Road Cycling - Fit - nobody ever talks about Top Tube

As I am reading about choosing the right size of the bike, no one ever mentions the length of the Top Tube, which I think is far more important, as it is fairly easy to adjust the height with the seat post (sloping technology clearly indicates that (Giant bikes)), but a bike too long or too short is far more of a problem?

So how to choose the right length of the top tube???

Thanks,
Blaz

The length of the top tube will change the frame length, which in turn make the bike ride differently. The longer the top tube, the more relaxed a ride you will get. A shorter top tube will make it a "quicker" bike. The length is important, but mostly for the ride characteristics. A shorter handlebar stem will sit you up a bit more... one key to the fit, is the distance of the seat vertically. So adjusting the seat to adjust height might be a critical mistake in your riding position.
I would recommend that you be more aware of frame height, than of frame length where fit is concerned.:)



Have an expert at a reputable bike shop fit you correctly (plumb bob, ect., ect.)... it REALLY will make you ride better.

Its pretty easy to adjust the height of the saddle so seat-tube height is not that critical. Your right that top tube length is important for correct fit. If your stem is too short (to fit with a long TT), the steering will be twitchy.
You also need to note how high the head tube is, depending on how high or low you like your bars.
More sophisticated fitting guides, such as Sheldon Brown or Peter White concur with your thoughts on TT.
TT also sets toe-clip overlap with the front wheel.

Have a look at the geometry charts eg from tek website, and compare the ratio of TT/seat tube for different sizes of bike.
The largest models have TT 100% of the seat tube. smaller models have TT 140% the length of the seat tube (ie shorter bikes are longer than bigger bikes). This is usually to provide toe clip clearance when using standard cranks, which are too big for the small rider, and nothing to do with rider fit.

Top tube length is the single most important parameter in sizing a bicycle. This is particularly true of bikes with sloping top tubes, for which standover height becomes a non-issue for the average-or-taller rider.

For a given saddle-to-handlebar distance, a longer-reach stem/shorter top tube combination will feel different to the rider.

Differences in frame geometry are the reason I advocate test-riding various bikes in one's price range, even though they all may have similar frame materials and component groupos.

What size TT to chose ??
How do you measure it ??
see

http://www.peterwhitecycles.com/fitting.htm

I think seat tube ANGLE is more important than that TT length. A 72degree seat tube angle allows you to balance correctly on the bike and get comfortable (It's a myth that shorter legs require steeper seat tube angles).

A 73 or 74 degree seat tube angle throws you forward (and off balance) onto the handlebar, causing hand, wrist and shoulder pain. Moving the seat back won't help because that makes the reach even longer.

See http://www.rivendellbicycles.com for more information about bike fit and why non-racers should use a 72 degree seat tube angle.

These days frame angle is less important, since seatposts are available in many different laybacks. If your legs are proportioned oddly (eg longer femur) then you will need to move your saddle back, and that can be achieved with a slack seatpost angle, or a seatpost with plenty of layback.
I have very short femurs and ride a very standard road bike (73 angle) with no layback in the post. I find it fits me perfectly, but it probably wouldn't fit you.
You also need to hand out over the back more if you like a low racing position, so your butt can act as a counterbalance.
If I want to compare a frame to my current one, I mark the bottom bracket position on the top tube, then measure forwad and backwards from that point.