Priority when choosing a new frame?

I guess the taller stack is accommodating the Isospeed?

Anyway, do you know your fit numbers? I know they're not the be all and end all, but I had my custom frame built around being able to run the frame with no spacers under a 100mm stem.

I would look at what stem and spacer setup each results in and see if you're happy. There's a good website I found which calculates all this for you: http://www.bikegeo.net/

Classic example of why reach and stack are next to useless on their own.

54 cm and 56 cm treks have almost exactly the same reach (as does the Giant) - you aren't going to notice 3 mm - there is more variation between different brands of bar tape. So if you go by stack and reach, the only difference should be that you are more upright on the 56 cm, right? You can fix that with fewer spacers, you'd think.

But reach doesn't take into account what's going on behind the vertical line with the BB. With the Treks, the 56 cm has slacker frame angles, so your bum will end up quite a bit further back, and the distance from the top of the seat post to the bars will be somewhere around 1.5-2 cm range longer, which is quite a bit.

So, assuming your two bikes fit nicely, I'm going to say the 56 is too big for you. You will be forever trying to push the saddle further forward and lower the bars - by about 1-2 cm each.

And if you think you are between sizes, the smaller one is the size to go for.

The seat mast thing might be a red herring, btw - the previous owner may have cut it to size.

Years ago I got a couple of second hand bikes. I needed a traditional 56 (normally comes up as a 55 on more modern charts). Convinced myself a 58 would do. Felt fine in the shop and round the car park I thought. Different matter 30 miles into a hilly ride. I used that bike all through uni with an 8 cm stem and the saddle rammed forward, because I needed a 56. So, I'd question the wisdom of being like me!!

DeVlaeminck said:

In an ideal world wouldn't you set saddle fore /aft with reference to the bottom bracket so a slacker seat tube angle shouldn't put you any further back?

However adding spacers to the 54 would shorten the reach for an equivalent stack to the 56.

Very marginally the reach would be shorter I agree. But do the trig for a 3mm spacer.

In practice you end up adjusting saddle to the bars, that's my point.

Re to the part of the bike behind the BB was merely to point out that reach does not take it into account at all. Which is fine unless you are one of those cyclists who sit down.

And the line between the BB and the seat is a hell of a lot longer than a couple of spacers. So there's a bit fat unknown in terms of fit if you only look at stack and reach.

Personally, I'm not very sensitive to differences in reach, as between two bike that fit. Perhaps some people are, I don't know.

Stack seems to make a bit more sense for comparisons though, I'll admit. And that 56 looks a lot more upright to me.