I'm building up a bike for a friends 12 year old daughter. The donor frame is a 15" 2009 Specialized rockhopper pro with a 70Deg head angle. This is a frame that she is going to have to grow into. I'm wondering how short a stem I can realistically fit before the handling goes a bit weird.
My guess is 50mm but interested in other opinions. The stock bike came with a 60mm stem.
If "guess" = test ride, then go for it.
How will the handling go weird?
If she needs a 30mm put one on. As low as possible and with a flat bar.
My assumption was that a very short stem with a steep (for 2015) head angle might make the steering very sharp.
Would love to be able to test ride but being 6ft it's only going to be around the car park.
put on the stem that's long/short enough for the bike to fit her now.
that head angle is fine.
My assumption was that a very short stem with a steep (for 2015) head angle might make the steering very sharp.
Your assumption is correct, surprised no ones reccomend 800mm bars yet.
BMX bikes have (or used to at least) 72 or 73 degree head angles and short stems, so should be OK.
My assumption was that a very short stem with a steep (for 2015) head angle might make the steering very sharp.
A total load of guff. Perpetuated on here more than anywhere.
Go with a 30mm or 35mm I'd say. It'll steer nicely, she'll most likely not be charging rooty singletrack at speed so high speed stability in the rough isn't a big issue I wouldn't think. Regardless, it'll be easier to ride with manageable reach and quick steering than with the bars too far away.
As above, also consider how to get the bars as low as possible and check out whether the brake lever reach can be reduced.
Can you push the seat forward a bit?
Straight post?
I reckon seat geometry changes are just a matter of habit until you're grown, but shorter stems can be odd.
A total load of guff. Perpetuated on here more than anywhere.
Have you tried to ride an XC bike with XC angles with a 40-50mm stem?
I wouldn't say it was unrideable, because I did it to lend my bike to a short person and they rode it, but it was definately the wrong side of 'involving'.
On the other hand if the bike had a 60mm stem originaly I guess the lower COG and shorter torso of shorter people helps keep things stable.
Have you tried to ride an XC bike with XC angles with a 40-50mm stem?
Yup, I maybe ride oddly but I notice very little difference in handling with stem length changes, mainly just the bars feel nearer or further away.
Have you tried to ride an XC bike with XC angles with a 40-50mm stem?
It's all more weight behind the front axle. There is only benefit regardless of head angle.