measuring frame size
 

I just bought a1997 gary fisher tassajra with a19.5 frame size. After i got it home and rode it around some it seemed a bit larger than my 19 in gaint escape. When i measured from the center of bb to the top of the seat post its over 21 in. I do get 19.5 from center of bb to center of cross brace. I thought frame measurements were taken to the top of the seat post.

Several ways , not just One. C toT is center of BB axis To top edge of the Seat Tube; C to C is BB to center line of the top tube (1/2 it's Diameter. )

When bikes all had flat top tubes the measurement was from the centre of the bottom bracket to the centre of the top tube, measured along the seat tube. With compact frames they will sometimes (or used to) measure the same way but nowadays they generally measure from the centre of the BB to where an imaginary top tube WOULD have been if it was perfectly horizontal. This second method is more useful as measuring the actual seatpost length, centre to centre is wildly misleading on a compact frame.

I've never heard of measuring centre of BB to seatpost clamps, however, on a compact frame, this may have accidentally worked out.

Anthony

In my original post i should have said it was over 21 in. to top of the seat tube. Not seat post. My giant is
19 in. center of bb to top of tube. Although its larger than i would like it will b ok for my intended use.