I'm 5'9.7 and have longer legs than my torso. My 1st bike I bought from a friend and it was a 56', my 2nd bike was a 56', and now I bought a cannondale evo super six in a '56, however reading the sizing charts it appears maybe I should've looked at a '54.

My bike feels fine to me. I don't have the time in saddle or experience to know if a bike is too big/small.

thoughts?

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...if it slams your nuts on the dismount, it's too big.

that isn't a problem. lol

You are probably fine with a 54 or a 56. Most people really have two sizes that can work. You may be on the shorter side by a hair for the 56, but that's very common.....if it feels great, chances are is a good fit. You can always get fitted professionally.

+1

I'm about an inch taller and had a CAAD9 that was 54 and fit me great. I currently ride a 54 with similar geometry. But I could fit a 56. A lot is personal preference and the fact that people who are the same height don't necessarily have the same proportions. I know people shorter than OP who prefer a 56. And most people can make two sizes fit about the same.

I read the question as not could the bike be POSSIBLY the right size, but rather how can OP tell if it is or isn't. Some folks above are advising OP that he is likely okay. But his question was how can he know for himself. To answer OP's question, check for the tell-tale signs. Do you need a stupid short stem, like 80 mm or less? Are you showing hardly any seat post even though the frame is compact? Would you like your bar tops to be lower, but already have the stem flipped down without any spacers? If the answer to any or all of these questions is YES, then perhaps the frame is too big for him. If the answers to more than one of these questions is YES, then I would be pretty sure the frame is too big.

FWIW I rode a 54 for years and it felt really good. Then I got a Masi as a birthday gift and it is a 58. After a bit of adjustment I was comfortable and efficient on it. A couple years ago I got a killer deal on a steel Guru that is a 55 and it feels "just right." Point is there is a range of sizes most of us can ride well. I have lost 1 1/2-2 inches in height thanks to age and injury and am now down to 5'9" with a 32" inseam.

****.

Besides stand over height, seat post height is usually where I look when sizing a new bike. I shoot for the post to be 3/4 of the way out of the frame, if your seat post is more than half way down then its probably too big. That being said, I ride a 60cm frame but could make a 58cm work easily in a pinch with the post all the way up and possibly a longer stem.

If you can, take it to your LBS for a proper fit & measurement.

I am exactly the same height as you.

I ride a 55 and it is perfectly fine, but leaning towards the big side. A 56 would really be pushing it to the big side...54 would be more perfecter.

I'm 5'8" and was previously on a 58, and I experienced tremendous upper back pain after rides. I moved to a 53 and am much more comfortable.

I rode 58s until I test rode my LeMond Versailles, it's a 53, what an eye opener. I bought it and have sold the 58cm bikes, including a LeMond Zurich. Now the largest frame I ride is a 56, my '87 Trek 560.
It's easier to adapt a small frame to fit than a large frame. IMHO.

Mostly a matter of the position you want. Larger frame, more relaxed and upright. More aggressive, leaning down low riding fast, smaller frame. So it's not only subjective but depends on how we want to ride, and in some cases how well we're capable of riding that way.

My opinion only. You'd get a completely different answer from fitters and in the bike fit subforum, but they're wrong

If it feels fine ignore the charts. They are for baseline only. If you start to experience a new pain or discomfort then there are many adjustments that will correct the fit on most bikes one size above and below the ideal size. Nuts or no that top tube is uncomfortable if you inadvertently make contact.

I'm guessing that he is a she, or had testicular cancer.

I have to agree with this advice. You usually know when a bike is too big or too small. I used to like my mtb on the small end when I was in my 20's. That same bike is way too small for me now though. I have stayed in good shape but for some reason the small bike no longer appeals to me. My road bikes, I have always been more comfortable on the larger side of the scale. I am 6'02 and ride a 59 Sloping. I used to ride a 61cm. A regular 59cm has always seemed a little small.
Either way, charts are not perfect. They are just guides. They are usually pretty accurate within a range. A good fitting will dial in the ultimate comfort.

Check your saddle to bar drop. Your bars should be lower than your seat.

Out of shape/overweight/elderly- typically 2 inches +/-

Medium fitness/comfort- typically 3 to 4 inches

Fit cyclist- should be 5 to 8 inches. (I am overweight and slow, so my drop is currently 5 inches)

On a non compact frame, your seatpost will typically protrude 5.5 to 6 inches. On a compact, typically around 9 inches +/- (this is from clamp to seat rails).

Every bike is different, but these are basic guidelines that would indicate at least the start of a correct fit. If you are handicapped or very slow, then bars level or even higher than the seat are deemed acceptable.

...
...I have to interject that I am very hurt by someone calling me handicapped or very slow. What ever happened to charity and kindness ?

That is definitely a matter of opinion. Having no drop - top of handlebars even horizontally with the top of the seat - works very well for most people. 5-8" sounds more like "serious racer" or "serious racer wannabe" (lol) than "fit cyclist".

I'd say that if your seat is down as far as it can go in order for you to ride your bike comfortably then the bike is to big.

I think that's mostly a matter of fashion, and of course fashion knows no kindness ... the bike with high handlebars makes the rider slower, not the other way around.

I am exactly 5' 10", so similar to your height. Also, my legs are proportionally longer than my torso. My cycling inseam is exactly 34". I have had quite some experience owning Cannondale road and mountain bikes for a very long time. The Cannondale road geometry is kinda sketchy for men with our torso/leg proportions. I have owned Cannondale road frames in both 54 and 56. In my view, the 54, with a slightly longer stem (say 110mm) can work somewhat reasonably, but you have to watch your handle bar to saddle drop carefully. The 56 also can work, but with a shorter stem (90mm), and the drop differential is actually more decent. So, try both sizes and see which you prefer, but for me the 56 with 90mm stem was actually the better fit because of the lesser drop.

To be honest though, there are bikes that have different road geometries than Cannondales that can be better fit for you. The 56 fit I had with my Cannondales was still a "settled for fit." This is why I don't ride Cannondale road frames anymore, although hands down, they were some of the best road frames I owned and rode.

It is just not solely about his height. How that height is distributed between his torso and legs is key.

I'm 5'7" and I ride a 54. I can also ride a 55.

You can ride larger as long as you can safely dismount the bike. In my youth, I rode a 60 cm Peugeot Super Vitus 980 with no problems.

If you can get off and on the bike, any size can work. What you don't want is a bike that is too small!

The long top tube of a steel Lemond will have you feeling stretched out even if its your nominal size. Go down a size for a better fit.