What size road bike?

[Gixxerman]

I have a 17.5 inch Trek 7.3 FX hybrid that is a spot on fit for me.
I am wanting to buy a road bike (Trek Madone 5.X). What size should I get?
I was thinking that a 54cm one would be the correct size. But alas, there are very few of that size for sale a fleabay. Most are 56cm. Would that fit me OK? I am 5'7" tall.

 

[HLaB]

I have a 17.5 inch Trek 7.3 FX hybrid that is a spot on fit for me.
I am wanting to buy a road bike (Trek Madone 5.X). What size should I get?
I was thinking that a 54cm one would be the correct size. But alas, there are very few of that size for sale a fleabay. Most are 56cm. Would that fit me OK? I am 5'7" tall.

At first guess I'd say a 56cm is too big for you. I'm 5'9&1/4" and bike fits for me have suggested 56cm for me but due to manufacturers sizes I actually ride, a 54cm bike, an effective 54cm bike and a 55cm bike. I'd think a smaller bike feels more manoeuvrable and can be adjusted more easier to the larger rider but if a frame is too big it can not be adjusted easily.

 

[brockers]

1. Your center-to-center seat tube will equal (inseam x 0.65).
2. Your center-to-top seat tube will equal (inseam x 0.67).
3. Your bb-saddle height will equal (inseam x 0.883).

taken from here

Though I'd say you need Trek's equivalent of a 52cm, Trek do size funny as Lance uses a 58, and he's, what, 5'10''? Meaning he'd be using a 53-54 for a normal race fit (racers tend to size down compared to recreational cyclists). My guess is go for their 54, but why not pop into an Evans and try one out for size?

 

[slowmotion]

You have to sit on the bike and do a short test ride after the bike shop have done a basic set up. Height and in-seam dimensions are a starting point, but what really matters is how comfortable you feel, and you really won't find that out until you go for a test ride. Good luck.