I'm just starting to get into bike mechanics. I recently bought a Schwinn Fastback 2004 (in mint condition... and the price was right). The frame feels too big for me... and I bought it with the intention of swapping the frame out for a Nashbar aluminum frame... but I'm confused as to what size to get. The too big Schwinn frame has a 21"/53.3cm top tube, 99cm wheelbase, 31.5" standover. My friend's ladies' Trek 1000 fits me perfectly and says it is a 54cm... but the top tube measures at 50cm, 99cm wheelbase, and 30" standover height. Any opinions? I have both bikes here if you need more measurements.

The link to the nashbar frames -> https://www.nashbar.com/bikes/Product_10053_10052_173108_-1_201511_10000_200450)"](https://www.nashbar.com/bikes/Product...1_10000_200450)[/URL]

Trek sizes their frames from the center of the BB to the top of the seat clamp. Nashbar says their measurement is to the center of the top tube seat tube intersection.
The Nashbar top tube is going to be a little long. You can compensate by using a shorter stem.
You can also buy the 1000 frame and fork from a Trek dealer.

I could provide some help, if you can post your overall height and an actual saddle height. I don't like using cycling inseam because it so often gets measured wrong and saddle height can easily vary by 2-3cm, even among riders who claim to have the same cycling.

If yu look at the geometry charts, there is little difference in the two smallest sizes that have very short head tubes. The 52cm size finally gets a head tube length increase from 103mm to 125mm.