Originally Posted by Road Fan

If your concerned about how far out you are reaching, it's not enough to look at bike size which is normally based on seat tube length. First you need to clarify seat tube length. It can be (in order of more length) from BB center to center of the seat lug, along the seat tube to the top of the top tube, and along the seat tube to the top of the top tube. Since you are starting to be sensitive to differences as small as 1 cm, be aware these three different measures can differ by from 12.5 mm to another 20 mm. So a 52 and a 54 frame can easily be identical in the vertical direction, since these measurements are not standardized.

Since you are concerned about length, there's a trick in here, too. The TT is generally measured from the center of the seat lug to the center of the upper head lug. This at least seems pretty common, but no firm standard exists. The trick is in the seat tube angle. Two frames with the same top tube might reach very different when the angle is different. The setback of the frame is the portion of the TT which is behind the BB. You can measure this by dropping a plumb line from the top tube to point at the center of the BB shell, the BB axis.

Given that on the two contrasting bikes you'll set up the same saddle for the same fore-aft position, the steeper seat tube will put the head tube farther away from you. This distance is very similar to "reach" invented by Slowtwitch.com, and used by a number of small to medium sized frame makers. So if you want to compare how a frame will reach for you with the same cockpit, you need to look at this reach value, or the part of the TT that is ahead of the BB axis.

You like your Lemond. If you still have it, make a plumb line, locate line to point exactly at the axis, and mark the string position on the TT - a bit of electrical or even cello tape is a good marker, with the tape edge at the string, for precision. Then measure forward to the center of the head tube and backwards to the center of the seat tube. Write down those two and add them up. Added up they should be about the same as the direct measurement of the TT.

Now you can use the forward top tube length as an indication of reach, assuming you can set up the saddle for the same knee to pedal orientation for both bikes.

Why is this setback compensation issue important? You are becoming more sensitive and buying/selling other frames sight unseen or with questionable measurements is expensive and a pain in the neck, and even if you like it you pay shipping. And even though it's only a few degrees, the difference in setback (the rear top tube length) is about 1 centimeter (10 mm) per degree. For a 72 degree frame versus a 75 degree frame that's about 30 mm, which you have already used as a length compensation criterion, with your very beneficial stem change. This all fits together.

So to control reach you need TT length AND seat tube angle. I especially raise this point if you are looking at a LeMond, because they are known to have numerically lower seat tube angles, compared to say a Mondonico (which I have) which, while designed for "stage-race" comfort, has a 75 degree seat tube in a 52 cm size (and is rather comfy!). The LeMonds are that way because that was how Greg LeMond liked to set up his frames. But on the Mondonico I need to use a seat post and saddle with LOADS of setback for it to work for me, so it's more challenging than say a similar sized vintage steel Trek with a 73 degree angle. Overall I care more about long-term comfort than about absolute max power. At my current low level of fitness I need to be out on the bike putting in hours, not breaking speed records -- yet.