Originally Posted by
HardyWeinberg
size for effective toptube, you essentially never straddle the bike flatfooted on both sides with it bolt upright.
Originally Posted by
Sirrus Rider
I say go with the smaller frame and fit it with a longer stem. The larger frame will have you stretched out more and have less capabilities to compensate for it. (Plus if you dismount in a hurry on a large frame you'll crunch your junk.)
Effective top tube is not always a good indicator. It is a good starting point though. The rest of the bike's geometry plays a major factor in comfort. For example, I have a couple of bikes with the same top tube length, but different seat tube angles. Some are more aggressive than others.
The aggressive ones are not comfortable for slow fully loaded touring. I will put too much weight on my hands with them. But, when I take them out for fast, hard rides, they are great because less pressure is on my hands and more on my feet. The more relaxed geometry bikes are great for long slow rides. I have the weight balanced nicely between my butt, my feet and my hands. But these bikes are not well suited for hard riding.
Peter White has an article on his site about the different things to look for in a fit.
http://www.peterwhitecycles.com/fitting.htm But there is no hard rule for what is correct. Even two people with the same height, leg length, arm length, torso length can have different needs. I have short legs and a long torso, I also have a pretty big upper body. A bike with aggressive angles will cause a lot of weight on my hands, unless I am pedaling very hard.
If I had to make a guess, the best way to find out is to ride the bike in the manner you intend, fast riding, slow speeds, etc. You want your seat tube/ saddle position to be as aggressive as possible but still be able to remove your hands from the bars without your facing falling down on the bars.
For me, The difference between the 46 and 50 LHT is the seat tube angle. The 46 has a 74.5 degree and the 50 has a 74 degree. I like a bike with much more seat tube angle than this. The differences between the top tube is .6". That is not much, so I would go with the 50cm bike and maybe a little shorter stem. But, you might be fine with the steeper seat tube, maybe you push harder than me or your upper body isn't as big and the 46 cm would be a better fit.
But don't worry about the stand over height. It really is meaningless. I have never hit the top tube in all of my riding and I can't stand flat footed over any of my bikes with any clearance. It just doesn't come into play.