Bike size help!

[Howmanysleepstilchristmas]

Dd is 4’10”.according to Halfords bike sizer she’s too big for kids bikes but not big enough for the smallest adult bike. Does anyone have a 4’10 dc and know their bike size? Please?

That's a bit taller than my DS who is now happily riding a small frame Specialized man's MTB. Thing to check is the frame geometry, especially step over height, reach (effective top tube length) and how low the saddle can be dropped (saddles are a relatively cheap component so you can always saw a bit off the tube to get it lower then if she grows loads buy a new full length one).

16 y/o DD is 5 foot & rides an Islabikes Beinn 27 - so the largest of their children's bikes. She's had it a fair while so I'd guess was around 4' 10" or a little under when she got it.

I'm 5'1" and generally take the smallest adult frame size. But - it does also depend on age - IIRC children tend to be longer in the leg / shorter in the body than an adult of equivalent height, so even a small adult bike is quite likely to not fit because the reach may be wrong. Can you take her to a bike shop to try some out?

It’s for Christmas and I’d like it to be a surprise so I’m trying to avoid having to try them

Could you make an excuse to measure her inside leg (e.g. trouser fitting or some such) then go to a proper bike shop (not Halfords - Halfords are very variable: some of their shops have people who genuinely know a bit about bikes, but most do not, they're just general salespeople) and ask for advice about frame sizes?

www.specialized.com/gb/en/womens-pitch/p/154424?color=236367-154424

Not suggesting you spend 400 quid on a bike, but if you scroll down this page you will get to the diagram of frame geometry (which is helpfully "roll over" and will colour in the relevant bits for you to look at).

Top priority - the step over/ stand-over height -vertical height from the cross bar to the ground. This is why you need her inside leg. I think you need about 5 to 10cm less than the inside leg measurement to avoid "ouch" moments if she has to hop off the bike suddenly.

Reach and effective top tube length (horizontal distance from saddle to handlebars) also matters. You don't want to end up with her in the "superman pose" as she tries to ride it.

Finally you need to make sure the seat post can be dropped far enough. Roughly speaking the seat tube (the hollow tube the seat post goes into) plus the crank length plus the depth of the saddle should be less than her inside leg. (Remember if the saddle doesn't go all the way down you can saw a bit off the seat post, the bit that goes inside the seat tube).