I am right between sizes on the LHT and I would like to get advice on what the best compromise is. I am 5'7" and am long in the torso. I can barely stand over the 50cm size frame - I can only lift the front wheel about a cm without jamming the top tube into my junk. When I'm on the bike, it is otherwise very comfortable. On the 46cm frame, I can comfortably stand over the bike, and lift the front wheel an inch or more. When I'm on the bike, it feels shorter, but that may be a matter of setup.

I will be using the bike for commuting (12 mi round trip, 3-4 days a week) and weekend rides / light touring. I generally like a more upright position.

What are the main trade offs in going to a larger size? There is virtually no difference in wheelbases between the two sizes.

Which do you plan on doing the most, standing over the bike or riding it? I would recommend getting the bike that is most comfortable while riding and don't worry about how it is when stopped. I too have SLLTS and if I worried about stand over height, there wouldn't be any bikes out there that I could ride, except for some small kids bikes.

OK, let me ask a more specific question - If I can make the smaller size "feel" more comfortable with longer stem and setback seat, what are the other trade offs?

There will be inevitable tweaks over the first 1,000 miles, but I don't want to create problems I can't solve by buying the wrong frame size. Just because something feels right in the store doesn't mean that it will be comfortable after a full day of riding. I have a bike in the garage that I am replacing with this bike that will remind me of this lesson.

I want a more educated answer than "buy what feels right".

size for effective toptube, you essentially never straddle the bike flatfooted on both sides with it bolt upright.

Thanks for the tip - what's the rule of thumb?

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.)

It is true that a ride around the store will not generally get a good fit. Riding for a couple of hours would be better. See if the store will allow for a long test ride. Unfortunately, without seeing you, I could not really guess which would be best.

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. https://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.

Yeah, that's my dilemma. I'm trying to get a better idea of what is the most critical criteria for LTSL.

I do apologize if I sounded critical of your post; Your experience is exactly what I'm looking for - "with x body type, you are better off buying based on y dimension"

aggiegrads asked "What's the rule of thumb" -- somewhat moot because you are a self-professed mutant and therefore you need a specific answer.

I recommend trying any of several online fit calculators to get an informed baseline that is semi-custom to you. competitvecyclist.com, et al. Yes, all the little details matter, blah blah. But within that, I strongly favor the top tube as the cardinal measurement. It has served me well in my situation, long legs, short upper body -- opposite problem you have where I often wind up feeling too stretched out unless I get a relatively short TT.

My hunch is that you'll be better served by the smaller frame, but ONLY IF you can get by with a 130-135 (are 140s out there, stable on that small a frame?) or so stem in place of the 110 or so it probably comes with. But that's just a hunch, no one can tell anything definitive from the vagaries posted to date. Except this: do NOT compensate for upper body length with seat setback. Seat setback positions your legs/butt/etc. (basically knee) relative to the crank, or your pedalling efficiency (+/- comfort) suffers. You adjust for your upper body with stem length within reason, and top tube length for the macro.