i finally got a friend to buy an entry level road bike. Now the question. He is 5'6" and with his inseam the calculation say he should start at 53cm for a frame size. He was sold a 51cm, without, mind you, a fitting of anykind. Using the standard formulas, we can get his seat height where it should be. The next size offered on the bike is a 54cm. Which would give him about 5 more inchs of top tube. Sitting on the 51cm now he is sitting at about a 45 degree aggle. Though I think he should be in a 54 wthe additional top tube length of a 54cm and with the length of the handle bar stem he might be sitting much more horizontal than a new rider might like.
Five inches of top tube is a long distance. Are you sure you didn't mean centimeters?
There are many factors that go into a good cycle fit, too many and too complicated without seeing the rider on the bike or conversing directly with the rider regarding what he likes or dislikes about his choice. He sounds happier with his purchase than you do, 51 cm is not too far off, and he'll find a way to make it fit if he gets serious about it.
I think either or would work. The smaller frame might suit him better anyway. but the purest in our group still say the 54cm would be a better choice even with the marginal difference in size......
If it's between those sizes he may be better of with the smaller. An almost 55.5cm top tube is on the long side for someone 5'6". If he's long armed or long in the torso maybe it is the right size. A frame on the small side can often be remedied by flipping the stem or adding headset spacers, and /or buying a longer stem. A larger frame with a shorter stem and/or removing spacers. I'm 5'9" with relatively normal proportions and wouldn't go above a 54cm (and certainly nothing longer than a 55cm effective top tube). But people are different.
FWIW. The 51cm frame for your friend sounds right to me. I'm 5'9" and my preferred Road frame size is 52x54.5 (cm, c-c) with a 120mm stem. I have other frames of varying sizes, but the combined length of the top tube + stem for all of them is in the 66.0cm to 66.5cm range.
