They are the measurements but you have to use them correctly to compare two bikes.

As i said 17mm shorter top tube does not mean 17mm less reach. Almost all manufacturers quote horizontal top tube length measured from centre line of seat tube to centre line of the top of the head tube. They also quote head tube length

If you look at the sketch you can see that frame B with shorter top tube also has a shorter head tube. Therefore to calculate the the change in reach you need to take off the 17mm but then add on the reach associated with dropping 20mm at 73 degrees. This can be a significant change in reach particularly if comparing a high end race bike with a tourer for example. Then you need to consider the height of the headset on both bikes and the spacers used which all adjust height and reach sometimes as much as flipping a stem or changing length by a size or two.

Then factor in bar reach, stem angle and reach, and you can compare the fit of your known bike and another (details taken from online tables) without ever even seeing one in the flesh. Of course you need to know your current bike dimensions and this is only half the story as you need to make sure your seat is in the right place too. Bikes come with different set back posts and often the manufacturer is more interested in telling you the post is carbon than telling you it has 20mm set back where last years model had no set back etc.

By the time you get into different frame angles and wondering just how many spacers you need bla bla bla you will find a spreadsheet is very useful. Like I said, I just walked in the shop, told them exactly what i wanted, no questions asked and nailed it firts go with identical fit to my other bike



I am a super geek tho