I'm looking for my first road bike in many years. Want to do what I can to get the best fit.

What is the correct (or optimal) way to determine if a particular frame geometry is suitable for your body? I ran the Competitive Cyclist fit calculator and don't really understand the results, other than learning I need a top tube of 51-52cm. I realize there's bike "fitting" and bike "sizing", and it's the latter that I'm looking for. I don't know how to measure the ideal stack and reach or other pertinent measurements.

Thanks,
Robert

Honestly, there is a lot of voodoo to your question. Generally, sizing should work out in such a way that height will be a good predictor of somewhat "standard" frame sizes, while the Stack or headtube length allows options of sitting up or leaning over more. If you are at a height that is well within a the range for a particular size, and you get an appropriately high or low front end (Stack), then the rest of your fit should come from selecting proper bars, stem, etc.

Recommending a top tube length makes some sense, but if it comes on a bike that is too low, you haven't accomplished a good fit choice.

That said, real 51cm top tubes are rare and are very hard to find without "faking it" by the maker leaning the seat tube angle too far forward. Are you 5'3" or shorter? If not, or you don't have really short arms, I'd suspect the measurements you submitted might not be spot on.


I think what is likely to work for the most people is a size based on height, and a choice of low or tall head tube based on rider posture preference. If you're 5'10 and are moderately flexible, most everyone's 56cm frame should be a reasonable choice.

I'm 5'6" and my inseam is 30". Irrespective of this, where do I go to find my ideal stack and reach? Are there online tools? I'm in Los Angeles, and Helen's does bike sizing for free, but I'm not looking for something that's part of the sales process. Bike fitters are, I believe, are something you do AFTER you already have the bike.

How do experienced cyclists get sized correctly?

By getting experience. Experience includes bad decisions you learn from.

Seriously, there is no magic rule. Just find the bike that seems to fit you based on what you already know and the advice of others that seems plausible. Road bike fit as you perceive it, is going to change as your body gets more used to the new things you are doing with it. How hard you intend to ride your road bike is going to have a lot to do with how any advice giving will help or be adverse to your perception of the fit.

If you are willing to change the things out or have them adjusted as your body changes, then that bike that fit you when you bought it can be made to remain comfortable up to a point with some money thrown at it for what you don't know yourself.

There are plenty of online tools (just do a search), and sometimes they'll agree.

Bike fit really needs to start before you buy. It's like buying a car; how big a trailer do you think you can tow with a convertible? It may sound like a truism, but the bike you're getting fit to you needs to be a size that can be fit to you. The person selling you a bike needs to be your friend, someone you can trust to help you get the right size. If you don't have that feeling, it's time to go to another shop.

I've been searching for online tools, with no luck. Do you know one, besides Competitive Cyclist?

Bike fitters absolutely can fit you BEFORE you buy a bike, and can make specific sizing recommendations for particular models.

Everybody else just uses rules of thumb. If you look at many companies fit charts, you'll see height recommendation ranges for sizes:

https://www.trekbikes.com/us/en_US/b...meset/p/16701/

Trek says someone 5'6" should be on a 52 or 54. You have a shortish inseam for your height (if it was measured accurately), so it seems odd that you would need a 51cm top tube, which are generally only found on 49cm frames and smaller.


If you're going to get fit anyway, get fit first - the numbers should be easily transferable to a new bike of the correct size. When I worked for a fitter he provided this service all the time using a shop bicycle to get numbers and make a sizing recommendation among whatever model choices. If the patron decided to buy a brand we didn't carry, we'd look at the charts for that model and recommend a size, and they would still bring it back for a final fitting where we dialed it in.

If you just want an answer - buy a 52 (or 53 if odd sizes). And that's a "real" 52, not a Colnago 52S or other non-standard sizing systems. Look for models with moderately tall headtubes as they will make it easiest to get the bars as high or as low as you want, if you don't already know what you want.

Here's a whole thread that shows a great degree of correlation between height and frame size:
How tall are you and what frame size do you ride?

5' 9.5"/34" here. My experience was exactly the same when I first went through the bike shop sales thing. Everyone said 54-56; they just looked at your height.
My ideal is a 360mm reach; generally found in a 52.

You can look up the Stack and Reach Calculator, but its really a tool for fine tuning your fit at the front end once you know the ideal stack and reach.
I'd say the CC fit measurements give you a ball-park framework (e.g. the ETT you mentioned) from which to set the bike up to suit your proportions.
I found their Race/Eddy measurements are pretty much bang on what I ride now. The French option gave me a too long bike.

Your ideal stack really comes down to how flexible you are and how high or low at the front end you find comfortable. This can change with time on the bike.
Reach, to me, is the critical number.

So how do you find ideal reach? The bikes I'm looking at (CAAD12 and Super Six Evo) aren't available for rent in my area, and test rides aren't long enough to really evaluate the finer points of the geometry. I'm happy to pay to have stack and reach calculated by a fitter, as long as the cost is reasonable (around $100-$150 or so).

There is no such thing as ideal Reach - Reach is dependant on Stack. Your ideal reach with 1cm more Stack than what ever you're talking about would be 357mm, for instance.

The range of stem lengths is designed to make up for minor reach differences. The difference in real top tube length between a 52 and 54 is less than the difference between a 100 and 110mm stem - around 1cm or less. The potential problem with going small for frame size is that it can make the front end particularly low, but some bikes with tall head tubes can more than make up for that, and are a good solution for those with long inseams for their heights.