What size frame should I get?
 

I'm ordering a new frame on line to build up. I'm debating between a 53 and 55 size frame. I'm 5'10 and I have a bike that is a 54 that fits me better then any other bike I've ever had. I know the suggested theory is base off effective top tube length.

My current , great fitting bike has a 543mm eff tt. The new frame I'm looking at in 53 has 540mm level top tube/535 actual. The 55 has a 557mm level top tube/550 actual.

I have another bike with a 565mm effective top tube that definitely feels too big.

I like the feel of a smaller frame. 540mm level top tube should feel pretty close to 543mm effective top tube...right?

By only looking at the effective top tube of the bicycles, you are looking at only one dimension that approximates the reach of the bicycle. Bicycles that have the same reach can have different heights. The height of the bicycle's frame affects the height of the handlebars; consequently, you can end up with a bike that has a decent reach but an unsatisfactory drop to the bars. The following article explains the stack and reach of bicycle frames http://www.slowtwitch.com/Bike_Fit/C...er_One_95.html and the following article explains how to measure your bicycle's stack and reach http://velonews.competitor.com/2012/...tically_216035

If it were me, I would measure the bicycle whose fit I like the best and then compare the stack and reach measurements with those listed in the geometry chart of the bicycles that I am interested in. You would select size on the basis of which bicycle required the least amount of adjustment compared to the one that you currently like. Be aware that many bicycles have their stack and reach listed in their respective geometry charts however some companies such as Giant have yet to do so. The stack and reach measurements provide the easiest way to compare different bicycles and different sizes.

how high a bar can you stand over?

Without knowing how and why your 54 fits and is beloved, it's going to be a crap shoot guessing which of the new frame sizes is going to work best. ETT, stack, reach, etc., are only useful insofar as you know how they relate to meeting your needs. While we can use the numbers to put ourselves in a position that works on one bike, it's not true that the same position will let us get the most out of another bike; we must understand the overall design and handling traits, and rider objectives.

In the end, it's not numbers that make a frame fit, it's design and use. To illustrate this point, consider these two bikes:

http://farm8.staticflickr.com/7314/9...e491eed1_c.jpg

http://farm4.staticflickr.com/3728/9...7db17127_c.jpg

By the numbers, one or the other doesn't, or shouldn't, "fit," but they're both mine and they both are comfy and well suited to their intended uses.

Obviously, that's an extreme example, as they are very different bikes which perform very different duties. Even within the same category, though, compare the red Lemond above to this:

http://farm6.staticflickr.com/5446/9...377f724a_c.jpg

Again mine, and my favorite road bike. Again, by the numbers when compared to the Lemond, one of these shouldn't fit; 585ETT vs. 570ETT, for example.

So what's going on? Fit is mutable, and it takes a concert of numbers and insights to bring bike and rider into harmony.

More info about the bikes in question, and what the OP's goals are with the new frame, will help us address the question of which size to go with, or at least help bring to light some of the considerations germane to the decision.

If the OP is a 'gut felling' kind-of -guy, my gut says go with the 53. ;)

Sorry for the long post, but if a pic is worth a thousand words, this was actually the short version!