Aero road bike frame - frame size number vs actual frame size? I'm encountering some confusion on frame size and any clarification would be greatly appreciated.  I've always been under the impression that a stated frame size was the distance, usually in cm, from the top of the seat tube to the center of the BB opening.  For example looking at any of the road bikes on the Trek site:

http://www.trekbikes.com/us/en_US/bikes/road-bikes/c/B200/

For all the bikes the "Frame Size Number" and the "Actual Frame Size" (see the chart at the bottom of the page for any of the bikes) are the same, even for the aero Madone.

I'd like to purchase a Dengfu FM098 frame for a bike I'm planning on building:

http://dengfubikes.com/ROAD_FRAME/129.html

The available sizes are listed as "49/52/54/56/58/61cm", but upon going to the Geometry tab this chart is shown:



It would appear that the first column (which is oddly untitled) would correspond to the available size, but dimension "A" is different than the frame size on each line!

At first I figured this was a mistake or an oddity for this particular frame, however the Dengfu R02 (choosing the "Geometry" tab again):

http://dengfubikes.com/ROAD_FRAME/131.html

and the Hongfu FM098:

http://www.hongfu-bikes.com/html_Products/Road_Frame/Aero_Road_Frame/2016/1113/2017%20new%20aero%20road%20frame%20FM098.html

Have the same frame size number vs actual frame size difference.

Can somebody clarify this?  Is this standard in the industry for aero frames or frames with a curved or non-flat top tube?  Is the idea that I should know what size I should be in a flat top tube frame, for example 52cm, then purchase the frame with the name "52cm" even though it is not actually 52cm?  Please advise, thanks!
The answer is the top tube is not straight but curves down at the seat stem.

The idea behind the difference is that somebody who need a 49cm size frame would have the seat adjusted to their leg length and somebody who as a slightly smaller leg length can't lower the seat to suit. So you have got 10 mm up and 10 mm down otherwise your seat would be set at the top of the seat tube all the time. Same on the next size if you bought  a 49cm frame but your leg length is that bit longer you buy the next size up and lower the seat to the top of the seat tub.

You have a 49cm frame with +/-10mm, and the next size frame is a 51cm with plus and minus 10mm. Plus you can work out your crank length from 165 mm to 175 mm to suit your leg length.

So this mean you have some 40 mm to play with in various leg lengths. And the same with your cage length. Or top tube as some people say. If you've got short arms you use a small handle bar stem or long arms a longer handlebar stem.

I hope this may give some of you some better idea and thought to buying your frame. Measure your leg length and subtract 30mm from that and work from there.

I know this as a fact as I design cranks for a living. Hope you have something of a better idea now.