As others have said that may be true, but it may not as well. Unless it's a custom frame it's made to fit the most common body proportions. But there are those of us who have different body proportions who sometimes have to make do with a stock frame. I usually need a zero offset seatpost even on a correctly sized frame.

Frame size has NOTHING to do with setback, it's the seat tube angle that matters, but in this case, the small size uses a very common 74 degree STA and the medium uses a rather slack 73.

The answer to the size question depends on your height and your saddle height. If you have short legs, the the frame may be perfectly sized, but the short saddle height does not produce enough setback. The small Defy is easily tall enough for a saddle height of 73cm. That's my saddle height and the head tube is 20mm taller on the Defy, than the LOOK frame I ride.

A lot of smaller frames use STAs that are even steeper, at 74.5-75 degrees. I have to use a 25-32mm of setback even with a tall saddle height. If I had legs that were only 3cm shorter, the setback would be about 9mm less.

Where did you find these figures?

http://www.giant-bicycles.com/_uploa.../2009/DEFY.jpg

http://www.giant-bicycles.com/en-US/...fy/7307/44047/

The geoemetry of any model may change slightly from year to year.

Currently, there are far fewer sizes than the previously posted chart suggests.

This is really interesting to know. I'm not a tall guy (5'7.5), but have short legs for my size, a long torso, and short arms. How would taller people with longer legs ever get the right seat fore/aft position if that stayed relatively the same on different sized frames? I'm sure there's something simple that I'm just not understanding right.

But good to know...I always second guess myself on the small vs. medium frame. Looks like I can put a stop to that now.

So that's why! :lol:

Adjust the set back by feel. Set back and seat height are the most important fit adjustments. Start with these first. Get the seat height in the ball park and now start moving the seat set back towards the back until you can feel very little pressure on the hands and you can easily take your hands off the bars (hoods and drops) and you don't fall forward. Readjust seat height as it will change with set back changes.

Now look at your reach and if its way off, go with a different stem.

A seat tube angle only defines a certain amount of setback for each unit of saddle height. The amount is the cosine of the STA, times the saddle height. For a 73 degree angle, the cosine is .29, but for 75 degrees, it's only .26.

As riders get taller, with longer legs, the amount of setback increases. If everyone had the same leg proportions, there would be one angle that works for everyone, but that's not the case. Some people have short upper legs and some have much longer ones, for a given total leg length. Many fitting theories work on the idea that placing the knee directly over the pedal (KOP) is best, but others say to pay no attention to KOP and concentrate on the rider's weight balance over the saddle. I follow the later idea and ignore KOP.

IMO, those with short legs and along torso should need even more saddle setback that someone at the opposite extreme, like me. I'm 5'-6.5" inches or 169cm in height with an 83cm cycling inseam and 73cm saddle height. I still use a lot of setback to keep the weight off my hands. I pay no attention at all to KOP.