boys both had a Trek Marlin with upgrades at that size.