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I might be over thinking this.
I went to collect my daughters new bike, with her, last night.
Its currently getting in the way in the kitchen until the planned ride this weekend.
Before we use it and forego any option to swop it, is there such a thing as an ideal fit.
For context, it is a Trek Roscoe 17” frame, chosen as per the size guide, albeit she was bottom end of 17 and top end of 15.
She is a gangly 5’4”.
Old bike is a 17” 26” wheel frame HT spesh Myka.
She can ride the bike ok on the car park test and it doesn’t look gate like or too large.
Standover height is around 2-3” but it’s difficult to gauge as it is such a steep top tube.
She needs around 6.5” of seatpost out, that from top of clamp to seat rails.
The shop said it was ok for her, any smaller and her knees might clip the bars when turning.
I can’t get my head around that she is on 17” frame and is 5” shorter than me and I’m on a 19” frame.
Its not that simple I get but I’m niggled that she would be better on a 15” with more seat tube out.
Or is just because it looks a big bike anyway, the front is very tall, plus tyres on 650 rims.
I suspect we could ask for a 15” to be ordered to try if need be.
My wife is 5'3" and I can't imagine her on a 17" frame, it'd be enormous.
I can’t get my head around that she is on 17” frame and is 5” shorter than me and I’m on a 19” frame.
Assuming you are both the same proportions and 1/2 the height difference is legs and 1/2 upper body then that's 2.5" shorter legs and 2" smaller frame. If her legs are longer then ...
She needs around 6.5” of seatpost out, that from top of clamp to seat rails.
Both my droppers are nearly slammed 125mm ...
Standover height is around 2-3” but it’s difficult to gauge as it is such a steep top tube.
Don't know or care what mine is... pretty irrelevant when on ST. Anywhere you would put boy feet down at the same time is probably the last place you should... (unless you fancy careering down a slope)... (OK other than pics at trail heads)
Are you trying to size her for a modern mountain bike or a more traditional style? Obviously with kids who are probably still growing and will probably use the bike for more than just ‘mountain biking’ it becomes complicated, quickly but generally ideas about standover and seat tube length determining size are completely at odds with modern frame design. As you’re not buying a BSO I’ll assume ‘mountain biking’ is on the list of intended uses!
There’s been so much written about this I won’t repeat it, but the basics now probably should be ‘what will the bike be used for’ and ‘how do the designers intend it to fit?’. Second guessing the frame designer and product manager could easily take a good bike and make it ‘meh’...
For reference, I’m 178/5’10” and always seem to go for ‘medium’/17.5ish. Every ‘large’/19” frame I’ve ever had has turned out to be a mistake. Leg/arm ratio probably comes into that as much as personal preference though.