Why do some road bikes have a slanting top tube & what's its advantage from a level or horizontal top tube?

Question

Why do some road bikes have a slanting top tube & what's its advantage from a level or horizontal top tube?

Answer 1

One advantage of the sloping tube is that it allows those of us with short legs and long torsos to get a better fitting bike, as the "reach" from the seat to the handlebars is the same as a bike with a straight top tube.

Answer 2

I'm not sure there is any real advantage. Like most things there is a bit of a tradeoff. The sloping tube design can be slightly lighter. Also since the tube is lower there is more standover height. The design is a compromise between mens/womens classic design and therefore allows them to be sold as unisex bikes (!). On the other hand it's not quite as stiff/strong.

Answer 3

Road bikes with a sloping top tube have a slightly more relaxed geometry.  They are not quite as twitchy in the handling department and they allow for a more upright riding position.  

The advantage is a more comfortable riding position for some cyclists.  The disadvantage is a nominal loss of aerodynamics.

The choice for non-competition riders is really one of personal preference.