Bicycle sizing for my girlfriend..

[1150A]

the bike feels too big for her. The top tube is 54cm. That might just be too long for her reach!
what's wrong with the smaller bike you bought her?

It has track ends and not enough clearance to run knobby tires.

[Andrea]

I'm 5'6" w/a 30" inseam and I ride a 46cm crosscheck. I was trying to decide between that and the 50cm, but went with the 46 because the standover on the 50cm is 30.3" (yeah, I'm mixing my metric & imperial units ) The reach on it is definitely shorter than my road bike (surly TT: 52.8cm Blue Rc7 TT: 54cm), but it doesn't bother me for CX racing.

In other words, the Surly's 50cm standover was barely too big for me, but the TT was OK, and I'm a little bigger than your GF, so the 50cm is likely going to be too large for her.

[Triskeliongirl]

Height is only one paramater. I am 5'4.5" tall, and have long femurs and a short upper body. My best fitting bike is a 19" terry road bike with a 49cm TT. I also need a slack STA (73 degree or less) to fit my long femurs, and even then my saddle is all the way back on a very laid back seat post. This is another reason I need such a short top tube, I sitting further back than most, increasing my reach.

I could never ride a bike with a 54 cm top tube!

[aicabsolut]

I'm 5'7" with a 31.5" inseam. I've got long arms and a flexible lower back. I ride a 54cm road bike with a 54.8cm effective top tube and a 110mm stem. I can make a 52cm fit (with a 53something TT), but I prefer a 54. A guy I know who is about 5'10" rides the same bike with almost the same setup, only his seat is a lot higher.

I think that based on all our stats you can get a good starting point for her, but she's going to need to sit on a bunch to figure out what size is best. Different brands have different geometry, so maybe she'd like a 52 in one but a 49 or 50 in another.

[kie_fujo]

i don't know a whole lot but i'm 5'4" and i ride a 48cm. i had a 50cm and it was too big causing me a lot of shoulder pain. i also ride a road bike though.