# Stack & Reach Primer: Chapter Two

Let us consider two tri-geometry bikes, made by competing manufacturers. Both come in 700c in 58cm. Both bikes have 76-degree seat angles. Each of these bikes has a top tube measurement of between 58cm and 58.5cm. Carbon copies, right?

And this is not a hypothetical construct, these are two bikes that until recently were offered for sale in these geometries. Since they both measured 58cm, with almost identical top tubes and seat angles, only a closer inspection would've uncovered how incredibly differently these bikes fit. But, if you knew the stack and reach of these two bike frames you'd be able to tell precisely what the differences were and be prepared for them.

Simply as a function of it, the seat angle makes absolutely no difference in any of this. Let's take Trek's Team Time Trial bike as an example. Its virtue is in its very low front end. It's head tube measures 11.3cm in its large size. But it's got a very slack seat angle of 73.7 degrees. The nose of my saddle needs to be straight over the bottom bracket on my tri bike. At 73.7 degrees, and at 80cm of seat height, my saddle is going to be roughly 7cm behind the BB. Obviously, I can't ride this bike in triathlons. Or can I?

If I find the right set-back seat post, and flip it around forward, and move the saddle forward on its rails, perhaps I can find this 7cm for forward saddle movement. All I really care about is, once it's there, is the bike going to fit me from the BB forward? It's certainly low enough for me, but is it long enough?

In fact, it won't work for me very easily. While it's low enough -- it's got a small enough stack -- it hasn't got enough reach. You might find this funny, since it's got a 57.7cm top tube length and my own tri bike only has 56.0cm of top tube. But in fact the reach of this bike is quite small, because its top tube starts so far behind the BB. If you normalized the seat angle at 78 degrees -- that is, if you draw an imaginary line through the BB at a 78 degree angle, and measured the to tube length of the Trek at the point it intersected with this imaginary line -- that 57.7cm top tube might only be 52cm or 53cm. It was this problem that caused Trek to revamp the design, and come out with the much better designed Equinox TTX.